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Title: Consequences of cool-season drought induced plant mortality to Chihuahuan Desert grassland ecosystem and soil respiration dynamics

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

Submitted to: Ecosystems
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/25/2013
Publication Date: 5/21/2013
Citation: Hamerlynck, E.P., Scott, R.L., Barron-Gafford, G. 2013. Consequences of cool-season drought induced plant mortality to Chihuahuan Desert grassland ecosystem and soil respiration dynamics. Ecosystems. DOI: 10.1007/s10021-013-9675-y.

Interpretive Summary: The Southwest US is expected to be increasingly drier and warmer over the next century, in part due to an increase in the severity and frequency of drought, especially during the winter and spring. Strong droughts can induce wide-spread plant death, which in turn can affect ecosystem carbon dynamics. At a desert grassland that experienced perennial grass die-off twice in the last six years, we quantified ecosystem net carbon dioxide exchange (NEE), and its constituent fluxes, ecosystem respiration (Reco) and gross ecosystem photosynthesis (GEP) responses to spring and summer rains across these six years. In addition, we measured soil respiration (Rsoil), the dominant part of Reco, over the summer rainy season in high- and low-mortality plots following the most recent plant mortality event. We found that springtime GEP was sensitive to precipitation, and that bad springtime conditions limited this grassland’s ability to respond to subsequent summer rainfall. Reco was not as sensitive to spring or summer rain as GEP across all the years, but within the two years with widespread plant death, Reco responded very strongly to the limited spring rains that fell, much more so than GEP, showing additional soil carbon resulting from plant death was quickly burned off by soil microbial activity. Overall, total carbon respired by the soil did not depend on overall plant mortality, but rather depended on the health of the surviving plants surrounding the plot. These findings suggest future cool-season conditions may severely affect the productivity and health of these important rangeland ecosystems.

Technical Abstract: Global climate change is predicted to increase the severity and frequency of cool-season drought across the arid Southwest US. We quantified net ecosystem carbon dioxide exchange (NEE), ecosystem respiration (Reco), and gross ecosystem photosynthesis (GEP) in response to interannual seasonal precipitation (P) at semidesert grassland watershed that experienced repeated grass mortality events; in addition, we quantified daily total soil respiration (Rsoil) over the summer rainy season following the most recent mortality event. We found extensive grass mortality coincided with exceptionally dry spring-time conditions, and that there was a close relationship between springtime GEP and P, and that this modulated summer rainy season GEP:P relationships. In addition, springtime drought dramatically increased Reco:P and Reco:GEP ratios, suggesting available carbon following plant mortality was rapidly decomposed following limited spring rains. Our summer monsoon Rsoil study showed monthly mean Rsoil was higher in low-mortality plots under drier conditions through the summer rainy season, likely reflecting higher root contributions. However, seasonal cumulative Rsoil did not depend on the amount of mortality surrounding the plot, but on the condition of proximate surviving plants. Mean plant mortality and plant condition across the grassland watershed was within the range of values of such high-yielding Rsoil plots, suggesting these areas were the dominant influence on Reco. This, coupled with springtime drought constraining GEP, led to positive NEE over the summer monsoon season. These findings suggest that current and future predictions of reduced cool season precipitation may strongly and negatively affect monsoon-season carbon balance in these southwest U.S. grasslands.