Land-Use Legacies and Climate Change as a Double Challenge to Oak Forest Resilience: Mismatches of Geographical and Ecological Rear Edges

Global change challenges ecosystems in xeric locations transformed by intensive human use. Resilience to drought of relict Mediterranean Quercus pyrenaica populations in the southern Iberian Peninsula was analyzed in relation to historical records of land use, combining dendroecological growth of adult trees and greenness (EVI) as proxies for secondary and primary growth. The growth trends reflected a strong influence of old land-use legacies (firewood removal) in the current forest structure. Trees were highly sensitive to moisture availability, but both primary growth and secondary growth expressed high resilience to drought events over the short and the long term. Resilience and the tree growth response to climate followed a water-stress gradient. A positive growth trend since the late 1970s was particularly evident in mesic (colder and wetter) high-elevation stands, but absent in the most xeric (warmer and drier) stands. The high values of resilience observed suggest that the studied Q. pyrenaica populations are located in a geographical but not a climatic or ecological rear edge. Resilience of oak stands to drought events was not spatially homogeneous across the mountain range, due to differences in ecological conditions and/or past management legacies. This is particularly relevant for rear-edge populations where topographic and biophysical variability can facilitate the existence of refugia.