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CARBON ISOTOPE DISCRIMINATION AND GROWTH RESPONSE TO STAND DENSITY REDUCTIONS IN OLD PINUS PONDEROSA TREES
McDowell, N., J R. Brooks, S. Fitzgerald, AND B. J. Bond. CARBON ISOTOPE DISCRIMINATION AND GROWTH RESPONSE TO STAND DENSITY REDUCTIONS IN OLD PINUS PONDEROSA TREES. Presented at 3rd International Conference on Applications of Stable isotope Techniques to Ecological Studies, Flagstaff, AZ, April 29-May 1, 2002.
Carbon isotope ratios ( 13C) of tree rings are commonly used for paleoclimatic reconstruction and for inferring canopy water-use efficiency (WUE). However, the responsiveness of carbon isotope discrimination ( ) to site disturbance and resource availability has only rarely been examined, and never in the old trees typically used for paleoclimatic reconstruction. We examined the response of >250-year-old ponderosa pine trees (Pinus ponderosa) to site disturbance in the form of stand density manipulations in Oregon, USA. We hypothesized that reductions in stand density increase soil moisture availability, thus alter canopy WUE and hence . We measured 13C from cellulose of annual tree rings, soil water availability, photosynthetic capacity and stem basal growth in three paired forests of thinned and un-thinned stands. The three stands were thinned in 1985, 1987, and 1993. Carbon isotope discrimination of thinned trees increased by 0.89 ? (? 0.15?), whereas did not change for un-thinned trees (0.00? ? 0.04?). Significant differences were observed in the growing season after the thinning took place but it took 6 years before full 0.89 ? difference was observed. Increases in stem growth lagged behind the changes in . The response of thinned trees indicated that WUE, which is the ratio of photosynthesis to stomatal conductance, decreased after thinning. Photosynthetic capacity, as indexed by foliar nitrogen ([N]) and by the relationship between photosynthesis and internal CO2 (A-Ci curves), was unchanged by thinning, suggesting that the WUE decline was due to a relative increase in stomatal conductance rather than decreased photosynthesis. Soil water availability is positively correlated with stomatal conductance in many forests, and we found soil water availability was greater at the thinned stands. Pre-dawn soil water potential averaged 0.11 MPa (? 0.03 MPa) less negative for the thinned compared to the un-thinned stands. There was a strong relationship between stem basal growth and , suggesting that changes in water availability and WUE have a significant effect on growth of these old trees. Our data indicate that old trees used in paleoclimate studies are highly responsive to site disturbances. Thus, future use of 13C in treerings for paleoclimatic inferences should take site disturbance into account when possible.