Large Uptake of Atmospheric OCS Observed at a Moist Old Growth Forest: Controls and Implications for Carbon Cycle Applications

Abstract

Diurnal and vertical patterns of carbonyl sulfide (OCS) and CO 2 mixing ratios above and within a 60-m-tall old-growth temperate forest are presented. Canopy air from four different heights was sampled in situ using a continuous integrated cavity output spectroscopy analyzer during August-September 2014. Measurements revealed large vertical gradients in OCS, from which we inferred ecosystem fluxes. The diurnal cycle of OCS mixing ratios at all heights exhibited a typical pattern characterized by nighttime drawdown, an early morning minimum, and a maximum of OCS around midday. Daytime increase in the upper canopy is attributed to entrainment of planetary boundary layer air into the canopy. The ecosystem was found to be a large daytime sink of OCS (mean maximum daytime flux~À75 pmol · m À2 · s À1 ). Mean leaf relative uptake (concentration normalized uptake of OCS flux to CO 2 uptake) was found to be 6.9. We discuss this high leaf relative uptake in the context of the presence and distribution of epiphytes at the site. While epiphytic uptake of OCS has been studied before, we show for the first time that this may contribute significantly to ecosystem fluxes under humid or moist conditions. We test this theory using a chamber experiment measuring epiphytic fluxes for two species of lichen and one moss species (in situ and in a laboratory). We suggest that the role of epiphytes should be explicitly considered when using OCS as a tracer of ecosystem-scale photosynthesis in forest ecosystems with abundant epiphytic cover and biomass.Plain Language Summary The resilience of old growth forests in changing climates is less well understood, in part due to difficulties in measuring forest productivity. Here we apply a new method using measurements of carbonyl sulfide (OCS) to understand the same in a tall old growth forest in the Pacific northwestern United States. We find that OCS is taken up by plants, soil, and epiphytes (lichens and mosses) and provides biophysical controls on OCS uptake and its utility in estimating productivity in similar forests.

Seasonal dynamics of the COS and CO<sub>2</sub> exchange of a managed temperate grassland

“…The extremely low COS canopy mole fractions we observed within the canopy have also been reported by Rastogi et al (2018), who measured a mean χ COS minimum of 152 ppt at 1 m above the soil within an old-growth forest. Compared to the consistent decrease in COS below the canopy level during daytime and nighttime, the gradient for CO 2 reverses during nighttime due to ongoing respiration processes while plants are not photosynthetically active.…”

“…S12; Campbell et al, 2017). A similarly steep increase until midday has also been observed by Rastogi et al (2018). Even though CO 2 and COS share a similar pathway into plants, reflected by their respective decrease in the mole fractions within the canopy, we saw a difference at the lower levels of our gradient analysis during daytime.…”

“…developed by Sandoval-Soto et al (2005), where F COSmodel is the modeled COS flux (pmol m −2 s −1 ); χ COS (ppt) and χ CO 2 (ppm) are the measured ambient mole fractions of COS and CO 2 , respectively; and LRU (-) is the leaf relative uptake rate:…”

“…F COS is the COS leaf flux (pmol m −2 s −1 ), F CO 2 is the gross CO 2 uptake on the leaf level (µmol m −2 s −1 ), and χ COS and χ CO 2 are the ambient COS and CO 2 mole fractions in parts per trillion and parts per million, respectively. Leaf level studies for C 3 plants have estimated the LRU to be around 1.7, with a 95 % confidence interval between 0.7 and 6.2 (Whelan et al, 2018;Seibt et al, 2010;Sandoval-Soto et al, 2005). The large spread of the LRU most likely originates from differences between plant species, for example, leaf structure and plant metabolism (Wohlfahrt et al, 2012;Seibt et al, 2010), which questions the applicability of the concept of LRU in real-world ecosystems under naturally varying environmental conditions.…”

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

“…We ignored the evaporation of dew since there was no direct way of estimating dew quantity in the absence of leaf wetness measurements throughout the canopy profile. However, H 2 O vapor profile measurements from a recent study at the site demonstrate that dew is not a substantial part of the measured ET fluxes in this forest, particularly during the daytime (Rastogi et al 2018a).…”