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Title: The carbon balance pivot point of southwestern U.S. semiarid ecosystems: Insights from the 21st century drought

item Scott, Russell - Russ
item Biederman, Joel
item Hamerlynck, Erik
item BARRON-GAFFORD, G.A. - University Of Arizona

Submitted to: Journal of Geophysical Research-Biogeosciences
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/24/2015
Publication Date: 12/31/2015
Citation: Scott, R.L., Biederman, J.A., Hamerlynck, E.P., Barron-Gafford, G. 2015. The carbon balance pivot point of southwestern U.S. semiarid ecosystems: Insights from the 21st century drought. Journal of Geophysical Research-Biogeosciences. 120:2612-2624.

Interpretive Summary: Semiarid regions are very important for regulating the amount of atmospheric carbon dioxide taken up by the global land surface. However, we lack a detailed understanding of how climate shifts, such as the ongoing decadal-scale drought in the Southwest US, impact the uptake of carbon in semiarid ecosystems and how this response may vary across ecosystems with differing vegetation composition. Therefore, we used measurements of land-atmosphere water and carbon dioxide exchange from the last ten years, and we investigated the response of the ecosystem carbon uptake to changes in water availability in four Southwest US ecosystems varying in relative shrub, tree and grass abundance. We identified a precipitation-induced “pivot point” in the annual carbon balance where the carbon uptake turned from carbon uptake in wet years to carbon loss in dry years. At sites with larger amounts of grass cover, pivot points were closer to the drought-period mean precipitation, suggesting that these grassier ecosystems have more quickly adjusted to the decadal-scale drought than the ecosystems with shrubs. As the climate in Southwest US is expected to become drier in this century, our results suggest that the carbon uptake of ecosystems with more grass and less woody vegetation will adjust quicker to these changes and result in a faster return to carbon sequestration and the mitigation of climate change.

Technical Abstract: Global-scale studies indicate that semiarid regions strongly regulate the terrestrial carbon sink. However, we lack understanding of how climatic shifts, such as decadal drought, impact carbon sequestration across the wide-range of structural diversity in semiarid ecosystems. Therefore, we used eddy covariance measurements to quantify how net ecosystem production of carbon dioxide (NEP) differed with relative grass and woody plant abundance over the last decade of drought in four southwest US ecosystems. We identified a precipitation “pivot point” in the carbon balance for each ecosystem where annual NEP switched from negative to positive. Ecosystems with greater grass abundance had pivot points closer to the drought-period precipitation than the pre-drought average, making them more likely to be carbon sinks (and a grass-free shrubland, a carbon source) during the current drought. One reason for this is that grasslands supported higher leaf area than shrublands at a given water availability. Higher leaf area was associated with a greater proportion of evapotranspiration being transpiration (T/ET), and therefore with higher ecosystem water use efficiency (GEP/ET). Another reason for the greater carbon sequestration in grasslands could be the larger and more recalcitrant above and below-ground biomass in shrublands, decreasing the rate at which these pools could adjust to the lower productivity during drought. While water availability is recognized as the primary driver of semiarid NEP, our findings illustrate that structural differences contribute to the speed at which ecosystem carbon cycling adjusts to climatic shifts.