Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 2006. Ecosystem processes are tightly linked to the structure and function of plant communities; therefore changes in the dominant vegetation will have important consequences for ecosystem function. Non-native plants, such as cheatgrass (Bromus tectorum L.), have aggressively invaded the Western United States. Promoted by overgrazing and fire, this invasion is rapidly converting large expanses of native sagebrush to post-fire invasive communities. The objective of this research was to test the general hypothesis that plant cover controls ecosystem function by quantifying the effect of land cover change on carbon, water and energy resources. Net carbon exchange (NCE), evapotranspiration (ET) and energy partitioning were measured in sagebrush and adjacent post-fire communities in the northern Great Basin Desert using a gas-exchange chamber and micrometeorological techniques. Temporal and spatial patterns of NCE and ET were correlated with changes in leaf area. Net C exchange and ET in post-fire communities dominated by perennial bunchgrasses was similar or greater than sagebrush. However, communities dominated by annuals (Bromus tectorum and Sisymbrium altissimum L.) had significantly lower NCE than sagebrush and became sources of carbon to the atmosphere with increased severity of the summer drought. Bunchgrasses partitioned more available energy into latent heat early in the season, resulting in greater ET, lower sensible heat flux and more rapid declines in surface soil moisture compared to sagebrush. These differences disappeared as surface soil moisture declined below 10%. DAYCENT, a daily time-step process-based ecosystem model, agreed well (R2 > 0.63, P < 0.01) with field-based measurements of NCE, ET and soil water content. While total annual ET was similar between sagebrush and post-fire Sisymbrium communities, increased evaporation in the annual community caused decreases in surface soil moisture. Lower productivity of annuals caused lower soil carbon storage compared to sagebrush. Replacement of native sagebrush with post-fire communities severely disrupted temporal and spatial patterns of carbon and water exchange, as well as long-term storage of ecosystem resources in these arid ecosystems. Understanding the disruption of carbon and water cycling in invaded ecosystems provides insight into controls on ecosystem function and provides a foundation for predicting future impacts of global change on arid ecosystems. U of I Only Restricted to the U of I community idenfinitely during batch ingest of legacy ETDs