A major foundation of trait-based ecology is that traits have an impact on individual performance. However, trait-growth relationships have not been extensively tested in trees, especially outside tropical ecosystems. In addition, measuring traits directly related to physiological processes (‘hard traits’) remains difficult and the differences between inter- and intraspecific relationships are seldom explored. Here, we use individual-level data on a set of hydraulic, leaf and stem traits to explore which traits are the best predictors of basal area increment (BAI) and growth efficiency (BAI per unit of tree leaf area, GE) among and within species for six dominant tree species along a water availability gradient under Mediterranean climate (Catalonia, NE Spain). Measured traits include: leaf mass per area (LMA), leaf nitrogen concentration (N), leaf C isotopic composition (d13C), stem wood density (WD), branch-level estimates of the Huber value (Hv), the sapwood-specific hydraulic conductivity (KS), the leaf-specific hydraulic conductivity (KL) and resistance to xylem embolism (P50), and the leaf water potential at turgor loss (Ptlp). Traits were better predictors of GE than BAI and significant relationships were largely driven by differences among species means. Contrary to our initial hypotheses, high values of both growth metrics were associated with ‘conservative’ leaf and hydraulic traits. In particular, BAI was negatively associated with wood density and hydraulic efficiency per unit leaf area (KL), while GE increased with LMA, allocation to sapwood relative to leaves (Hv) and resistance to xylem embolism (P50). Climate effects on BAI and GE were indirectly mediated by changes in traits, stand structure and tree size. Overall, these results suggest that maintaining functionality over extended periods of time may be more important that maximum gas exchange or hydraulic capacity to achieve high radial growth under Mediterranean climates. Our study reveals that the relationships between ‘functional’ traits and tree performance along environmental gradients are complex and do not necessarily conform to simple hypotheses based on our understanding of organ-level processes. Trait integration along common axes of variation together with a revaluation of the variables that better reflect whole-tree performance can greatly improve our understanding of trait-growth relationships.
|Date made available||2 Dec 2020|