Foliar morphology varies within the crowns of Sequoia sempervirens trees as shoot mass:area (SMA) increases with height. This plasticity is a response to light and hydraulic constraints. If height-related factors constrain foliar structure, foliar physiological functions may be compromised in tall trees. SMA is often inversely correlated with leaf internal conductance, gi, the diffusion of CO2 from the substomatal chamber to the sites of carboxylation. Thus, the increase in SMA with height might limit photosynthesis by reducing gi. Increased SMA may also impact gas exchange by affecting the balance of photosynthesis and respiration, as high SMA is associated with decreased photosynthesis and higher respiration. We examined relationships between foliar morphology and gas exchange as they vary with height within and among crowns of S. sempervirens trees ranging from 29 m to 113 m in height. We addressed the following questions: 1) Does leaf internal CO2 conductance decrease with height? 2) Does dark respiration (Rm) increase with height? We measured gas exchange with a Li-Cor 6400 in cut-hydros taken from treetops ranging from 30 to 110 m in height. Cut-hydros were also taken from the low- and mid-crown of 110 m trees for a within-tree gradient.
Results/Conclusions
SMA increased with height and was less responsive to changes in light availability as height increased, suggesting a transition from light to water relations as the primary determinant of morphology with increasing height. Mass-based rates of maximum photosynthesis (Amax,m), standardized photosynthesis (Astd,m), and internal CO2 conductance (gi,m) decreased with height and SMA, while mass- and area-based rates of dark respiration (Rm) increased with height and SMA. Thus, the morphological and anatomical changes that occur along the vertical gradient in redwood trees have photosynthetic consequences. Results of this study suggest that hydrostatic tension contributes to structural changes at the shoot level that indirectly reduce net photosynthesis via increased respiration rates and decreased internal CO2 conductance. Among foliage from different heights, much of the variation in standardized photosynthesis was explained by variation in gi. The syndrome of lower internal conductance to CO2 and higher respiration may contribute to reductions in upper crown growth efficiency with increasing height in S. sempervirens trees. Higher respiration and lower internal conductance are two costs associated with increasing height that limit net photosynthesis near the tops of the tallest trees.
