Evolution of Bioclimatic Belts in Spain and the Balearic Islands (1953–2022)

Abstract

This study examines the spatio-temporal evolution of bioclimatic belts in peninsular Spain and the Balearic Islands from 1953 to 2022 using the World Bioclimatic Classification System and data from 3668 meteorological stations. Findings indicate a shift toward warmer and more arid conditions, with thermotypes showing an increase in mesomediterranean and thermomediterranean types and a decrease in mesotemperate and supratemperate types. Ombrotype analysis revealed a rise in semiarid types and a decline in humid and hyperhumid types. Significant changes occurred in climate transition zones and mountainous regions, where a process of “Mediterraneanisation”—a process characterised by the expansion of warmer and drier conditions typical of Mediterranean climates into previously temperate areas and/or an altitudinal rise in thermotypes—has been observed. The spatial variability of changes in ombrotypes was greater than that in thermotypes, with regions showing opposite trends to the general one. These results highlight the need for adaptive conservation strategies, particularly in mountainous and climate transition areas, where endemic species may face increased vulnerability due to habitat loss and fragmentation. The results of this study provide insight into how climate change is affecting bioclimatological conditions in the Iberian Peninsula and the Balearic Islands.

1. Introduction

Climate change is one of today’s greatest challenges, generating serious impacts on socioeconomic and ecological systems globally. Among other things, climate change has been associated with an increased frequency and severity of natural disturbances such as forest fires, droughts, heat waves and extreme weather events [1]. These disturbances have significant ecological and socio-economic impacts [2,3]. For example, rising temperatures and prolonged droughts have led to more frequent and intense forest fires, altering vegetation patterns, contributing to soil degradation and threatening biodiversity [4]. Similarly, changes in precipitation patterns have led to both severe droughts and torrential rains, resulting in floods and landslides [5]. These phenomena not only disrupt ecosystems, but also affect human communities [5], agriculture [3] and water resources [6].

This phenomenon is manifested, among other aspects, through the alteration of temperature and precipitation patterns [7]. This affects the distribution and survival of plant species, which depend on fixed climatic parameters for their development [8,9]. In Europe, the Iberian Peninsula and the Balearic Islands have been classified as “hotspots for climate change”, as they are areas sensitive to its impacts [10]. The complex orography and the influence of various air and water masses generate a high climatic diversity in the Iberian Peninsula, giving rise to numerous plant communities, some of which are endemic [11,12]. For their part, the Balearic Islands, due to their insular nature and variety of microclimates, also possess great taxonomic and phylogenetic biodiversity, highlighting their mountainous regions [13,14].

In the context of climate change, the study of thermotypes (temperature-based bioclimatic categories) and ombrotypes (precipitation-based bioclimatic categories)—bioclimatic units defined in the Worldwide Bioclimatic Classification System (WBCS), a system developed by Rivas-Martínez et al. [15] to classify climates globally according to their influence on vegetation using bioclimatic indices and parameters—is a very effective indirect tool for understanding and quantifying their respective impacts on ecosystems as a consequence of altered temperature and precipitation regimes, permitting the development of bioclimatic models [16]. Such models have proven useful in estimating species responses to climate change, reducing the uncertainty associated with predictive models [17]. These bioclimatic units, thermotypes and ombrotypes, allow the characterisation of what are known as bioclimatic belts, which are fundamental for understanding the altitudinal distribution of vegetation belts in a region [18,19].

Bioclimatic belts are areas where similar climatic conditions determine the presence of certain types of vegetation and ecosystems. They have a high predictive value for the plant composition of the terrain and can therefore be used in biodiversity conservation studies and programmes. They allow a quantitative assessment of climatic conditions associated with temperature and precipitation values, providing a basis for the study of the impacts of climate change on biota and spatial distribution, allowing the characterisation and modelling of local ecosystems through time, and facilitating the study and monitoring of climatic variations associated with climate change [18].

The predictive power of bioclimatic belts derives from the combination of a specific thermotype and an ombrotype, which in turn are defined in terms of a series of bioclimatic indices. Bioclimatic indices are tools that combine climatic variables, such as temperature and precipitation, to characterise in detail the climate of a region and predict how it affects ecosystems. Examples of bioclimatic indices are the Annual Ombrothermal Index (Io), which measures water availability to plants through the relationship between annual precipitation and annual temperature, and the Compensated Thermicity Index (Itc), which measures the adaptability or affinity of plants to heat by combining the values of the mean annual temperature with the maximum and minimum temperatures of the coldest month, adjusted for continentality [15].

Due to the intrinsic relationship of these indices with precipitation and temperature respectively, ombrotypes and thermotypes are indicators of climatic variations, thus acting as predictors of climate change impacts on ecosystems. The changes observed in the temperature and precipitation patterns of Mediterranean regions over the last decades, both at a general [20,21] and local level [22,23,24,25,26], suggest potential changes in their bioclimatic conditions. This may affect their floristic biodiversity, especially in the case of endemic species that have little adaptive capacity and/or are already at their respective distribution limits [26]. This situation is of particular concern in mountainous regions, where plant species are unable to move to higher altitudes, thus threatening with local extinction [27]. For motile species (altitudinal and/or latitudinal), these changes cause these species to seek more favourable climatic conditions, affecting the local composition of the ecosystem and fragmenting their habitat [28].

In the mountain systems of the Iberian Peninsula, several endemic species show a special vulnerability to changes in bioclimatic conditions. In the Pyrenees and Cantabrian Mountains, species such as Aster pyrenaeus show a high sensitivity due to their specific microclimatic requirements, including precise humidity regimes and specific needs for accumulation of chilling hours [29]. Similar sensitivity is shown by the species Borderea chouardii and Borderea pyrenaica—adapted to rock walls and screes, respectively—whose survival depends on a specific water balance maintained by the gradual melting of snow. These conditions are at risk due to increasing temperatures and reduced snow accumulation, which affect both the water supply and the stability of their microhabitats [30].

In montane forest ecosystems, species such as Fagus sylvatica are particularly vulne-rable in transition zones between temperate and Mediterranean macrobioclimates. Their sensitivity is directly related to ombrothermal indices, which determine water availability during their growing season [31,32]. In addition, other forest species such as Quercus petraea and Quercus robur face similar risks in these areas, as decreasing water availability could further restrict their already limited distribution in the Iberian Peninsula [33].

On the other hand, mountain conifers such as Pinus sylvestris, Pinus uncinata and Abies alba also show specific patterns of vulnerability: Pinus sylvestris is particularly sensitive to the lack of accumulation of winter chilling hours, which may affect its reproductive cycle and development, while Pinus uncinata and Abies alba show greater vulnerabilities to thermal increase and altered precipitation regimes [33,34]. Similarly, Juniperus thurifera and Quercus pyrenaica, which inhabit submediterranean transition zones, face important range reductions due to drought intensifications and extreme climatic variations [33].

As a consequence, bioclimatic studies need to be regularly updated in order to provide an accurate and recent picture of the implications of climate change. Considering the complex climatology and orography of the Iberian Peninsula and the Balearic Islands, the use of a dense network of climate data is also essential, as this allows for a more detailed analysis and, consequently, a more accurate estimation of the climatic changes that have occurred. These estimates are particularly relevant for the Balearic Islands, for which there are fewer studies on bioclimatic conditions.

Over the last decade, there have been multiple studies applying the WBCS in the Iberian Peninsula region. Some of these studies have addressed the detailed analysis of limited geographical areas, as for example is the case of Miró et al. [24] or Del Río et al. [35], while others have performed a general study of the region at a large scale, analysing several bioclimatic units simultaneously, like López-Fernández et al. [36], López et al. [37] or Andrade and Contente [38]. Several of these studies already identified changes in the extent of various bioclimatic units in the Iberian Peninsula both in thermotypes and ombrotypes [24,36,37,38].

Although bioclimatic changes due to climate change have been widely documented in the Iberian Peninsula, similar patterns have been observed on a global scale. Recent studies demonstrate widespread changes in the composition of bioclimatic belts as a consequence of climate change in regions such as Central Asia [39], California [16], the Himalayan region [40], Mexico [41], and southern Iran [42]. Broadly speaking, these studies indicate a consistent pattern characterised by increasing thermicity and aridity. Projections suggest a significant loss of bioclimatic diversity, with the potential disappearance of up to 50% of current isobioclimates—synthetic bioclimatic units integrating macrobioclimate, bioclimate, bioclimatic variant, thermotype and ombrotype—in some regions as well as altitudinal shifts in thermotypes and ombrotypes. These changes are manifested mainly through the expansion of warmer and drier conditions, the reduction of temperate and humid areas, and an overall increase in climatic contrasts between coastal and inland regions [16,39,40,41,42].

However, so far, no research has been carried out with such an extensive and up-to-date database as the one presented in this study, which allows more precise information on climatic conditions to be obtained, as well as to capture in detail the temporal and spatial variations produced on the respective bioclimatic units.

Against this background, the following study aims to analyse the evolution of the bioclimatic belts of peninsular Spain and the Balearic Islands over the last decades. To this end, the changes produced in the spatial distribution of the corresponding thermotypes and ombrotypes throughout the defined study period (1953–2022) have been quantified, and the implications of these changes at the ecological level have been evaluated. In addition, the study addresses possible environmental conservation measures to mitigate the impacts of these bioclimatic alterations, highlighting the need for adaptive strategies in the management and protection of the affected ecosystems, as well as proposing future lines of research in relation to the results obtained.

2. Materials and Methods

2.1. Study Area

The area analysed comprises peninsular Spain and the Balearic archipelago (see Figure 1), covering a total of approximately 498,884 km². This area is situated between 36° and 43.5° N, and between 9.5° W longitude and 3° E, and is characterised by a remarkable geographic and climatic diversity. This variability in terrain and atmospheric conditions gives rise to a wide range of ecosystems, from the humid forests of the north to the arid and semi-desert regions of the southeast, also featuring Mediterranean ecosystems, alpine environments and island ecosystems. The complex orography, marked by the mountain systems of which it is composed, contributes significantly to the important climatic diversity of the region.

2.2. Database: Processing and Analysis

The study is based on an extensive climate database covering a period of seven decades, from 1953 to 2022. To facilitate the comparative analysis of the evolution of thermotypes and ombrotypes, this interval is divided into two periods of 35 years each: the first period spans from 1953 to 1987, known as Period 1; the second period goes from 1988 to 2022 and is known as Period 2. A total of 3668 weather stations provided the meteorological dataset used in this study, containing daily measurements of precipitation (mm) and maximum and minimum temperatures (°C). This network of stations provided an average measurement coverage of one station per 136 km². Most of these stations (3616) belong to AEMET (Agencia Estatal de Meteorología), while the remaining 52 stations belong to AVAMET (Asociación Valenciana de Meteorología).

Gaps in the data were addressed by applying advanced statistical methodologies, namely NLPCA-EOF-QM (nonlinear principal component analysis—empirical orthogonal functions—quantile Mapping) for rainfall, and VBPCA-QM (variational Bayesian PCA—quantile mapping) for temperatures [43,44,45]. The choice of these methods for data processing is based on their proven effectiveness in previous studies conducted in the Iberian Mediterranean context [25,46,47,48].

These methods have shown remarkable accuracy in cross-validation, with correlation coefficients above 0.9, and mean absolute errors (MAE) on average below 3 mm for daily rainfall, and bias (reflected in monthly and annual totals) that is almost negligible for rainfall [43]. For temperatures, the correlation was close to 1 (>0.96) with an error (MAE) for estimated data ≤ 1 °C and a root mean square error (RMSE) ≤ 1.4 °C. The error and bias obtained for monthly and annual means were negligible (always < 0.1 °C) [43].

To ensure data reliability and chronological stability, a comprehensive validation and standardisation protocol was carried out. The analysis included only stations containing a minimum of 7 years with complete observation records. This threshold represented an optimal balance between statistical reliability and spatial coverage, as demonstrated through extensive validation analyses within the region [43]. The validation process revealed that stations with ≥7 years of data maintain high correlations with cross-validation data (>0.9), while not showing a significant greater improvement in error metrics (RMSE and MAE) when increasing this threshold [43]. This was thanks to the ability of these methods for establishing the necessary relationships between a variable (target observatory series) and the variability components of the whole information set, even with ≥5 years of record of the target series.

Furthermore, reconstructed series from stations meeting this criterion have demonstrated the preservation of essential statistical properties, including cumulative distribution functions, wet/dry day ratios, and both monthly and annual variability patterns [43]. This criterion was particularly crucial for mountainous areas with lower station density, where maintaining adequate spatial coverage was essential for capturing local climatic patterns while ensuring statistical robustness. The validation of the method was carried out through a cross-validation process where 5% of the observed data, equally distributed among all series, was removed for subsequent estimation and comparison with the real values. Finally, the ACMANT method, developed by Domonkos [49,50], was applied to homogenise the completed climate data series obtained.

The result dataset was generated through these processes. These files contained daily climatological records along with station metadata, encompassing location coordinates, elevation data, and ID codes.

2.3. Methodology

We used the bioclimatic classification known as Worldwide Bioclimatic Classification System (WBCS), developed by Rivas-Martínez et al. [9]. This classification is based on the correlation between climatic variables and plant community distribution patterns, quantified through hierarchically classified bioclimatic units. Over the last decade, multiple researchers have applied this classification to examine variations in bioclimatic units over different time intervals, providing relevant information on their respective spatiotemporal dynamics in different geographical areas. Examples are Andrade and Contente [38], López-Fernández et al. [39] or González-Pérez et al. [51].

At the top level of this hierarchy are macrobioclimates, which are broad climatic regions characterised by similar temperature and precipitation conditions, which influence the types of ecosystems and vegetation that develop there. They represent the highest-ranking typological unit in the WBCS. Rivas-Martínez et al. [15] identify five macrobioclimates: Mediterranean, temperate, tropical, polar, and boreal.

The determination of the macrobioclimates, as well as the rest of the bioclimatic units (bioclimates, bioclimatic variants, thermotypes, ombrotypes, etc.) is based on the quantification of various climatic parameters. These parameters are obtained from monthly precipitation and temperature data (maximum and minimum). In turn, from these parameters, a series of bioclimatic indices are obtained, including those mentioned above, which allow the definition of bioclimatic units.

The bioclimatic parameters and indices used for the elaboration of this study are detailed in the works of Rivas-Martínez et al. [15], Rivas-Martínez et al. [18] and Lorente et al. [52]. From a bioclimatic perspective, as indicated by the previous studies, our study area contains two macrobioclimates: Mediterranean and Temperate. The respective ranges of values of the thermotypes and ombrotypes of the macrobioclimates present are detailed below (

Table 1. Thermotypes of the Mediterranean macrobioclimate and their respective ranges of Itc/T_p. Source: Rivas-Martínez et al. [15].

Thermotype	Itc *	Tp **
Inframediterrenean (ime)	450–580	>2400
Thermomediterrenean (tme)	350–450	2100–2400
Mesomediterrenean (mme)	220–350	1500–2100
Supramediterrenean (sme)	<220	900–1500
Oromediterrenean (ome)	-	450–900
Crioromediterreneano (cme)	-	1–450
Gelid	-	0

* Itc is the Compensated Thermicity Index. It is calculated as Itc = It ± C, where It = (T + M + m) × 10 and C is a compensation value applied when the continentality index (Ic) is higher than 18 or lower than 8. In the It formula, T is the mean annual temperature, M is the mean of the maxima of the coldest month, and m is the mean of the minima of the coldest month. The methodology is detailed in Rivas-Martínez et al. [15]. ** T_p is the positive annual temperature. It is the sum of the average temperature (in tenths of °C) of the months whose average temperature is above zero degrees Celsius. The methodology is detailed in Rivas-Martínez et al. [15].

,

Table 2. Thermotypes of the Temperate macrobioclimate and their respective ranges of Itc/T_p. Source: Rivas-Martínez et al. [15].

Thermotype	Itc	Tp
Infratemperate (ite)	>410	>2350
Thermotemperate (tte)	290–410	2000–2350
Mesotemperate (mte)	190–290	1400–2000
Supratemperate (ste)	<190	800–1400
Orotemperate (ote)	-	380–800
Criorotemperate (cte)	-	1–380
Gelid	-	0

and

Table 3. Ombrotypes and their respective Io ranges. The ombrotypes are common to all macrobioclimates. Source: Rivas-Martínez et al. [15].

Ombrotypes	Io *
Ultrahyperarid (uha)	0.0–0.2
Hyperarid (ha)	0.2–0.4
Arid (ar)	0.4–1.0
Semiarid (sa)	1.0–2.0
Dry (se)	2.0–3.6
Subhumid (su)	3.6–6.0
Humid (hu)	6.0–12.0
Hyperhumid (hh)	12.0–24.0
Ultrahyperhumid (uhu)	>24.0

* Io is the Annual Ombrothermic Index. It equals the quotient of the positive annual precipitation value (P_p) and the positive annual temperature (T_p), multiplied by 10. The methodology is detailed in Rivas-Martínez et al. [15].

).

3. Results

3.1. Thermotypes

The following thermotypes have been identified in the stations of the study area: mme, tme, mte, ste, sme, tte, ote, ime, ite, and ome. The ite and ome thermotypes were only identified in Period 2, with both showing small percentages of occupation. The spatial distribution of thermotypes in Periods 1 and 2, as well as the changes in their distribution, can be seen in Figure 2.

During Period 1, the spatial distribution of thermotypes shows a clearly differentiated pattern. The thermotype mme dominates a large part of the territory, occupying most of the central peninsular region, including the Ebro valley. The thermotype tme is mainly located in the coastal areas of the south and southeast of the Iberian Peninsula, as well as in the Balearic Islands, while the thermotype sme is concentrated in the mountainous areas of the interior, especially in the Iberian and Central systems.

During this period, the thermotype ime has a very limited presence, appearing only in some very localised points on the southern coast of the Iberian Peninsula. The temperate thermotypes show a more restricted distribution linked to latitude and altitude: the thermotype mte predominates in the northern fringe of the Iberian Peninsula, the thermotype ste is found in the highest areas of the Cantabrian Mountains and the Pyrenees, while the thermotype ote appears in a very localised manner in the highest elevations of these mountain systems. The thermotype tte is scattered along the north coast.

During Period 2, a different configuration is observed. Broadly speaking, the thermotype tme shows a notable expansion towards the interior of the peninsula, occupying areas that previously corresponded to the thermotype mme, especially in the south and southeast, as well as in the Balearic Islands. The thermotype ime also increased its range, appearing in new locations on the southeast coast.

In the northern half, there is a clear regression of the coldest temperate thermotypes: thermotype ote significantly reduces its presence in the highest areas, while the thermotypes ste and mte give way to tte in coastal and mid-mountain areas. The occasional appearance of the thermotype ite is also observed on the north coast, and of the thermotype ome in high mountain areas. The thermotype sme shrinks in inland mountain systems and is replaced mainly by the thermotype mme, which—despite giving way to the thermotype tme in warmer areas—maintains its dominance in the central peninsular region.

Changes in thermotypes of different macrobioclimates are concentrated in the climatic transition zones, mainly in the northern fringe and in the mountainous areas. Specifically, thermotype changes are observed in the Cantabrian Mountains and their foothills, in the Pyrenees (especially on their southern slopes and in the pre-Pyrenean valleys), in the Iberian System (in its eastern and southern sectors), and in the Central System (particularly on its northern slopes). There is a general trend to change from temperate to Mediterranean thermotypes, indicating a trend towards warmer conditions.

These changes are shown in Figure 3, which shows transitions between thermotypes throughout the study period. The main regions where changes between Mediterranean thermotypes are observed are in the southern and eastern half of the Iberian Peninsula, as well as in the Balearic Islands.

In the southern region, there is an expansion of the thermotype tme towards the interior, replacing areas previously dominated by the thermotype mme. This change is also observed in the Baetic System.

In the southeast of the Iberian Peninsula, we observe the occasional appearance of the thermotype ime in coastal areas previously classified as tme, which represents the change towards the warmest thermotype within the Mediterranean macrobioclimate.

In the centre of the peninsula, a transition from the thermotype sme to mme is detected. Changes from the thermotype sme to mme are also observed in mid-mountain areas in the interior. In the Balearic Islands, an expansion of the thermotype tme to the detriment of mme is observed, especially in inland and high-altitude areas.

The changes within temperate thermotypes are mainly concentrated in the northern fringe of the Iberian Peninsula, although some mountainous areas of the Central and Iberian systems also stand out. In the Cantabrian Mountains, a transition from the thermotype ste to mte can be observed in the mid-mountain areas, while in the valleys and lower altitude areas a change from the thermotype mte to tte can be observed. In the coastal regions, a change from the thermotype tte to ite is observed in some stations.

In the northeastern region, in the western and central sectors of the Pyrenees, as well as in some regions of the Iberian System, an altitudinal ascent of thermotypes is detected, with areas previously classified as the thermotype ste becoming mte. In this sector, an expansion of the thermotype tte is also observed in lower altitude areas, which were previously classified as mte. In some localised high mountain areas, mainly in the Pyrenees and some regions of the Central System, a transition from the thermotype ote to ste is observed.

A total of 840 stations have experienced changes in their thermotype, accounting for 22.9% of the total.

Table 4. Changes in thermotypes identified throughout the study period in terms of number of stations, as well as the transitions produced. Meanings of abbreviations: inframediterranean (ime), thermomediterranean (tme), mesomediterranean (mme), supramediterranean (sme), oromediterranean (ome), infratemperate (ite), thermotemperate (tte), mesotemperate (mte), supratemperate (ste), and orotemperate (ote).

Initial Thermotype	Stations (1953–1987)	Stations (1988–2022)	Percentage of Change Between Both Periods	Transition	Number of Stations in Which This Transition Has Taken Place	Percentage with Respect to the Total Number of Stations in the Study Area
ime	18	46	155.56%	-	-	-
tme	653	847	29.71%	From tme to ime From tme to mme	28 5	0.76 0.14
mme	1315	1407	7.00%	From mme to tme	199	5.43
sme	304	243	−20.07%	From sme to mme From sme to mte	118 1	3.22 0.03
ome	0	1	-	-	-	-
ite	0	1	-	-	-	-
tte	256	292	14.06%	From tte to tme From tte to mme From tte to ite	28 24 1	0.76 0.65 0.03
mte	617	498	−19.29%	From mte to mme From mte to sme From mte to tte From mte to ste	142 9 89 1	3.87 0.25 2.43 0.03
ste	463	314	−32.18%	From ste to mte From ste to sme From ste to mme	121 49 2	3.30 1.34 0.05
ote	42	19	−54.76%	From ote to ste From ote to ome	22 1	0.60 0.03
Total	3668	3668	-	17	840	22.90%

shows substantial changes in the distribution and evolution of thermotypes during the study period, with a clear trend towards those of warmer conditions both in Mediterranean and temperate macrobioclimates. This indicates a generalised warming process.

The most notable changes are observed in the extreme thermotypes: the thermotype ime shows the largest percentual increase (+155.56%), while the thermotype ote shows the largest percentual decrease (−54.76%).

The thermotypes tme and tte show remarkable increases (+29.71% and +14.06%, respectively), while the thermotypes ste and sme show significant decreases (−32.18% and −20.07%). On the other hand, the thermotype mme, despite already being the dominant thermotype, shows a moderate increase (+7.00%), while the thermotype mte shows a notable decrease (−19.29%).

It is also remarkable the appearance of two new thermotypes in Period 2: ome and ite, although with a very limited presence. In terms of transitions, different types of changes have been identified affecting approximately one fifth of the total number of stations analysed. The most frequent transitions reflect a warming pattern, with changes from mme to tme, from mte to mme, from ste to mte, and from sme to mme standing out.

From a statistical perspective, 69.40% of the stations that have undergone changes have experienced a shift between thermotypes belonging to the same macrobioclimate, which represents 15.92% of the total number of stations. Of this percentage, 41.67% (9.54% of the total) corresponds to changes within Mediterranean thermotypes, while the remaining 27.73% (6.38% of the total) corresponds to changes within temperate thermotypes.

It is worth noting that in 99.4% of the stations where these changes have taken place (15.82% of the total), the change has been towards warmer thermotypes. On the other hand, with regard to changes between thermotypes of different macrobioclimates, it can be observed that in 39.48% of the stations that have undergone changes (6.98% of the total) there has been a permutation from a temperate to a Mediterranean thermotype. The opposite occurred in only 0.12% of the stations that experienced changes (0.03% of the total).

As can be seen in Figure 4, in the study area as a whole, the thermotypes that have undergone the greatest percentage change are tme, ste, mte, mme and sme. Specifically, the thermotypes that have increased their percentage of occupied stations, ordered from highest to lowest, are tme, mme, tte, ime, ome and ite, while the thermotypes that have experienced the opposite trend, ordered in the same way, are ste, mte, sme and ote. The thermotype tme stands out as it has shown the greatest increase in terms of number of occupied stations, increasing by 5.29%. On the other hand, the thermotype ste has shown the greatest decrease in terms of the number of occupied stations, decreasing by 4.06%.

3.2. Ombrotypes

All the ombrotypes defined by Rivas-Martínez et al. [9] have been identified in all the stations distributed in the study area, except for the uha and ha ombrotypes; those present are the ar, sa, se, su, hu, hh, and uhu ombrotypes. Their spatial distribution in Periods 1 and 2, as well as the changes throughout the study period, can be seen in Figure 5.

During Period 1, the ombrotype se dominates a large part of the territory, occupying the central peninsular region, including the Ebro valley, as well as a large part of the Mediterranean coast. The ombrotype sa is mainly concentrated in the southeast of the peninsula, while the ombrotype ar appears in a very localized manner at some points on the southeast coast. The ombrotype su forms a transition zone between dry and humid areas, being distributed in a dispersed manner throughout the mountainous areas of the interior of the peninsula and forming an irregular strip in the north. The most humid ombrotypes show a clear northern distribution: the ombrotype hu predominates in the north, especially in the Cantabrian Mountains, while the ombrotype hh is concentrated in the highest areas of the Pyrenees and the Cantabrian Mountains. The ombrotype uhu appears very occasionally in the extreme northwest of the peninsula. Finally, in the Balearic Islands, the ombrotype se predominates, with some areas of the ombrotype su in the highest areas.

During Period 2, a different configuration is observed. Broadly speaking, the ombrotype sa shows a notable expansion both in the southeast of the peninsula and in the Ebro valley, occupying areas that previously corresponded to se. The ombrotype ar also increases its presence in the southeastern coast. In the northern peninsular region, there is a clear regression of the more humid ombrotypes: hh significantly reduces its presence in the highest areas, while the ombrotype hu gives way to su in large areas of the north coast.

The ombrotype uhu maintains a punctual presence in the extreme northwest, although it shows variations in its location. In the central region, the ombrotype se, although predominant, shows a more fragmented configuration, with a greater presence of areas of the ombrotype su in the mountainous systems of the interior. In the Balearic Islands, an expansion of the ombrotype sa is observed to the detriment of se, especially in coastal and lower altitude areas, while the ombrotype su reduces its presence in mountainous areas. Consequently, the distribution of the ombrotypes in the study area has been altered throughout the two defined study periods, with transitions towards both wetter and drier ombrotypes. The spatial distribution of these transitions can be seen in Figure 6.

The distribution of the changes in the ombrotypes reveals a complex and heterogeneous pattern, with a general trend towards drier conditions, but with multiple regional exceptions. In the northern region, particularly in the Cantabrian Mountains and the Pyrenees, there is mainly a transition from hh to hu and from hu to hu conditions. The latter transition is also observed in the northern coastal strip and the northwest of the Iberian Peninsula. However, in some areas of this region, some transitions towards more humid conditions are also observed—although much less frequently—at several stations, with transitions from su to hu, two from hu to hh, and one from hh to uhu.

In the northwest region, frequent transitions to drier ombrotypes are observed, namely from hu to su and from su to se. The region of the Ebro valley stands out, which shows a trend towards more arid conditions, concentrating points with changes from se to sa conditions.

The central areas also show a marked, although more dispersed, trend towards drier conditions, showing transitions from hh to hu, from hu to su, and from su to se in multiple points of the Central System. However, some exceptions to this trend are observed in certain areas, where there are localised changes towards wetter conditions, with transitions from se to su and from su to hu.

These changes towards more humid conditions are also observed in the eastern peninsular strip, along the Baetic System, where they predominate, together with the transition from sa to se, which is present to a lesser extent. Similarly, in the southeast of the peninsula, in the area of the Iberian System, changes towards more humid conditions can be detected, with transitions from sa to se and from se to su, which predominate in contrast to certain changes towards more arid conditions observed in the nearby coastal areas.

The southern zone, on the contrary, shows the most pronounced transformation towards more arid conditions, with a clear predominance of changes from su to se, from se to sa, and to a lesser extent from hu to su.

In the coastal areas of the southeast, transitions from sa to ar are observed, representing the most extreme changes towards more arid conditions in the whole peninsula. However, points showing a transition from ar to sa are also identified, albeit isolated. Likewise, in some mountainous areas of the Baetic System, localised changes towards more humid conditions are identified, with transitions from se to su ombrotype.

In the west of the Iberian Peninsula a smaller variety of changes are identified, although these mainly tend towards drier conditions. Lower altitude areas show transitions from su to se ombrotypes, while in mountainous areas changes from hu to su are observed. However, in some areas of the Sierra Morena, changes towards wetter conditions are observed, with several stations showing a transition from se to su and from su to hu. Finally, the Balearic Islands exhibit a trend towards aridification, with an altitudinal gradation in the transitions: from the ombrotype hu to su and from su to se in inland and elevated areas and from the ombrotype se to sa in coastal and lower-elevation areas.

A total of 1066 stations have experienced ombrotype changes, representing 34.78% of the total. Within this group of stations, 71.66% have shifted from a wetter to a drier ombrotype (that is, with a lower Io value), while the remaining 28.34% have shifted from a drier to a wetter ombrotype (that is, with a higher Io value). In total, 14 types of transitions have been identified, affecting more than one-third of the total number of stations. The most frequent transitions reflect a pattern of aridification, with changes from the ombrotype se to sa, from su to se, and from hu to su. However, transitions towards more humid ombrotypes are also observed, although less frequently, such as changes from sa to se and from se to su ombrotypes. These transitions are detailed in

Table 5. Changes in ombrotypes throughout the study period in terms of occupied stations, as well as the transitions produced. Meanings of abbreviations: arid (ar), semiarid (sa), dry (se), subhumid (su), humid (hu), hyperhumid (hh), ultrahyperhumid (uhu).

Initial Ombrotype	Stations (1953–1987)	Stations (1988–2022)	Percentage of Change Between Both Periods	Transition	Number of Stations in Which This Transition Has Taken Place	Percentage with Respect to the Total Number of Stations in the Study Area
ar	15	30	100%	From ar to sa	2	0.07
sa	290	465	60.34%	From sa to se From sa to ar	83 17	2.71 0.55
se	1733	1608	−7.21%	From se to sa From se to su From se to hu	271 174 1	8.84 5.68 0.03
su	769	832	8.19%	From su to se From su to hu From su to sa	238 38 2	7.76 1.24 0.07
hu	692	629	−9.10%	From hu to su From hu to hh	167 3	5.45 0.10
hh	166	101	−39.16%	From hh to hu From hh to uhu	68 1	2.22 0.03
uhu	3	3	0.00%	From uhu to hh	1	0.03
Total	3668	3668	-	14	1066	34.78%

and in Figure 7.

The analysis of transitions between ombrotypes reveals a complex pattern of changes in moisture conditions. These patterns evidence a general trend towards more arid conditions, although with some spatial heterogeneity in the direction of change. The most significant transition occurred from se to sa conditions, affecting 271 stations, followed by the shift from su to se conditions in 238 stations. This pattern of transitions towards drier conditions is particularly noteworthy as it represents the most substantial changes observed in terms of number of stations affected.

In the lower-middle range of the moisture gradient, there is a notable bidirectional pattern of changes. While 174 stations transitioned from se to su conditions, 238 stations showed the opposite trend, moving from su to se. Similarly, while 83 stations shifted from sa to se conditions, 271 stations experienced the reverse transition. This bidirectional pattern suggests spatial heterogeneity in the changes of moisture conditions across the study area, although with a clear predominance of transitions towards drier conditions.

At the extremes of the recorded moisture gradient, transitions were less frequent but still significant. In the drier spectrum, 17 stations shifted from sa to ar conditions, while only 2 stations showed the opposite trend. In the wetter spectrum, 68 stations transitioned from hh to hu conditions, while only 3 stations shifted from hu to hh. The latter recorded transitions were particularly rare, with only one station switching from uhu to hh and only one other station experiencing a change from se to hu conditions.

As shown in Table 5, the most notable changes are observed in the ombrotype ar, which shows the greatest percentage increase (+100%), followed by sa (+60.34%); the ombrotype hh shows the greatest percentual decrease (−39.16%). The intermediate ombrotypes also experienced changes: su showed a moderate increase (+8.19%), while hu showed a similar decrease (−9.10%). There was also a moderate decrease in the se ombrotype (−7.21%), which, however, remains the dominant ombrotype. The case of the ombrotype uhu is also noteworthy, as it experienced a major redistribution in stations without altering the total number, indicating a reconfiguration of the wettest areas of the study area.

Within the set of stations that have experienced changes in their ombrotype, 71.66% (24.92% of the total number of stations) have changed from a wetter ombrotype to a drier ombrotype, while the remaining 28.34% (9.86% of the total) have changed from a drier ombrotype to a wetter ombrotype. As a consequence, as shown in Figure 8, the ombrotypes se, hu, and hh have decreased their percentage of stations, with se standing out with a decrease of 3.41%. On the contrary, an increase is observed in the ombrotypes sa, su, and ar—especially sa, which shows an increase of 4.77%.

4. Discussion

4.1. Thermotypes

The analysis of the evolution of thermotypes throughout the study period shows significant changes in their abundance and spatial distribution, both at the general level and within their respective macrobioclimates. These changes are indicative of alterations in temperature patterns, being consistent with documented trends on changes in thermal regimes in the area [53], reflecting the effects of climate change in the region.

Specifically, there is a trend towards warmer conditions, manifested by an increase in thermotypes associated with higher temperatures (mme, tme, ime, and tte), and a decrease in those associated with lower temperatures (mte, ste, ote, and sme).

This contraction of colder thermotypes can affect species adapted to these conditions, especially those with restricted distribution and low dispersal capacity, making them vulnerable to local extinction [54]. Mountainous areas are particularly susceptible, as the loss of cold environments that can impact their biodiversity [55,56]. Likewise, the warming of traditionally cooler regions could lead to an alteration of ecological communities as a consequence of colonisation by species adapted to warmer climates, including among them invasive species [57].

There is evidence of a significant reduction of temperate conditions, characterized by more moderate temperatures, in favour of Mediterranean conditions, with warmer and drier conditions. These results support future climate projections, which indicate that this process of “Mediterraneanisation” will continue to increase as global warming continues throughout the 21st century [38].

From an ecological perspective, this phenomenon of “Mediterraneanisation” can alter the composition and structure of ecosystems, favouring species adapted to warm and dry conditions over those of cool climates [58]. Ecotones, with are transition zones between two different ecosystems where characteristics of both can be observed and which are often areas of high biodiversity, could be particularly affected, as they are highly sensitive to climate change [59,60]. Plant species adapted to specific thermotypes are likely to be forced to migrate towards cooler areas. However, migratory capacity varies between species [61], which can lead to imbalances in ecological communities and the potential breakdown of key biotic interactions [62]. For example, the expansion of warmer thermotypes, such as tme or ime, could favour the proliferation of thermophilic plant species such as Pistacia lentiscus [63].

Such ecological alterations, as well as others resulting from the expansion of warmer thermotypes, could have significant implications for the ecosystem services that ecosystems provide. Schlutow and Schröder [64] have shown that alterations in thermal ranges can compromise key ecosystem processes, highlighting the need for methodologies to link bioclimatic dynamics with ecosystem services at risk.

Another noteworthy aspect is the appearance of the ite and ome thermotypes in Period 2. Although they show a minimal occupation, their appearance suggests that the increase in temperatures is allowing the expansion of previously non-existent thermal conditions in the study area, which could have significant implications for the distribution of species and the composition of ecosystems. For example, these new thermotypes could act as catalysts for significant shifts in the geographic distribution of species.

Evidence suggests that species are responding to climate change by shifting their ranges [8] and that the emergence of these new bioclimatic conditions could accelerate this process. For example, in the case of the ite thermotype in coastal areas of the northern peninsular, we could observe the expansion of thermophilic species from more southerly latitudes, while the appearance of the ome thermotype in high mountain areas could generate new thermal refugia for some species [65], although it could also threaten others adapted to previously existing conditions [66].

A particularly relevant aspect is the possible formation of what are known as “novel ecosystems” [67,68]. These new bioclimatic environments may harbour combinations of species that have not historically coexisted, which could lead to new ecological interactions and alterations in ecosystem processes. Managing and conserving these emerging ecosystems represents a significant challenge, as traditional strategies may not be effective in these new contexts [69]. Endemic and highly specialised species are particularly vulnerable to these bioclimatic changes. As noted by Garcia et al. [70] and Pacifici et al. [71], species with restricted ranges and specific adaptations may face a high risk of local or total extinction if they are unable to adapt or migrate in response to new thermal conditions. This is of particular concern for high-mountain species, where the emergence of the ome thermotype could significantly alter the conditions necessary for their survival [56].

Furthermore, these changes in thermotypes could facilitate the spread of invasive species that find these new bioclimatic conditions a favourable environment for their establishment [72]. The potential proliferation of invasive species represents an additional challenge for local biodiversity conservation and ecosystem management [57], especially in areas where new thermotypes may favour their establishment and expansion.

From a management and conservation perspective, the emergence of these new thermotypes suggests the need for more dynamic and adaptive approaches. As noted by Stein et al. [73] and Morecroft et al. [74], traditional conservation strategies may not be sufficient to protect biodiversity under these new bioclimatic conditions. It is necessary to consider the creation of ecological corridors, the protection of climate refugia, and the implementation of adaptive management measures to respond to ongoing and future changes.

The impacts of these new thermotypes may also extend to ecosystem services. Changes in bioclimatic conditions can affect fundamental processes such as water regulation, soil fertility, and carbon sequestration [75], with potential implications for human activities such as agriculture and forestry. Disruption of these ecosystem processes could affect the resilience of natural systems to additional disturbances [76].

The evolution of the distribution of thermotypes throughout the study period reveals complex and heterogeneous patterns, reflecting the geographic and climatic diversity of the region. These changes evidence ongoing bioclimatic transformations, in line with previous studies such as those conducted by López-Fernández et al. [36] or López et al. [37]. It is observed that the alteration of thermotypes has occurred mainly in the transition zones between Mediterranean and temperate macrobioclimates, as well as in mountainous areas and the peninsular interior, in addition to the Balearic Islands.

In the climatic transition zones, mainly in the north and high elevations, there is a predominant trend to change from temperate to Mediterranean thermotypes. This “Mediterraneanisation” is particularly evident in the Cantabrian Mountains, the Pyrenees, the Iberian System, and the Central System. In these mountainous regions, an altitudinal ascent of thermotypes is also detected. This phenomenon has been previously described in the study area. For example, Miró et al. [24] observed that warmer thermotypes have expanded towards higher altitude areas and inland in the region of the Valencian Community (located in the eastern coast), reducing the extent of cooler thermotypes.

This process of altitudinal ascent of thermotypes occurs as a consequence of increasing temperatures along the different altitudinal regimes [77]. The result is a migratory shift of plant species in search of more optimal conditions [78]. This phenomenon has been documented in several mountain systems around the world, and similar patterns of upward shifts of plant communities in response to climate warming have been observed. For example, Martin et al. [79] documented significant upward shifts in plant communities in French mountain ranges, with species moving to higher elevations based on their thermal preferences. Similarly, Liu et al. [80] observed clear altitudinal migrations of maple species in the mountains of southern China, particularly in climate transition zones.

Plant species have been shown to be able to modify their altitudinal distribution in response to increasing temperature [81]. However, their respective ability to follow climate change through altitudinal migration varies significantly, with some species showing rapid responses, while others show considerable delayed effects [82].

This variation in migration capacity is particularly relevant in Mediterranean mountain systems, where complex interactions between temperature and precipitation gradients can create unique challenges for species redistribution [83]. As noted by Pecl et al. [8], these redistribution patterns can have cascading effects on ecosystem function and services, especially in mountain areas, where space for upward movement is limited [56].

The changes observed in our study area align with global patterns of species redistribution identified in the past by Neilson et al. [84], who emphasised the importance of mountain systems as early warning indicators of climate change impacts on biotic communities. This is especially evident in our results, which show the regression of cooler thermotypes at higher altitudes and their replacement by warmer ones, a pattern suggesting an active altitudinal migration of bioclimatic conditions.

It should also be noted that the altitudinal ascent of warmer thermotypes could significantly reduce the habitat available to plant species living on the edges of mountain systems, as they are unable to access higher altitudes [26,27]. This phenomenon could therefore have particularly serious consequences for high mountain species, as has already been documented in other European mountain regions [56]. Ultimately, certain plant species could experience local extinction, affecting the rest of the food chain and leading to a loss of biodiversity [85].

These consequences could be severe in the context of Mediterranean mountains due to their rich biodiversity and high level of endemism [14,86].

In the southern and eastern half of the Iberian Peninsula, as well as in the Balearic Islands, there is a clear expansion of the thermotype tme towards the interior, replacing areas previously dominated by mme. This transition is particularly noticeable in the Baetic System and in the inland areas of the Balearic Islands. Likewise, the occasional appearance of the thermotype ime in coastal areas of the southeastern peninsular implies a transition towards more extreme conditions within the Mediterranean macrobioclimate. These changes reflect a thermal intensification in the region, consistent with projections of increased temperatures [87] and aridity [88] in the Mediterranean due to climate change.

These changes could affect biodiversity and Mediterranean ecosystems by allowing the expansion of thermophytes (that is, those species that prefer or thrive best in warm climates) and xerophytes (that is, those species that are adapted to dry environments with low water availability) into areas previously dominated by mesophytes. For example, Chamaerops humilis, a species of the thermotype tme, could expand its distribution towards the peninsular interior and/or higher altitude areas. In fact, recent studies have recorded an increase in the abundance and altitudinal range of this species in the southeast [89]. This expansion could alter the structure and composition of plant communities, affecting biotic interactions and ecosystem processes.

Changes in thermotypes could also have significant non-ecosystemic impacts. In agriculture, the expansion of warmer conditions could alter the suitability of certain areas for traditional crops, requiring adjustments in agricultural practices [3,90]. In the forestry sector, these changes could increase the vulnerability of forests to disturbances such as fires and pests [91]. Finally, the tourism sector could also be affected, with possible changes in seasonal patterns and in the attraction of tourist destinations [92].

4.2. Ombrotypes

The results on ombrotypes show complex patterns of variation in moisture conditions in the study area, reflecting heterogeneity in terms of bioclimatic changes over the study period related to precipitation, in line with trends identified in previous studies on this region [93,94,95].

A predominant trend towards more arid conditions is observed in the territory, reflected in an increase in the ombrotypes ar and sa. This suggests an expansion of more arid conditions in the study region, which is consistent with the aridification projections estimated for the Mediterranean basin in the context of climate change [96,97]. In parallel, a decrease in the ombrotypes associated with increased water availability is observed, with reductions in the ombrotypes hu and hh.

These results are in agreement with those obtained by Andrade and Contente [38], who observed similar trends in the change of ombrotypes under climate change projections in the peninsular region, with an increase in drier ombrotypes and a decrease in wetter ones. Likewise, these results complement those previously obtained in relation to thermotypes since increases in temperature reduce water availability due to increased evapotranspiration [98].

From an ecological perspective, this trend could have negative impacts on ecosystems adapted to high humidity, particularly in the mountainous regions of the northern peninsular, where humid ombrotypes are more frequent. The reduction in humid conditions in these areas could lead to the contraction of habitats linked to water availability and could affect species with limited dispersal capacity or with very specific ecological requirements [99].

It should be noted that this trend towards drier conditions does not seem to be uniform throughout the territory. In fact, the results seem to indicate a redistribution of moisture conditions within the temperate macrobioclimate, with the trend towards aridification of the Mediterranean macrobioclimate being more marked and evident, which is in agreement with results obtained previously by Andrade and Corte-Real [100].

Specifically, and at a global level, transitions towards wetter ombrotypes have been observed at 28.34% of the stations that have experienced changes in their initial ombrotype. These transitions are mainly observed in certain specific areas, such as parts of the Iberian and Baetic systems. A priori, these transitions towards wetter ombrotypes derive from an increase in precipitation since temperature patterns have shown no trends towards cooler conditions (and therefore towards a lower trend to evapotranspiration), as analysed above.

Looking at the changes from a spatial perspective, the most pronounced transformation towards drier conditions has occurred in the south of the Iberian Peninsula and in the Balearic Islands. The predominant transition from the ombrotype su to se and from the ombrotype se to sa, together with the appearance of areas that have shifted to the ombrotype ar in the southeastern coastal areas, represents the most significant change towards more arid conditions observed in the study area. This trend towards aridification of the southeast peninsular has been observed in previous studies [101,102] and could have significant implications for ecosystems and agriculture in these areas that are already threatened by water scarcity [103]

In the northern region, especially in the Cantabrian Mountains and the Pyrenees, there is a generalised transition towards less humid conditions. This pattern has also been observed in some recent studies, such as Lastrada et al. [104], Bonsoms et al. [105], and Sigro et al. [106]. This trend could be related to changes in atmospheric circulation patterns that affect the arrival of frontal systems to the Iberian Peninsula [107].

It should be noted that in some high mountain areas, specially of the Pyrenees, an opposite trend is observed, with a localised increase in hyperhumid conditions and even the occasional appearance of the ombrotype uhu. This could be associated with an increase in orographic precipitation in high-altitude areas [108].

The northeastern region of the peninsula also shows a clear trend towards drier conditions. In particular, the Ebro valley emerges as one of the areas with the most pronounced changes, showing a marked trend towards drier conditions. An expansion of the ombrotype sa to the detriment of the ombrotype se is observed, suggesting an intensification of arid conditions in this region, corroborating the results obtained by de Luis et al. [109], who observed a general decrease in annual precipitation in this region from the mid-20th century to the beginning of the 21st century.

The Balearic Islands show a trend towards drier conditions, with predominant shifts from the ombrotype hu to su and from the ombrotype su to se in inland and higher-elevation areas and from the ombrotype se to sa in coastal and lower-elevation areas. This trend is consistent with projections of increasing aridity in the Mediterranean islands [110].

These changes in distribution of ombrotypes can affect ecosystems and biodiversity. The expansion of drier conditions can lead to changes in the composition of plant communities [111], favouring more drought-tolerant species [112]. For example, in areas that have shifted from the ombrotype su to se or from the ombrotype se to sa, we might expect an expansion of sclerophyte (that is, with tough, leathery leaves adapted to dry conditions) and xerophyte (that is, adapted to very arid environments) plant species to the detriment of mesophyte plant species (that is, those that require moderate levels of humidity) [113].

Changes in ombrotypes can have a negative impact on phenology, affecting processes such as flowering, fruiting and senescence. These alterations can, in turn, trigger temporal decouplings between interdependent species, such as plants and their pollinators or seed dispersers. For example, in a study conducted in Mediterranean ecosystems, Gordo and Sanz [114] found that changes in precipitation patterns were associated with alterations in the phenology of several plant species, which could have cascading consequences for the entire ecological community.

However, it is important to note that the response of ecosystems to these changes will not be uniform or immediate. Ecological inertia, the adaptive capacity of species and the existence of climatic micro-refugia may modulate the speed and magnitude of changes in vegetation. In this context, areas that have shown a trend contrary to the general trend—such as parts of the Baetic and Iberian Systems where increased humidity has been observed—could play a crucial role as refuges for species threatened by aridification in other areas [67].

4.3. Ecosystem Protection Management Implications and Study Contributions

The findings of this study have important practical implications for the management and protection of ecosystems in the Iberian Peninsula and the Balearic Islands. Evidence of changes in bioclimatic belts suggests that ecosystems are undergoing alterations that may affect their structure, function and services. The implications of these results are explored below and recommendations for environmental management are proposed.

First, the trend towards warmer and drier conditions implies that plant species and communities adapted to cooler and wetter thermotypes and ombrotypes may face environmental stress, reducing their ability to survive and reproduce. This is of particular concern in mountain areas and climate transition areas, where species are limited in their ability to migrate to more favourable altitudes or latitudes. The potential loss of biodiversity in these ecosystems highlights the need to implement specific conservation strategies adapted to the particularities of each area.

The provision of up-to-date information to policy mechanisms is essential to enable them to establish and implement adaptive conservation measures that take into account the latest scientific evidence. To this end, the possible creation of an inventory of endemic plants, especially those belonging to mountainous areas, is proposed. Such an inventory should also include the particularities of each species as well as their current conservation status. On the other hand, it is also proposed to create an updated cartography of the critical areas where climate change may affect these species to a greater extent.

Another direct implication is the need to identify and protect climatic refuges, areas that maintain more stable conditions and can serve as refuges for vulnerable species. Protected area management must consider these refuges and ensure their connectivity to facilitate species migration and dispersal. In addition, it is essential to update and adapt management plans for nature reserves and national parks to reflect the bioclimatic changes identified.

The results also suggest that invasive species adapted to warmer and drier conditions may expand their distribution, which may threaten native species and alter ecological communities. It is therefore crucial to strengthen invasive species monitoring and control programmes, especially in areas that have experienced shifts towards warmer and drier thermotypes and ombrotypes.

In the field of ecological restoration, projects need to adapt to the new bioclimatic conditions. This implies selecting species and restoration practices that are resilient to expected future conditions. The use of predictive models based on the trends identified in this study can improve the effectiveness of these projects by anticipating changes in environmental conditions.

Water resource management is another area affected by bioclimatic changes. The trend towards aridification may exacerbate water scarcity, affecting both ecosystems and human activities. It is essential to develop sustainable water management plans that take into account projections of decreasing precipitation and increasing evapotranspiration. This includes the implementation of efficient water use practices and the protection of watersheds to maintain ecological flows. Likewise, in the agriculture and forestry sector, changes in bioclimatic belts can affect the productivity and suitability of certain species and crops. Farmers and forest managers need to be informed about these trends in order to adapt their management practices, such as selecting more drought-resistant crops or implementing agroforestry systems that improve resilience to climate change.

In addition, this study contributes to the body of scientific knowledge by providing a detailed and up-to-date assessment of bioclimatic changes in the region. The results can serve as a basis for future research and for the development of public policies aimed at climate change adaptation. The integration of these findings into territorial planning and environmental management can improve the resilience of communities and ecosystems to climate challenges.

Finally, it is essential to promote environmental education and awareness within society. Understanding how climate change is altering local ecosystems can motivate individual and collective actions to reduce greenhouse gas emissions and support conservation initiatives.

4.4. Limits of the Study

Despite the important findings presented in this study, there are several limitations that should be taken into account when interpreting the results and that offer opportunities for future research.

First, the study is based on data collected at 3668 discrete-point weather stations. No spatial interpolation has been used to generalise the results to the whole study area. This methodology, although accurate at the measurement points, may limit the spatial representativeness of the results, especially in areas with lower station density, such as mountainous or remote areas. The absence of interpolation implies that there are areas between stations whose bioclimatology has not been directly assessed, which could mask important local variations.

Secondly, the study area is limited to the Spanish Iberian Peninsula and the Balearic Islands, excluding Portugal and the Canary Islands. This geographical delimitation implies that the results are not representative of the whole Iberian Peninsula or other Spanish territories of great bioclimatic interest. The exclusion of Portugal prevents a complete picture of bioclimatic changes in the Iberian Peninsula, while the omission of the Canary Islands limits the understanding of bioclimatic processes in other Spanish ecosystems with unique climatic characteristics.

Thirdly, although an extensive network of weather stations has been used, their spatial distribution is not completely homogeneous. There are areas where the density of stations is lower, which may affect the accuracy of bioclimatic estimates in these regions. This is especially relevant in mountainous areas and regions of difficult access, where bioclimatic conditions can vary significantly over short distances.

Another aspect to take into account is that the study is based on historical climate data and does not include future projections. Although a warming and aridification trend has been shown for the period analysed, it is essential to incorporate projective climate models to understand how the bioclimatic belts could evolve in the coming decades under different climate change scenarios.

On the other hand, the bioclimatic classification used, WBCS, although widely accepted, has its own limitations. The determination of thermotypes and ombrotypes is based on indices that may simplify the complexity of ecosystems and do not take into account other relevant factors, such as intra-annual variability of precipitation, extreme events, changes in land use, or ecological interactions.

Finally, although this study examines changes in thermotypes and ombrotypes in detail, it analyses these bioclimatic units separately rather than exploring their combined ecological effects. This approach, while methodologically sound for establishing baseline patterns of change, may not fully capture the complex interactions between temperature- and precipitation-based changes in specific ecosystems. The ecological impact of simultaneous changes in both bioclimatic units could vary significantly between regions: some areas may experience synergistic effects of changes in both units, while others may show compensatory patterns. Understanding these combined impacts would require detailed ecological data and analysis at the local scale, which was beyond the scope of this study.

This limitation presents an opportunity for future research focused on specific regions or ecosystems, where the interactive effects of changes in both thermotypes and ombrotypes could be examined in detail.

4.5. Future Lines of Research

In order to deepen our understanding of the effects of climate change on bioclimatic belts and overcome the limitations outlined above, several lines of research are proposed.

First, it is essential to incorporate spatial interpolation techniques to estimate bioclimatic conditions in the areas between stations. The application of interpolation methods would provide a more continuous and detailed representation of bioclimatic belts, improving the spatial accuracy of the results and allowing finer patterns and trends to be identified. This is especially important in high mountain or high-altitude areas, where the availability of real climate data is lower.

Furthermore, the extension of the study area to include the Portuguese territory and the Canary Islands in future work would provide a more complete picture of bioclimatic changes in the Iberian Peninsula and other relevant Spanish ecosystems. This extension would facilitate the analysis of bioclimatic processes on a regional scale and contribute to a better understanding of climate dynamics across the Iberian territory.

Another crucial line of research is the integration of future climate projections using regional climate models. The incorporation of these models will make it possible to project how bioclimatic belts might change under different greenhouse gas emission scenarios, anticipating possible impacts and developing more effective adaptation strategies.

It is also essential to conduct analyses at local and microclimatic scales, carrying out more detailed studies in specific areas, especially in mountainous regions and climate transition zones. Here, microclimates can play a crucial role in preserving climatic refugia for vulnerable species.

The integration of additional variables is another area of interest. Environmental factors such as changes in land use, habitat fragmentation, the impact of human activities and extreme weather events are crucial to take into account, as they may interact with bioclimatic changes and affect species distributions.

Another particularly promising avenue for future research is to examine the combined effects of changes in thermotypes and ombrotypes on ecosystem dynamics, as mentioned above. This would involve the development of integrated analytical frameworks to understand how simultaneous changes in temperature- and precipitation-based bioclimatic units affect specific ecosystems.

Future research could focus on selected regions or species of particular interest, incorporating detailed ecological data to examine possible synergistic effects or compensatory mechanisms between these changes in bioclimatic conditions. This approach would be particularly valuable in areas experiencing significant changes in both units, as it would help to identify ecosystems at greatest risk and inform more targeted conservation strategies.

In addition, it is of great importance to develop detailed ecological studies that investigate how plant species and communities respond in practice to changes in thermotypes and ombrotypes. This includes monitoring changes in the distribution, phenology, and conservation status of key or endemic species.

Assessing the impact on ecosystem services is another relevant line of research. Analysing how alterations to bioclimatic belts may affect essential ecosystem services such as water supply, soil protection, carbon sequestration, and socio-economic activities such as agriculture and tourism is crucial to understanding the wider implications of climate change.

Finally, the application of more integrated models combining climate, ecological and socio-economic data will provide a more holistic view of climate change impacts and support decision-making in environmental management and land use planning.

5. Conclusions

The present study evidences the existence of changes in the bioclimatic conditions of peninsular Spain and the Balearic Islands during between 1953 and 2022 in terms of their respective bioclimatic belts.

Firstly, in terms of thermotypes, there is a clear trend towards warmer conditions throughout the study area, with transitions towards warmer thermotypes having occurred in 99.17% of the stations that have undergone changes. This high percentage of change in a relatively short period of time (70 years) underlines the speed and magnitude of warming in the region and is in line with global observations of rapid climate change.

In the study area, this trend has been manifested mainly through an increase in the thermotypes mme and tme, and a decrease in the thermotypes mte and ste. The most pronounced changes have been identified in climate transition zones, including the northern peninsular fringe and mountainous areas. These areas could be considered particularly sensitive to the effects of climate change.

Likewise, a process of “Mediterraneanisation” by which previously cooler and wetter regions begin to exhibit climatic characteristics typical of a Mediterranean climate, that is, warmer and drier conditions-of the territory is observed, with multiple weather stations showing a change from a Temperate thermotype to a Mediterranean one. This trend of expansion of Mediterranean thermotypes, therefore towards warmer conditions, is consistent with the climate projections estimated for the Mediterranean region and suggests the need for adaptation strategies for biodiversity conservation and natural resource management.

The emergence of ite and ome thermotypes, albeit in limited areas, indicates the emergence of new bioclimatic conditions. This could pose challenges for the adaptation of native species and potentially lead to the formation of new ecotopes, with implications for biodiversity and ecosystem functioning.

As for the ombrotypes, a predominant trend towards more arid conditions is observed in the study area, although with marked spatial variability. Of note is the significant increase in the ombrotype sa, which has almost doubled its presence, and the decrease in the ombrotypes hu and hh.

However, although the general trend is towards drier conditions, especially in the south and east of the peninsula, there are areas that show trends towards wetter conditions, particularly in mountainous areas. This spatial heterogeneity underlines the complex interaction between topography, atmospheric circulation and precipitation patterns. These patterns underline the need to develop differentiated and locally specific adaptation strategies for biodiversity conservation and sustainable water resource management in the context of climate change.

The alterations observed in this study suggest significant impacts on vegetation distribution and biodiversity. Species and communities adapted to specific conditions, especially in mountain and climate transition areas, could face increasing risks of habitat loss and altitudinal displacement. This poses important challenges for the conservation and management of natural areas, underlining the need for dynamic and adaptive conservation approaches.