Ecosystem Consequences of Cheatgrass Invasion in the Great Basin
Investigators (most current known information)
For over a century, sagebrush ecosystems of western Nevada have been aggressively invaded by cheatgrass (Bromus tectorum L.), a winter annual native of western Europe. Propelled by overgrazing and fire, widespread transformation from diverse shrub-perennial grass steppe to annual grassland dominated by a single species is well underway, and this transformation may alter a myriad of ecosystem functions. The objective of this research was to quantify the ecosystem-level consequences of cheatgrass invasion, and remediation practices, on carbon and water resources in the Great Basin Desert.
To determine how this invasion may disrupt ecosystem function, carbon and water fluxes were quantified with two types of static chambers. A smaller 1-m3 gas exchange chamber (cube) was used to quantify spatial patterns of carbon and water exchange in different sagebrush and post-fire communities; and a large dome chamber (4-m diameter; 16.4 m-3), that captured all ecosystem elements, was used to construct annual budgets. The spatial and temporal variability in soil moisture was measured directly with a time domain reflectrometry (TDR) system and indirectly by isotopic analysis of the dominant plant species.
During the spring and summer in a relatively wet year gas exchange was measured with the cube in three, paired sagebrush and adjacent post-fire invaded communities in the northern Great Basin. Despite inter-annual variation, temporal and spatial patterns of net carbon exchange (NCE) and evapotranspiration (ET) of sagebrush measured with the cube differed in post-fire communities and were correlated with changes in leaf area and phenology in each community. Intact native sagebrush communities maintained positive carbon balance (mean maximum NCE 0.4 - 4.3 µmol m-2 s-1) throughout the growing season, driven by significantly greater (p < 0.05) NCE of shrubs despite large inter-shrub spaces (~70% ground cover). Post-fire community net C exchange was controlled by the dominant species in each of the three communities. Net C exchange in the perennial bunchgrass community was similar to sagebrush. However, cheatgrass and mustard communities had significantly lower NCE and became net sources of carbon (NCE declined to –0.5 µmol m-2 s-1) with the onset of the summer drought. Seasonal patterns of ET in post-fire communities differed from sagebrush communities and led to lower surface soil moisture content and increased soil temperatures in the cheatgrass-dominated community.
During a drought year, measurements made at one site during the first two dry years after wildfire with the larger dome gas exchange chamber indicated that both NCE and ET were higher in a post-fire successional ecosystem (-0.9 to 2.6 µmol CO2 m-2 s-1) and (0.0 to 1.0 mmol H20 m-2 s-1, respectively) than in the an adjacent intact sagebrush ecosystem (-1.2 to 2.3 µmol CO2 m-2 s-1 and –1.0 to 0.8 mmol H20 m-2 s-1, respectively). Higher NCE in the post-fire ecosystem appears to be caused by lower rates of aboveground plant respiration while higher ET appears to be caused by higher surface soil temperatures and increased soil water recharge after rains. These patterns disappeared or were reversed when conditions became even drier. During a drought year NCE was negative in the sagebrush community because of high respiration rates during winter and near zero for the post-fire community. >/p>
Continuous measurement of soil water content using a segmented TDR system in an intact sagebrush and in an adjacent post-fire ecosystem from March 2001 to October 2002 indicated that soil water contents in the upper 75 cm of the soil were similar and very low during summers, but that substantially lower water recharge in post-fire ecosystems after large snowfalls – possibly due to decreased snow deposition and higher sublimation – caused large differences in water storage between the two ecosystems in winter. These differences declined with the onset of the vegetation period in March, probably due to higher plant water uptake in the more densely vegetated intact sagebrush ecosystem. The presence of patchiness of near-surface soil water (0–20 cm), quantified among 80 points of nested grid systems placed in each ecosystem, appears to benefit native perennial shrub re-generation in the intact ecosystem.
Plant cover has a strong influence on carbon and water balance of ecosystems, and replacing the dominant plant functional type in a community alters many ecosystem processes. Our results indicated that conversion of native sagebrush to post-fire communities disrupts seasonal patterns of carbon and water exchange, particularly during times when moisture is available during the growing season. As the extent of post-fire cheatgrass communities increases, decreases in NCE and ET are likely to become more pronounced and have a greater impact on ecosystem function. Additionally, this conversion alters both the seasonal availability and spatial distribution of soil water, which may impede the establishment of native perennials. The capacity of Great Basin Desert ecosystems to uptake carbon and the regional water balance may be severely affected by this landscape transformation.
Articles in Journals
Obrist , D., D. Yakir and J.A. Arnone, III. 2004. "Temporal and spatial patterns of soil water following wildfire-induced changes in plant communities in the Great Basin." Plant and Soils (in press).
Arnone J.A. and D. Obrist. 2003. "A large daylight geodesic dome for quantification of whole ecosystem CO2 and water vapor fluxed in Arid Ecosystems." Journal of Arid Environments 55:629-643.
Prater, M.R., J.A. Arnone, III and E.H. DeLucia. 2003. "Net carbon exchange and evaportranspiration in post-fire and intact sagebrush communities in the Great Basin." Ecology.
Obrist, D., E.H. DeLucia and J.A. Arnone, III. 2003. "Consequences of wildfire on ecosystem CO2 and water vapor fluxes in the Great Basin." Global Change Biology 9:563-574.
Prater, M.R. and E.H. DeLucia. 2003. "Ecosystems consequences of Cheatgrass invasion in the Great Basin." Presented, Program in ecology and evolutionary biology, The University of Illinois, 4th annual graduate student symposium. Urbana IL.
Prater, M.R. and E.H. DeLucia. 2003. "Patterns of ecosystem carbon exchange in sagebrush and post-fire communities in the Great Basin." Presented, Program in ecology and evolutionary biology, University of Illinois, 5th annual graduate student symposium. Urbana, IL.
Prater, M.R., J.A. Arnone, III and E.H. DeLucia. 2003. "Conversion of sagebrush to non-native communities following fire: Impacts on ecosystem carbon and water exchange." Presented, The Ecological Society of America 88th annual meeting. Savannah GA.
Prater, M.R., J.A. Arnone, III and E.H. DeLucia. 2003. "Net carbon exchange and evapotraspiration in post-fire and intact sagebrush communities in the Great Basin." Poster, Invasive plants in aatural and manage systems: linking science and management - 7th international conference on the ecology and management of alien plant invasions. Ft. Lauderdale FL.
Obrist , D. and J.A. Arnone. 2002. "Effects of wildfire-induced changes in plant community composition on ecosystem CO2 and water vapor fluxes in the Great Basin." Presented, The Ecological Society of America 87th annual meeting. Tucson AZ.
Prater, M.R., D. Obrist, J.A. Arnone and E.H. DeLucia. 2002. "Post-fire effects on ecosystem gas exchange patterns in Northern Great Basin communities." Presented, The Ecological Society of America 87th annual meeting. Tucson AZ.
Obrist, D. and J.A. Arnone. 2001. "A large daylight geodesic dome for quantification of whole-ecosystem carbon dioxide and water vapor fluxes." Poster, Eos. Trans. AGU, 82(47), Fall Meeting. Suppl., Abstract B42A-0107.
Larsen, J. Title of thesis? Environmental & Resources Sciences Program. M.S. Thesis. University of Nevada-Reno. Reno NV.
Obrist, D. 2002. Title of dissertation? Hydrologic Sciences Program. Ph.D. Dissertation. University of Nevada-Reno. Reno NV.
Prater, M. Title of dissertation? Department of Plant Biology. The University of Illinois. Ph.D. (pending).