Variation in leaf structure and function in Quercus douglasii trees differing in root architecture and drought history

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-ψ_pd" trees, included trees with low amounts of fine root biomass in the upper 50 cm of soil and high predawn xylem pressure potentials (ψ_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-ψ_pd" trees, had three to five times higher fine root biomass in the upper 50 cm of soil and low ψ_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 ψ_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-ψ_pd trees than for trees with high ψ_pd. Nitrogen per leaf area did not differ between the groups. Net photosynthetic capacity at 2000 μmol 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 ψ_pd. Since differences existed between new cohorts of leaves produced in the spring before summer drought, when ψ_pd was similar, we suggest that some leaf characteristics of Q. douglasii trees are determined by the degree 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 ψ_pd than in trees with high ψ_pd, so that from August to the beginning of leaf senescence in October, leaves of high-ψ_pd trees had higher A_max/leaf area, A_max/leaf mass, and total leaf chlorophyll/leaf mass than those of low-ψ_pd trees. Overall, variations in root architecture and summer ψ_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.