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

Diurnal and vertical patterns of carbonyl sulfide (OCS) and CO2 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 ~-75pmol m( -2) s( -1)). Mean leaf relative uptake (concentration normalized uptake of OCS flux to CO2 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.