The acid taste of climate change: 20th century acidification is re-emerging during a climatic extreme event

The current state of an ecosystem results from the complex interaction of abiotic and biotic drivers jointly influenced by their dynamics and the legacy of a systems' history. Negative synergies between emerging climatically extreme events and past environmental impacts are expected to shift ecological communities to alternative stable states or towards hysteretic successional trajectories. However, knowledge on mutual effects of environmental stressors is scarce especially for not experimentally controlled, natural ecosystems. We investigated the effect of a prolonged drought and heat wave occurred during 2003 on the short-term vegetation responses of forest springs, a waterlogged type of ecosystem which is highly abundant in Central European siliceous mountains. These landscapes experienced strong impairment by 20th century atmospheric acidification. For different levels of acidification, we investigated plant community composition and water chemistry of 57 springs before (1996) during (2003) and after (2004-2006) the summer of 2003 and quantified ecological resilience and elasticity related to this extreme event for single plant species and species assemblages of whole communities. The extreme dry and hot summer 2003 significantly decreased discharge, increased water temperature and affected water chemistry of the investigated springs. Ecological resilience and elasticity against the climatic extreme event differed significantly between communities dependent on their previous impact by acidification. Springs which were less affected by acidification performed higher resilience and elasticity than strongly acidified springs. Our study shows that strong negative synergies between emerging climatic extreme events and past environmental impairments occur on landscape scale. Hitherto, such interactions between climate change, hydrochemistry, and the responses of ecosystems have been neglected.