Homeostatic gas-exchange parameters inferred from 13C/12C 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 dependent on the C3 photosynthetic pathway indicate that an increase of this magnitude would enhance net photosynthesis, reduce stomatal conductance, 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 the 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 differences were detected among Douglas-fir (Pseudotsuga menziesii), ponderosa pine (Pinus ponderosa), or western white pine (Pinus monticola). The values of c a-c i were inferred from stable carbon isotope ratio (δ13C) of tree ring holocellulose adjusted for the 0.6-2.6‰ difference between holocellulose and whole sapwood. The cellulose extraction removed contaminants deposited in the tree ring after it formed and the adjustment corrected for the enrichment of cellulose relative to whole tissue. The whole sapwood values were then adjusted for bublished estimates of past atmospheric δ13CO2 and CO2 concentrations. To avoid confounding tree age with CO2, cellulose deposited by saplings in the 1980s was compared to cellulose deposited in the inner rings of nature trees when the mature trees were saplings, between 1910-1929 and 1941-1970; thus saplings were compared to saplings. In a separate analysis, the juvenile effect, which describes the tendency for δ13C to increase in the first decades of a tree's life, was quantified independent of source CO2 effects. This study provides evidence that conifers have undergone adjustments 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 efficiency, should be considered a homeostatic gas-exchange set point for these conifer species.