Comparative whole-plant and leaf-level gas exchange of invasive and native desert grassland bunchgrasses

Background/Question/Methods

Since its introduction in the 1930s, the South African C₄ bunchgrass Lehmann lovegrass (Eragrostis lehmanniana) has steadily spread throughout the semiarid grasslands of Arizona.  In contrast to reports of 2- to 4-fold higher net primary productivity under E. lehmanniana dominance, short-term experimental rainfall studies have shown E. lehmanniana net ecosystem carbon exchange (NEE) is less efficient at utilizing precipitation pulses than native bunchgrasses, primarily due to higher ecosystem respiration (R_eco) following rainfall.  We measured biweekly volumetric soil moisture (θ), leaf-level photosynthesis (A_net) and stomatal conductance (g_s), and whole plant net ecosystem exchange (NEE), respiration (R_eco), and whole-plant photosynthesis (GEE; NEE-R_eco) of E. lehmanniana and the native bush muhly (Muhlenbergia porteri) through the 2008 summer monsoon on the Santa Rita Experimental Range in SE Arizona to see if seasonal soil moisture and ecophysiological dynamics may reconcile these findings, and provide better insight into how higher productivity is attained in this highly successful exotic grass.

Results/Conclusions

θ was 3-7% higher under E. lehmanniana throughout the monsoon; concurrently, g_s and A_net were consistently higher compared to M. porteri.  However, linear regressions of g_s and A_net were identical between species, suggesting g_s limitation was the primary control to leaf-level gas exchange.  Despite these differences at the leaf-level, early in the monsoon, when θ was high, NEE were similar, even though R_eco was markedly lower in E. lehmanniana.  Under drier late-season conditions, NEE and GEE was more negative in E. lehmanniana, due to markedly lower R_eco, which, unlike M. porteri, also up-regulated rapidly following rain. We believe these findings show that E. lehmanniana canopy attributes facilitate higher θ, which in turn might allow more rapid canopy development, lower stomatal limitations to photosynthesis, and more sustained seasonal activity.  Lower R_eco in this exotic species might indicate reduced belowground allocation, which may facilitate higher aboveground productivity.  In addition, rapid R_eco response to late-season rains suggest ecosystem respiratory kinetics may be altered under E. lehmanianna, possibly resulting in grasslands that are more sensitive to inter-annual variation in rainfall.