Plant guilds drive biomass response to global warming and water availability in subalpine grassland

1. The consequences of global warming and changes in resource availability were investigated in subalpine grasslands in the Pyrenees. These communities are considered to be especially vulnerable to climate change because of their position at the south-western edge of the semi-natural grassland biome in Europe. 2. Changes in patterns of above- and below-ground biomass were assessed for different plant guilds in two experiments, in which turves were transplanted from upland to lowland locations. The first experiment aimed to evaluate general responses to warming and drought, and the second to disentangle the effects of possible underlying mechanisms through resource manipulation by means of a nitrogen x phosphorus fertilization experiment. 3. The increased above-ground biomass in grassland turves transplanted to lowlands suggested that biomass production was more temperature-limited than water-limited. The enhancement effect found in the upland turves following phosphorus addition supported the hypothesis of a strong limitation arising from reduced nutrient availability, confirming the central role played by phosphorus in these grasslands and its potential importance in the response to global change. 4. Nitrogen addition did not stimulate total biomass but affected guild composition. Grasses dominated the uplands and at high resource levels, while forbs dominated the lowlands and when water and nutrients decreased. The counterintuitive effect of increased biomass with decreased water in the lowlands was related to shifts in dominance from grasses to forbs, probably enabled by decreased nutrient availability under drought conditions. 5. Synthesis and applications. Environmental factors interacted in complex ways, producing changes in biomass distribution and guild proportions in subalpine grassland. In addition, the results suggested that the capability of high-altitude grasslands to provide quality forage in summer time could be threatened in the northern Mediterranean region under climate change conditions because of: (i) a decrease in their reliability as a result of complex biomass interactions with temperature, water and nutrient dynamics; (ii) expected feedback mechanisms; and (iii) compositional shifts.