Changes in regional climate across the southern United States, which are hypothesized to occur in response to increasing greenhouse gases in the atmosphere, have raised concerns that southeastern forests might be vulnerable to future drought conditions. To provide experimental information related to such concerns a catchment-scale (19,000 m2) manipulation experiment was initiated on the Walker Branch Watershed (Oak Ridge, Tennessee) in July of 1993 to modify throughfall (precipitation passing through the forest canopy) to an upland oak forest over multiple growing seasons. The Throughfall Displacement Experiment (TDE) is being conducted by ORNL scientists with funding from the U.S. Department of Energy's (DOE's) Program for Ecosystem Research. The project seeks to identify the potential range of adjustments in physiological processes, water use, and aboveground and belowground growth by overstory and understory species and to evaluate changes in nutrient cycling and decomposition processes in the context of changing precipitation inputs.
Using a passive experimental design, the TDE treatments are implemented by a gravity-driven displacement of throughfall precipitation from one treatment plot to another. The manipulation yields experimental plots that have normal rainfall inputs or one-third more or less rainfall than typical forests in eastern Tennessee. The TDE experiment is one of the largest terrestrial manipulation experiments ever conducted.
Multiyear growth and physiological observations through six years of manipulation show that the small-stature vegetation is the most sensitive to changing precipitation. Shallow-rooted dogwood saplings showed reduced growth, water use, and increased mortality in our dry-treatment scenario, while mature canopy trees did not. Unlike the imposed dry treatment, periodic natural drought events in 1993, 1995, and 1998 have resulted in mature-tree growth reductions of 30-50%, but the net impact on aboveground stand production over six years of manipulation is limited. Foliar litter production by the canopy has not been impacted by the TDE treatments, and is very stable from year to year at approximately 500 g m-2 y-1. Belowground root production showed a significant increase in the dry treatment, suggesting a potential stand-level adjustment to the reduced precipitation scenario.
The sensitivity of overstory tree species to soil water deficits is ranked as follows: Yellow Poplar > Blackgum > Maple > White oak = Chestnut oak. This ranking is consistent among a variety of measured properties, including growth, canopy water use, foliar biochemistry, and CO2 and water vapor exchange. In the understory, dogwood is more sensitive than red maple saplings, and dramatic increases in mortality of dogwood under the TDE's dry-treatment scenario suggest potential future changes in forest species diversity if such species die out.
Direct measurements of litter layer mass show a significant 20-30% increase following six years of sustained precipitation reductions. Litter mass accumulation indicates reduced decomposition and carbon accumulation for a dry future, but carbon accumulation in the forest litter layer might increase future fire hazards and long-term reductions in nutrient availability. The suite of TDE measurements has led us to conclude that differences in seasonal timing of rainfall will have a greater impact on plant productivity (or carbon sequestration) than changes in rainfall applied equally throughout a year.
A key implication of this conclusion is that accurate predictions of plant, forest-stand, and ecosystem responses to changing regional climates will require a concomitant understanding of future climate dynamics. The temporal resolution of current precipitation change scenarios (i.e., more or less rainfall annually) must be improved to include scenarios of rainfall periodicity. This requirement is essential because it is the balance between rainfall inputs and ecosystem water use that lead to quantification of soil water status.
The TDE is slated to operate for two additional growing seasons (1999 and 2000) to complete observations of long-term cumulative tree and soil responses, fill out mechanistic plant/ecosystem response relationships, and support ongoing and externally funded collaborative research efforts. Final results from TDE research will yield a hierarchical integration of physiological response mechanisms into ecosystem models required for long-term assessments of the influence of future precipitation change on forest productivity and carbon sequestration. Original data sets covering the experimental environment, plant growth, physiological characteristics, and soils will be archived by CDIAC for alternative analyses and modeling.
Contact Paul Hanson (hansonpj@ornl.gov) for additional information and a list of published research papers.
kng 09/99