Interactive effects of elevation and canopy affect bilberry performance in a temperate coniferous region

Shrubs are key components of temperate and boreal ecosystems. Field experiments performed along environmental gradients are useful to study how these species may cope with new climatic conditions. We analyzed the effects of habitat, elevation, and canopy on the performance of the bilberry Vaccinium myrtillus, a key resource for numerous species in boreal and temperate regions of Eurasia. We estimated bilberry dry mass and measured vegetative (ramet height, stem diameter, and shoot production) and reproductive (fruit and seed production) performance of bilberry shrubs in thirty plots located at six study sites established in coniferous forests and subalpine meadows along a 600-m elevation gradient in Tatra National Park, Poland. Although we detected more ramets per plot in the meadows, bilberry dry mass was larger inside the forest, whereas ramets were heavier and taller and produced more annual shoots. A higher proportion of ramets produced fruits in the meadows, where we additionally found a higher proportion of mature seeds per bilberry fruit. When only plots located inside the forest were considered, we detected a negative relation between elevation and bilberry ramet dry mass. Canopy cover affected the number of ramets and the dry bilberry biomass per plot differently depending on the elevation, demonstrating interactive effects of these two variables in bilberry vegetative performance. Fruit production inside the forest was negatively affected by both elevation and canopy cover, while interactive effects of these two variables determined seed development. Bilberry management actions must consider habitat heterogeneity even at small spatial scales and possible interactive effects of environmental variables such as those analyzed in this study.