ARS | Publication request: Growing season ecosystem and leaf-level gas exchange of an exotic and native semiarid bunchgrass

Submitted to: Oecologia
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 24, 2009
Publication Date: June 17, 2010
Citation: Hamerlynck, E.P., Scott, R.L., Moran, M.S., Keefer, T.O., Huxman, T.E. 2010. Growing season ecosystem and leaf-level gas exchange of an exotic and native semiarid bunchgrass. Oecologia. 163:561–570.

Interpretive Summary: The dominance and spread of Lehmann lovegrass into desert grasslands is in part due to its higher annual productivity and standing biomass compared to native grasses. However, field studies have shown that carbon uptake in plots dominated by Lehmann lovegrass are less efficient than native grasses at utilizing rainfall pulses. This study seeks to reconcile these conflicting findings by tracking soil water content, leaf level gas exchange, and ecosystem-level evapotranspiration (ET), net ecosystem carbon exchange (NEE) and its components, ecosystem respiration (Reco) and ecosystem photosynthesis (GEP) in Lehmann lovegrass and a native bunchgrass, bush muhly, over the course of a monsoon growing season. We found that soils under lovegrass always had higher water contents, yet ET were similar, possibly reflecting differences in canopy structure that affect rain interception and transmission to the soil. Moister soils likely allowed lovegrass to rapidly develop leaves, have lower stomatal limitations to photosynthesis, allowing greater carbon uptake, especially with drier soils later in the season. Lower and more dynamic Reco in lovegrass plots, which might reflect lower allocation to root growth, may allow more aboveground growth, which could facilitate its spread and dominance in semiarid grasslands

Technical Abstract: The extensive spread of the South African grass, Lehmann lovegrass (Eragrostis lehmanniana) may potentially alter ecological and hydrological processes across semiarid grasslands and savannahs of western North America. We compared volumetric soil moisture (Q), ecosystem (i.e. whole-plant and soil) and leaf-level gas exchange of Lehmann lovegrass and the native bush muhly (Muhlenbergia porteri) over the 2008 monsoon season in a semiarid grassland in southern Arizona, USA to see if seasonal soil moisture and ecophysiological dynamics were consistent with reports of greater productivity associated with invasive success of lovegrass. Q across 0-5 and 0-25 cm was higher while evapotranspiration (ET) was similar between lovegrass and bush muhly plots, except shortly (1-3d) after rainfall, when ET was 32 to 81% higher in lovegrass plots. When early season Q was high, net ecosystem CO2 exchange (NEE) was similar (-5.43 umol m-2 s-1 +/- 0.564 SE and -7.42 umol m-2 s-1 +/- 1.397 SE, for lovegrass and bush muhly, respectively), but as storm frequency and Q declined, NEE was more negative in lovegrass (-0.69 to -3.00 umol m-2 s-1) than in bush muhly (+1.75 to -1.55 umol m-2 s-1), indicating greater carbon uptake in lovegrass-dominated plots. Ecosystem respiration (Reco) responded quickly to early monsoon rains, and was lower in lovegrass (2.44 to 3.74 umol m-2 s-1) compared to bush muhly (3.60 to 5.3 umol m-2 s-1) across the season. Unlike bush muhly, lovegrass Reco rapidly up-regulated following late-season storms; at these times, gross ecosystem photosynthesis (GEP) was greater in Lehmann lovegrass, with concurrent higher rates of leaf-level photosynthesis and greater stomatal conductance. We conclude that phenological and canopy structural characteristics facilitate higher Q under Lehmann lovegrass, allowing rapid leaf growth and lower stomatal limitations to photosynthesis, which affect plot-level ET and increase net carbon uptake through the short monsoon growing season.