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

item Hamerlynck, Erik
item Scott, Russell - Russ
item BARRON-GAFFORD, G.A. - University Of Arizona
item Cavanaugh, Michelle
item MORAN, M.S. - Retired ARS Employee
item HUXMAN, T.E. - University Of Arizona

Submitted to: Plant Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/13/2012
Publication Date: 7/3/2012
Citation: Hamerlynck, E.P., Scott, R.L., Barron-Gafford, G., Cavanaugh, M.L., Moran, M., Huxman, T. 2012. Cool-season whole-plant gas exchange of exotic and native semiarid bunchgrasses. Plant Ecology. 213:1229-1239.

Interpretive Summary: The South African bunchgrass, Lehmann lovegrass, dominates large areas of Southwest U.S. grasslands. High productivity is important to the invasive success of this grass, yet short-term pulse studies and single season ecosystem-level studies have resulted in contradictory findings regarding the mechanisms underlying higher productivity, and the consequences to ecosystem water and carbon processes to U.S. rangelands. To address this, we measured water balance (i.e. soil water content, evapotranspiration and transpiration) and carbon-balance components (net ecosystem carbon exchange, and its parts, ecosystem respiration and photosynthesis) and their interaction (water use efficiency, or photosynthesis per transpiration) over a forty week period for lovegrass and two important native grasses. We found that lovegrass dominance increased soil evaporation throughout the year, and that cool season conditions limited lovegrass and cottontop net carbon exchange throughout the year, while bush muhly effectively used cool season rain, and subsequently fixed more carbon during the summer monsoon. Soil water was always lower under bush muhly, but this likely reflected differences in soil bulk density, not high plant water use or differences in canopy structure that could influence rainfall interception. Water use efficiency in lovegrass increased, as did bush muhly WUE, while cottontop WUE did not. These findings showed that the invasive lovegrass did not use cool season soil water to achieve higher productivity, as past researchers have conjectured, but that its ability to rapidly maximize WUE following early monsoon season rain likely enhances its productivity compared to most native grasses. Future climate predictions suggest warmer temperatures and infrequent, though larger, rainfall events across the arid Southwest, and these conditions are likely to favor the spread of this invasive grass across arid and semi-arid U.S. rangelands.

Technical Abstract: The success of invasive aridland plants may depend on their utilization of precipitation not fully exploited by native species, which could lead to seasonally altered ecosystem carbon and water fluxes. We measured volumetric soil water across 25-cm profiles (h25cm) and springtime whole-plant water- and carbon-fluxes of the exotic Lehmann lovegrass (Eragrostis lehmanniana) and a native bunchgrass, bush muhly (Muhlenbergia porteri), following typical (55 mm in 2009) and El Nin˜o-enhanced accumulations (154 mm in 2010) in a SE Arizona savanna. Across both years, h25cm was higher under lovegrass plots, with similar evapotranspiration (ET) between lovegrass and bush muhly plots. However, in 2010 transpiration (T) was higher in bush muhly than lovegrass, implying higher soil evaporation in lovegrass plots maintained similar ET. Net ecosystem carbon dioxide exchange (NEE) was similar between lovegrass and bush muhly plots in 2009, but was more negative in bush muhly plots following El Nin˜o, indicating greater CO2 assimilation. Ecosystem respiration (Reco) and gross ecosystem photosynthesis (GEP) were similar between lovegrass and bush muhly plots in 2009, but were higher in bush muhly plots in 2010. As a result, lovegrass plots reduced ecosystem water-use efficiency (WUEe = NEE/ET), while bush muhly WUEe remained constant between 2009 and 2010. Concurrent whole-plant WUE (WUEp = GEP/T) did not change in lovegrass plots, but increased in bush muhly plots between these years. We concluded that cool-season precipitation use is not a component of Lehmann lovegrass invasive success, but that the change in ET partitioning and attendant shifts in cool-season WUEe may increase interannual variation in ecosystem water- and carbon-exchange dynamics in the water-limited systems it dominates.