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Article

Riparian Forests of Alnus Species Communities and Their Role in Sustainability in the Romanian Carpathians and Adjacent Regions

by
Daniela Mogîldea
,
Tiberiu C. Sahlean
and
Claudia Biță-Nicolae
*
Department of Taxonomy, Ecology and Nature Conservation, Institute of Biology Bucharest, Romanian Academy, 296 Spl. Independentei, P.O. Box 56-53, 060031 Bucharest, Romania
*
Author to whom correspondence should be addressed.
Sustainability 2024, 16(10), 4130; https://doi.org/10.3390/su16104130
Submission received: 1 March 2024 / Revised: 10 May 2024 / Accepted: 10 May 2024 / Published: 15 May 2024
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Abstract

:
Alnus riparian communities are important for ecological stability. Data on Alnus species were gathered from herbaria and literature, revealing that 54.39% of the information refers to Alnus glutinosa, 39.42% to Alnus incana, and 6.18% to Alnus pubescens. This information highlights the widespread occurrence of A. glutinosa and A. incana, as described in distribution maps, contrasting with the more limited range of the hybrid A. pubescens. Principal component analysis (PCA), utilizing standardized factors, was carried out on 217 relevés of the communities of the studied species. In these relevés, we identified a total of 169 plant species, categorized as herbaceous (83%), trees (11%), and shrubs (6%). Three distinct forest communities, Stellario nemorum-Alnetum glutinosae, Alnetum incanae, and Telekio speciosae-Alnetum incanae, emerged from relevé groups. The distribution maps of the three taxa were overlaid on maps of the region’s protected areas, revealing a substantial presence of these taxa within the protected zones. The research aims to highlight the sustainability and conservation importance of Alnus communities in the Romanian Carpathians, to contribute to ongoing conservation efforts and promote the viability and resilience of these ecologically important wetland habitats.

1. Introduction

Riparian vegetation, found along the edges of rivers and water bodies, represents an important terrestrial ecosystem identified on Earth [1,2] These areas are defined by terrestrial vegetation interacting with both permanent and temporary aquatic systems like rivers, streams, lakes, and wetlands [1,2,3]. Hence, vegetation displays significant diversity due to microclimate variations, soil differences, and land use types [4]. This diversity is evident regionally and along a single river, influenced by altitude changes and human activities [5]. Riparian systems function as transition zones between land and water ecosystems, forming interconnected networks across various landscapes [6]. The zones differ significantly in characteristics based on the size of the river, ranging from narrow strips along smaller streams to extensive floodplains along major rivers [7].
The deciduous tree species A. incana is native to Europe, Asia, and North America [8]. Typically reaching medium-sized heights of 15 to 25 m, it exhibits smooth, gray bark when young, transforming into a rougher, darker texture as the tree matures. Its distinctive leaves are rounded, serrated, and dark green on the upper surface, with a lighter, slightly hairy underside. Thriving in moist environments like riverbanks, wetlands, and forests, A. incana is well-suited for nutrient-poor soils and is often utilized in reforestation projects due to its nitrogen-fixing ability [9]. This tree fosters a symbiotic relationship with nitrogen-fixing bacteria, enriching the soil and supporting the growth of other plants in the area.
On the other hand, A. glutinosa is another deciduous tree native to Europe, western Asia, and northern Africa. With a height of about 25 m, it features dark green, oval-shaped leaves that are slightly sticky to the touch, hence, its species name “glutinosa” [10]. Found near rivers, streams, and wetland areas, A. glutinosa thrives in moist soils and tolerates waterlogged conditions [11]. Similar to A. incana, it plays a role as a nitrogen-fixing species, enhancing soil fertility and contributing to reforestation efforts while supporting biodiversity in wetland ecosystems [12].
Both A. incana and A. glutinosa are widespread in Europe, with A. incana ranging across Central Europe into Russia, and A. glutinosa extending from mid-Scandinavia to the Mediterranean and North Africa [13]. While hybridization between these alder species is relatively rare due to differences in flowering time, occasional hybridization can occur under anomalous conditions such as prolonged cold springs [14,15]. First-generation hybrids exhibit morphological variations, particularly in luster, leaf form, petiole length, and vein characteristics, posing challenges in identifying pure species during introgression when hybrid alders backcross with parent species [16,17].
In temperate and boreal forest zones, abandoned agricultural lands are frequently reclaimed by various alder species, which notably dominate these secondary successions. Reports of their encroachment on former arable lands and meadows have been documented in several regions, including Sweden [18], Finland [3], Russia [19], Norway [20], Estonia [21], Korea [22], France [23], the Swiss Alps [24], Latvia [5], among others. Alders, as well as pioneering species, have proven successful in the restoration of mining areas. [1,2,21,25,26].
Previous studies show that in Norway, A. glutinosa grows optimally in certain regions along the west coast, where it plays a key role as a pioneer species in secondary successions [27]. Previous studies have also focused on structural, coenological, and syntaxonomical features of forest fragments occurring along the watercourse [28]. Some authors have highlighted the negative effect of sudden changes in groundwater levels, which cause extensive water clumps on alder growth [29], while others also pointed to a clear genetic distinction between morphology-based groups of A. glutinosa and A. incana [30]. The probability values for the supposed hybrid group varied considerably, suggesting complex relationships within these alder species. A study in Croatia indicates a structured distribution of A. incana influenced by both geographical isolation and environmental factors [31].
The Carpathians were investigated by authors that present the classification of alder forests in the Southern Carpathian Mountains, revealing that only a small percentage (3.6%) meet the criteria for smart forests, located in humid, nutrient-rich soils [32]. Recently, a study focused on the influence of site-specific factors, such as moisture, hydrology, and soil variables on local plant diversity and species composition in floodplain Alnus forests [33]. Additionally, another study provides the initial evaluation of upland alluvial black maple forests in Georgia’s Greater Caucasus, identifying three new vegetation types in distinct regions, indicative of climatic and biogeographic variations [34]. Determining the optimal conditions for the emergence and natural growth and regeneration of A. glutinosa (L.) Gaertn. was also the objective of the research of some stands in Ukrainian Polis [35]. The distribution of various communities in the Romanian Carpathians was reported in the recent year [36,37]
Understanding the global spatial distribution of species is important for understanding ecological and evolutionary processes and effectively managing the impact of climate change [38]. Alnus communities play a significant role in maintaining sustainability and major functions, as pioneer species in secondary successions. Our study aims to highlight different aspects of Alnus species in the context of the Romanian Carpathians, focusing on understanding their ecological significance. First, we investigate the historical interest in the study of Alnus species over time, aiming to elucidate the evolution of the importance and relevance of these taxa in scientific research. In addition, we attempt to provide a comprehensive characterization of the plant communities of these trees. By focusing on A. glutinosa, A. incana, and their hybrid A. pubescens, we try to contribute valuable information to the sustainable management and conservation of riparian areas. This research is intended to provide essential data that can inform conservation strategies and improve our understanding of the dynamics of these wetland habitat species, namely A. glutinosa and A. incana, commonly found in riparian habitats within the study area.

2. Materials and Methods

2.1. Study Area

The Romanian Carpathians, part of the broader Carpathian Mountain range in Central and Eastern Europe, feature a varied landscape [39]. Inter-mountain depressions like the Transylvanian Depression contrast with higher mountainous terrain [40]. Alluvial plains and river valleys, shaped by rivers like the Danube, offer fertile land for agriculture. This dynamic region, crisscrossed by numerous rivers, exhibits a diverse interplay of mountains, basins, and plains, contributing to the physical geography of the area [41]. A map for the location of the study area can be found below Figure 1 [42].

2.2. Species Mapping

The Universal Transverse Mercator (UTM) grid system is a widely used method for representing locations on the Earth’s surface. It divides the world into a series of zones, each 6 degrees of longitude in width, numbered from 1 to 60, starting at the international date line in the Pacific Ocean [43]. In the case of the UTM grid system, skipping “O” avoids confusion between the letter “O” and the number “0”, especially in situations where handwriting or printing might not be entirely clear. Moreover, for our studied area, the omission of the letter “H” in the UTM grid system is related to a specific geographical area of the studied area like a feather.
Within each zone, a Cartesian coordinate system is used to pinpoint locations. It employs easting and northing values, which measure distances east-west and north-south, respectively, from an origin within each zone. These values are presented in meters, providing a precise way to locate and navigate across various terrains and maps. The UTM system is commonly utilized in mapping, surveying, and navigation applications [44,45]. A UTM grid contains several data points depending on the number of records made. A ‘data point’ refers to the geographical location where a species has been recorded.
To read a point on the map using the alphanumeric code: locate the 100 × 100 km square, and find the two capital letters that represent the square the point is in.
Then identify the letter of the first vertical grid line west of the location: determine the letter that represents the position of the point in terms of the vertical grid lines at the western edge of the square. Finally, add the letter of the first horizontal grid line south of the locality: Similarly, determine the letter that represents the position of the point in terms of the horizontal grid lines at the southern edge of the square.
Maps overlaying the distribution of the species were generated in ArcGIS 10.7 [42] using the spatial data available from the Ministry of Environment [46].
The distribution maps of Alnus species were generated also in RoBioAtlas, 2023 [45]. The software allows for the graphical representation of syntaxa chorology at a scale of 1:6,000,000, utilizing a map displaying the multiyear average temperature per annum [40,45]. This study compiles data sourced from Romania’s national literature and herbaria [47]. The consulted herbaria are as follows:
FRE—Flora Romaniae Exsiccata. FOE—Flora Olteniae Exsiccata FEF—Flora Exsiccata Forestry, FMDE—Floras Moldaviae et Dobrogeae Exsicalcata BUCA—Herbarium of the Bucharest Institute of Biology, BUCM Mycological Herbarium of the Bucharest Institute of Biology, BUC Bucharest Botanical Garden Herbarium, CL—Cluj Botanical Garden Herbarium, CLA—Herbarium of the Cluj Agronomic Institute, CRHM—Herbarium of the Craiova County Museum, I—Herbarium of the Faculty of Biology Iasi, IASI—Herbarium of the Iaşi Agronomic Institute, IAGB—Iași Botanical Garden Herbarium, IANB—Herbarium of the Bucharest Agronomic Institute, ICAS—Herbarium of the Institute of Forestry Research Bucharest SIBIU—Herbarium of the Brukenthal Museum of Sibiu, SMHM—Herbarium of the Satu-Mare County Museum, SVHU—Herbarium of the University of Suceava BVHU—Herbarium of the University of Braşov GLHM—Herbarium of the Galaţi County Museum, BNHM—Herbarium of the Bistriţa—Năsăud County Museum, PLHM—Herbarium of the Piatra Neamţ County Museum, BCHM—Herbarium of the Bacău County Museum, CVHM—Herbarium of the Covasna County Museum, PTHM—Herbarium of the Piteşti County Museum, BRHM- Herbarium of the Brăila County Museum, HDHM Herbarium of the Hunedoara County Museum, TMMJ—Herbarium of the Târgu Mureş County Museum ROHM—Herbarium of the Roman County Museum FOHM Herbarium of the Vrancea County Museum, TULCEA—Herbarium of the Tulcea County Museum.

2.3. Vegetation Investigation

Vegetation in the study area was analyzed using phytosociological methods aligned with the Braun-Blanquet School (Zurich-Montpellier) [48]. The investigation specifically targeted plant communities where Alnus species predominate. Angiosperms were taxonomically classified following the Euro + Med PlantBase [49]. We relied on national literature [48] and gathered vegetation data from both our database and existing literature [47]. Each species in the studied communities was assessed on five indices—soil moisture (M), soil nitrogen (N), soil reaction (R), light (L), and temperature (T)—based on European Ecological Indicator Values (EIVE) [50,51] and national literature [47]. The EuroVegChecklist [52], a standardized reference system widely adopted in phytosociology, guided vegetation classification based on syntaxonomy.

2.4. Data Analysis

The data obtained through the Braun-Blanquet quantitative method was subjected to statistical analyses without further transformation. The species’ degrees of coverage in the relèves were interpreted as their abundance relative to the total surface percentage of 100%. The number of species and their abundances were utilized in subsequent analyses. Statistical processing was carried out using the PAST 4.16c software [53]. Principal component analysis (PCA) was employed to visually examine the primary patterns of plant communities under investigation and their correlations among factors. The data were normalized before PCA analysis. We performed PCA analysis using standardized factors. In the PAST software the “Eigenvalue scale” was applied [53]. The data points were scaled by 1/√dk and the eigenvectors by √dk. We obtain the correlation biplot of Legendre & Legendre [53].

3. Results

3.1. Field Results

The vegetation data were gathered from our own database, herbaria as well as from the literature [47].
Through the collection of multiple relevés from various sites, we were able to construct a more comprehensive overview of the plant communities under investigation. In total, the study identified 169 plant species within these communities. Herbaceous species constituted the majority (83%), followed by trees (11%) and shrubs (6%) (Figure 2). The predominant plant families of angiosperms are Asteraceae (15.9%), Lamiaceae and Rosaceae (13.8%), and Ranunculaceae (10.6%) (Figure 3). A detailed list of species can be found in the annex (Table S1).
The data were classified into herbaria, literature, and both herbaria and literature sources. Specifically, the data from literature and herbaria account for around 42% of available information for A. glutinosa, 26% for A. incana, and 3% for A. pubescens (Figure 4). A substantial amount of data from herbaria remains unpublished for A. pubescens (29%) and A. incana (24%).

3.2. Mapping of Alnus Species

Using the UTM code database gathered from the literature and herbaria, the distribution maps were elaborated for each of the two Alnus species studied and the hybrid (Figure 5, Figure 6 and Figure 7). The analysis indicates that 54.39% of the information is associated with A. glutinosa, 39.42% with A. incana, and 6.18% with A. pubescens.
The entirety of the study area comprises 2595 UTM grids. Analysis of our database revealed a wide distribution of A. glutinosa, with 1052 data points spread across 787 UTM grids (Figure 5a), and A. incana, with 925 data points spread across 511 UTM grids (Figure 5b). Less extended is A. pubescens, with 75 data points spread across 66 UTM grids (Figure 5c).
Historical data dating back to the 19th century were uncovered, specifically for the A. glutinosa and A. incana species (Figure 6). Notably, the oldest records pertain to A. glutinosa and are associated with site/UTM grid LM.
Moreover, grid/LL, where A. incana was collected in 1887 (preserved in the BVHU herbarium) was exclusive to that particular time frame and was not represented in subsequent collection intervals. A. incana specimens from KL and LL were recorded from 1856 and 1846 with data available up to 1983–2003 (Figure S1).
The investigation of A. glutinosa reveals that there were two UTM grids exclusively explored before 1919, with an additional ten UTM grids exclusively examined between 1983 and 2003. Moreover, 14 UTM grids were scrutinized both before 1919 and during the period from 1983 to 2003 (Figure 7a). In the case of A. incana, the study indicates the exploration of five UTM grids exclusively before 1919 and an extensive examination of 79 UTM grids exclusively from 1983 to 2003. Additionally, seven UTM grids were investigated both before 1919 and during the period from 1983 to 2003 (Figure 7b). For A. pubescens, the earliest recorded data dates back to 1907 for the MN UTM grid (Figure S1). Subsequently, there were 11 UTM grids exclusively investigated between 1983 and 2003 (Figure 7c).
Data were grouped in 20-year time intervals (Figure 8). Most of the data were acquired between 1962 and 1983 for A. incana and A. glutinosa. Regarding A. pubescens the number of records was low, for all time intervals, except for a small increase in 1941–1961.
While surveys for A. glutinosa were conducted in two different years across most UTM grids, only one survey on the UTM grid was conducted for A. incana and A. pubescens. However, it is noted that certain UTM grids were the object of multiple surveys, with some reaching up to 17 records in different years (Figure 9). Particularly, sites with a high frequency of A. glutinosa collections include MN (27 different years of records), and MM, (24 different years of records). Similarly, for A. incana, notable sites with extensive investigation intervals are LN (27 different years of records), LL (13 different years of records). Although the literature provides a riches of easily accessible data for all three species, herbaria collections remain a significant but underutilized source (Figure S1). The distribution of A. incana mainly overlaps in the mountainous area of the Romanian Carpathians. Since 1941, A. glutinosa has been the most studied species of the genus Alnus in the area under study. The hybrid A. pubescens was the least studied, with no more than 33 records in the period 1941–1961.

3.3. Statistical Analysis of Alnus Communities

A total of six primary components were obtained (Table 1) from which, the first two cumulative contributions were 67.3% and the first three were 85.94%. This all accounts for most of the information provided by the initial variables.
The PCA analysis reveals that plant species were grouped into two clusters in the PCA biplot (Figure 10a). The numbers in the analysis represent the species of plants studied (Supplementary Materials Table S1).
By considering PC3 to achieve 85.9% data variability, the second cluster was further subdivided into two distinct groups (Figure 10b).
The factor coordinates (arrows) provide insights into which original variables are responsible for the variability observed in each principal component. Principal component 1 is positively influenced by soil reaction and soil nitrogen, countered by a negative contribution from light. Principal component 2 is shaped positively by soil reaction and light, while soil nitrogen exerts a negative influence. Principal Component 3 was positively influenced by light and soil nitrogen (Figure 11).
The distribution maps of the three taxa were juxtaposed with the distribution of protected areas in the Carpathians and adjacent regions (Figure 12). A total of 787 grids exhibited the occurrence of A. glutinosa, with 189 grids situated within designated protected areas. Similarly, within the 511 grids where A. incana was identified, 146 were located within the confines of protected areas. For A. pubescens, observed in 66 grids, 26 were situated within designated protected areas.

4. Discussion

4.1. Distribution of Alnus Species

The distribution of two species of Alnus commonly found in the Carpathians as well as a hybrid were mapped, focusing on the Romanian Carpathians and their surroundings. The Romanian Carpathians, characterized by an abundance of permanent and intermittent rivers and streams, have a wide range of plant species in the riparian areas [53,54]. However, the survival of many of these species is seriously threatened [55,56]. In this context, our comprehensive search of the Romanian literature provided results for A. glutinosa, A. incana, and A. pubescens. In addition, herbaria contributed supplementary records for these species. The UTM code database was used for the data from the literature and herbaria and distribution maps were generated for each Alnus species, A, glutinosa, and A. incana and their hybrids A. pubescens (Figure 5, Figure 6 and Figure 7). We observed A. glutinosa in 30.32% of the total surface area of the studied region, A. incana in 19.69%, and A. pubescens in 2.54%.
Overall, the study revealed the extensive geographical range of A. glutinosa and A. incana, while A. pubescens is much less common. Moreover, for A. pubescens data are not available both before 1919 and in the period 1983–2003, highlighting a significant gap in our understanding of its distribution during these periods. Distribution patterns were associated with temperature variations between regions.
The distribution maps (Figure 6 and Figure 7) provide a broad picture of both historical and recent collection grids. Emphasis on regions explored exclusively before 1919 is important because these data may be linked to changes in vegetation types. Regions exclusively explored between 1983 and 2003 indicate new sites surveyed. Overlapping regions surveyed both before 1919 and between 1983 and 2003 provide comparative data that indicate changes over the years. Maps describing species richness serve as the foundation for applied research, conservation planning, and theoretical investigations exploring patterns of richness and the underlying processes influencing these patterns [56,57].
Generating distribution maps for the three taxa on the map describing the isotherms of mean annual temperatures indicates that most populations are concentrated between the 2 °C and 8 °C isotherms. This range corresponds closely to the ecological requirements of these taxa in terms of moisture, as determined by Ellenberg [51,58].
The study identified a total of 169 plant species, with herbaceous species (83%), trees (11%) and shrubs (6%). The predominant plant families of angiosperms include Asteraceae (15.9%), Lamiaceae, Rosaceae (13.8%), and Ranunculaceae (10.6%). There are data obtained only from herbaria or only from literature but also from both herbarium and literature. Approximately 42% of the available information is for A. glutinosa, 26% for A. incana, and 3% for A. pubescens and comes from the literature and herbaria. In particular, a significant amount of data from herbaria represents unpublished data for A. pubescens (29%) and A. incana (24%).
When analyzing the communities of the species studied, there are 217 surveys and we have identified the species found in each community from the data we have. The analysis was performed on 152 surveys where we found A. incana as the dominant species and 65 surveys where we found A. glutinosa as the dominant species. We have provided a synoptic table in Table S1.
More recently, detailed distribution maps of several significant wetland communities in the same geographic region have been produced [36,37]. With our study, we complement the knowledge of wetland plant species diversity with the riparian habitats of the study area. A. glutinosa and A. incana represent a common occurrence in the studied area, which is consistent with all previous observations [34,35,36]. However, the hybrid, with rare records in the study area and documented only in a limited number of years, appears to be considerably less widespread. Distribution maps for Alnus species include mean annual temperature data as shown in Figure 5, Figure 6 and Figure 7. The inclusion of this ecological indicator serves the purpose of elucidating the temperature preferences shown by Alnus species. The same ecological indicator has also been used to highlight the distribution of some Salix species in riparian zones [37]. Upon close examination of these maps (Figure S1a–c), it becomes evident that these species predominantly occur in communities that fall within the average temperature range a relationship supported by the values of the ecological indicators listed in Table 1. Therefore, agree with the decision to use mean annual temperature as an indicator for distribution maps, which was based on the idea that this is an important factor in understanding the distribution of communities of some species, such as Salix or Alnus.

4.2. Statistical Results

Numerical classification revealed the significant influence of soil reaction, soil nitrogen, and light on plant species and principal components.
The studied communities, classified under the Alnetea glutinosae class, thrive in swamp forests and scrub, specifically as European mesotrophic regularly flooded alder carr and birch wooded mires. Further characterization places them within the Alnetalia glutinosae alliance and Alnion glutinosae order. Two distinct forest communities emerge from relevé groups: Stellario nemorum-Alnetum glutinosae and Alnetum incanae and Telekio speciosae-Alnetum incanae. Group A features amphi-tolerant species, while group B, encompassing Alnetum incanae and Telekio speciosae-Alnetum incanae, is marked by mesothermophilous species. Recent investigations emphasize the significant influence of habitat quality and landscape configuration on species composition in riparian forests dominated by Alnus sp. [59,60,61]. Altitudinal gradients play a crucial role, especially in wetland communities, notably riparian alder forests [60,61,62,63]. Soil reaction, particularly its influence on the variability of Alnus forests, is noteworthy, with the acidity gradient playing a key role in delineating communities of two Alnus species [62,63,64,65]. Our PCA findings regarding the influence of soil reaction reaffirm existing research highlighting its influence on the variability of Alnus forests, is noteworthy, with the acidity gradient playing a key role in delineating communities of two Alnus species [62,63,64,65]. Unique environmental factors characterize Finnis black alder forests, setting them apart from Central Europe, as observed by Mäkinen [66].
The distinctive boundaries of alder forests, encompassing both flora and ecology, take shape through micro-relief features, a high water table, and marshy soils enriched with organic substrates [63]. Black alder showcases a broader ecological range in West Norway, with exceptions in the most humid and cool areas of the British Isles [67].
However, further on, according to the 3D graph, two clusters can be observed, both represented by distinct communities dominated by A. incana.

4.3. Communities of Alnus

Studies have shown a clear transition from plants favoring eutrophic habitats at lower altitudes (Acer campestre, Circaea lutetiana, Geum urbanum) to mesotrophic and slightly oligotrophic species in submontane and montane sites (Equisetum sylvaticum, Picea abies). They also pointed to a shift from mesophytic substrate-adapted forest species (Asarum europaeum, Geranium robertianum, Primula elatior) to spring taxa (Cardamine amara, Galium palustre, Lycopus europaeus) [64].
On the other hand, some studies show a gradient from eutrophic and mesotrophic forests at lower altitudes to oligotrophic vegetation on acidic sites at higher altitudes [59]. This gradient correlated positively with altitude and negatively with nutrients and soil response.
Based on these observations, we identify and analyze the phytosociological associations occurring in the study area, thereby improving our understanding of the complex interactions within the plant communities of the region [52,59,61,65].
Stellario nemori—Alnetum glutinosae (Kästner 1938) Lohm. 1957: Forests characterized by the dominance of Alnus glutinosa are specifically found in expansive valleys and meadows where water flow is gentler. The accumulation of water during the spring season in subtle land gradients creates an environment conducive to the presence of various meso-hygrophilic elements [68], including Iris pseudacorus, Caltha laeta, and Mentha longifolia. Alnus glutinosa prevails in the tree layer, accompanied by Salix alba, Fraxinus pallisiae, Fraxinus excelsior, and Acer campestre. While the shrub layer is generally not highly developed, Corylus avellana may occasionally cover substantial areas. The herbaceous layer features species such as Brachypodium sylvaticum, Aegopodium podagraria, Geum urbanum, Allium ursinum, and Rubus caesius.
Telekio speciosae—Alnetum incanae Coldea (1986) 1990: Alnus incana communities thrive on gravel or silt deposits, primarily in the lower and middle sections of rivers. Telekia speciosa becomes increasingly sparse towards extinction in stands located in the upper reaches of the valley, where the relief’s energy is more pronounced. Petasites hybridus dominates river bends with substantial alluvial deposits. Pulmonaria rubra, Symphytum cordatum, Campanula patula ssp. abietina, Carduus personata, Chaerophyllum hirsutum, Viola biflora, Geum rivale, Heracleum palmatum, and Delphinium elatum also contribute to the diverse flora in these areas.
Alnetum incanae Aichinger et Siegrist 1930: A. incana communities represent mature stands, occasionally featuring rare specimens of A. glutinosa, Fraxinus angustifolia, and Ulmus minor in the tree layer. The herbaceous layer is rich in meso-hygrophilous species such as Aegopodium podagraria, Angelica sylvestris, Alliaria petiolata, Caltha palustris ssp. laeta, and Circaea lutetiana.

4.4. Management and Conservation of Riparian Alder Communities

We assessed the ecological significance of riparian Alnus communities based on their widespread occurrence, highlighting their major functions as pioneer species in secondary successions. Distribution maps highlight their widespread presence in the area considered, contrasting with the more limited distribution of the hybrid. We found studies with important implications for predicting successional rates that contribute to soil fertility-focused management for aboveground biomass production [66].
Maps of three Alnus taxa in the Romanian Carpathians offer insights into alder abundance and distribution patterns, aiding researchers, and conservationists in understanding the region’s ecology and safeguarding critical habitats in the context of environmental changes [10,12].
To do this, we have overlaid the distribution maps of three taxa: A. glutinosa, A. incana, and their hybrid, A. pubescens, on a map showing designated protected areas. We note that a substantial part of the populations for each of the three taxa are found in these protected areas. Specifically, we see of the entire area inhabited by A. glutinosa, 24.01% lies within a protected area, while for A. incana, 28.57% of its occupied area is within such designated zones. As for A. pubescens, 39.39% of its occupied area is found in protected areas.
Climate change has a major impact on these communities, with rising temperatures leading to higher evapotranspiration, making them susceptible to disease and insect invasion [66]. Changes in precipitation further disrupt water availability, affecting the growth and survival of the alder forests [67]. Effective management strategies are imperative to preserve these communities, which are essential for stabilizing riverbanks and providing diverse habitats [62].
Globally recognized for their value, wetlands of the Alnus forests face varying conservation statuses across European regions. Central Europe’s fragmented wetlands contrast with the Baltic states [68] and eastern Europe’s nearly intact stands [69,70], demanding region-specific conservation strategies. The riparian alder community responds clearly to environmental changes in the Oregon Coast Range [70]. Factors like altitude and soil pH influence alien richness in European regions, with varying impacts observed in the Pannonian region and the Western Carpathians [71,72].
In this context, sustainability means maintaining the health and functionality of these ecosystems while taking into account regional variations and challenges [73,74]. In essence, sustainability in this context involves not only addressing immediate threats and challenges but also promoting ecosystems that are resilient and adaptable to dynamic environmental conditions [75], ensuring the long-term health and functionality of Alnus riparian communities and wetland forests [75,76]. We agree that accurate geographic data on species distribution is important for effective decision-making in areas such as land management, climate change, and biodiversity conservation and our study also had this aim [77].

4.5. Conservation Status

According to Mandžukovski et al. [78], the Alnus communities are identified within various classifications with a significant role in conservation efforts.
Emerald: G1.12—Boreo-alpine riparian galleries.
Emerald: G1.21—Riverine Fraxinus—Alnus woodland, wet at high but not at low water.
Annex 1: 91E0 Alluvial forests with Alnus glutinosa and Fraxinus excelsior (Alno-Padion, Alnion incanae, and Salicion albae).
Emerald: G1.41—Alnus swamp woods not on acid peat.

5. Conclusions

This research addresses knowledge gaps by providing comprehensive data on the distribution of three Alnus taxa in the riparian zone, covering a significant study area. We emphasize the importance of our findings by highlighting the distinct distribution patterns observed among the taxa studied. In particular, while two species exhibit a wide distribution, the hybrid species show a limited range.
An important aspect of this study involves compiling distribution data from a variety of sources, including literature reviews and herbarium records, thereby incorporating historical data to provide a robust understanding of temporal changes in species distribution. By synthesizing these datasets, we provide valuable insights into riparian Alnus communities’ spatial and temporal dynamics various.
This investigation identifies three distinct forest vegetation types associated with Alnus communities, each characterized by specific ecological attributes such as water regime, soil properties, and microrelief heterogeneity. By delineating these communities, we contribute to a deeper understanding of the ecological dynamics driving riparian ecosystems.
The overlap of distribution maps of the three taxa with maps of protected areas reveals the importance of conservation efforts in protecting Alnus populations. This analysis highlights the importance of protected areas in biodiversity conservation and highlights the need for specific conservation strategies tailored to the specific ecological requirements of the Alnus taxon.
In addition, our study highlights the importance of comprehensive information on alder forests for effective conservation and management strategies.
In conclusion, this study contributes valuable information on the distribution, ecology, and conservation of the Alnus taxa in riparian ecosystems. By clarifying distribution models, ecological attributes, and conservation implications, this research contributes to the development of evidence-based management strategies to preserve the ecological integrity and sustainability of riparian habitats [79].

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/su16104130/s1.

Author Contributions

Conceptualization, C.B.-N.; methodology, D.M. and C.B.-N.; software, D.M. and T.C.S.; validation, D.M. and C.B.-N.; formal analysis, D.M. and C.B.-N.; investigation, D.M. and C.B.-N.; writing—original draft preparation, D.M. and C.B.-N.; writing—review and editing, D.M. and C.B.-N.; visualization, D.M., T.C.S. and C.B.-N.; supervision, C.B.-N. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by project numbers RO1567-IBB01/2024, RO1567-IBB04/2024, and RO1567-IBB09/2024 from the Institute of Biology Bucharest of the Romanian Academy.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in this study are included in the article; further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Map for the location of the study area.
Figure 1. Map for the location of the study area.
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Figure 2. The life forms found in the studied communities. H-herbs, S-shrubs, T-trees.
Figure 2. The life forms found in the studied communities. H-herbs, S-shrubs, T-trees.
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Figure 3. The families of the studied communities.
Figure 3. The families of the studied communities.
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Figure 4. Number of records of the three studied taxa.
Figure 4. Number of records of the three studied taxa.
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Figure 5. Distribution of A. glutinosa (a) A. incana (b) A. pubescens (c) in the Carpathian region and its vicinity.
Figure 5. Distribution of A. glutinosa (a) A. incana (b) A. pubescens (c) in the Carpathian region and its vicinity.
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Figure 6. The distribution map of A. glutinosa (a) A. incana (b) according to the annual means of temperatures (1980–1989) in °C.
Figure 6. The distribution map of A. glutinosa (a) A. incana (b) according to the annual means of temperatures (1980–1989) in °C.
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Figure 7. The distribution map of A. glutinosa (a) A. incana (b) and A pubescens (c) according to the annual means of temperatures (1980–1989) in °C.
Figure 7. The distribution map of A. glutinosa (a) A. incana (b) and A pubescens (c) according to the annual means of temperatures (1980–1989) in °C.
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Figure 8. Number of records in time intervals.
Figure 8. Number of records in time intervals.
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Figure 9. Number of records of studied taxa.
Figure 9. Number of records of studied taxa.
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Figure 10. PCA correlation biplot (a) and 3D scatter PCA (b). ● The plant species; component 1 and PC1—principal component 1; component 2 and PC2—principal component 2; PC3—principal component 3; C—plant coverage; M—soil moisture; N—soil nitrogen; R—soil reaction; L—light; and T—temperature (T).
Figure 10. PCA correlation biplot (a) and 3D scatter PCA (b). ● The plant species; component 1 and PC1—principal component 1; component 2 and PC2—principal component 2; PC3—principal component 3; C—plant coverage; M—soil moisture; N—soil nitrogen; R—soil reaction; L—light; and T—temperature (T).
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Figure 11. Principal components loadings. C—plant coverage; M—soil moisture; N—soil nitrogen; R—soil reaction; L—light; and T—temperature (T).
Figure 11. Principal components loadings. C—plant coverage; M—soil moisture; N—soil nitrogen; R—soil reaction; L—light; and T—temperature (T).
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Figure 12. The distribution map of A. glutinosa (a) A. incana (b) and A pubescens (c) over the distribution of protected area in Romania.
Figure 12. The distribution map of A. glutinosa (a) A. incana (b) and A pubescens (c) over the distribution of protected area in Romania.
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Table 1. Eigenvalue and variance of principal factors.
Table 1. Eigenvalue and variance of principal factors.
PCEigenvalue% Variance
13.2039.88
22.2027.46
31.4918.61
40.668.23
50.425.22
60.050.61
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Mogîldea, D.; Sahlean, T.C.; Biță-Nicolae, C. Riparian Forests of Alnus Species Communities and Their Role in Sustainability in the Romanian Carpathians and Adjacent Regions. Sustainability 2024, 16, 4130. https://doi.org/10.3390/su16104130

AMA Style

Mogîldea D, Sahlean TC, Biță-Nicolae C. Riparian Forests of Alnus Species Communities and Their Role in Sustainability in the Romanian Carpathians and Adjacent Regions. Sustainability. 2024; 16(10):4130. https://doi.org/10.3390/su16104130

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Mogîldea, Daniela, Tiberiu C. Sahlean, and Claudia Biță-Nicolae. 2024. "Riparian Forests of Alnus Species Communities and Their Role in Sustainability in the Romanian Carpathians and Adjacent Regions" Sustainability 16, no. 10: 4130. https://doi.org/10.3390/su16104130

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