Growth Responses of European Beech (Fagus sylvatica L.) and Oriental Beech (Fagus orientalis Lipsky) Along an Elevation Gradient Under Global Climate Change

Abstract

European beech (Fagus sylvatica L.) and Oriental beech (Fagus orientalis Lipsky) are ecologically and economically important tree species in European and western Asian forests, which are currently significantly affected by global climate change. To assess their response to climate variability, tree-ring data from 12 permanent research plots (PRPs) were analyzed in Central Europe and Turkey, covering an elevational gradient from 360 to 1430 m above sea level (a.s.l.). Using 360 tree cores, the relation between air temperature, precipitation, and climate-related stress on radial growth was investigated, alongside production potential and carbon sequestration. The results show that European beech is more sensitive to both air temperature and precipitation than Oriental beech. Carbon sequestration of forest stands ranged from 37 to 361 t·ha⁻¹ depending on the elevational gradient, with European beech storing, on average, 33.1% more carbon than Oriental beech stands. Radial growth was related to both air temperature and precipitation, with the strongest effects observed at mid-elevations (740–950 m a.s.l.). In European beech, both current and previous year temperatures significantly related to growth, whereas in Oriental beech, only the current year was critical. July was the most influential month for tree growth in both species. On PRPs located at lower elevations, radial growth over the past 20 years decreased by 13.1%–19.3% compared to the previous 20-year period, whereas in mountainous areas, it increased by 5.6%–9.8%. Low precipitation during the growing season was the main limiting factor for growth in lowland areas, whereas low temperatures were the primary constraint in mountainous regions, and vice versa. In recent years, the frequency of negative pointer years with extremely low radial growth has been increasing, reflecting a rising occurrence of climate extremes. These findings highlight species-specific climate sensitivities, emphasizing the importance of adaptive forest management strategies for mitigating global climate change impacts and increasing carbon sequestration.

1. Introduction

Forest ecosystems are a key component of the global carbon cycle, serving as significant reservoirs of atmospheric carbon, especially under the conditions of ongoing global climate change (GCC) [1,2]. The GCC is characterized by rising air temperatures, shifts in precipitation patterns, and increased frequency of extreme weather events [3,4]. This phenomenon not only heightens forests’ vulnerability to abiotic and biotic stressors and affects their productive capacities, including carbon sequestration, but also causes significant shifts in the distribution ranges of tree species [5,6]. Among the tree species that have been suggested to potentially adapt to changing conditions are European beech (Fagus sylvatica L.) and Oriental beech (Fagus orientalis Lipsky) [7,8]. Both species rank among the most important broadleaved trees in Europe and Southwest Asia [9], possessing significant economic [10,11,12] and ecological functions [13,14,15], and playing a pivotal role in long-term sustainable forest management [16,17,18,19].

In the context of GCC, which induces substantial changes in the structure and stability of forests [20,21,22,23], shifts in the elevational gradients of tree species distribution deserve particular attention. These gradients are sensitive indicators of the impact of climatic factors and changes on the growth and stability of forest ecosystems [24,25,26]. Research on elevational gradients is essential for understanding tree species’ long-term adaptive strategies, which are crucial for designing sustainable forms of forest management [27]. For this reason, species such as European and Oriental beech require increased attention, as they enable detailed studies of adaptive strategies in response to progressing GCC [6].

European beech predominantly occurs in regions with oceanic and sub-oceanic climates, but it also extends into areas with a more continental climate, provided that moisture availability is sufficient [28]. It is sensitive to drought and late frosts [29,30]. European beech stands form one of the most significant forest ecosystems of Central Europe, particularly at elevations between 100 and 900 m above sea level (a.s.l.) [31], where it is the most widespread and abundant species in natural forest vegetation [9,32]. It is rarely found in warm and dry conditions [33], as it is highly susceptible to prolonged drought periods [34,35,36,37]. In mountainous regions, the upper limit of its occurrence is primarily determined by the frequency and intensity of late spring frosts [38], as European beech is highly frost-sensitive during bud break [39,40]. In contrast, Oriental beech originates from warmer and drier areas, particularly regions surrounding the Black, Caspian, and Marmara Seas [41]. At the overlap of their distribution ranges, in the Balkans and northwestern Turkey, Oriental and European beeches often co-occur and hybridize, forming Fagus × taurica [42]. In these areas, Oriental beech prefers valley sites, while European beech occupies higher slopes due to its greater frost sensitivity [42,43,44]. Even in higher elevations, Oriental beech can form pure stands [15].

Regarding radial growth, European beech is better adapted to climatic fluctuations at lower and mid-elevations on nutrient-rich sites [45,46]. In lower elevations, it is more vulnerable to drought or heat during the growing season. At higher elevations, its growth is limited by low air temperatures and late frosts, which can cause significant damage during bud break [47,48]. Growth limitation due to drought is also evident in the southern range of its distribution, such as in the Mediterranean mountains [49,50,51,52,53], while in the northern range, growth is constrained by low air temperatures [40]. Under ongoing GCC, European beech in Europe is expanding northward in lower elevations and into mountainous regions in southern areas [54], with a similar trend observed for Oriental beech [55,56]. Beech stands are also related to cyclic phenomena under GCC, which subsequently impact their growth. One such recurring cyclic influence is the solar cycle, confirmed through tree-ring analyses in beech stands across Europe [57,58].

The marked rise in air temperature, coupled with reduced or uneven precipitation during the growing season in Central Europe and Southwest Asia, will increase the likelihood of more frequent summer droughts, significantly affecting vegetation in natural and managed forest ecosystems [6,59,60,61]. Many current forest management practices, particularly those relying on even-aged monocultures, inappropriate thinning methods, and low species diversity, in many parts of Europe and Asia may become unsustainable due to their reduced resilience to drought, pests, and other climate-induced stressors in the future [4,6,55,62,63,64]. For this reason, it is essential to understand the types of forest stands and their elevational optima that will be most suitable under future climatic conditions—those with the greatest adaptability—while ensuring the preservation of their key functions [6,65,66]. The study thus aimed to comprehensively evaluate (i) the production potential, including carbon sequestration, and (ii) the dynamics of radial growth, including the effects of climatic factors, of European beech in Central Europe (an elevational gradient in Czechia and Poland) and Oriental beech in Southwest Asia (an elevational gradient in Turkey) under the conditions of progressing GCC, as a basis for creating beech stands with greater adaptability and for developing sustainable forest management practices. We hypothesize that (a) with increasing elevation, both production and carbon sequestration change, and the positive relationship between precipitation and growth shifts to negative, while the impact of temperature reverses; (b) European beech is more sensitive to weather variables and climate change than Oriental beech; (c) precipitation patterns, particularly long-term droughts, have a greater impact on beech radial growth than air temperature; and (d) the frequency of years with significantly reduced growth, driven by climatic extremes, continues to increase in the context of ongoing climate change and forest productivity.

2. Material and Methods

2.1. Study Site

The study area focusing on European beech was located along an elevational gradient (510–1310 m) within the Krkonoše Mountains National Parks, spanning both the Czech Republic and Poland, specifically within the Krkonoše/Karkonosze Transboundary Biosphere Reserve (Figure 1). The average annual air temperature in this area ranges between 2.6 and 7.5 °C, with 590–1260 mm of annual precipitation depending on elevation [67]. Over 44 years (1975–2019), there has been an increase in temperature by 1.4 °C and an increase in precipitation by 30 mm [68]. According to Köppen’s climatic classification [69], the area of interest falls within three climate zones: Cfb—temperate oceanic climate, Dfb—warm-summer humid continental climate, and Dfc—boreal continental climate. The predominant soil types are Cambisols, Leptosols, and Cryptopodsols. The bedrock is highly diverse and consists of biotite granite, schist, phyllite, and gneiss. The research plots in Krkonoše were established and selected in 1980 for long-term studies. The growing season lasts from 130 days in Poland to 65 days in mountainous areas in Czechia. All plots were chosen based on the significant representation of European beech, with admixed species including Norway spruce (Picea abies [L.] Karst), sycamore maple (Acer pseudoplatanus L.), mountain pine (Pinus mugo Turra), and rowan (Sorbus aucuparia L.).

Table 1. Overview of basic stand and site characteristics of permanent research plots (age was derived from forest management documentation for 2025; the name of the plots is derived from the abbreviation of the Latin name of the tree species and the elevation).

PRP	Name of PRP	Latitude (N)	Longitude (E)	Species	Stand Age (Years)	Elevation (m)	Aspect	Slope (°)	Clim. Class.
Fagus sylvatica
1	FagSyl510	50°50′11.8′′	15°38′32.5′′	FS	193	510	NW	15	Cfb
2	FagSyl620	50°44′21.8′′	15°25′15.1′′	FS, PA	195	620	SW	22	Cfb
3	FagSyl760	50°39′57.7″	15°53′05.2″	FS, AP	162	760	NE	35	Dfb
4	FagSyl940	50°44′06.1′′	15°32′21.0′′	FS, PA	245	940	E	24	Dfb
5	FagSyl1170	50°44′42.7′′	15°32′46.9′′	FS, PA	208	1170	SW	17	Dfb
6	FagSyl1310	50°44′46.4′′	15°32′58.9′′	FS, PM, SA	146	1310	SW	21	Dfc
Fagus orientalis
7	FagOri360	40°54′43.9′′	31°12′27.6′′	FO, QP, CB	119	360	NW	23	Cfb
8	FagOri570	40°51′42.5′′	31°18′10.8′′	FO, QP,	189	570	N	30	Cfb
9	FagOri740	40°57′15.0′′	31°14′28.5′′	FO, QP	124	740	SE	5	Cfb
10	FagOri950	40°49′37.7′′	31°25′05.6′′	FO	180	950	W	21	Cfb
11	FagOri1150	40°46′52.8′′	31°28′00.3′′	FO, AN, CB	198	1150	SW	13	Dfb
12	FagOri1430	40°47′41.8′′	31°28′16.2′′	FO, AN	239	1430	NW	16	Dfb

Notes: PRP—permanent research plot, Cfb—temperate oceanic climate, Dfb—warm-summer humid continental climate, and Dfc—boreal continental climate; FS—Fagus sylvatica, PA—Picea abies, AP—Acer pseudoplatanus, PM—Pinus mugo, SA—Sorbus aucuparia, FO—Fagus orientalis, QP—Quercus petraea, CB—Carpinus betulus, AN—Abies nordmanniana.

provides basic data about the study site.

The study of Oriental beech was initially carried out across an elevational gradient (360–1430 m a.s.l.) in the forest stands of the Black Sea Region (BSR) in Turkey, specifically in Düzce Province. This province is located in the western part of the BSR, within the Euro-Siberian Floristic Region. The average annual temperature in this area is 13.4 °C, with more than 820 mm of precipitation annually [70]. Soil texture varies, ranging from clay and clay loam to loamy clay. The forest floor has a depth of 3–5 cm, while the A horizon reaches around 10 cm, and the B horizon is between 40 and 60 cm thick. The stoning of the soil is moderate, ranging from 10% to 30% by volume. The total soil profile depth is 110 cm, although rooting can extend deeper in rocky areas. The soils in this region are classified as Inceptisols, specifically Typic Haplumbrepts [71]. The measurements were conducted in mixed oak–beech forests, pure beech stands, and mixed fir–beech stands. In the dominant beech forests, the admixed tree species (comprising up to 9%) include sessile oak (Quercus petraea [Matt.] Liebl.), common hornbeam (Carpinus betulus L.), and Nordamann fir (Abies nordmanniana Spach). According to Köppen’s climate classification [69], the area falls within two climate zones: Cfb—temperate oceanic climate, and Dfb—warm-summer humid continental climate.

2.2. Data Collection

To determine the structure and production parameters of the tree layer, FieldMap technology, version 21 (IFER, CR) was used during the establishment of 12 permanent research plots (PRPs), each measuring 40 × 40 m (0.16 ha). Six plots were established for the analysis of European beech and six for Oriental beech. The selected PRPs were established as part of long-term monitoring programs, with some in the Czechia dating back to 1980 (see, e.g., [72]). The plots were chosen to encompass the widest possible elevational gradient while maintaining comparable elevation ranges and an equal number of plots on both sides of the gradient.

Using this FieldMap system, version 21 (IFER, CR), the positions of all individuals in the tree layer with a diameter at the breast height > 4 cm, along with their crown projections, were mapped in at least four perpendicular directions. Breast-height diameters, tree heights, and heights to the live crown base were measured within the tree layer. Breast-height diameters were measured to the nearest 1 mm using a Matax Blue metal caliper (Haglöf, Långsele, Sweden), and tree heights were measured to the nearest 0.1 m using a Vertex laser hypsometer (Haglöf Sweden). The accuracy or minimum DBH threshold for target trees measurement was set according to standardized forest inventory protocols to ensure consistency with previous studies and long-term monitoring frameworks (see, e.g., Vacek et al. [70]).

To analyze the relationship between climatic factors and the radial growth of beech, increment cores were extracted from each PRP using a Pressler borer (Haglöf, Långsele, Sweden) at breast height (130 cm), perpendicular to the trunk axis, both downslope and upslope. The sample collection for Oriental beech was conducted in the spring of 2015, while the sampling for European beech took place in 2021 during the most recent data inventory. At each plot, the 30 samples of beech were randomly selected (using the RNG function in Excel) from dominant and co-dominant trees, as classified by [73], due to their significant growth responses compared to sub-dominant and suppressed trees [74]. Tree-ring width was measured on all the increment cores using a LINTAB 5 (Rinntech, Heidelberg, Germany) measuring table equipped with an Olympus microscope. The measuring table has a precision of 0.01 mm, and the TSAP-Win 4.6 software (Rinntech, Heidelberg, Germany) was utilized to record the chronologies of each tree-ring width. Subsequent cross-dating of the measured tree-ring cores was carried out using Cdendro 9.6 software (Cybis Elektronik & Data AB, Saltsjöbaden, Sweden). The cross-correlation index (CCi), which measures the degree of similarity between the tree-ring samples, for the measured tree-ring sample was greater than CCi > 25 compared to the other samples, setting the threshold for inclusion of the sample in the analyses [48].

Climate behavior related to air temperature and precipitation conditions was evaluated based on data from the meteorological station in Pec pod Sněžkou (815 m a.s.l.; period 1962–2020) for Krkonoše Mts., respectively, from the station Bolu (750 m a.s.l.; period 1962–2014). The analysis of air temperature and precipitation trends was based on data for mean annual air temperature, air temperature during the growing season, monthly air temperatures, total annual precipitation, total precipitation during the growing season, and monthly precipitation totals.

2.3. Data Analyses

The basic production parameters of the tree layer of beech forest stands were evaluated by the SIBYLA Triquetra 10 software [75]. The volume of European beech was calculated according to Petráš and Pajtík [76], and, respectively, Carus [77] and Vacek et al. [70] for Oriental beech. Based on the measured stand parameters, crown closure (CC) [78] and the relative stand density index (SDI) were determined. The relative SDI expresses the proportion of the actual stand density index to its theoretical maximum. The SDI itself represents the estimated number of trees per hectare assuming a mean quadratic diameter of 25 cm [79]. The maximum SDI value was derived from the model of yield tables (for beech 1050 trees per ha [80]). The slenderness quotient, as an indicator of stand stability, was calculated as the ratio of tree height to its diameter at breast height [81]. The above-ground and below-ground biomass of beech trees was estimated using species-specific allometric models. Foliage biomass was determined using the model proposed by [76,82,83,84]. Branch biomass was derived according to the model by [76,82,83,84], while stem biomass was estimated following the approach of [76,82,83,84]. The conversion of biomass to dry mass was performed using the dry wood and bark density values provided by Speight [85]. The biomass of tree roots was calculated using a model by Drexhage and Colin [86]. The content of carbon (C) in beech trees was calculated according to Bublinec [87] using the unit content of elements in 10 mg kg⁻¹ of dry matter. The carbon sequestration varies across different tree components, including roots and stump, stem wood, stem bark, branches, and assimilatory organs.

Beech dendrochronological data were analyzed in R software 4.4 (Team R Core, Vienna, Austria) using the “dplR” package [88]. To enhance climate-growth analysis, the ring width index (RWI) was computed by standardizing the raw tree-ring measurements, allowing for the removal of non-climatic influences while preserving interannual variability in growth patterns. Each tree-ring series was detrended by fitting a negative exponential curve with a 0.67n spline, which removed age-related trends and preserved low-frequency climate signals [89]. An expressed population signal (EPS) of ≥0.85 was set as a threshold to ensure reliability in climate analyses [90]. The analysis of negative pointer years (NPYs) was accomplished by [91]. For each tree, a pointer year was identified as an extremely narrow tree ring that does not reach 40% of the increment average from the four preceding years. The occurrence of the negative year was proved if a strong reduction in increment occurred in at least 20% of trees on the plot. To quantify the statistical relationship between climate characteristics (monthly average air temperatures and sum of precipitation in particular years in the period 1962–2014 for F. orientalis, respectively, 1962–2020 for F. sylvatica) and radial growth, the DendroClim 2002 software (Tree-Ring Laboratory, University of Nevada-Reno, USA) was used. Bootstrapped confidence intervals were applied to assess the significance (p < 0.05) of the linear correlation, based on the Pearson correlation coefficient [92].

Differences in radial growth across PRPs along the elevation gradient for individual beech species were assessed using Statistica 13.4 software (TIBCO Software Inc., Palo Alto, USA). The normality of the data was first tested using the Shapiro–Wilk test. If the normality assumption was confirmed, the homogeneity of variances was subsequently tested using Bartlett’s test. In cases where the assumption of equal variances was not met, the non-parametric Kruskal–Wallis test was applied, and differences between tree species were compared using multiple comparisons of mean ranks. If the assumptions of normality and homogeneity of variances were satisfied, a one-way analysis of variance (ANOVA) was used to identify differences between variants. In cases of statistically significant differences, Tukey’s HSD test was performed for post-hoc comparisons between groups. Statistically significant differences between groups (plots) were indicated by different letters. The relationships between elevation, productivity, and growth parameters were evaluated using Pearson correlation also in this software.

Principal component analysis (PCA) was performed in the CANOCO 5 program (Microcomputer Power, Clover Lane, USA) to evaluate the relations between radial growth, climatic factors, site conditions, and stand parameters of individual research plots. Before the analysis, the data were standardized and centralized. The results of PCA were illustrated by unconstrained ordination diagrams of species and environmental variables projected.

3. Results

3.1. Production and Structure

In European beech, the stand volume (V) ranges from 362 to 822 m³·ha⁻¹, while in Oriental beech, it reaches values between 215 and 589 m³·ha⁻¹ (

Table 2. Structural and production characteristics of live trees on PRPs in 2020 for European beech and in 2015 for Oriental beech.

PRP	dbh	h	v	HDR	N	BA	V	PAI	CC	SDI	BIO	CBIO
	cm	m	m3		Trees·ha−1	m2·ha−1	m3·ha−1	m3·ha−1·y−1	%		t·ha−1	t·ha−1
						Fagus sylvatica
1	47.6	24.94	3.021	52.4	272	48.2	822	4.37	91.1	0.61	695	361
2	52.2	33.37	3.153	63.9	208	44.2	656	3.45	92.9	0.83	539	280
3	41.3	21.21	1.502	51.4	256	34.3	375	2.39	85.5	0.81	340	177
4	37.5	20.40	1.566	54.4	656	44.4	619	2.58	91.4	0.75	524	273
5	46.6	22.89	1.996	49.1	184	31.3	362	1.78	97.1	0.66	333	173
6	24.0	7.93	0.223	33.0	396	17.9	88	0.62	66.0	0.30	72	37
						Fagus orientalis
7	27.2	21.36	0.626	78.5	484	28.0	303	2.78	86.0	0.53	242	126
8	39.3	24.06	1.550	61.2	381	46.1	589	3.29	98.8	0.75	469	241
9	21.5	18.86	0.387	87.7	1080	39.2	418	3.67	94.6	0.81	318	166
10	35.5	19.31	1.302	54.4	276	27.2	359	2.11	78.7	0.46	306	160
11	29.2	19.05	0.793	65.3	284	18.9	225	1.20	79.7	0.35	173	91
12	26.9	14.04	0.596	52.1	360	20.5	215	0.93	84.3	0.40	168	87

Notes: PRP—permanent research plot, dbh—mean quadratic breast-height diameter, h—mean height, v—mean tree volume, HDR—slenderness ratio, N—number of trees per hectare, BA—basal area, V—stand volume, PAI—periodic annual increment; CC—canopy closure, SDI—relative stand density index, BIO—biomass in dry matter, C_BIO—carbon sequestration in biomass.

). With increasing elevation, a general decrease in stand volume is observed, which is evident in both European beech (PRP 1–6) and Oriental beech (PRP 7–12). The highest stand volume was recorded at PRP 1 and PRP 8, which are located in lower elevations for the respective species. A similar trend can be observed in other parameters, such as basal area (BA), which ranges from 17.9 to 48.2 m²·ha⁻¹ in European beech and from 18.9 to 46.1 m²·ha⁻¹ in Oriental beech, and especially as periodic annual increment (PAI 0.62–4.37 m³·ha⁻¹·y⁻¹ for European beech and 0.93–3.67 m³·ha⁻¹·y⁻¹ for Oriental beech). Trees at higher elevations also exhibit lower values of the slenderness ratio (HDR), indicating their adaptation to more challenging growth conditions, such as the increased occurrence of extreme meteorological events. Carbon sequestration (C_BIO) across both tree species ranges from 37 t·ha⁻¹ in lowland areas to 361 t·ha⁻¹ in mountainous regions. When comparing both species, European beech (217.0 t·ha⁻¹ carbon stored in biomass) exhibits, on average, a 33.1% higher potential for GCC mitigation compared to Oriental beech (145.2 t·ha⁻¹). On the other hand, the number of trees (N) did not correlate (r = −0.14; p = 0.67) with elevation, and a higher average number (by 166 trees/ha) was recorded for Oriental beech, which also reproduced vegetatively on the permanent research plots. The average tree height significantly (r = −0.67; p = 0.02) decreased with elevation, whereas the relationship between dbh and elevation was weaker (r = −0.33; p = 0.29).

3.2. Dynamics of Radial Growth

The maximum core age, according to dendrochronological analyses, was 260 years for European beech and 234 years for Oriental beech on PRPs (

Table 3. Characteristics of basic tree-ring chronologies of European beech (PRPs 1–6) and Oriental beech (PRPs 7–12) dominant trees on permanent research plots; significant differences (p < 0.05) in radial growth (RW), tested using the Kruskal–Wallis test, are indicated by different letters.

PRP	Cores (n)	Age Min–Max (Years)	RW Mean (mm)	RWI SD	EPS	Negative Pointer Years
1	29	82–124	1.993 c	0.174	0.94	2003, 2011, 2016, 2018, 2020
2	24	73–167	1.960 c	0.194	0.88	2011, 2020
3	30	128–249	1.018 b	0.221	0.92	1978, 2004, 2011, 2020
4	28	177–216	0.863 ab	0.278	0.89	1948, 1952, 1956, 1981, 1984, 1985, 1996, 2020
5	30	161–260	0.946 ab	0.286	0.93	1952, 1953, 1981, 1985, 1996, 2000
6	29	52–102	0.637 a	0.251	0.91	1981, 2018, 2020
7	28	62–104	1.796 c	0.198	0.92	1994, 2014
8	25	109–188	1.625 c	0.279	0.85	1993, 2008
9	25	66–104	1.067 b	0.288	0.88	1948, 1994, 2014
10	24	93–170	1.413 bc	0.291	0.85	1955, 1977, 1993, 1994
11	26	79–140	1.034 b	0.393	0.88	1963, 1993, 1994
12	23	155–234	0.679 a	0.371	0.86	1947, 1993, 2007

Notes: PRP—permanent research plot, Cores—number of analyzed core samples, Age—minimum and maximum age of cores, RW mean—mean tree-ring width, RWI SD—standard deviation of ring width index, EPS—expressed population signal, negative pointer years—years with significantly extreme low radial growth.

). The average annual ring width in European beech (1.236 mm) across the PRPs was comparable to Oriental beech (1.269 mm). In both beech species, the variability of radial growth (expressed by standard deviation—SD) significantly (r = 0.72; p < 0.01) increased with elevation. Significant differences (Kruskal–Wallis test, p < 0.001) in ring width were found for both beech species across plots concerning the elevation gradient. The significantly (p < 0.05) highest diameter increment was found on the lowest-elevation (PRP 1 and 2) (1.993–1.960 mm), while the lowest values were recorded on PRP 6 (0.637 mm) located at the upper forest boundary for European beech. Similar differences were also found in Oriental beech. On research plots located at lower elevations (PRPs 1–3), radial growth over the past 20 years decreased by 13.1% compared to the previous 20-year period, whereas in mountainous areas (PRPs 4–6), it increased by 5.6%. A similar trend was observed in Oriental beech (lowland −19.3%, mountains +9.8%).

In terms of growth variability dynamics between 1940 and 2014/2020, Oriental beech exhibited greater fluctuations in the ring width index (RWI) compared to European beech (Figure 2). On the other hand, European beech showed a higher number of narrow pointer years (NPYs), characterized by significantly reduced radial growth. When dividing the study period into two halves, a markedly higher RWI was observed in both species in the second half of the period, which can be attributed to extreme weather fluctuations driven by GCC.

For European beech, the highest number of NPYs was recorded at PRP 4 (940 m a.s.l.), while for Oriental beech, the highest number of NPYs was also found at a mid-elevation site, PRP 4 (950 m a.s.l.). In European beech, the most frequent NPY in 2020 was caused by extremely low precipitation during the growing season, particularly in July, which was historically the driest on record (27 mm, an average of 132 mm). Similarly, 2018 was characterized by low precipitation and its uneven distribution, with August receiving only 37 mm (an average of 121 mm), the lowest recorded for this month in the study period. Furthermore, 2018 recorded historically the warmest April (7.8 °C, an average of 3.5 °C) and May (12.7 °C, an average of 8.8 °C). Conversely, the NPY recorded in 1981 at the three highest-elevation PRPs coincided with the highest annual precipitation on record (1852 mm, an average of 1268 mm) and an exceptionally high snowpack.

A similar trend was observed in Oriental beech; for example, in the NPY year of 1993, when June and July received only 3.3% (0.5 mm) of the average monthly precipitation (15.1 mm), combined with an extreme heatwave event. The following year, 1994, was also exceptionally unfavorable in terms of precipitation during the growing season, with total rainfall reaching only 28.6% of the long-term average.

3.3. Effect of Climate Factors

Mean monthly air temperatures were more strongly related to radial growth than monthly precipitation for both beech species across PRPs (six more significant months for F. sylvatica and two for F. orientalis) (Figure 3). In terms of temperature sensitivity, European beech exhibited a higher number of statistically significant (p < 0.05) months compared to Oriental beech (32 vs. 23 months). For European beech, air temperatures in both the previous and the current years were equally important for radial growth, whereas for Oriental beech, growth was more strongly related to air temperatures in the current year. Specifically, July was identified as the most influential month in terms of air temperature impact on tree growth in both the previous and current years.

Figure 3 also illustrates how the effect of air temperature on radial growth changes over an elevational gradient for both species. At lower elevations, high air temperatures, particularly during the growing season, act as a limiting factor for growth. In contrast, in mountainous areas, temperature has a significantly positive effect on growth. Both tree species were most sensitive to air temperature fluctuations at mid-elevations (740–950 m a.s.l.), whereas the relationship between temperature and radial growth was weakest at the highest-elevation sites (1310–1430 m a.s.l.).

Similar to air temperature, European beech was more sensitive to the relationship of monthly precipitation with radial growth compared to Oriental beech (Figure 4). In European beech, precipitation from the previous year had a stronger association with growth; whereas, in Oriental beech, the dominant relationship came from the current year. For Oriental beech, the most important months in terms of precipitation effect were June and July of the current year, whereas for European beech, the response was less consistent.

Regarding elevation, the relationship of precipitation with radial growth followed a similar pattern to air temperature, transitioning from a positive association at lower elevations to a negative relationship in mountainous areas. This trend was more pronounced in European beech. Likewise, mid-elevation sites were the most sensitive to variations in monthly precipitation sums. However, unlike air temperature, the weakest relationship of monthly precipitation with radial growth was observed not at the highest-elevation sites but rather at the lowest-elevation plots.

3.4. Relationships Between Radial Growth, Climate, Site, and Stand Parameters

The complex relationships between radial growth, climatic factors, site conditions, and stand parameters of individual research plots can be visualized using principal component analysis (PCA). The PCA ordination diagram is shown in Figure 5. The first PCA ordination axis explains 39.7% of the data variability, the first two axes together explain 62.9%, and the first four axes account for 88.8%. The total variation is 168.0, with supplementary variables contributing 75.7% (adjusted explained variation: 46.5%). The horizontal axis represents stand volume and mean height, while the vertical axis reflects stand age and the NPY dataset. Radial growth variability increased with elevation, whereas radial growth itself, height, stand volume, and HDR decreased along the elevational gradient. The number of trees was negatively correlated with tree volume and DBH. The number of NPYs increased with the mean stand age. Among the observed variables, slope and precipitation (number of significant months) appeared to be the least influential factors in the ordination diagram. Overall, the highest number of NPYs was found in the middle-elevation plot (ALT4), high radial growth was characteristic of lowland plots (ALT1), and top-mountain plots (ALT6) exhibited the greatest variability in radial growth. When comparing tree species, higher stand density was observed for Oriental beech, whereas European beech exhibited greater tree volume and DBH.

4. Discussion

4.1. Productivity Potential and Stand Structure

The studied close-to-nature beech forests in the Krkonoše Mts. represent a rare remnant of autochthonous stands in the entire Sudeten system. The stand volume of European beech in the examined PRPs ranged from 88 m³·ha⁻¹ at the upper tree line to 822 m³·ha⁻¹ at lower elevations, which confirms our hypothesis (a). Similar values have been reported in other studies from the Krkonoše Mts. [93], the Orlické Mts [47], the Jizera Mts. [94], and Central Bohemian [95]. A broader European review by Fuchs et al. [19] cites stand volume from 64 m³·ha⁻¹ in Czechia [96] to 1237 m³·ha⁻¹ in Ukraine [97], with an average of 585 m³·ha⁻¹ [97,98].

For Oriental beech in the Black Sea region of Turkey, the stand volume ranged from 215 to 589 m³·ha⁻¹. Previous studies indicate that the total volume of uneven-aged stands in Turkey varies between 290–1482 m³·ha⁻¹ in pure stands and 172–924 m³·ha⁻¹ in mixed stands [94]. Other studies report values between 472 and 600 m³·ha⁻¹ [99] in Iran [100]. The lower growth performance of Oriental beech compared to European beech is influenced by ecological conditions [22] and genetic differences [101]. Growth is also affected by the slope aspect, with higher productivity on north-facing slopes [102].

The production and structural parameters of both beech species are significantly influenced by age, developmental stage, disturbance regime, soil conditions, browsing pressure, forest management practices, as well as elevation [22,38,101,103,104,105,106,107]. Elevation plays a crucial role in beech productivity, not as a direct ecological factor [108], but as a proxy for multiple environmental gradients [109]. The most critical factors include air temperature, precipitation, and solar radiation [110]. In high-elevation areas, snow cover can become a limiting factor [111], while in arid regions, increasing precipitation at higher elevations may enhance productivity [112]. With GCC and warming trends, the productivity of beech has been increasing even at the upper forest limit [38]. Various environmental gradients associated with elevation further influence beech forest productivity [113].

Carbon sequestration in the studied plots ranged from 37 t·ha⁻¹ in lowland areas to 361 t·ha⁻¹ in mountainous regions, similar to values observed in undisturbed or slightly thinned mature beech stands [114]. Carbon sequestration in forests plays a crucial role in mitigating GCC by capturing atmospheric CO₂, enhancing ecosystem resilience, and maintaining long-term carbon storage in biomass [6].

4.2. Radial Growth Trends and Variability

The average annual ring width of European beech ranged from 0.637 to 1.993 mm (min–max values across plots), while for Oriental beech, it varied between 0.679 and 1.796 mm (Table 3). In both species, radial growth variability generally decreased with increasing elevation. The highest radial growth in this study was observed at the lowest elevations, whereas the lowest values were recorded in the ecotone of the upper forest limit. A similar elevation-dependent pattern has been reported for European beech in the Krkonoše Mts. [38,48] and the Orlické Mts. [47].

Our findings also indicate that on PRPs at lower elevations, radial growth over the past 20 years has decreased by 13.1% compared to the previous 20-year period, while in mountainous areas, it has increased by 5.6%. A similar but even more pronounced trend was observed for Oriental beech. This pattern has also been documented for European beech in the eastern Krkonoše Mts. [48] and Oriental beech in Turkey and Iran [115,116]. Several studies predict that, in response to GCC, tree species will shift to higher elevations and more northern exposures [117,118,119,120].

Vannoppen et al. [121] reported an increasing radial growth trend for European beech in Belgium between 1927 and 2015, particularly after 1957. However, after 1983, the growth rate began to slow. More recent studies, particularly in low-elevation and southern-range regions, indicate a decline in radial growth due to GCC [122,123,124,125]. European beech is highly sensitive to climate variability [55,113].

Our results also highlight (hypothesis d) an increasing frequency of NPYs characterized by significantly low radial growth in recent years. From a forest management and adaptation perspective, it is crucial to consider the rising frequency of extreme weather events due to GCC [126,127]. These extreme conditions are affecting both European beech [107,128] and Oriental beech [106,129,130,131], emphasizing the need for climate-adaptive forest management strategies.

4.3. Impact of Climatic Factors on Growth

Mean monthly air temperatures had a significantly stronger relationship with radial growth in both beech species compared to precipitation, and hypothesis (c) was therefore rejected. This has also been confirmed for European beech [47,48] and Oriental beech [55]. However, [132] suggest that this relationship is highly dependent on local air temperature and precipitation conditions along elevational gradients and does not always hold. The complex climatic dynamics influencing this relationship vary between the Black Sea region of Turkey and the Caspian region of Iran [115,133,134]. Kehl [135] attributes this primarily to high precipitation variability (500–2100 m a.s.l.) and significant global warming, particularly at lower elevations in the eastern range of Oriental beech.

Our study indicates that European beech exhibited a higher number of months with statistically significant (p < 0.05) correlations between radial growth and monthly air temperature compared to Oriental beech (32 vs. 23 months), suggesting a stronger sensitivity of growth to temperature variability at the monthly scale. In European beech, both current and previous years’ temperatures were equally important for radial growth, whereas Oriental beech showed a stronger relationship with temperatures in the current year. July was identified as the most influential month for tree growth. Similar findings were reported in the Krkonoše Mountains [48] and other studies from Germany and Italy, where air temperature had a strongly positive effect on radial growth in May at high elevations (1560 m a.s.l.), while at low elevations (420–450 m a.s.l.), June temperatures had a negative impact [136].

While air temperature plays a dominant role, precipitation also affects radial growth. For example, [137,138] report a weaker correlation for European beech, while [132] found a statistically significant positive effect of precipitation on Oriental beech growth in June, but a negative impact in February and March. High June precipitation contributed to wider ring formation, whereas high precipitation in February–March and elevated July temperatures led to narrower rings. Oladi and Pourtahmasi [116] noted that rising mean air temperatures in recent decades generally increased radial growth unless extreme events occurred. However, above-average temperatures and prolonged drought conditions at the end of the growing season may increase respiration the following spring, reducing the amount of carbon compounds available for wood formation [139].

Beyond climate, radial growth is influenced by multiple ecological factors. Beech trees respond to mast years with reduced radial growth in the following year [140,141]. Additionally, they exhibit similar growth declines following air pollution damage (especially SO₂, O₃, and NO_X), insect infestations (beech scale, beech-leaf gall midge, Nectria spp.), and extreme weather events, particularly frost [47]. Late spring frosts damage leaves and cambial tissues, delaying or reducing growth [142]. Soil conditions also play a crucial role, as nutrient imbalances or compacted soils limit water and nutrient uptake, restricting growth [143]. Stand density and competition further affect growth, with higher competition reducing resource availability [144]. Additionally, genetic variability influences resilience to environmental stressors, causing differences in growth responses [145].

Many studies predict that global warming and decreasing precipitation will reduce growth at lower elevations while increasing growth at higher elevations within the beech range [19,28,55,117,132,146,147]. Our study strongly supports this pattern, confirming that insufficient precipitation and prolonged dry periods, combined with extremely high air temperatures, are the primary limiting factors for beech growth at lower elevations, whereas precipitation has a predominantly positive effect in mountainous areas.

4.4. Study Limitations and Recommendations

While this study provides valuable insights into the growth dynamics, carbon sequestration, and climate sensitivity of European and Oriental beech across elevational gradients, certain limitations should be acknowledged. First, the study was conducted on a limited number of PRPs without random sampling (selection of plots), which, despite being representative of different elevations and climatic conditions, may not fully capture the broader ecological variability across the entire distribution range of these species [148]. Another limitation could be the plot size, but small plots are sufficient for assessing basic production parameters, while larger plots (>1 ha) are required to properly evaluate tree spatial patterns [149]. Future research should aim to expand the number of study sites and integrate additional regions to enhance the generalizability of the findings. In particular, studies on Oriental beech should also focus on the Hyrcanian forests (not only the Black Sea region), one of its key habitats, to better understand its ecological requirements and adaptive potential under changing climatic conditions [150,151].

The observed differences in growth performance between European and Oriental beech under the same elevational gradient suggest species-specific physiological and ecological adaptations. European beech, generally exhibiting higher productivity and carbon sequestration potential, may benefit from its ability to utilize available water more efficiently and tolerate a wider range of climatic conditions [16,17,18,19]. In contrast, Oriental beech, although less productive in absolute terms, may exhibit greater resilience to specific climatic stressors, particularly in regions with pronounced seasonal droughts, including critical sites in Europe [8]. Further studies should investigate the underlying physiological traits, such as differences in water-use efficiency, wood anatomy, and photosynthetic capacity, to better understand the mechanisms driving these species-specific responses.

From a forest management perspective, the results emphasize the need for elevation-specific silvicultural strategies. At lower elevations, where increasing drought stress leads to reduced radial growth, adaptive measures such as selective crown thinning with strong intensive to reduce competition (stand density, number of trees) for water, or soil moisture conservation techniques [152] could be used. The promotion of mixed-species stands with drought-tolerant associates should be considered for higher resistance and resilience of beech [153]. Specifically, species evenness, in terms of admixture, plays a significantly more important role compared to species richness [68]. From the perspective of production potential and resistance to climate change, secondary diversity plays an important role, specifically in terms of structural development and variability, rather than species richness. In contrast, at higher elevations, where warming trends have enhanced growth rates, management should focus on maintaining vertical structural diversity and ensuring regeneration success to sustain productivity over the long term [154].

Moreover, given the increasing frequency of extreme climatic events, proactive measures should be taken, such as assisted migration of drought-resistant provenances, genetic selection for climate resilience, and updating of forestry legislation connected with close-to-nature silviculture [155]. Integrating these adaptive management strategies will be critical for maintaining the functional stability of beech forests and optimizing their role in carbon sequestration under ongoing global climate change [5,6].

5. Conclusions

This study examines biomass production, carbon sequestration, and the impact of GCC on the radial growth of European beech in Central Europe and Oriental beech in the Black Sea region. The results indicate a considerable variation in the production potential of both tree species, which was significantly influenced by elevation and stand structure under the effects of GCC.

The growth of both species was strongly affected by climate, with a significant decline in radial growth observed in lower elevations over the past 20 years due to long-term droughts, while warming in mountainous areas led to an increase in growth rates. Over the years, an increasing frequency of negative growth years associated with extreme climatic events has been observed.

The insights gained from this study on biomass production, carbon sequestration, and radial growth of European and Oriental beech in model forest plots are crucial for developing adaptive forest management strategies. These strategies should aim to maintain the functional diversity and ecological stability of beech forests while ensuring the provision of essential ecosystem services. Future research should expand study sites and investigate genetic variability and provenance suitability to enhance the representativeness of the results.