Stable Carbon Isotopes as Indicators of Limitations to Forest Growth Imposed by Climate Stress

Stable carbon isotopes have the potential to be sensitive indicators of carbon uptake limitations in forest trees. We assessed the ability of climate factors to explain the variation in δ13C in foliage and annual rings of Douglas-fir trees at six sites along a steep climatic gradient in Oregon. To examine the physiological basis of the relationship between climate and δ13C, we parameterized a process-level model (FOREST-BGC) and used the model to separate the daily limitations on stomatal conductance due to climate factors. We found that, of the climatic constraints that could affect stomatal conductance (freezing temperature, humidity deficit, and soil drought), only the cumulative growing season constraints on stomata imposed by humidity deficit explained the variability in the carbon isotope composition of cellulose in foliage. Over an 8-yr period at any one site, modelled stomatal limitation due to humidity deficit was significantly correlated with foliar δ13C. Over the same period, δ13C in annual growth rings was less well correlated with modelled stomatal constraints. This relation between δ13C and absolute humidity demonstrated the sensitivity of carbon uptake in Douglas-fir, a species with a broad range across the intermountain West, to moisture at the leaf–atmosphere interface. Furthermore, the parameters derived from the relationship offer a baseline from which to assess limitations imposed on Douglas-fir carbon uptake from sources other than climate.