HOMEOSTATIC GAS-EXCHANGE PARAMETERS INFERRED FROM C-13 C-12 IN TREE-RINGS OF CONIFERS/

The CO2 concentration of the atmosphere has increased by almost 30% in the past two centuries, with most of the increase (> 5 Pa) during the past 60 years. Controlled environment studies of crop plants dependen t on the C-3 photosynthetic pathway indicate that an increase of this magnitude would enhance net photosynthesis, reduce stomatal conductanc e, and increase the difference in CO2 concentration across the stomata , i.e., CO2 concentration outside the leaf to that within (c(a)-c(i)). Here we report evidence, based on stable isotope composition of tree rings from three species of field-grown, native conifer trees; that th e trees have indeed responded. However, rather than increasing c(a)-c( i), intercellular CO2 concentrations have shifted upward to match the rise in atmospheric concentrations, holding c(a)-c(i) constant. No dif ferences were detected among Douglas-fir (Pseudotsuga menziesii), pond erosa pine (Pinus ponderosa), or western white pine (Pinus monticola). The values of c(a)-c(i) were inferred from stable carbon isotope rati o (delta(13)C) of tree ring holocellulose adjusted for the 0.6-2.6 par ts per thousand difference between holocellulose and whole sapwood. Th e cellulose extraction removed contaminants deposited in the tree ring after it formed and the adjustment corrected for the enrichment of ce llulose relative to whole tissue. The whole sapwood values were then a djusted for published estimates of past atmospheric delta(13)CO(2) and CO2 concentrations. To avoid confounding tree age with CO2, cellulose deposited by saplings in the inner rings of trees when the mature tre es were saplings, between 1910-1929 and 1941-1970: thus saplings were compared to saplings. In a separate analysis, the juvenile effect, whi ch describes the tendency for delta(13)C to increase in the first deca des of a tree's life, was quantified independent of source CO2 effects . This study provides evidence that conifers have undergone adjustment s in the intercellular CO2 concentration that have maintained c(a)-c(i ) constant. Based on these results and others, we suggest that c(a)-c( i) which has also been referred to as the intrinsic water-use efficien cy, should be considered a homeostatic gas-exchange set point for thes e conifer species.