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The role of interannual, seasonal, and synoptic climate on the carbon isotope ratio of ecosystem respiration at a semiarid woodland


Jee H. Shim, tel. +1 505 665 9773, fax +1 505 665 3866, e-mail:


The terrestrial carbon cycle is influenced by environmental variability at scales ranging from diurnal to interannual. Here, we present 5-years of growing season (day 131–275) observations of the carbon isotope ratio of ecosystem respiration (δ13CR) from a semiarid woodland. This ecosystem has a large necromass component resulting from 97%Pinus edulis mortality in 2002, is dominated by drought-tolerant Juniperus monosperma trees, and experiences large variability in the timing and intensity of seasonal and synoptic water availability. Mean growing season δ13CR was remarkably invariant (−23.57±0.4‰), with the exception of particularly enriched δ13CR in 2006 following a winter with anomalously low snowfall. δ13CR was strongly coupled to climate during premonsoon periods (∼May to June), including fast (≤2 days) responses to changes in crown-level stomatal conductance (Gc) and vapor pressure deficit (vpd) following rain pulses. In contrast, δ13CR was relatively decoupled from Gc and environmental drivers during monsoon and postmonsoon periods (July–August and September, respectively), exhibiting only infrequent couplings of δ13CR to vpd and soil water content (SWC) with longer lags (∼8 days) and variable response slopes (both positive and negative). Notably, δ13CR exhibited consistent dynamics after rainfall events, with depleted δ13CR occurring within 1 h, progressive hourly δ13CR enrichment over the remainder of the night, and net δ13CR depletions over the multiple nights postevent in monsoon and postmonsoon periods. Overall this ecosystem demonstrated strong dependence of δ13CR on precipitation, with an apparent dominance by the autotrophic δ13C signal in premonsoon periods when deep soil moisture is abundant and surface soil moisture is low, and weaker coupling during monsoonal periods consistent with increasing heterotrophic dominance when deep soil moisture has declined and surface moisture is variable.