Southern Carpathian ultramafic grasslands within the central-southeast European context: syntaxonomic classification and overall eco-coenotic patterns

Abstract

Background: Previous investigations carried out in ultramafic habitats emphasized the greater importance of site conditions over soil toxic metal content for vegetation composition. Very little is known about the floristic structure of the Southern Carpathian ultramafic grasslands and there is no information on the local environmental drivers of their composition and coenotic features. Here, we aim to fill these knowledge gaps by referring to similar phytocoenoses described in the Balkan Peninsula and central Europe. In particular, we searched for: (i) floristic and ecological patterns supporting the classification and taxonomic assignment of these grasslands, and (ii) simple relationships between serpentine vegetation characteristics and its physiographic environment. A total of 120 phytosociological relevés, of which 52 performed in the Southern Carpathians, were analysed through cluster, ordination and regression procedures.

Results: Despite some floristic similarities with their Balkan counterparts, the Southern Carpathian ultramafic grasslands were clustered into four distinct groups, which were assigned to as many new syntaxa: Plantago serpentinae-Armerietum halleri, Asplenio serpentini-Achnatheretum calamagrostis, Minuartio frutescentis-Plantaginetum holostei and Sileno saxifragae-Plantaginetum holostei. The latter was best individualised through the occurrence of several Carpathian endemic taxa. The first two ordination axes were significantly related with the terrain slope/presence of xerophilous species and respectively, with site elevation/presence of calcifugous species. The total plant cover showed a unimodal relationship with respect to site elevation. While controlling for the effect of the sampled area, species richness showed a unimodal response to both elevation and slope of the terrain, although their effects were not singular.

Conclusions: The syntaxonomic distinctiveness of the Southern Carpathian ultramafic grasslands is mainly supported by their overall species composition rather than regional differential species. The main limiting factors driving the composition, cover and species richness of all studied ultramafic grasslands are the water deficit at low elevation and on steep slopes, and the low soil fertility at higher elevations. Our results confirm the previous findings according to which physiographic conditions and, to a lesser extent, soil base nutrients are more important than heavy metal concentrations in structuring the ultramafic vegetation.

Keywords: Balkan Peninsula; Elevation; Heavy metals; New syntaxa; Serpentine habitats; Soil pH; Southern Carpathians; Species richness; Terrain slope; Vegetation cover.

Fig. 1

Location of the Carpathians within Europe (A) and of the study sites within the Southern Carpathians (B) by reference to the 10 × 10 km UTM-grid cells (FQ = Mehedinți Mts.; FR = Retezat Mts.; KL = Cozia Mts.)

Fig. 2

A Optimal number of clusters determined by consensus among the maxima of four validation criteria calculated for all possible solutions ranging from 2 to 12 clusters; B Output dendrogram of the cluster analysis performed on the matrix of compositional dissimilarities between the 120 relevés under study (the percentage values indicate the stability of the nine retained clusters); C Distribution of the four Southern Carpathian herbaceous associations distinguished on ultramafic substrates

Fig. 3

Ordination of the 120 relevés of ultramafic grasslands in the space determined by the two most important NMDS axes, from the total of three axes extracted (final stress = 0.0987; linear R-square = 0.916); the length and orientation of the two environmental vectors stem from the outcome of the linear fitting of elevation and slope against the relevé scores (see Table 8). PA = Plantago serpentinae–Armerietum halleri; AsA = Asplenio serpentini–Achnatheretum calamagrostis; SP = Sileno saxifragae–Plantaginetum holostei; MP = Minuartio frutescentis–Plantaginetum holostei; Ah = Armerietum halleri; AaA = Artemisio albae–Achnatheretum calamagrostis; OF = Onosmo pavlovae–Festucetum dalmaticae; PP = Poo molinerii–Plantaginetum holostei; AP = Anthemido–Plantaginetum holostei

Fig. 4

Second-order polynomial regression of total plant cover by site elevation in all ultramafic communities considered (n = 120); all regression coefficients are significantly different from zero (p < 0.0001)