Impacts of tree height on leaf hydraulic architecture and stomatal control in Douglas-fir

This study investigated the mechanisms involved in the regulation of stomatal closure in Douglas-fir and evaluated the potential impact of compensatory adjustments in response to increasing tree height upon these mechanisms. In the laboratory, we measured leaf hydraulic conductance (K-leaf) as leaf water potential (Psi(l)) declined for comparison with in situ diurnal patterns of stomatal conductance (g(s)) and Psi(l) in Douglas-fir across a height gradient, allowing us to infer linkages between diurnal changes in K-leaf and g(s). A recently developed timed rehydration technique was used in conjunction with data from pressure-volume curves to develop hydraulic vulnerability curves for needles attached to small twigs. Laboratory-measured K-leaf declined with increasing leaf water stress and was substantially reduced at Psi(l) values of -1.34, -1.45, -1.56 and -1.92 MPa for foliage sampled at mean heights of approximately 20, 35, 44 and 55 m, respectively. In situ g(s) measurements showed that stomatal closure was initiated at Psi(l) values of -1.21, -1.36, -1.74 and -1.86 MPa along the height gradient, which was highly correlated with Psi(l) values at loss of K-leaf. Cryogenic scanning electron microscopy (SEM) images showed that relative abundances of embolized tracheids in the central vein increased with increasing leaf water stress. Leaf embolism appeared to be coupled to changes in g(s) and might perform a vital function in stomatal regulation of plant water status and water transport in conifers. The observed trends in g(s) and K-leaf in response to changes in Psi(l) along a height gradient suggest that the foliage at the tops of tall trees is capable of maintaining stomatal conductance at more negative Psi(l). This adaptation may allow taller trees to continue to photosynthesize during periods of greater water stress.