Abstract
Quantifying plant intraspecific trait variation and fitness across species distribution ranges is key to understand population dynamics and local extinctions in the face of global environmental change. Despite the large amount of primary data available in the ecological literature and publicly available data repositories, quantitative syntheses addressing generalities and dissimilarities of such patterns across different species worldwide are still lacking. In this PhD dissertation, I report three studies using meta-analytical approaches to quantify cross-species patterns of plant intraspecific trait variation within environmental and geographic space. A meta-analysis describing the intraspecific response of morpho-physiological leaf traits along elevational gradients is presented in Chapter I. Afterwards, I report a meta-analysis upon survival and intraspecific trait variation following reciprocal transplants within the elevational range of different plant species in Chapter II. Finally, in Chapter III we test whether the probability of occurrence modeled trough species distribution models (SDMs) is coupled with intraspecific trait variability in North American tree species so that individuals have higher fitness at the centre of their species environmental niche than at the edges, which we here define as ‘fitness-centre’ hypothesis. The meta-analysis reported in Chapter I reveals that leaf mass per area (LMA), leaf nitrogen per unit of area (Narea), leaf nitrogen per unit of mass (Nmass) and leaf carbon isotope composition (δ 13C) significantly increase with increasing elevation worldwide. Conversely, leaf area (LA) and leaf phosphorous per unit of mass (Pmass) shows no significant pattern with elevation. We also detect paired cross-species trends of covariation between leaf traits along elevation. By summarizing results of reciprocal transplant experiments in Chapter II, we found that individuals transplanted downward show larger biomass and height compared to their site of origin but failed to reach equal these traits and survival to that of local individuals. Individuals transplanted upward adjust their traits by decreasing plant growth and number of reproductive units to that of local individuals but nevertheless show lower survival. Importantly, we found that such patterns are moderated by temperature and precipitation difference between transplant sites at different elevations. Finally, in Chapter III we show that the ‘fitness-centre’ hypothesis does not hold true in none of the 66 North American tree species analyzed and that the lack of the relationship between individual fitness and the SDM-modeled probability of occurrence is pervasive across species differing in dispersal ability and climatic niche breadth. Because plant functional traits and individual fitness play a crucial role in plant shift and local persistence following altered environmental conditions, studies reported here have relevant implications for biological conservation and contribute to characterize abiotic constraints within geographic distribution ranges. These results overall highlight the importance of meta-analytical approaches to assess cross-species patterns of intraspecific trait variability.