Understanding trait interactions and their impacts on growth in Scots pine branches across Europe

Plants exhibit a wide variety in traits at different organizational levels. Intraspecific and interspecific studies have potential to demonstrate functional relationships and trade-offs amongst traits, with potential consequences for growth. However, the distinction between the correlative and functional nature of trait covariation presents a challenge because traits interact in complex ways. We present an intraspecific study on Scots pine branches and use functional multi-trait concepts to organize and understand trait interactions and their impacts on growth. Branch-level traits were assessed for 97 branches from 12 Scots pine sites across Europe. To test alternative hypotheses on cause-effect relationships between anatomical traits, hydraulic traits and branch growth, we measured for each branch: the tracheid hydraulic diameter, double cell wall thickness, cell lumen span area, wood density, cavitation vulnerability, wood-specific hydraulic conductivity, the leaf area to sapwood area ratio and branch growth. We used mixed linear effect models and path models to show how anatomical traits determine hydraulic traits and, in turn, how those traits influence growth. Tracheid hydraulic diameter was the best predictor of cavitation vulnerability (R 2=0·09 explained by path model) and specific conductivity (R 2=0·19) amongst anatomical traits. Leaf area to sapwood area ratio had the strongest direct effect on branch growth (R 2=0·19) and was positively associated with the tracheid hydraulic diameter (R 2=0·22). A number of bivariate correlations between traits could be explained by these functional relationships amongst traits. The plasticity in tracheid hydraulic diameter (10.0-15.1μm) and leaf area to sapwood area ratio (600-6051cm 2cm -2) and the maintenance of a minimum leaf water potential (between -2 and -2·5MPa) appear to drive the anatomical and hydraulic traits of Scots pine across Europe. These properties are major drivers of the functional trait network underlying the growth variation amongst pine branches and thus possibly contribute to the ecological success of pines at a local and continental scale.