Within the leaf economics spectrum, evergreens have lower specific leaf area (SLA), are more resistant to climate stress, but have reduced gas exchange rates compared to deciduous leaves. Genus Rhododendron contains over 900 species from sub-tropical to alpine environments and representing a range of SLA. As climate determines evaporative demand, and gas exchange differs across SLA, we hypothesized that Rhododendron should vary in xylem hydraulic supply to match leaf water demand. Also, high within-species variation may buffer against changes in climate, but past selection could differentially constrain leaf and wood traits, leading to differences in trait coordination across growth environments, or over evolutionary time.
Using 27 species grown in two common gardens with differing climates (Holden Arboretum, Kirtland, OH and Rhododendron Species Botanical Garden, Seattle, WA), we determine the effects of evolutionary history, climate of origin, and growth environment on leaf (SLA, leaf size, leaf C:N) and xylem traits (vessel density, vessel diameter, xylem hydraulic diameter(Dh)), and on leaf-xylem coordination. Closely related species were significantly more similar than expected by chance in mean vessel diameter, Dh, leaf size and SLA. Therefore, we used phylogenetic generalized least squares to analyze mean traits, while within-species variation was analyzed using generalized least squares.
We found that Rhododendronconforms to expected leaf economics relationships, with higher SLA associated with significantly lower C:N. For all species, xylem vessels were small compared to all other angiosperms. We detected a significant negative relationship between Dh and C:N, both across and within clades. Vessel diameter showed a positive relationship to SLA across clades, however, within clades trait relationships were not always as expected: SLA was negatively related to vessel diameter for deciduous and semi-evergreen clades. Growth environment and mean annual precipitation in the climate of origin significantly affected vessel density, leaf size and C:N in accordance with leaf economics predictions. Within-species variation in SLA was significantly negatively related to within-species variation in Dh. A t-test indicated that, compared to leaf traits, wood traits are significantly less variable among species and across growth environments.
Leaf gas exchange rates of Rhododendron are likely constrained by wood anatomy, however, clades with more constrained wood anatomy compensate through increased leaf variability. The clade-specific leaf-wood relationships suggest that the leaves of some species may be hydraulically under-supplied, with implications for embolism risk and climate tolerance in a warming world.