Environmental influences and coordination of Rhododendron carbon and water economics at leaf and species scales

The ratio of leaf surface area to dry mass, specific leaf area (SLA), relates function to carbon investment, but how the environment impacts SLA and whether SLA represents whole-plant resource acquisition remains debated. We tested two hypotheses using 12 Rhododendron species from four taxonomic sections with different leaf habits: (1) for leaves, we hypothesized that species, leaf position and light interception impact SLA, but higher SLA would be accompanied by higher net photosynthesis (A), stomatal conductance (gs), and maximum leaf hydraulic conductance (Kleaf), maintaining A/gs and Kleaf/gs across the canopy, and (2) for species, we hypothesized those from stressful climates would have lower SLA, higher Kleaf, higher stomatal density, and smaller stomata. At the leaf-scale, Kleaf was higher for species with lower SLA, contrary to predictions. We observed strong coordination of SLA and carbon to nitrogen ratio, but the relationship of Kleaf/gs to SLA was characterized by species replacement along the leaf economic spectrum, suggesting weak leaf-level trait coordination as a mechanism for low drought tolerance. Across species, lower SLA was associated with lower summer precipitation, lower precipitation seasonality, and larger guard cells. We show that leaf habit and habitat associations shape the functional significance of SLA, determining resource acquisition at leaf and species scales.