Leaf anatomical trends in a temperate evergreen dwarf shrub, Rhododendron myrtifolium (Ericaceae) along elevational and exposure gradients in the northeastern Carpathian Mountains

Species’ responses to climatic gradients are constrained by multiple internal (e.g. trait plasticity and genetics) and external (local habitat) factors and may vary considerably between co-occurring species. This study examines morphological trends associated with elevation and slope aspect (exposure) in the European declining species Rhododendron myrtifolium (Ericaceae) to assess its adaptive responses to spatially changing environment. Measurements of 23 leaf anatomical traits from 40 individuals sampled at 24 locations along a broad elevational range from 1,248 to 2,034 m a.s.l. in the Eastern Carpathians (Ukraine) were analysed using multivariate statistics. Significant intercorrelations between traits indicate a tendency towards leaf xeromorphy with increasing elevation. Trait shifts along a south-to-north aspect gradient parallel those observed with increasing elevation and correspond to presumed differences between the aspects in water availability. Different leaf traits display distinct clinal and non-clinal patterns of variation across elevational strata, with an overall decline in the range of variation at high elevations. Significant effects of growth form, vegetation cover and populational characteristics on leaf traits are also found. The results indicate that despite plastic response of R. myrtifolium to ecological gradients, the species appears well-adapted to a limited range of habitats (north-exposed sites at medium and high elevations) and is likely impacted by abiotic stresses and increased competition for resources at south-facing and low-elevation locations.