Decrease in climatic disequilibrium associated with climate change and species abundance shifts in Mediterranean plant communities

Climate change induces changes in plant communities according to species' climatic requirements. These changes can be assessed by community climatic disequilibrium (CD), which corresponds to the difference between the climate inferred from the climatic requirements of the species in a community (community-inferred climate, CIC) and the local observed climate. We assessed changes in CIC and CD during a long-term climatic manipulation (warming and drought treatments), embedded within the ongoing trends of climate change, in a Mediterranean shrubland (NE Iberian Peninsula) during 1999-2014. We used plant censuses, species distribution and climate layers since 1979 to create a multivariate environmental space where CIC and CD trends were estimated for 1999-2014. CD consistently decreased, concomitant with an overall climate change-derived increase in aridity (higher temperature and lower precipitation). CIC significantly changed during 1999-2014, reflecting the reshuffling of the community composition due to an increase in the abundance of species distributed in warmer, drier and more seasonal localities. Overall, treatments simulating greater climate change did not accelerate the decrease in CD. However, a trend of steeper diminution of CD was observed under warming treatment. In turn, under drought treatment the species less adapted to seasonality and aridity became less abundant. This community tracking of climate did not follow yearly climatic variability; instead, it was detected by the CD trend at the decadal scale. Synthesis. The procedure developed to measure CD reflects demographic behaviours, thus providing a reliable method to assess the impact of climate change on species and communities. The study demonstrates the current, rapid tracking of Mediterranean woody plant communities to climate change. This tracking results from changes in the abundance of species according to their climatic requirements, inferred from their distribution. However, inertia in demographic processes implies that plant communities do not immediately fit current climate change at local level, as supported by the minor effect of experimentally accentuated climate change.