SHIFTS IN PHENOLOGY AND PLANT ARCHITECTURE ACROSS GENUS RHODODENDRON HIGHLIGHT DIFFERENT WAYS TO BECOME MORE ACQUISITIVE DESPITE UNIVERSALLY CONSERVATIVE XYLEM ANATOMY

摘要

Premise of research. Across the leaf economics spectrum, leaves with higher specific leaf area (SLA) have higher transpiration rates, so xylem water transport capacity should be higher for species with more acquisitive leaf traits. Not individual leaves but rather whole-plant traits determine overall water balance, and past selection on xylem traits, phenology, or plant architecture could result in different patterns of leaf-xylem coordination across SLA. Methodology. For Rhododendron clades with evergreen (10 species), semievergreen (5 species), and deciduous (6 species) phenology, we used phylogenetic least squares to determine relationships among leaf economics, stem xylem anatomy, and plant architecture, including maximum plant height and branch-level variation in SLA (CV SLA). Pivotal results. Xylem anatomy indicates universally conservative stem water transport compared with most angiosperms, and leaf and xylem traits of closely related species showed significant similarity. Higher SLA was associated with a significantly lower ratio of leaf carbon to nitrogen and with a larger vessel diameter, but more acquisitive clades also had significantly lower branch leaf area and shorter maximum height. Within deciduous species, SLA increased significantly as stem water transport capacity declined, while for evergreens and semi-evergreens, CV SLA and leaf size were significantly larger for species with lower transport capacity. Last, evergreen water transport capacity per leaf area (Huber value) declined with increasing maximum height, but semievergreens had significantly higher Huber values, driven by small leaf size. Conclusions. Across Rhododendron clades, branch leaf area and plant height may be limited by past selection on xylem traits. Within evergreen and semievergreen clades, higher CV SLA may reflect turgor loss during growth or acclimation to the growth environment, which could promote rapid growth under good conditions without risking whole-plant water balance. Within the deciduous clade, more acquisitive species potentially operate closer to hydraulic limits, risking defoliation under stress. These clade-specific patterns of trait coordination suggest that, to understand the whole-plant economic spectrum, we must consider evolutionary history.