Root dynamics and global change: seeking an ecosystem perspective
Authors: Norby, RJ, and RB Jackson
Tansley (1935) in describing the concept of the ecosystem commented on "...the necessity for investigation of all the components of the ecosystem and of the ways they interact to bring about approximation to dynamic equilibrium. That is the prime task of the ecology of the future." With respect to the current mandate to scientists to unravel the complexity of terrestrial ecosystem responses to global change, the future is now. Whether the current research (or policy) question concerns the capacity for forests and grasslands to sequester carbon as the atmosphere becomes progressively enriched with CO2, or the resistance of communities to changing water and temperatures regimes, or the fate of anthropogenic nitrogen compounds deposited onto ecosystems, the key components of ecosystem response may reside out of sight-the belowground system of roots, soil, and associated microorganisms.
Changes in the production and turnover of roots in forests and grasslands in response to rising atmospheric CO2 concentrations, elevated temperatures, altered precipitation, or nitrogen deposition could be a key link between plant responses and longer-term changes in soil organic matter and ecosystem C balance. While Tansley's call for integrated studies of all ecosystem components have long been embraced, ecosystem-level observations of root production and mortality in response to global change variables are just starting to emerge. To highlight the current state of knowledge about root responses to atmospheric and climatic change and their interaction with ecosystem properties, a symposium was convened in Townsend, Tennessee, USA in October, 1999, on the theme: "Root Dynamics and Global Change: An Ecosystem Perspective."
The collection of papers arising from that symposium, as well as numerous unpublished poster presentations and spirited discussion, revolved around three central questions: (1) do elevated atmospheric CO2, N deposition, and climatic change alter the dynamics of root production and mortality? (2) what are the consequences of root responses to plant physiological processes? (3) what are the implications of root dynamics to soil microbial communities and the fate of C in soil? These questions are considerably easier to ask than they are to answer. Ecosystem-scale experiments on global change phenomenon are few and recent (or ongoing). Results are often ambiguous, contradictory, and confounded by methodological problems. Although the body of knowledge may be immature for a definitive synthesis, we have an opportunity to highlight the critical uncertainties, generate new hypotheses, and improve our techniques in current and future research programs.