Changes in drought response strategies with ontogeny in Quercus rubra: implications for scaling from seedlings to mature trees

We investigated scaling of physiological parameters between age classes of Quercus rubra by combining in situ field measurements with an experimental approach. In the in situ field study, we investigated changes in drought re sponse with age in seedlings, juveniles, and mature trees of Q. rubra. Thro ughout the particularly dry summer of 1995 and the unusually wet summer of 1996 in New England, we measured water potential of leaves (Psi(Leaf)) and gas exchange of plants at three sites at the Harvard Forest in Petersham, M assachusetts. In order to determine what fraction of the measured differenc es in gas exchange between seedlings and mature trees was due to environmen t versus ontogeny, an experiment was conducted in which seedlings were grow n under light and soil moisture regimes simulating the environment of matur e trees. The photosynthetic capacity of mature trees was three-fold greater than that of seedlings during the wet year, and six-fold greater during th e drought year. The seedling experiment demonstrated that the difference in photosynthetic capacity between seedlings and mature trees is comprised eq ually of an environmental component (50%) and an ontogenetic component (50% ) in the absence of water limitation. Photosynthesis was depressed more sev erely in seedlings than in mature trees in the drought year relative to the wet year, while juveniles showed an intermediate response. Throughout the drought, the predawn leaf water potential (Psi(PD)) of seedlings became inc reasingly negative (-0.4 to -1.6 MPa), while that of mature trees became on ly slightly more negative (-0.2 to -0.5 MPa). Again, juveniles showed an in termediate response (-0.25 to -0.8 MPa). During the wet summer of 1996, how ever, there was no difference in Psi(PD) between seedlings, juveniles and m ature trees. During the dry summer of 1995, seedlings were more responsive to a major rain event than mature trees in terms of Psi(Leaf), suggesting t hat the two age classes depend on different water sources. In all age class es, instantaneous measurements of intrinsic water use efficiency (WUEi), de fined as C assimilation rate divided by stomatal conductance, increased as the drought progressed, and all age classes halt higher WUEi during the dro ught year than in the wet year. Mature trees, however, showed a greater abi lity to increase their WUEi in response to drought. Integrated measurements of WUE from C isotope discrimination (Delta) of leaves indicated higher WU E in mature trees than juveniles and seedlings. Differences between years, however, could not be distinguished, probably due to the strong bias in C i sotope fractionation at the time of leaf production, which occurred prior t o the onset of drought conditions in 1995. From this study, we arrive at tw o main conclusions: 1. Different age classes of Q. rubra use different strategies for respondin g to drought. Seedlings resist drought by closing stomata early in the day at the expense of C uptake; mature trees avoid drought conditions by access ing deeper water reserves and adjusting WUE. 2. Only through studies which separate environmental differences from ontog enetic differences can parameters measured on seedlings be scaled to mature trees.