Associations between carbon isotope ratios of ecosystem respiration, water availability and canopy conductance

We tested the hypothesis that the stable carbon isotope signature of ecosystem respiration (delta(13)C(R)) was regulated by canopy conductance (G(c)) using weekly Keeling plots (n=51) from a semiarid old-growth ponderosa pine (Pinus ponderosa) forest in Oregon, USA. For a comparison of forests in two contrasting climates we also evaluated trends in delta(13)C(R) from a wet 20-year-old Douglas-fir (Pseudotsuga menziesii) plantation located near the Pacific Ocean. Intraannual variability in delta(13)C(R) was greater than 8.0parts per thousand at both sites, was highest during autumn, winter, and spring when rainfall was abundant, and lowest during summer drought. The delta(13)C(R) of the dry pine forest was consistently more positive than the wetter Douglas-fir forest (mean annual delta(13)C(R): -25.41parts per thousand vs. -26.23parts per thousand, respectively, P=0.07). At the Douglas-fir forest, delta(13)C(R)-climate relationships were consistent with predictions based on stomatal regulation of carbon isotope discrimination (Delta). Soil water content (SWC) and vapor pressure deficit (vpd) were the most important factors governing delta(13)C(R) in this forest throughout the year. In contrast, delta(13)C(R) at the pine forest was relatively insensitive to SWC or vpd, and exhibited a smaller drought-related enrichment (similar to2parts per thousand) than the enrichment observed during drought at the Douglas-fir forest (similar to5parts per thousand). Groundwater access at the pine forest may buffer canopy-gas exchange from drought. Despite this potential buffering, delta(13)C(R) at the pine forest was significantly but weakly related to canopy conductance (G(c)), suggesting that delta(13)C(R) remains coupled to canopy-gas exchange despite groundwater access. During drought, delta(13)C(R) was strongly correlated with soil temperature at both forests. The hypothesis that canopy-level physiology is a critical regulator of delta(13)C(R) was supported; however, belowground respiration may become more important during rain-free periods.