Differential aridity-induced variations in ecosystem multifunctionality between Iberian Pinus and Quercus Mediterranean forests

Abstract

Forests deliver multiple ecosystem functions and services (i.e. multifunctionality) of great importance to society. However, such multifunctionality capacity is threatened by global warming, especially in Mediterranean forests, which are at high risk of desertification according to climate projections. Certain studies suggest that rising aridity could drastically reduce ecosystem multifunctionality. Yet, the application of this pattern to forests remains uncertain due to their inherent structural complexity and the critical influence of species identity, which make the aridity-multifunctionality relationship not straightforward. Here, we studied 56 plots dominated by four key Mediterranean tree species from Pinus and Quercus genus (Pinus halepensis Mill., Pinus pinaster Ait., Quercus faginea Lam., Quercus ilex L.) along aridity gradients. In each plot, we measured ten ecosystem functions (EFs) such as primary productivity, forest regeneration, biodiversity, decomposition, and nutrient cycling. From these EFs, we assessed shifts in both single-EF and multifunctionality (calculated from averaging and multivariate approaches) across aridity gradients. Our findings indicated that while Pinus forests significatively suffer a multifunctionality loss with increasing aridity, Quercus-dominated forests showed fluctuations in multifunctionality across aridity levels. Forest productivity emerged as a key driver shaping the negative multifunctionality-aridity relationship in Pinus forests. However, the effect of primary productivity was not always synergistic with other ecosystem functions as it has been often assumed and its exclusion from the multifunctionality index calculation did not change the overall pattern of a decrease of ecosystem multifunctionality with aridity. Moreover, a notable loss of ecosystem functional covariation (i.e. interactions between EFs) was found with aridity, more pronounced in Pinus than in Quercus. The contrasting aridity-driven responses in multifunctionality and their EFs interactions between Pinus vs. Quercus forests highlight the fact that distinct management strategies will have to be employed to face climate change and ensure the capacity of these fragile forests to provide essential ecosystem services.