Vaccinium myrtillus stands show similar structure and functioning under different scenarios of coexistence at the Pyrenean treeline

Abstract

Plant-plant interactions are key drivers of treeline dynamics. At the Pyrenean treeline, the dwarf shrub Vaccinium myrtillus grows in pure stands or in mixed stands with Vaccinium uliginosum or Rhododendron ferrugineum. They form sparse shrub patches that colonize subalpine grasslands, having dramatic impacts on their structure and functioning. We investigated the role of the two co-occurring shrubs as possible modulators of the structure and performance of V. myrtillus in the Central Pyrenees. We analysed biomass, growth, functional parameters, age distribution, N and C concentrations and isotope compositions (d15N and d 13C) of V. myrtillus ramets in pure stands, and stands mixed with V. uliginosum or R. ferrugineum. Volume, above-ground biomass and age of the ramets did not differ between stand types. We found lower d13C values, indicative of lower water-use efficiency (WUE), in leaves and shoots of V. myrtillus in stands with R. ferrugineum than in pure stands. The N content and d15Nof V. myrtillus leaves and shoots in pure stands were higher than in mixed stands, pointing to a competition for N in mixed stands. Our results indicate that V. myrtillus competes for nutrients with its neighbours, but neither this competition nor the lower WUE affect its above-ground performance. Therefore, the interaction with co-occurring shrubs does not have a major effect on V. myrtillus structure and functioning at treeline and, consequently, should not be considered as a key driver on the dynamics of this species in the encroachment of subalpine grasslands.