Understanding tree sapling functional strategies for forest recovery

Understanding the spectrum of functional possibilities in plant organisms and the impacts of different trait combinations across environments is a central goal in comparative plant ecology. Identifying functional strategies overcomes the need for detailed information in all species, simplifying reality without assuming equal survival probabilities. The study of functional trait variability enables us to understand how plants respond to historical and spatial environmental diversity, and to predict whether they will keep pace with environmental changes. Furthermore, the distribution and assemblage of species with different functional properties ultimately define ecosystem productivity and resilience. For these reasons, functional traits have been proposed as a quantitative tool for defining ecological targets and overcoming current flaws in ecosystem restoration programmes. This thesis focused on functional responses of juvenile trees to drought, which is one of the primary environmental pressures hindering tree establishment. Under these conditions, leaf gas exchange, water transport, and carbon metabolism decline at different rates, depending on their sensitivity and the plant's functional strategy. It is therefore crucial to develop an integrated understanding of all these processes to comprehend their connections to tree mortality and the capacity of species to adjust to external conditions. This thesis aims to address current gaps in this regard, focusing on tree saplings at different taxonomic scales, to help inform management-oriented decisions in forest restoration. I defined specific objectives for each chapter of this dissertation. (1) Determine whether tree species with different water-use strategies exhibit different sensitivity of leaf gas exchange to environmental vapour pressure deficit and volumetric water content; assess whether this, in tandem with leaf shedding, translates into differential non-structural carbohydrate (NSC) accumulation; and whether NSCs are reduced at mortality. Results showed that the species had comparable sensitivity to vapour pressure deficit and volumetric water content, but different rates of leaf gas exchange, with consequences for non-structural carbohydrate allocation. NSCs were maintained at relatively high levels in recently dead trees compared to alive conspecifics, and starch was reduced, suggesting a reallocation before mortality. (2) Assess trait combinations explaining interspecific differences in drought-induced mortality, inter-annual mortality, and the effect of soil hydraulic properties in buffering plant hydraulic dysfunction, through a functional trait-enabled modelling approach. I found that resistance to embolism better explained time to mortality than minimum stomatal conductance in the assessed species during one summer season. The balance between crown development and rooting depth differently influenced interannual differences in the percentage loss of hydraulic conductivity across species. Plant hydraulic dysfunction was soil-specific and influenced by specific soil hydraulic parameters of the van Genuchten model that regulate soil water retention. (3) Understand how different oak plantations respond to contrasting temperature and precipitation regimes across a latitudinal gradient spanning from Temperate to Mediterranean climates in terms of water relations, non-structural carbohydrates, and morphology. I found evidence of greater seasonal fluctuations in sugar levels in the twigs and stems of Mediterranean plantations compared to Temperate conspecifics, as a strategy to cope with drought, along with increased root allocation and more acquisitive leaf function. Results also indicated distinct turgor loss point regulation in the two Mediterranean plantations at comparable levels of water stress. Despite greater NSC concentrations in summer, Mediterranean saplings showed reduced non-structural carbohydrate reserves than Temperate conspecifics at the end of the growing season, particularly in Q. ilex. Given the central role of NSCs in supporting drought recovery processes, this reduced accumulation may compromise their ability to respond to future and recurrent drought events. Furthermore, the greater xylem embolism and mortality in Mediterranean saplings suggest that functional adjustments were not sufficient to fully compensate for greater drought stress. Overall, findings shed light on the physiological mechanisms by which plants cope with drought at different scales, as well as on the functional trait combinations that may facilitate tree establishment. By integrating plant morphology, water relations, and carbon metabolism, this work provides novel insights which can help inform restoration decisions and define functional metrics of plant performance.