Comparative hydraulic architecture of early and late successional tropical tree species

Background/Question/Methods

Plant hydraulic architecture (PHA) has been linked to water transport sufficiency, photosynthetic rates, growth form and attendant carbon allocation. Despite its influence on traits central to conferring an overall competitive advantage in a given environment, few studies have examined whether key aspects of PHA are indicative of successional stage, especially within mature individuals. While it is well established that wood density (WD) tends to be lower in early- versus late successional tree species, and that WD can influence PHA, the interaction of WD, successional stage and the consequent implications on PHA have not been sufficiently explored. We studied the influence of WD and successional stage on PHA over a range of scale from vessels to stem segments in trunks and twigs to explore how hydraulic parameters are integrated to influence whole-tree hydraulic conductance in tree species from early versus late successional tropical forests in Panama. We hypothesized that mean WD would be lower in pioneer than late successional species and that this trend would be associated with greater vessel diameters in stems of pioneer species. We further hypothesized that the preceding properties would scale up to greater whole-tree hydraulic conductance in pioneer species than late successional species.

Results/Conclusions

Although the trunk WD was indistinguishable between the successional groups, the branch WD was lower in the early successional species. Across all species, WD correlated negatively with vessel diameter and positively with vessel packing density. The ratio of branch: trunk vessel diameter, branch sap flux and whole-tree leaf-specific conductance scaled negatively with branch WD across species. Pioneer species showed greater sap flux in branches than in trunks and a greater leaf-specific hydraulic conductance, suggesting pioneer species can move greater quantities of water at a given tension gradient. In combination with greater water storage capacitance associated with lower WD, these results suggest these pioneer species can save on carbon expenditures needed to build safer xylem and instead allow more carbon to be allocated to rapid growth.