PARENT SESSION
Symposium 14: Respiratory Control of the Global C Cycle in a Changing Environment
Organized by: M Gonzalez-Meler and W Schlesinger
Wednesday, August 6. 1:30 PM to 5:00 PM, SITCC Chatham Ballroom B.

Rate-based respiratory measurements without a system context are impeding a mechanistic understanding of the controls on respiratory fluxes.

Ryan, Michael^{*,1, 2}, ¹ mgryan@fs.fed.us, Fort Collins, CO 80526, USA² Graduate Degree Program in Ecology, Fort Collins, CO 80523, USA

ABSTRACT- Ecosystems scientists and ecophysiologists measure autotrophic and soil surface CO₂ efflux ('soil respiration') to understand how these fluxes vary among plant parts and with environmental variation. Such knowledge is essential to develop scaling models and to predict how fluxes and ecosystem carbon storage will change with changes in species, biomass, nutrition, climate, atmospheric decomposition and composition. Early measurements of respiration rates assumed that these rates would remain constant (once the sources of variability had been identified) and that inferences at the system level could be drawn or inferred from measurements of rates. The recent intense focus on the carbon cycle and its regulation has promoted a renewed interest in respiration. However, the fundamental question motivating these measurements remains the same: will environmental changes alter a plant's carbon balance (its ratio of respiration to photosynthesis, R:P) or the carbon balance of the ecosystem? My objective with this review is to convince ecophysiologists to place respiration measurements into the context of a larger plant or ecosystem carbon balance, because short-term measurements of respiration rates cannot be used to infer ecosystem behavior. Autotrophic respiration rates are highly variable and plastic – they vary with temperature, metabolic activity, substrate availability, phenology, and tissue type. Soil surface CO₂ efflux is strongly linked to plant processes through root and mycorrhizal respiration, and because much of the heterotrophic respiration consumes recently produced detritus. Both autotrophic and soil respiration rates 'acclimate' if conditions change. Finally, plants can shift carbon allocation in response to shifts in resource availability. This plasticity and structural changes make it difficult or impossible to infer how R:P or ecosystem C storage might change from short-term measurements of autotrophic or soil respiration rates. I provide examples of rates placed in the context of larger carbon balances, and some strategies for assembling such budgets.

Key words: respiration, carbon cycle, soil respiration