The two sides of carbon sink in mountainous mediterranean woodlandsclimate and diversity in new forests assemblages

Abstract

Forests absorb vast amounts of CO2 each year. Owing to the relatively inexpensive cost of planting trees, afforestation is seen as the most appealing solution to mitigate the rising trend of global temperature. In the future, these tree plantations will be new forests subject to the same ecological principles that are true for the few old-growth forests that remain in the globe. Even though the hope placed in afforestations resides fundamentally in the carbon sink produced by the land use change, it should be questioned whether this forest ability can be modified by biotic and abiotic factors. In classical ecology, stress and diversity both operate simultaneously as the two sides of the same coin to drive ecological functions, like biomass production. The aim of this thesis is to ponder the relative role of both drivers to sink carbon using the secondary forests established in the Sistema Central. The first chapter focuses on the exploration of species-specific growth responses to the main large-scale climatic patterns that operate in western Europe. Different species showed different affinities to climatic patterns that were contingent on the biogeographic character. However, all -Atlantic pattern at some extent, highlighting that secondary forests growth can be unstabilized just by the action of one climatic agent. Chapter two address the eternal debate in ecology: the effect of the scale. Sudden extreme droughts can undermine the ability of trees to grow years beyond the impact itself. Results confirmed that the presence of drought legacy effects is also true in an aggregated scale. By contrast, legacies were absent when computed using climate-growth models that accounted for the inherent individual variability of trees. It symbolizes the matter of addressing ecological processes at the same scale at which they operate; in this case, the tree level. The third and last chapter demonstrates the importance of structural diversity of forests in carbon sink. The results show that carbon storage in tree biomass and topsoil is dependent on the interactive effects between canopy age and tree-size heterogeneity. Furthermore, results reveal how secondary forests must develop diverse tree-size structures to maximize the temporal effect of ontogeny and development in carbon sink ability. This thesis increases the current understanding of two opposed biomass accumulation drivers climatic stress and diversity at the tree scale. First, it underlines the importance of the scale and individual variability to manage the uncertainties of the carbon cycle in forests. And second, it remarked the importance of diversity, as tree species diversity proved to generate rich climatic growth responses ready to ameliorate the stress of climate, while tree-size heterogeneity demonstrated to interact with the forest developmental grade to sink more Carbon. The knowledge generated in this work using mature secondary forests enable useful guidelines to increase the carbon sink ability of afforestations.