Tree Physiology Advance Access published online on December 19, 2008
Tree Physiology, doi:10.1093/treephys/tpn024
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Coordination of leaf structure and gas exchange along a height gradient in a tall conifer
1 USDA Forest Service, Forestry Sciences Laboratory, Corvallis, OR 97331, USA
2 Corresponding author (david.woodruff{at}oregonstate.edu)
3 Department of Wood Science and Engineering, Oregon State University, Corvallis, OR 97331, USA
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Abstract |
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The gravitational component of water potential and frictional resistance during transpiration lead to substantial reductions in leaf water potential (
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 mm2 m–1, respectively. Needle thickness and mesophyll thickness increased with tree height by 4.8 x 10–2 mm m–1 and 0.74 µm m–1, respectively. Mesophyll conductance (gm) and CO2 assimilation in ambient [CO2] (Aamb) decreased by 1.1 mmol m–2 s–1 per m and 0.082 µmol m–2 s–1 per m increase in height, respectively. Mean reductions in gm and Aamb of foliage from 5 to 55 m were 47% and 42%, respectively. The observed trend in Aamb was associated with gm 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 
13C values with height (0.042
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.
Keywords: growth limitation, leaf anatomy, mesophyll conductance, photosynthesis, Pseudotsuga menziesii
Received June 6, 2008; Accepted October 3, 2008
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