Ecosystem fluxes of carbonyl sulfide in an old-growth forest: temporal dynamics and responses to diffuse radiation and heat waves

Abstract

Abstract. Carbonyl sulfide (OCS) has recently emerged as a tracer for terrestrial carbon uptake. While physiological studies relating OCS fluxes to leaf stomatal dynamics have been established at leaf and branch scales and incorporated into global carbon cycle models, the quantity of data from ecosystem-scale field studies remains limited. In this study, we employ established theoretical relationships to infer ecosystem-scale plant OCS uptake from mixing ratio measurements. OCS fluxes showed a pronounced diurnal cycle, with maximum uptake at midday. OCS uptake was found to scale with independent measurements of CO2 fluxes over a 60 m tall old-growth forest in the Pacific Northwest of the US (45∘49′13.76′′ N, 121∘57′06.88′′ W) at daily and monthly timescales under mid–high light conditions across the growing season in 2015. OCS fluxes were strongly influenced by the fraction of downwelling diffuse light. Finally, we examine the effect of sequential heat waves on fluxes of OCS, CO2, and H2O. Our results bolster previous evidence that ecosystem OCS uptake is strongly related to stomatal dynamics, and measuring this gas improves constraints on estimating photosynthetic rates at the ecosystem scale.

Seasonal Evolution of Canopy Stomatal Conductance for a Prairie and Maize Field in the Midwestern United States from Continuous Carbonyl Sulfide Fluxes

“…It is now more common to use EC to derive the OCS flux (e.g., Asaf et al, 2013), but the analyzer used here did not have sufficient temporal resolution to use this approach. However, previous studies have effectively used the flux gradient method for OCS fluxes (Bartell et al, 1993;Rastogi et al, 2018), and we have confidence in the approach based on the diurnal and seasonal cycles in the flux that emerged from the analysis (as discussed below).…”

Differential responses to two heatwave intensities in a Mediterranean citrus orchard are identified by combining measurements of fluorescence and carbonyl sulfide (COS) and CO2 uptake

“…Under stressful conditions such as high temperature and high VPD, LRU values can increase dramatically as well (e.g. Stimler et al ., 2010; Rastogi et al ., 2018; Wohlfahrt et al ., 2018). Leaf relative uptake can, therefore, serve as a useful physiological indicator for differential effects on conductance and other intrinsic biochemical processes, as is shown here.…”

“…Previous studies have demonstrated that the COS daily peak tends to occur in the morning hours (Whelan et al ., 2018; Yang et al ., 2018a; Kooijmans et al ., 2019) and is correlated with stomatal opening during the hours of optimal activity. Increasing VPD levels are known to trigger rapid stomatal closure in angiosperm plants through ABA production (McAdam & Brodribb, 2015) leading to a reduction in F cos (Rastogi et al ., 2018). In our system, the daily patterns of COS uptake and VPD were not tightly coupled, indicating that the reduction of stomatal opening on both HW and INT days occurred only after the initial increase in VPD, extending the pre‐stress assimilation rates.…”

“…Stimler et al ., 2010; Spielmann et al ., 2019; Kooijmans et al ., 2019). While our knowledge regarding the effect of heatwaves on COS uptake flux is still limited, evidence suggests that heatwaves can lead to a reduction in COS uptake fluxes in old‐growth forests and semi‐arid pine forests (Rastogi et al ., 2018; Wohlfahrt et al ., 2018) and after extended heatwave stress can be concomitant with increasing ecosystem relative uptake (ERU), reflecting decreasing GPP and effects that go beyond reduced conductance (Wohlfahrt et al ., 2018).…”

Trends and controls on water-use efficiency of an old-growth coniferous forest in the Pacific Northwest

“…In other words, higher light use efficiency occurs in lower incident PAR days (Wharton et al 2009). Consequently, although photosynthetic rates are lower at the canopy top, the rates increase at lower levels of the canopy due to less shading, thereby increasing overall canopy photosynthesis on cloudy days (Rastogi et al 2018b). This is particularly true for forests like Wind River where leaf area is high and distributed in a complex manner along the vertical axis .…”