Understanding the role of plant hydraulic traits in ecological processes

Abstract

Among all ecosystems tropical forests play a disproportionate role in Earth’s carbon cycle as they account for one third of total primary productivity. Global environmental change is causing novel precipitation patterns, shifts in the timing of rainfall events and an increase of the atmospheric water demand in tropical regions. Such changes pose a threat to the ecosystem services provided by tropical forest ecosystems as there is great uncertainty of how tropical forests will respond to warmer and drier climatic conditions. Plant responses to drought occur at different biological and temporal scales, ranging from responses at the tissue level to responses that define species and populations. Plant hydraulic traits have proven to be key in the characterization of how plants respond to drought, as they allow to group species in those that avoid drought and those that resist drought. Therefore, quantifying the sources of variation in plant hydraulic traits presents a promising way to move forward our understanding of how tropical forests will respond to climate-change. The main objective of this research was to study the aspects that drive variation in plant hydraulic traits in tropical plant species, and how nutrient availability affects tropical forest responses to drought. In Chapter 1, I performed a study in which I investigated if it is possible to generalize tree species drought tolerance and growing strategies using their leaf habit (i.e., deciduous, evergreen and semi-deciduous). In this study I found that in the tropical dry forest deciduous species tend to share similarities in their physiological properties, contrary to evergreen species that tend to be physiologically different across sites. Generally deciduous species will show a hydraulically risky growing strategy, and evergreen species a hydraulically safe growing strategy. In Chapter 2, I carried out a large-scale ecosystem manipulation experiment to study how both water and nutrient availability control primary productivity in a tropical dry forest. I found that nutrient availability had a stronger but weak effect on forest productivity that a decrease in soil moisture. Forest productivity was more sensitive to inter-annual variation in climatic conditions, as we observed large decreases in productivity in warmer and drier years when compared to the effect of the experimental treatments. In Chapter 3, I performed a data synthesis to study how environment and evolutionary history shape plant hydraulic trait variation among evergreen and deciduous species across the world tropics. Through this pantropical data synthesis, I was able to point out biases in the assessment of tropical plants drought tolerance and that environment play a stronger role in shaping plant hydraulic traits than species’ evolutionary history. I also demonstrated that deciduous species tend to have similar vulnerability to drought across environmental conditions, while evergreen species hydraulic traits vary as a function of water availability. Collectively, these three studies help us to understand the drivers of drought tolerance in tropical plants and how environmental variation shapes responses to drought in the tropical forests.