Hydraulic architecture and tracheid allometry in mature Pinus palustris and Pinus elliottii trees.

摘要

Pinus palustris Mill. (longleaf pine, LL) and Pinus elliottii Engelm. var. elliottii (slash pine, SL) frequently co-occur in lower coastal plain flatwoods of the USA, with LL typically inhabiting slightly higher and better-drained microsites than SL. The hydraulic architecture and tracheid dimensions of roots, trunk and branches of mature LL and SL trees were compared to understand their role in species microsite occupation. Root xylem had higher sapwood-specific hydraulic conductivity (k_s) and was less resistant to cavitation compared with branches and trunk sapwood. Root k_s of LL was significantly higher than SL, whereas branch and trunk k_s did not differ between species. No differences in vulnerability to cavitation were observed in any of the organs between species. Across all organs, there was a significant but weak trade-off between water conduction efficiency and safety. Tracheid hydraulic diameter (D_h) was strongly correlated with k_s across all organs, explaining >73% of the variation in k_s. In contrast, tracheid length (L_t) explained only 2.4% of the variability. Nevertheless, for trunk xylem, k_s was 39.5% higher at 20 m compared with 1.8 m; this increase in k_s was uncorrelated with D_h and cell-wall thickness but was strongly correlated with the difference in L_t. Tracheid allometry markedly changed between sapwood of roots, trunks and branches, possibly reflecting different mechanical constraints. Even though vulnerability to cavitation was not different for sapwood of roots, branches or the trunks of LL and SL, higher sapwood to leaf area ratio and higher maximum sapwood-specific hydraulic conductivity in roots of LL are functional traits that may provide LL with a competitive advantage on drier soil microsites.