Seasonal evolution of canopy stomatal conductance for a prairie and maize field in the Midwestern United States from continuous carbonyl sulfide fluxes

Authors: BERKELHAMMER, MAX - University Of Chicago; ALSIP, B - University Of Chicago; MATAMALA, ROSER - Argonne National Laboratory; COOK, DAVI - Argonne National Laboratory; WHELAN, M - Rutgers University; JOO, EVA - University Of Illinois; Bernacchi, Carl; MILLER, JESSE - University Of Illinois; MEYERS, TILDEN - National Oceanic & Atmospheric Administration (NOAA)

Submitted to: Geophysical Research Letters
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/25/2020
Publication Date: 3/28/2020
Citation: Berkelhammer, M., Alsip, B., Matamala, R., Cook, D., Whelan, M.E., Joo, E., Bernacchi, C.J., Miller, J., Meyers, T. 2020. Seasonal evolution of canopy stomatal conductance for a prairie and maize field in the Midwestern United States from continuous carbonyl sulfide fluxes. Geophysical Research Letters. 47(6). https://doi.org/10.1029/2019GL085652.
DOI: https://doi.org/10.1029/2019GL085652

Interpretive Summary: Leaf stomata play an important role in how carbon and water are cycled through terrestrial ecosystems. A lot of information on stomata has been derived from measurements taken on individual leaves. However, it is difficult to make predictions about how the stomata of an entire canopy, that may contains thousands of leaves, will behave. In other words, how can we be confident that measurements made at the leaf level capture the behavior of a complex canopy? In this manuscript, we use measurements of a trace gas called carbonyl sulfide, which is consumed when it diffuses into leaves, to study how the stomata of an entire canopy behave. We measured this gas over a tall grass prairie and corn field in Illinois over a series of growing season and used this to study how the collective stomata of these critical ecosystems behave. We found that during periods when stomata were most active, the cycling of carbon and water remained constant. This was a surprising result and showed how measurements at the leaf level may not capture the complex processes within a canopy. We hypothesize that this occurs because air gets trapped within canopies and plants deplete the available CO2.

Technical Abstract: There are inherent challenges in scaling stomatal conductance (gs) from the leaf to canopy. Here, we address this gap in understanding using measurements of carbonyl sulfide (OCS) fluxes from a predominantly C3 prairie and C4 maize field in the midwestern US. We document a well-defined temperature optimum for gs at the grassland of 25-31oC and found that nighttime gs was typically an order of magnitude higher than for the maize site. We also observed that as gs rose above a threshold of ~0.15 mol m-2s-1, gross primary production and transpiration remained stable. We hypothesize that this behavior emerged from a feedback where high humidity and low CO2 in the canopy during elevated gs periods limited leaf-atmosphere exchange. This latter result particularly highlights how constraints on canopy gs from OCS fluxes can be used to account for the combined effects of leaf physiology and canopy structure on land-atmosphere exchange.