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Title: Cool-season whole-plant gas exchange of exotic and native desert semiarid bunchgrasses

item Hamerlynck, Erik
item Scott, Russell - Russ
item Cavanaugh, Michelle

Submitted to: Plant Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/13/2012
Publication Date: 8/1/2012
Citation: Hamerlynck, E.P., Scott, R.L., Barron-Gafford, B., Cavanaugh, M.L., Moran, M.S., Huxman, T. 2012. Cool-season whole-plant gas exchange of exotic and native desert semiarid bunchgrasses. Plant Ecology. 213:1229–1239. DOI 10.1007/s11258-012-0081-x.

Interpretive Summary: Lehmann lovegrass invasion is a serious ecological disturbance to Southwestern US desert grasslands. One attribute that has been considered as key to the invasive success of this South African species is its ability to maintain green biomass during the winter, which may give it access to a wider array of seasonal water than native grasses. However, several important native bunchgrasses also have green foliage during the winter. To determine if Lehmann’s lovegrass gains a competitive edge in using winter rainfall, we compared its whole-plant carbon dioxide and water exchange to a native winter-active grass, bush muhly, to see if this invasive grass was better able to use winter rainfall. We found that both species effectively used winter rains to achieve significant carbon uptake through the cool-season, but that Lehmann’s lovegrass was less temperature sensitive and performed much better than bush muhly during the coldest periods of the winter, having greater carbon uptake and transpiring more water during these times. These findings suggest that winter rainfall is more effectively used by Lehmann’s lovegrass, but also that conversion of native grasslands to lovegrass-dominated systems could seriously affect surface soil water dynamics and patterns of inter-annual productivity in these semiarid grasslands.

Technical Abstract: Cool season precipitation may be an overlooked component in ecosystem water and carbon fluxes in summer rainfall dominated grasslands, and may contribute to the invasive success of the South African bunchgrass, Lehmann lovegrass (Eragrostis lehmanniana). To address this, we measured soil water, plant canopy temperature, and ecosystem and whole-plant water and carbon fluxes of Lehmann lovegrass and a native bunchgrass, bush muhly (Muhlenbergia porteri), across the winter season in a SE Arizona semidesert grassland. Volumetric water content (q) was consistently higher under lovegrass plots across 5cm and 25cm profiles, and this was accompanied by higher levels of plant transpiration (T) and plot-level ET. There was more net carbon uptake in Lehmann lovegrass plots, due to distinct differences in lower ecosystem respiratory fluxes (Reco) at higher temperatures and higher gross ecosystem photosynthesis (GEP) under cold, wet conditions when Reco in both species was suppressed. Transpiration (T) and aboveground plant respiration (Rabove) showed greater increases in bush muhly going from cold to warmer conditions, while lovegrass T and Rabove were relatively invariant. These findings show that cool-season precipitation can induce significant ecosystem-level carbon uptake, but also that the invasive lovegrass is better able than native species to utilize winter rains. This suggests that shifts to lovegrass dominance may alter seasonal surface soil water balance and carbon exchange dynamics in these water-limited grasslands.