Effects of age-related increases in sapwood area, leaf area, and xylem conductivity on height-related hydraulic costs in two contrasting coniferous species
Résumé
Knowledge of vertical variation in hydraulic parameters would improve our understanding of individual trunk functioning and likely have important implications for modeling water movement to the leaves. Specifically, understanding how foliage area (A (l)), sapwood area (A (s)), and hydraulic specific conductivity (k (s)) vary with canopy position to affect leaf-specific conductivity (LSC) and whole-tree leaf-specific hydraulic conductance (K (l)) may explain some of the contrasting patterns of A (l)/A (s) reported in the literature. The general aim of the study was to characterize and compare the aboveground relationships between cumulative A (l), A (s), and k (s) for two Pacific Northwest coniferous species with contrasting sapwood areas to give insight into size-related design of trees for water transport through changes in LSC and K (l). The 230-year-old ponderosa pine (Pinus ponderosa) trees had slightly smaller basal diameters than the 102-year-old Douglas-fir (Pseudostuga menziesii) trees, but contained 85% sapwood at the base compared to 30% in Douglas-fir. At the tree base, there was no evidence that A (l)/A (s) decreased with tree age or with tree size. In both species, A (l)/A (s) of branches was significantly higher than A (l)/A (s) at the tree base, but it was not different from A (l)/A (s) measured in the trunks at the top of the tree. Douglas-fir had higher A (l)/A (s) at the base than did ponderosa pine (0.42 vs. 0.24 m(2) cm(-2)), similar patterns of change in A (l)/A (s) with height, and similar values of k (s), such that LSC in Douglas-fir was 77% the value of LSC in ponderosa pine. Compensating changes to increase LSC between short and tall trees occurred through an increased in k (s) in tall trees but not through a reduction in A (l). LSC increased logarithmically with branch path length or trunk path length whereas K (l) decreased significantly from top to base of old trees, but not between sections from old and young trees of similar cambial age. Even though ponderosa pine had three times more sapwood than Douglas-fir, this study revealed a common relationship of declining K (l) with increasing tree height and diameter between the two species, within age classes and among trees. There was no compensating decrease in A (l)/A (s) as trees got taller, which showed that a homeostasis in K (l) was not maintained during growth. The trend of higher allocation of biomass to sapwood over leaves in ponderosa pine is consistent with this species' tendency to inhabit drier sites than Douglas-fir.
Origine : Accord explicite pour ce dépôt
Loading...