title: Homeostatic gas-exchange parameters inferred from13C/12C in tree rings of conifers.
abstract: The CO2concentration 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 C3photosynthetic pathway indicate that an increase of this magnitude would enhance net photosynthesis, reduce stomatal conductance, and increase the difference in CO2concentration across the stomata, i.e., CO2concentration outside the leaf to that within (ca-ci). 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 ca-ci, intercellular CO2concentrations have shifted upward to match the rise in atmospheric concentrations, holding ca-ciconstant. No differences were detected among Douglas-fir (Pseudotsuga menziesii), ponderosa pine (Pinus ponderosa), or western white pine (Pinus monticola). The values of ca-ciwere 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 δ13CO2and CO2concentrations. 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 CO2effects. This study provides evidence that conifers have undergone adjustments in the intercellular CO2concentration that have maintained ca-ciconstant. Based on these results and others, we suggest that ca-ci, 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.
authors.affiliation: Department of Forest Resources, University of Idaho, 83843-1133, Moscow, ID, USA.
authors.forename: John D
authors.initials: JD
authors.lastname: Marshall
authors.affiliation: Department of Agriculture, US Forest Service, Intermountain Research Station, 83843, Moscow, ID, USA.
authors.forename: Robert A
authors.initials: RA
authors.lastname: Monserud
keywords: Carbon isotope discrimination; Pinus monticola; Pinus ponderosa; Pseudotsuga menziesii; Water-use efficiency