Land-use practices (coppices and dehesas) and management intensity modulate responses of Holm oak growth to drought

Abstract

Last decades increase in reported events of drought-induced tree mortality evidences how climate-change is transforming forest ecosystems all over the world. The parallel increase in human pressure over the land is also causing major changes in forest functioning but it remains unclear how these two driving forces interact between them. We combined tree-ring data with aboveground cover, leaf area index (LAI), soil variables, and the standardized precipitation evapotranspiration index (SPEI) as water availability indicator to disentangle the existence of linkages between contrasting Holm oak (Quercus ilex L. subsp. ballota [Desf.] Samp) land-use practices and its drought-induced decline and mortality. We selected ten sites covering different soil and climatic gradients, land-use practices (i.e., declining dehesas, DH; declining coppices, FRd; and healthy coppices, FRh), and tree vigour classes (i.e., living, affected, and dead trees) in Spain. DH sites presented lower tree coverage, soil water holding capacity and soil pH than coppice (FR) sites. Dead Holm oaks from DH sites were younger than living ones, whereas dead trees from FRd sites were smaller and showed lower growth rates than living ones. We also found that conservation of traditional land-use practices in FR sites, resulting in less understorey cover but more soil erosion (less nutrients and microbial biomass and more bare soil), may positively affect the growth plasticity and sensitivity to drought of Holm oak trees by alleviating inter-specific competition, but in detriment of vegetation cover and soil health. Further studies should evaluate whether what holds true for FRh sites regarding the maintenance of traditional land-use practices might also apply for healthy DH. In the face of drier and hotter scenarios, our results add robust evidences on how the modulation of the intensity of the traditional uses could be a useful tool to optimize ecosystem services in Mediterranean systems highly vulnerable to climate change.