Plant Uptake of Atmospheric Carbonyl Sulfide in Coast Redwood Forests

Abstract

The future resilience of coast redwoods (Sequoia sempervirens) is now of critical concern due to the detection of a 33% decline in California coastal fog over the 20th century. However, ecosystem‐scale measurements of photosynthesis and stomatal conductance are challenging in coast redwood forests, making it difficult to anticipate the impacts of future changes in fog. To address this methodological problem, we explore coastal variations in atmospheric carbonyl sulfide (COS or OCS), which could potentially be used as a tracer of these ecosystem processes. We conducted atmospheric flask campaigns in coast redwood sites, sampling at surface heights and in the canopy (~70 m), at the University of California Landels‐Hill Big Creek Reserve and Big Basin State Park. We simulated COS atmosphere‐biosphere exchange with a high‐resolution 3‐D model to interpret these data. Flask measurements indicated a persistent daytime drawdown between the coast and the downwind forest (45 ± 6 ppt COS) that is consistent with the expected relationship between COS plant uptake, stomatal conductance, and gross primary production. Other sources and sinks of COS that could introduce noise to the COS tracer technique (soils, anthropogenic activity, nocturnal plant uptake, and surface hydrolysis on leaves) are likely to be small relative to daytime COS plant uptake. These results suggest that COS measurements may be useful for making ecosystem‐scale estimates of carbon, water, and energy exchange in coast redwood forests.

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

“…OCS measurements have not been previously reported for old‐growth forests, although a recent study using flask samples inferred large OCS uptake in coastal redwood forests of northern California (Campbell et al, ). Old‐growth forests exhibit dynamics that are very different from those of other forest ecosystems due to a high LAI and complex forest structure (Hollinger et al, ).…”

Response to Reviewer 1

“…However, the global warming potential of OCS roughly balances whatever global cooling effect it might have (Brühl et al, 2012). Abiotic hydrolysis in the atmosphere plays a small role: while snow and rain were observed to be supersaturated with OCS Mu et al, 2004), even in the densest supersaturated clouds the OCS in the air would represent 99.99 % of the OCS present (Campbell et al, 2017b). Multiple lines of evidence support uptake by plants as the dominant removal mechanism of atmospheric OCS (e.g., Asaf et al, 2013;Berry et al, 2013;Campbell et al, 2008;Glatthor et al, 2017;Hilton et al, 2017;Launois et al, 2015b;Mihalopoulos et al, 1989;Montkza et al, 2007;Protoschill-Krebs and Kesselemeier, 1992;Sandoval-Soto et al, 2005;Stimler, 2010b;Suntharalingham et al, 2008).…”

“…The OCS tracer approach is particularly useful in high-humidity or foggy environments like the tropics, where traditional estimates of carbon uptake variables via water vapor exchange are ineffective. Additionally, OCS observing towers upstream and downstream of large forested areas could resolve the synoptic-scale variability in forest carbon uptake (Campbell et al, 2017b). OCS observations can address the need for additional studies on primary productivity in grassland ecosystems.…”

“…12). OCS would be especially useful in humid environments or at night, when transpiration is too small to use other methods that rely on sap flow or heat flux (Campbell et al, 2017b). However, an independent estimate of CA activity and mesophyll conductance would be required.…”

“…Global OCS concentrations have been retrieved from several satellite instruments, including NASA's TES (Kuai et al, 2014), the Canadian Space Agency's Atmospheric Chemistry Experiment-Fourier Transform Spectrometer (ACE-FTS) (Boone et al, 2005), and the European Space Agency's MIPAS (von Clarmann et al, 2003;Glatthor et al, 2017) and Infrared Atmospheric Sounding Interferometer (IASI) (Camy-Peyret et al, 2017;Vincent and Dudhia, 2017). Among these instruments, TES and IASI are nadir-viewing instruments (i.e., looking downwards from space towards the surface), while ACE-FTS and MIPAS are limb scanners (i.e., looking through the atmosphere tangentially).…”