Response of the carbon isotopic content of ecosystem, leaf, and soil respiration to meteorological and physiological driving factors in a Pinus ponderosa ecosystem

Understanding the controls over ecosystem‐respired δ 13 C (δ 13 C R ) is important for applications of isotope‐based models of the global carbon budget as well as for understanding ecosystem‐level variation in isotopic discrimination (Δ). Discrimination may be strongly dependent on synoptic‐scale variation in environmental drivers that control canopy‐scale stomatal conductance ( G c ) and photosynthesis, such as atmospheric vapor pressure deficit (vpd) photosynthetically active radiation (PAR) and air temperature ( T air ). These potential relationships are complicated, however, due to time lags between the period of carbon assimilation and ecosystem respiration, which may extend up to several days, and may vary with tissue (i.e., leaves versus belowground tissues). Our objective was to determine if relationships exist over a short‐term period (2 weeks) between meteorological and physiological driving factors and δ 13 C R and its components, soil‐respired δ 13 C (δ 13 C R‐soil ) and foliage‐respired δ 13 C (δ 13 C R‐foliage ). We tested for these hypothesized relationships in a 250‐year‐old ponderosa pine forest in central Oregon, United States. A cold front passed through the region 3 days prior to our first sample night, resulting in precipitation (total rainfall 14.6 mm), low vpd (minimum daylight average of 0.36 kPa) and near‐freeze temperature (minimum air temperature of 0.18°C ± 0.3°C), followed by a warming trend with relatively high vpd (maximum daylight average of 3.19 kPa). Over this 2‐week period G c was negatively correlated with vpd (P < 0.01) while net ecosystem CO 2 exchange (NEE) was positively correlated with vpd (P < 0.01), consistent with a vpd limitation to conductance and net CO 2 uptake. Consistent with a stomatal influence over Δ, a negative correlation was observed between δ 13 C R and G c measured 2 days prior (i.e., a 2‐day time lag, P = 0.04); however, δ 13 C R was not correlated with other measured variables. Also consistent with a stomatal influence over discrimination, δ 13 C R‐soil was negatively correlated with G c (P < 0.01) and positively correlated with vpd and PAR measured one to 3 days prior (P = 0.01 and 0.04, respectively). In contrast, δ 13 C R‐foliage was not correlated with vpd or G c , but was negatively correlated with minimum air temperature measured 5 days previously (P < 0.01) supporting the idea that cold air temperatures cause isotopic enrichment of respired CO 2 . The significant driving parameters differed for δ 13 C R‐foliage and δ 13 C R‐soil potentially due to different controls over the isotopic content of tissue‐specific respiratory fluxes, such as differing carbon transport times from the site of assimilation to the respiring tissue or different reliance on recent versus old photosynthate. Consistent with G c control over photosynthesis and Δ, both δ 13 C R‐soil and δ 13 C R‐foliage became enriched as net CO 2 uptake decreased (more positive NEE, P < 0.01 for both). The δ 13 C value of Pinus ponderosa foliage (−27.1‰, whole‐tissue) was 0.5 to 3.0‰ more negative than any observed respiratory signature, supporting the contention that foliage δ 13 C can be a poor proxy for the isotopic content of respiratory fluxes. The strong meteorological controls over G c and NEE were associated with similar variation in δ 13 C R‐soil but only minor variation in δ 13 C R , leading us to conclude that δ 13 C R is not controlled solely by either canopy and belowground processes, but rather by their time‐dependent interaction.