Are leaf, stem and hydraulic traits good predictors of individual tree growth?

A major foundation of trait-based ecology is that traits have an impact on individual performance. However, trait–growth relationships have not been extensively assessed in trees, especially outside tropical ecosystems. In addition, measuring traits directly related to physiological processes 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 assess their ability to predict 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 (δ¹³C), the leaf water potential at turgor loss (P_tlp), stem wood density (WD) and branch-level estimates of the Huber value (Hv), sapwood- and leaf-specific hydraulic conductivity (K_S and K_L) and resistance to xylem embolism (P₅₀). Trait–growth associations were generally weak, particularly for BAI and within species. High values of both growth metrics were associated with ‘conservative’ leaf and hydraulic traits. In particular, BAI was negatively associated with K_L (and wood density), 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 basal area. Overall, these results suggest that maintaining functionality over extended periods of time may be more important than maximum gas exchange or hydraulic capacity to achieve high radial growth under Mediterranean climates. Our study reveals that widely used ‘functional traits’ may be poor predictors of tree growth variability along environmental gradients. Moreover, trait effects (when present) do not necessarily conform to simple hypotheses based on our understanding of organ-level processes. An improved understanding of trait coordination 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. A free Plain Language Summary can be found within the Supporting Information of this article.