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 δ¹³C (δ¹³C_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 δ¹³C_R and its components, soil-respired δ¹³C (δ¹³C_R-soil) and foliage-respired δ¹³C (δ¹³C_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 CO₂ exchange (NEE) was positively correlated with vpd (P < 0.01), consistent with a vpd limitation to conductance and net CO₂uptake. Consistent with a stomatal influence over Δ, a negative correlation was observed between δ¹³C_R and G_c measured 2 days prior (i.e., a 2-day time lag, P = 0.04); however, δ¹³C_R was not correlated with other measured variables. Also consistent with a stomatal influence over discrimination, δ¹³C_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, δ¹³C_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 CO₂. The significant driving parameters differed for δ¹³C_R-foliage and δ¹³C_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_ccontrol over photosynthesis and Δ, both δ¹³C_R-soil and δ¹³C_R-foliage became enriched as net CO₂uptake decreased (more positive NEE, P < 0.01 for both). The δ¹³C 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 δ¹³C 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 δ¹³C_R-soil but only minor variation in δ¹³C_R, leading us to conclude that δ¹³C_R is not controlled solely by either canopy and belowground processes, but rather by their time-dependent interaction.