PARENT SESSION
Contributed Oral Session 154: Photosynthesis and Water Relations: Climate Effects
Friday, August 12, 8:00 AM - 11:30 AM, Meeting Room 515 A, Level 5, Palais des congrès de Montréal

Disentangling ecosystem responses to external and intrinsic drivers on multiple time scales.

Stoy, Paul^*,1, Katul, Gabirel¹, Siqueira, Mario¹, Juang, Jehn-yih¹, Oren, Ram¹, ¹ Nicholas School of the Environment and Earth Sciences, Durham, NC, USA

ABSTRACT- Long-term eddy covariance records can address questions of general ecological interest, particularly those that involve system responses to forcings that act on multiple temporal scales. For example, how do ecosystems with different internal characteristics (in our case vegetation) respond to and mediate common external drivers (e.g. precipitation and radiation) that vary on time scales from hours to years, and how is the temporal heterogeneity of observed ecosystem response best modeled? We address these questions using fifteen total site-years of net ecosystem CO₂ exchange (NEE) and evapotranspiration (ET) measurements at a successional gradient of adjacent old-field, Pinus taeda dominated mid-rotation planted pine forest, and mature oak-hickory forest in the Duke Forest, NC. The ecosystems experience identical edaphic and climatic conditions, which during the measurement period included severe drought and major ice storm events to which the ecosystems exhibited differential immediate and lagged responses. We employ a combination of low-dimensional physiological models, wavelet decomposition, and sensitivity analyses to deconvolve the relationship between forcing and response at time scales from hours to years with a focus on longer time scales and the mechanisms that give rise to interannual variability in NEE and ET. Results of note include an observed decoupling of the carbon and water cycles along the successional gradient; the grass ecosystem switched from C sink to C source during drought, pine NEE decreased by over 50%, but hardwood NEE was drought insensitive. Combining these results with ice-storm impacts suggests that the pine ecosystem represents a high risk / high gain management option for C sequestration, but mature hardwood forests represent an underutilized alternative that may sequester more C than plantations depending on rotation length. At a broader spatial scale, the land cover area of managed pine plantations in the SE is predicted to nearly double over the next half century, and our results suggest that increasing the fractional cover of pine plantation will make the future magnitude and variability of the southeastern US C sink more sensitive to projected shifts in the variability of the hydrologic cycle.

Key words: eddy-covariance, net ecosystem exchange, evapotranspiration, temporal scale