The South African grass, Lehmann lovegrass (Eragrostis lehmanniana), may alter ecosystem processes across extensive semiarid grasslands and savannahs of western North America. We compared volumetric soil moisture (theta), total and green tissue leaf area index (LAI), 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 savanna in southern Arizona, USA, to see if these were consistent with high productivity associated with lovegrass invasive success. theta across 0-5 and 0-25 cm was higher while evapotranspiration (ET) was similar between lovegrass and bush muhly plots, except shortly after rainfall, when ET was 32-81% higher in lovegrass plots. Lehmann lovegrass had lower, quickly developing LAI with greater leaf proportions than bush muhly. When early season theta was high, net ecosystem CO(2) exchange (NEE) was similar, but as storm frequency and theta declined, NEE was more negative in lovegrass (-0.69 to -3.00 micromol m(-2) s(-1)) than bush muhly (+1.75 to -1.55 micromol m(-2) s(-1)). Ecosystem respiration (R (eco)) responded quickly to monsoon onset and late-season rains, and was lower in lovegrass (2.44-3.74 micromol m(-2) s(-1)) than bush muhly (3.60-5.3 micromol m(-2) s(-1)) across the season. Gross ecosystem photosynthesis (GEP) was greater in Lehmann lovegrass, concurrent with higher leaf-level photosynthesis and stomatal conductance. We conclude that canopy structure facilitates higher theta under Lehmann lovegrass, reducing phenological constraints and stomatal limitations to whole-plant carbon uptake through the short summer monsoon growing season.