International Journal of Plant Sciences
Seasonal changes in leaf specific mass, nitrogen, chlorophyll, and photosynthetic properties were measured for two groups of spatially intermixed Quercus douglasii trees with different drought histories and apparently different root architectures. One group, referred to as ''high-psi(pd) trees, included trees with low amounts of fine root biomass in the upper 50 cm of soil and high predawn xylem pressure potentials (psi(pd)) during summer drought. These two characteristics indicate that trees in this group have deep roots, which may reach the water table. The second group, referred to as ''low-psi(pd) trees, had three to five times higher fine root biomass in the upper 50 cm of soil and low psi(pd) during summer drought. These two characteristics indicate that these trees may not have access to the water table and are dependent on shallow soil moisture, which decreases rapidly during the rainless summers of central California. In the spring, after the full expansion of new leaves, but prior to significant divergence in psi(pd) between the groups, leaf area per leaf, leaf specific mass, chlorophyll per leaf area, incident quantum yield, leaf respiration rate, and irradiance at light compensation were lower for low-psi(pd) trees than for trees with high psi(pd). Nitrogen per leaf area did not differ between the groups. Net photosynthetic capacity at 2000 mu mol m(-2) s(-1) (A(max)) per leaf area was similar among all trees in the spring, but A(max)/leaf mass during the spring was higher for trees that eventually would develop low seasonal psi(pd). Since differences existed between new cohorts of leaves produced in the spring before summer drought, when psi(pd) was similar, we suggest that some leaf characteristics of Q. douglasii trees are determined by the de ree of drought exposure experienced in previous years, or by genetic variation within the species. During the rainless summer and fall seasons, A(max)/leaf area, A(max)/leaf mass, and total leaf chlorophyll/leaf mass decreased more rapidly in trees with low psi(pd) than in trees with high psi(pd), so that from August to the beginning of leaf senescence in October, leaves of high-psi(pd) trees had higher A(max)/leaf area, A(max)/leaf mass, and total leaf chlorophyll/leaf mass than those of low-psi(pd) trees. Overall, variations in root architecture and summer psi(pd), for Q. douglasii were correlated with substantial differences in morphological and physiological leaf characteristics. This apparent coordination of aboveground and belowground organs may explain, in part, how Q. douglasii tolerates the exceptionally broad range of topography and soil moisture conditions in which it occurs.
© 1996, University of Chicago Press. View original published article at 10.1086/297329.