A stable carbon isotope approach to distinguish climate stress from other imposed stresses in coniferous forests

To understand the effect of human-induced stresses on forests, there is a need for a method to separate effects of imposed stress from effects of natural climate stress. I developed an approach to predict forest response to climate stress using as indicators stable carbon isotopes in tree foliage and growth-rings. This approach required understanding and modeling the relation between climate and isotope abundance in tree tissue. Isotope abundance is highly variable within and between trees. Before modifying, it was necessary to identify important sources of this isotope variability to ensure that I included the major components in the model. Six study sites across a climate gradient in Oregon incorporated the broad range of climate types necessary to explore 8'3C variability, and to establish and test the model. Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) was common to all sites across the gradient. Patterns of 8'3C variability within the canopy of Douglas-fir trees implied that xylem hydraulics pose important limitations to carbon uptake. Branch length was significantly correlated with 8'3C in the foliage at branch tips, which suggested that stem hydraulics were involved in the relation, as branch length is a measure of path length of water movement. The importance of hydraulic properties in '3C variability was confirmed by measuring branch hydraulic, specific, and leaf-specific conductivity of the same branches in which '3C was measured. A strong inverse relation between specific and leaf-specific conductivity and foliar '3C was found, as predicted by theory, and confirmed on several age-classes of foliage. The model which best predicted annual variability in '3C in foliage and tree rings over a range of climate types included environmental constraints to stomata and xylem hydraulic properties. The model predicted '3C in foliage west of the Oregon Cascade Mts. extremely well. East of the Cascades, site means were well-characterized, but annual variability was not. Annual variability in tree-ring 13C was poorly characterized by the model, probably because annual variation in whole-canopy hydraulics was inadequately described by the hydraulic measure. Refinements for improving the tree-ring '3C relation are suggested