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 ( δ 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 δ 13 C R from a wet 20‐year‐old Douglas‐fir ( Pseudotsuga menziesii ) plantation located near the Pacific Ocean. Intraannual variability in δ 13 C R was greater than 8.0‰ at both sites, was highest during autumn, winter, and spring when rainfall was abundant, and lowest during summer drought. The δ 13 C R of the dry pine forest was consistently more positive than the wetter Douglas‐fir forest (mean annual δ 13 C R : −25.41‰ vs. −26.23‰, respectively, P =0.07). At the Douglas‐fir forest, δ 13 C R –climate relationships were consistent with predictions based on stomatal regulation of carbon isotope discrimination (Δ). Soil water content ( SWC ) and vapor pressure deficit ( vpd ) were the most important factors governing δ 13 C R in this forest throughout the year. In contrast, δ 13 C R at the pine forest was relatively insensitive to SWC or vpd , and exhibited a smaller drought‐related enrichment (∼2‰) than the enrichment observed during drought at the Douglas‐fir forest (∼5‰). Groundwater access at the pine forest may buffer canopy–gas exchange from drought. Despite this potential buffering, δ 13 C R at the pine forest was significantly but weakly related to canopy conductance ( G c ), suggesting that δ 13 C R remains coupled to canopy–gas exchange despite groundwater access. During drought, δ 13 C R was strongly correlated with soil temperature at both forests. The hypothesis that canopy‐level physiology is a critical regulator of δ 13 C R was supported; however, belowground respiration may become more important during rain‐free periods.