The Hydraulic Design of Mesomorphic Tree Leaves : Hydraulic Resistance, Anatomical Correlates, and the Effects of Damaging Venation
Cowan, Peter D.
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The leaf hydraulic design of sun and shade leaves was investigated for several species of trees at the Harvard Forest in Petersham, MA. Leaf feature measured included leaf hydraulic resistance and stomatal density, and pressure-volume (PV) curves, which provide leaf parameters such as turgor loss point (TLP), leaf capacitance near TLP (Cd), and osmotic potentials. Using vacuum infiltration several other characteristics were measured, including specific leaf area (SLA), bulk density, and leaf water content (LWC). A total of 16 aspects of leaf design were measured, ranked, and correlations were sought. Species differences and the effects light regime (sun vs. shade) during development on the leaf characteristics were tested. Significant correlations were found between leaf parameters. Species differed significantly and the effect of light regime was considerable on leaf resistance, SLA, leaf water content, and turgor loss point. This information will facilitate the formation of predictive models and deeper ecological understanding of niche selection of leaf design features. The study was expanded to investigate leaf vascular design by cutting the different leaf vein orders and measuring effects on transpiration rate. Leaves of Quercus rubra and Acer saccharum were cut to restrict flow to certain veins, experimental leaves were paired with controls. In Acer most of the leaves died, while in Quercus they survived for four weeks, though, there was a marked drop off in transpiration compared to the controls. This suggest that leaves vary greatly in their ability to withstand damage, that vein systems are not redundant and that low vein order may be responsible for most of a leafs resistance.