Submitted to: Journal of Geophysical Research-Biogeosciences
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/1/2010
Publication Date: 9/22/2010
Citation: Scott, R.L., Hamerlynck, E.P., Jenerette, G.D., Moran, M.S., Barron-Gafford, G.A. 2010. Carbon dioxide exchange in a semidesert grassland responding through drought-induced vegetation change. Journal of Geophysical Research-[Biogeosciences]. 115: G03026. Interpretive Summary: Global warming is predicted to increase drought severity and duration. Severe drought can lead to a change in plant community structure, which, in turn, may yield differences in how water and carbon dioxide are cycled in ecosystems. We report on how the exchange of carbon dioxide between the atmosphere and a grassland in southern Arizona responded to a severe drought. When the drought ended the native grass species were replaced by an invasive African grass. The grassland was a source of carbon dioxide to the atmosphere during the drought and then became a sink when the drought ended and the exotic grass moved in. When another dry growing season occurred after the invasion, the grassland still took in more carbon than was released to the atmosphere. This study shows that invasive species may lead to more carbon sequestration in certain ecosystems and environments.
Technical Abstract: Global warming is predicted to intensify the hydrological cycle, thus increasing drought severity and duration. Severe drought can lead to a change in plant community structure, which, in turn, may yield differences in how water and carbon dioxide are cycled. We report on how the net ecosystem exchange of carbon dioxide (NEE) of a semidesert grassland responded to a severe drought immediately followed by succession from native bunchgrasses, to forbs, to an exotic grass species invasion. We monitored NEE and energy fluxes using eddy covariance along with meteorological and soil moisture variables for six years at a semidesert grassland site in southeastern Arizona, USA. During this time the ecosystem experienced a severe drought in the first two years of monitoring that caused widespread mortality of most perennial bunchgrass species. This resulted in a transition to a composition dominated by forbs for one growing season, followed by a near monoculture of the African bunchgrass Eragrostis lehmanniana in the following three years. The seasonal patterns of NEE revealed a springtime growing season following winter-spring periods with average rainfall and a much larger summer growing season in response to monsoon rains. NEE was markedly dampened in the drought years of 2004 and 2005, resulting in a net release of carbon dioxide (25 g C) over this time. Above average summer rains in 2006 alleviated the severe drought conditions and drove cumulative NEE downward to a net sink (-55 g C yr-1). The ecosystem dominated by E. lehmanniana in 2007 through 2009 was a net sink of carbon dioxide (-47 to -98 g C yr-1) but with very different annual patterns in NEE. In this semiarid grassland, precipitation was the dominant driver of respiration, gross ecosystem exchange and their net effect, NEE, in both the spring and summer growing seasons. When normalized by evaporation, respiration did not vary significantly between the years. In contrast, normalized gross ecosystem exchange (i.e., water use efficiency) was low during drought years and then increased as water returned to the system and the non-native grass invasion progressed. Thus, when dry summer conditions returned in 2009, the potential for the ecosystem to accumulate carbon was increased and the ecosystem remained a net sink when dominated by the exotic grass.