Topography and aridity influence oak woodland bird assemblages in southern Europe

The increasing frequency and severity of drought spells in southern Europe has been associated with reduced growth and increased defoliation and mortality of several tree species, consequently affecting woodland productivity, structure and associated biodiversity. However, topographic variation and associated microclimatic features may buffer against the negative effects of drought at a local level. Here, we aimed to evaluate the influence of topographic variation on oak woodland communities along an aridity gradient. For this, we quantified cork oak woodland structure, bird species richness, abundance, and caterpillar biomass on north- and south-facing slopes along an aridity gradient across the south-western Iberian Peninsula. Drier microclimates on south-facing slopes had lower tree cover and bird richness and abundance than north-facing slopes, and the magnitude of these differences increased markedly from wetter (tree cover and woodland bird abundance 5–15% lower on south facing slopes) to drier regions (50–60% lower). This reduction in woodland bird richness and abundance is mainly driven by tree foraging species and likely a response to lower tree cover and resource availability, as caterpillar
abundance was also 50% lower on south-facing slopes. The topographic complexity of the Mediterranean Basin seems to be shaping woodland community-level and trophic chain responses to increasing aridity in Southern Europe. Our results suggest that topographic and microclimatic conditions on north-facing slopes can potentially act as refugia for the high biodiversity characteristic of cork oak woodlands and other Mediterranean landscapes. We discuss cork oak woodland management measures needed to help maximize the resiliency of these Mediterranean socio-ecological systems in the face of climate change.