Growth maximization trumps maintenance of leaf conductance in the tallest angiosperm.

Structural and physiological changes that occur as trees grow taller are associated with increased hydraulic constraints on leaf gas exchange, yet it is unclear if leaf-level constraints influence whole-tree growth as trees approach their maximum size. We examined variation in leaf physiology, leaf area to sapwood area ratio (L/S), and annual aboveground growth across a range of tree heights in Eucalyptus regnans. Leaf photosynthetic capacity did not differ among upper crown leaves of individuals 61.1-92.4 m tall. Maximum daily and integrated diurnal stomatal conductance (g s) averaged 36 and 34% higher, respectively, in upper crown leaves of ~60-m-tall, 80-year-old trees than in ~90-m-tall, 300-year-old trees, with larger differences observed on days with a high vapor pressure deficit (VPD). Greater stomatal regulation in taller trees resulted in similar minimum daily leaf water potentials (Ψ L) in shorter and taller trees over a broad range of VPDs. The long-term stomatal limitation on photosynthesis, as inferred from leaf δ (13)C composition, was also greater in taller trees. The δ (13)C of wood indicated that the bulk of photosynthesis used to fuel wood production in the main trunk and branches occurred in the upper crown. L/S increased with tree height, especially after accounting for size-independent variation in crown structure across 27 trees up to 99.8 m tall. Despite greater stomatal limitation of leaf photosynthesis in taller trees, total L explained 95% of the variation in annual aboveground biomass growth among 15 trees measured for annual biomass growth increment in 2006. Our results support a theoretical model proposing that, in the face of increasing hydraulic constraints with height, whole-tree growth is maximized by a resource trade-off that increases L to maximize light capture rather than by reducing L/S to sustain g s.

Effects of height on treetop transpiration and stomatal conductance in coast redwood (Sequoia sempervirens).

Treetops become increasingly constrained by gravity-induced water stress as they approach maximum height. Here we examine the effects of height on seasonal and diurnal sap flow dynamics at the tops of 12 unsuppressed Sequoia sempervirens (D. Don) Endl. (coast redwood) trees 68-113 m tall during one growing season. Average treetop sap velocity (V(S)), transpiration per unit leaf area (E(L)) and stomatal conductance per unit leaf area (G(S)) significantly decreased with increasing height. These differences in sap flow were associated with an unexpected decrease in treetop sapwood area-to-leaf area ratios (A(S):A(L)) in the tallest trees. Both E(L) and G(S) declined as soil moisture decreased and vapor pressure deficit (D) increased throughout the growing season with a greater decline in shorter trees. Under high soil moisture and light conditions, reference G(S) (G(Sref); G(S) at D = 1 kPa) and sensitivity of G(S) to D (-δ; dG(S)/dlnD) significantly decreased with increasing height. The close relationship we observed between G(Sref) and -δ is consistent with the role of stomata in regulating E(L) and leaf water potential (Ψ(L)). Our results confirm that increasing tree height reduces gas exchange of treetop foliage and thereby contributes to lower carbon assimilation and height growth rates as S. sempervirens approaches maximum height.

Physiological consequences of height-related morphological variation in Sequoia sempervirens foliage.

This study examined relationships between foliar morphology and gas exchange characteristics as they vary with height within and among crowns of Sequoia sempervirens D. Don trees ranging from 29 to 113 m in height. Shoot mass:area (SMA) ratio 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 (A(max,m)), standardized photosynthesis (A(std,m)) and internal CO(2) conductance (g(i,m)) decreased with height and SMA, while the light compensation point, light saturation point, and mass and area-based rates of dark respiration (R(m)) increased with height and SMA. Among foliage from different heights, much of the variation in standardized photosynthesis was explained by variation in g(i,) consistent with increasing limitation of photosynthesis by internal conductance in foliage with higher SMA. The syndrome of lower internal and stomatal conductance to CO(2) and higher respiration may contribute to reductions in upper crown growth efficiency with increasing height in S. sempervirens trees.