Fine-scale vegetation mosaics in Pyrenean mires are driven by complex hydrological regimes and threatened by extreme weather events

Holarctic mires are systems of great ecological value and limited alpine distribution. Their southernmost European boundary borders the Mediterranean region. Understanding how environmental factors influence the plant community distribution and its response to disturbances, especially in the water regime, is a key step for their management and conservation. Here, we report our study of the main ecological gradients that determine the distribution of different plant community types in the Trescuro mire system, and their response to variations in the environmental conditions such as summer drought. We set up 30 sampling points across the vegetation mosaic where pH, electrical conductivity, water table depth and ion concentration were recorded during the 2014 and 2015 summers. The results revealed that variation in the water table level is what most drives the vegetation distribution at the local scale, followed by the ion concentration. Soligenous communities, which are highly dependent on water runoff, were the most affected by the drought event in 2015. Since these communities are generally situated in the marginal parts of mire systems, summer drought periods may facilitate invasion by trees into the areas they occupy, which may eventually lead to a reduction of mire area. This study contributes to the understanding of the dynamics of these unique and complex systems through elucidating the relationship between the vegetation and climatic variations.