Submitted to: Journal of Agricultural Science
Publication Type: Peer reviewed journal
Publication Acceptance Date: 7/8/2010
Publication Date: 1/25/2011
Citation: Polley, H.W., Morgan, J.A., Fay, P.A. 2011. Application of a conceptional framework to interpret variability in rangeland responses to atmospheric CO2 enrichment. Journal of Agricultural Science. 149:1-14. Interpretive Summary: Plant growth (productivity) is expected to increase as a result of the global increase in carbon dioxide (CO2) concentration in air. Yet, CO2 effects on productivity have been found to vary widely among grasslands and other rangeland ecosystems. We applied a conceptual framework to address the question of why CO2 effects on plant productivity vary among rangeland ecosystems. The conceptual framework is based on the premise that the influence of CO2 on productivity is determined by four “internal” variables, regional climate, soil resource supply, the types of organisms present, and disturbance regimes. These variables both regulate CO2 effects on productivity and respond to CO2-caused changes in ecosystems. Changes in internal variables resulting from CO2 enrichment may feed back to dampen or amplify the initial effect of CO2 on productivity. We contend that most feedbacks from controlling variables will dampen CO2 effects. By stimulating plant growth and demand for nitrogen, for example, CO2 enrichment may reduce available nitrogen in soil and limit future growth responses to CO2. The most common source of positive feedbacks appears to be through changes in plant species composition. Productivity of plant stands will increase through time when species that realize little benefit from high CO2 are replaced by those that experience a large boost in growth at elevated CO2, for example. We conclude that a shift in plant composition will be required on most rangelands if CO2 enrichment is to lead to a sustained increase in plant productivity.
Technical Abstract: Plant productivity and other ecosystem processes vary widely in their responses to experimental increases in atmospheric carbon dioxide (CO2) concentration. We adapt a conceptual framework first suggested by Chapin et al. (1996) to define conditions that sustain ecosystems to address the question of why CO2 effects on primary productivity vary so greatly among rangelands and among years for a given ecosystem. The “interactive controls” framework is based on the premise that the influence of CO2 on productivity is governed by a set of internal variables that interact dynamically with ecosystem processes. These interactive controls, which include regional climate, soil resource supply, major functional groups of organisms, and disturbance regimes, both regulate CO2 effects on ecosystems and respond to CO2 effects. Changes in interactive controls resulting from CO2 enrichment may feed back to dampen or amplify ecosystem responses to CO2. Most feedbacks from interactive controls will be negative and dampen CO2 effects on ecosystems. Negative feedbacks promote homeostasis in ecosystem processes and reduce CO2 effects on plant productivity. Positive feedbacks on CO2 responses are fewer, but can sustain or even increase benefits of CO2 enrichment for productivity. The most common source of positive feedbacks on CO2 responses appears to through changes plant species and functional group composition. We suggest that understanding positive and negative feedbacks on CO2 responses is one key to predicting consequences of CO2 enrichment for rangeland productivity and other processes.