Negative synergistic effects of land-use legacies and climate drive widespread oak decline in evergreen Mediterranean open woodlands

Abstract

Evergreen oak woodlands in the Western Mediterranean exploited as agrosilvopastoral systems have often been considered as a sustainability paradigm. Yet, these ecosystems show profound symptoms of degradation with widespread tree decline and increased mortality, making them a paradigmatic example of overexploited ecosystems threatened by global change. Understanding the biotic and abiotic, environmental and management factors involved in the current decline of these open woodlands is key to derive sustainable management options. Our goal was to evaluate the potential role of climate and land-use legacies as drivers of tree decline in Quercus ilex open woodlands at the regional level in western Spain. We analysed tree recruitment and health as proxies to the key processes implied in ecosystem decline. Overall, tree health was poor. Levels of tree decline followed a latitudinal gradient, with cooler stands in the North exhibiting better health (i.e. fewer, less severe decline symptoms) and higher sapling and seedling abundance than warmer stands in the South. Warmer conditions and more intense human management, indirectly expressed by stands with lower canopy cover and larger trees, were directly related to both worse plot health and lower tree regeneration. The widespread tree decline and very low recruitment abundance observed in the open oak woodlands studied may be the consequence of negative synergistic effects of a more limiting climate and land-use legacies from human overexploitation of a fragile ecosystem. These results warn of the negative impacts that land-use practices can exert on similar agrosilvopastoral ecosystems with the added risks of ongoing climatic changes, threatening ecological and economical sustainability.