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Syntaxonomy of the xero-mesophytic oak forests in the Republic of Tatarstan (Eastern Europe)

Aims: To develop a syntaxonomic classification of the xero-mesophytic broad-leaved oak forests of the Republic of Tatarstan with a preliminary analysis of their unique ecological features. Study area: The Republic of Tatarstan (European part of the Russian Federation). Methods: A total of 91 relevés were processed. Most of them (73.6%) were sampled in Tatarstan during 2016 and 2017, the remaining ones (26.4%) were historical published data. They were classified by means of a modified TWINSPAN algorithm using total inertia as a heterogeneity measure. Diagnostic, constant, and dominant species were identified using analytical tools in the JUICE 7.0 program. Results: The xero-mesophytic forests of the study area were assigned to four clusters. We describe two of them as new associations: Astragalo ciceri-Quercetum roboris ass. nova and Sanguisorbo officinalis-Quercetum roboris ass. nova. We classify them within the class Quercetea pubescentis. Conclusions: Our study is the first attempt to classify thermophilous and xero-mesophytic oak forests of the Republic of Tatarstan using the Braun-Blanquet system. Taxonomic reference: Czerepanov (1995). Syntaxonomic reference: Mucina et al. (2016) unless stated otherwise in the text. Abbreviations: GIVD = Global Index of Vegetation-Plot Databases; NMDS = Non-metric multidimensional scaling.


Introduction
The xero-mesophytic broad-leaved forests of the Republic of Tatarstan (hereafter referred to as Tatarstan) are of interest for several reasons. These forests are characterized by high biodiversity and host many rare and protected plant species. Quercus robur, a canopy-forming tree species of these ecological communities, is found here near the northeastern boundary of its native range (Gorchakovskij 1968). The communities of this type form an ecotone between forest and steppe, which has long attracted researchers, starting with the works of Korzhinsky (1888) and Markov (1935).
Until recently, the classification of plant communities of Tatarstan has been performed using the dominance approach (Rogova and Shajhutdinova 2000;Pozdnyak 2005). The syntaxonomic position of the xero-mesophytic oak forests of Tatarstan in the Braun-Blanquet system is still unclear.
The westerly distributed analogues have been attributed to the alliance Aceri tatarici-Quercion (Semenishchenkov and Poluyanov 2014) and the eastern analogues to the alliance Lathyro pisiformis-Quercion roboris (Yamalov et al. 2004;Willner et al. 2016). However, Semenishchenkov and Panchenko (2012) suggested that some associations previously assigned to the Aceri tatarici-Quercion should be classified in the Quercion petraeae. They also pointed out that the xero-mesophytic oak forests of Tatarstan are distinct from both of the aforementioned alliances. In a recent revision of the thermophilous oak forests of the steppe and forest-steppe zones of Ukraine and Russia, Goncharenko et al. (2020) described the eastern part of the Aceri tatarici-Quercion as a new alliance Scutellario altissimae-Quercion roboris and the eastern part of the Quercion petraeae as Betonico officinalis-Quercion roboris.
The aim of this article is to address the following research questions: 1. Are there communities in Tatarstan that may be assigned to the order Quercetalia pubescenti-petraeae? 2. To which lower-level syntaxa can they be assigned? 3. What are the compositional, ecological, and chorological characteristics of these syntaxa?

Study area
The Republic of Tatarstan is located in the eastern part of the East European Plain at the confluence of the largest European river Volga with the rivers Kama and Belaya ( Figure 1). The northwesternmost point is approximately 56.67°N, 047.26°E, the southeasternmost one 53.97°N, 054.27°E. The total area is 67,600 km 2 . The territory is divided by the rivers into clearly separated natural and geographical parts: Cis-Volga region (west and south of the Volga valley), Cis-Kama region (north of the Kama and Volga valleys), Trans-Kama region (south of the Kama valley) (Butakov 1994).
Large uplands alternate with lowland areas across the study area. The lowest elevation in the territory is along the line of the Kuibyshev Reservoir with an average of 53 m, while the maximum elevation of 380 m is reached in the south-east of the study area (Butakov 1994). Being located within the Sarmatian mixed forests and the East European forest-steppe (Dinerstein et al. 2017), the study area has high biodiversity, particularly regarding its vegetation cover (Bakin et al. 2000). The heterogeneity of site conditions due to climatic and soil characteristics, as well as the long-term human impact on vegetation (Bakin et al. 2000), has determined the complexity and diversity of the vegetation cover. The territory is comprised of 18% forests, 21.5% grasslands and 6% water bodies (Shadrikov 2019). The remaining 54.5% of the territory is agricultural and urban land. Young forest stands prevail in the forest vegetation (secondary birch, aspen, and lime coppice), whereas the ancient forests are small and fragmented. Steppe communities occupy very small territories. They are represented by meadow steppes along the edges of deciduous forests and gentle slopes. The steep slopes of southern exposure in the southeastern part of the Tatarstan are occupied by petrophytic steppes (Bakin et al. 2000).

Vegetation data
All relevés of the oak forests of Tatarstan were previously classified and analyzed to exclude hygrophytic and mesophytic communities (Kozhevnikova et al. 2018). For the present study, a total of 91 relevés of xero-mesophytic oak forests were compiled from the study area. The majority of relevés (n = 67), was sampled in the field during the field seasons of 2016 and 2017, with the aim of investigating the communities of thermophilous oak forests following the construction of a model of their potential distribution (Kozhevnikova et al. 2019). Further 24 relevés were historical data retrieved from the literature (Markov 1935).
The newly collected relevés were sampled using the standard phytosociological methodology (Dengler et al. 2008). In most cases, the plot size was 400 m 2 . For each vegetation plot, all vascular plant species were recorded with indications of their layer and abundance based on the Drude scale (Drude 1896). In addition, the geographical coordinates, altitude, exposition, and slope were recorded for each relevé.
The published relevés of Markov (1935) include information on all species of vascular plants, their abundance on the Drude scale and the geographical position, which we georeferenced with an accuracy of 200 m.

Analysis
The relevés of xero-mesophytic communities were exported from the information system "Flora" with simultaneous translation of the Drude abundance grades into cover percentage (soc -95%, cop 3 -75%, cop 2 -50%, cop 1 -25%, sp -3%, sol -2%, un -0.5%). This file was then imported into the JUICE 7.0 program (Tichý 2002) with the transformation of cover percentage into the Braun-Blanquet scale. The relevés were classified by applying the modified TWINSPAN algorithm (Roleček et al. 2009). For optimizing the number of clusters, the procedure OptimClass proposed by Tichý et al. (2010) was used. The resulting clusters were analyzed by calculating the species frequency and by identifying diagnostic, constant and dominant species. The following threshold values were used: for diagnostic species, a phi value > 0.6, for constant species, a frequency > 60%, and for dominant species, average cover > 80%.
The resulting clusters were compared with the aforementioned associations of the alliances Aceri tatarici-Quercion and Lathyro pisiformis-Quercion by combining them into a single constancy table. For all vegetation units, the frequency sum of diagnostic species of the following syntaxa was calculated: Lathyro pisiformis-Quercion roboris, Betonico officinalis-Quercion roboris, and Scutellario altissimae-Quercion roboris. Diagnostic species follow Goncharenko et al. (2020).
The names of classes, orders and alliances follow Mucina et al. (2016), except for those newly described in Goncharenko et al. (2020). The newly described associations follow the ICPN, 4 th edition (Theurillat et al. 2021). Biogeographic characteristics of the species are given according to Bakin et al. (2000).
The TWINSPAN clusters were compared with the other associations by calculating a distance matrix. As a distance metric we used 1 -Jaccard coefficient following the recommendations of Legendre and De Cáceres (2013). As vectors for paired comparison, we used the species list of each group and the frequency of the species. The results are visualized using a "heat map" combined with a dendrogram, which is computed by complete-linkage clustering method. We also used non-metric multidimensional scaling (NMDS) as a "dimensional reduction" method (Kraemer et al. 2018).
Cluster 1 contained five relevés located at the single site on the high and steep slope of the Volga terrace. Species identified as diagnostic for this cluster included ruderal and meadow plants (Asparagus officinalis, Crepis tectorum, Melandrium album, Phleum phleoides, Polygonatum odoratum, Rumex acetosella, Tanacetum vulgare), which indicates the derivative nature of these communities.
Cluster 3 contained 37 geographically widespread plots, which indicates a regular occurrence of this community type. Only one species was identified as diagnostic -Laser trilobum. When the phi value threshold was decreased from 0.6 to 0.3, Astragalus cicer, Adonis vernalis, Campanula rapunculoides, and Xanthoselinum alsaticum also became diagnostic.
In the following, we describe clusters 3 and 4 as new associations. We refrain from describing clusters 1 and 2 formally as new syntaxa because of the small number of relevés and their presumable derivative nature.
Geographical range: Communities assigned to this association are found in the southeast of Tatarstan, Cis-Volga region, and the western part of Tatarstan. The most typical of these communities were described from the Central Cis-Volga region, Kamskoe Ust' e and Apastovo districts (a distribution map and a photo of the community are provided in Suppl. material 3).
Floristic composition: These communities represent a sparse open forest. The first tree layer is dominated exclusively by Quercus robur, which also occurs in the shrub layer. In the second tree layer, Betula pendula, Tilia cordata and Sorbus aucuparia are found along with oak. The shrub layer is not dense and mainly consists of Euonymus verrucosa, Corylus avellana, Rhamnus cathartica, Sorbus aucuparia, and Lonicera xylosteum. The proportion of shrubs in these communities increases if there are signs of fire impacts. In case of intensive grazing, the undergrowth density is reduced, and the proportion of herbs increases. The floristic composition is homogeneous; only 94 plant species were recorded at the 37 plots of this association (with most com-monly 20-30 species per plot). The composition of dominant species is determined by quite high light availability. Among the dominant species, Brachypodium pinnatum, Carex muricata, Fragaria viridis and Laser trilobum prevail.
Habitat characteristics: These communities grow on the middle parts of gentle (5-15°) slopes of southwestern exposure at altitudes less than 150 m a.s.l. The flat surfaces adjacent to the tops of these slopes are usually plowed up or, more rarely, occupied by meadow steppes with a large number of grasses (including Stipa species) and legumes. The lower parts of the slopes are most often occupied by a strip of shrubby vegetation with Cerasus fruticosus, Genista tinctoria and Spiraea species. The soils are generally rich in nutrients. The parent rocks are characterized by high content of calcium. Geographical range: The communities assigned to this association occur in the southeast of Tatarstan, within the western slope of the Bugulma-Belebey Upland at the territories of the Bugulma, Leninogorsk, Bavly, Aznakaevo and Almetyevsk districts of Tatarstan (a distribution map and photos of the community are provided in Suppl. material 3).
Floristic composition: The communities are characterized by an extremely high species diversity. The total number of species is 293, while the average number of species per relevé is 50. In the tree layer, Betula pendula, Pinus sylvestris, Populus tremula, Tilia cordata and Ulmus glabra are found in addition to the dominant Quercus robur. Trees are distributed unevenly within the plots: some of them grow close to each other, while others are separated and form open areas (meadows) with sparse tree stands. In the meadow areas, heliophytes are abundant. The shrub layer is not dense, being characterized by high species diversity (total number of species 21) without any clear dominance among them. The most abundant species is Euonymus verrucosa. The herb layer is multilayered, polydominated, with tall forest-steppe herbs (Campanula trachelium, Euphorbia semivillosa, Heracleum sibiricum, Lilium pilosiusculum and Pleurospermum uralense).
Habitat characteristics: In Tatarstan, the communities of this type occur at altitudes of 250-300 m a.s.l. They occupy areas near the water divide and middle parts of the gentle (up to 5°) slopes of mostly southeastern exposure. The soils are leached and typical chernozems. The parent material can be Permian bed rocks, Permian eluvial clays and loams, deluvial deposits on the gentle slopes, and post-Pliocene loess-like loams.

Comparison with associations in other regions
The comparison of the identified syntaxa and previously described associations of the Lathyro pisiformis-Quercion and Aceri tatarici-Quercion (sensu lato) are given in Table 1. The analysis of the table reveals significant differences in both floristic composition and combinations of characteristic species between identified syntaxa and previously described associations. Compared to the Chamaecytiso ruthenici-Quercetum roboris, Pyro pyrastris-Quercetum roboris, Vicio pisiformis-Quercetum roboris and Lathyro nigri-Quercetum roboris associations, the Astragalo ciceri-Quercetum roboris has a higher proportion of Euro-West Asian species (41.5% against 30% in the above-listed associations, on average) and a lower number of European species (9.6% against 16%).
Based on the floristic composition, the Sanguisorbo officinalis-Quercetum roboris is most similar to the Filipendulo vulgari-Quercetum roboris, but it differs from the latter by the absence of such characteristic species as Galatella biflora and Artemisia armeniaca, as well as because of the lower proportion of Carex praecox, Veronica spuria and Campanula bononiensis. Compared to the Sanguisorbo officinalis-Quercetum roboris, the Filipendulo vulgari-Quercetum roboris has a much lower proportion of European species (3.4% against 8.5%) and more Eurasian species (23.3% against 19%).
An analysis of "heat maps" shows that all associations have a low similarity. The largest number of pairs being compared has a distance between 0.4 and 0.8 ( Figure 2).
The newly identified associations are clustered in the dendrogram into one group with the associations of the Aceri tatarici-Quercion. This clustering is generally consistent with the analysis of the composition of diagnostic species. Cluster 2 was grouped with the new associations from the territory of the Republic of Tatarstan, and cluster 1 was grouped with the association Vicio pisiformis-Quercetum roboris.
However, the NMDS ordination ( Figure 3) shows that the Sanguisorbo officinalis-Quercetum is intermediate between the Lathyro pisiformis-Quercion and Aceri tatarici-Quercion and is closer to the Filipendulo vulgaris-Quercetum than to the newly described Astragalo ciceri-Quercetum roboris. The left group of points unites the "western" (in relation to the territory of the Republic of Tatarstan) associations of the Aceri tatarici-Quercion. Clusters 1 and 2 adjoin them, together with the Astragalo ciceri-Quercetum roboris. The right part unites the "eastern" associations of the Lathyro pisiformis-Quercion. It is also noticeable that the "eastern" associations are less homogeneous and may require a revision of their syntaxonomic position.
In clusters 2, 3 and 4, the frequency sum of the diagnostic species of the Lathyro pisiformis-Quercion is higher than the frequency sum of the diagnostic species of the Betonico officinalis-Quercion roboris and Scutellario altissimae-Quercion roboris alliances (Table 2).

Discussion
The NMDS ordination diagram shows distinct floristic and ecological composition of the identified syntaxa ( Figure 3).
Our results suggest that the communities of the Sanguisorbo officinalis-Quercetum roboris are close to the group of associations of the Lathyro pisiformis-Quercion roboris alliance. However, they differ from the latter by their preference for warmer sites with more light availability. Sanguisorbo officinalis-Quercetum roboris includes the diagnostic species of this alliance  such as Geranium sylvaticum, Heracleum sibiricum, Lathyrus pisiformis, L. sylvestris, Pleurospermum uralense, Rosa majalis, Seseli libanotis. Some of diagnostic species of Lathyro pisiformis-Quercion are absent: Caragana frutex, Carex macroura, Lathyrus gmelinii, and L. litvinovii.
We conclude that the xero-mesophytic oak forests in the Republic of Tatarstan can be assigned to the alliance Betonico officinalis-Quercion roboris (ass. Astragalo ciceri-Quercetum roboris), and to the alliance Lathyro pisiformis-Quercion roboris (ass. Sanguisorbo officinalis-Quercetum roboris). However, a syntaxonomic revision of the entire phytocoenotic material of xero-mesophytic oak forests in Europe, including the European part of Russia, is necessary to clarify the exact delimitation of these alliances.