Application of a conceptual framework to interpret variability in rangeland responses to atmospheric CO2enrichment

Abstract

SUMMARYPlant productivity and other ecosystem processes vary widely in their responses to experimental increases in atmospheric carbon dioxide (CO2) concentration. A conceptual framework first suggested by Chapinet al.(1996) was adapted to address the question of why CO2effects 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 elevated CO2on 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 CO2effects on ecosystems and respond to CO2effects. Changes in interactive controls resulting from CO2enrichment may feed back to dampen or amplify ecosystem responses to CO2. Most feedbacks from interactive controls will be negative and dampen CO2effects on ecosystems. Negative feedbacks promote homeostasis in ecosystem processes and reduce the response of plant productivity to CO2. Positive feedbacks on CO2responses are fewer, but can sustain or even increase benefits of CO2enrichment for productivity. Positive feedbacks on CO2responses occur most frequently through changes in plant species and functional group composition. Understanding positive and negative feedbacks on CO2responses could be one key to predicting consequences of CO2enrichment for rangeland productivity and other processes.