Resumen de Integrating hydraulics into the functional traits framework to understand plant adjustments along a water availability gradient
One of the main contributions of forest ecologists, in the 21st century, is to provide ecological theory and tools to describe and predict forests ecosystem changes caused by the ongoing global change. Over the last decade, ‘functional trait-based ecology’ has emerged as a refreshed discipline with the promise to turn ecology from a primarily descriptive science into a more mechanistic and predictive discipline. However, several foundational assumptions of trait-based ecology have not been rigorously tested. It is presumed that organ-level traits can be easily scaled-up to whole-plant traits, that intraspecific trait variability (ITV) can be largely overlooked and that traits affect individual demographic outcomes and thus, are functional. Additionally, most trait-based approaches study ‘soft’ traits which are relatively easy and quick to measure for a large number of samples although they are not directly linked to specific physiological mechanisms. We argue that plant hydraulic traits can provide useful insights to the understanding of plant ecological strategies. Water transport throughout the plant affects both photosynthetic rate and growth. Plant hydraulics allow linking water to the carbon/nutrient economics and determine plants’ drought resistance and thus, are key factors when assessing forest vulnerability to climate change.
The main aim of this thesis is to integrate plant hydraulics into a functional trait-based framework, to assess trait variability, relationships and trade-offs at different ecological scales and to use this information to define strategies to cope with drought stress. To achieve this objective, two different study approaches were followed: one based on compiling a global dataset for 1149 species worldwide (Chapter 2), and another based on field data collection of a set of leaf, stem and hydraulic traits along a water availability gradient for six of the dominant tree species in Catalonia (NE Spain) (Chapter 3 and Chapter 4). Specifically, in Chapter 2 we test a new framework relating hydraulic and more ‘standard’ traits across species at the global scale. In Chapter 3 we investigate the adjustments and coordination of hydraulic, leaf and stem traits along a water availability gradient at the interspecific and intraspecific levels. Finally, in Chapter 4 we test the functional importance of traits studied in the previous chapter, exploring the strength of the association between traits and tree growth also at the interspecific and intraspecific levels.
A significant finding to emerge from this thesis is that we do not find support for a world-wide ‘fast-slow’ plant economics spectrum that integrates across organs and resources (carbon, nutrients and water). Thus, scaling-up from organ level traits to whole-plant traits and resource use strategies may be more challenging than commonly anticipated because of compensatory responses within individuals. We also show that the ITV is especially relevant for integrative traits that involve more than one organ and that accounting for ITV is a necessary step forward towards improving our understanding of plant adjustments to environmental changes. Finally, we also show that our understanding of trait-growth (and by extension trait-performance) relationships can be greatly improved by selecting traits closely related to physiological functions and context-specific environmental drivers, integrating them along common axes of variation, and re-assessing the variables that are used to reflect whole-tree performance
