Coordination of leaf structure and gas exchange along a height gradient in a tall conifer

The gravitational component of water potential and frictional resistance during transpiration lead to substantial reductions in leaf water potential (Psi(l)) near the tops of tall trees, which can influence both leaf growth and physiology. We examined the relationships between morphological features and gas exchange in foliage collected near the tops of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) trees of different height classes ranging from 5 to 55 m. This sampling allowed us to investigate the effects of tree height on leaf structural characteristics in the absence of potentially confounding factors such as irradiance, temperature, relative humidity and branch length. The use of cut foliage for measurement of intrinsic gas-exchange characteristics allowed identification of height-related trends without the immediate influences of path length and gravity. Stomatal density, needle length, needle width and needle area declined with increasing tree height by 0.70 mm(-2) m(-1), 0.20 mm m(-1), 5.9 x 10(-3) mm m(-1) and 0.012 mm(2) m(-1), respectively. Needle thickness and mesophyll thickness increased with tree height by 4.8 x 10(-2) mm m(-1) and 0.74 microm m(-1), respectively. Mesophyll conductance (g(m)) and CO(2) assimilation in ambient [CO(2)] (A(amb)) decreased by 1.1 mmol m(-2) s(-1) per m and 0.082 micromol m(-2) s(-1) per m increase in height, respectively. Mean reductions in g(m) and A(amb) of foliage from 5 to 55 m were 47% and 42%, respectively. The observed trend in A(amb) was associated with g(m) and several leaf anatomic characteristics that are likely to be determined by the prevailing vertical tension gradient during foliar development. A linear increase in foliar delta(13)C values with height (0.042 per thousand m(-1)) implied that relative stomatal and mesophyll limitations of photosynthesis in intact shoots increased with height. These data suggest that increasing height leads to both fixed structural constraints on leaf gas exchange and dynamic constraints related to prevailing stomatal behavior.