Mechanisms of forest ecosystem adjustment to altered precipitation -- Throughfall Displacement Experiment (TDE)
Principal investigator: Paul J. Hanson
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Mechanisms of forest ecosystem adjustment to altered precipitation -- Throughfall Displacement Experiment (TDE) Principal investigator: Paul J. Hanson |
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Project goal
To develop a mechanistic understanding of the adjustments of forest organisms and ecosystems to changes in precipitation amounts that could accompany global warming. Ecosystem being studied An upland oak forest on Walker Branch Watershed in eastern Tennessee. The experimental site is representative of temperate deciduous forests that comprise about 40% of U.S. forests. Such forests are among the most productive natural ecosystems, store substantial amounts of carbon, are a key source of clean water, and are a repository of biodiversity. An assessment of the similarity of the research site to other locations in the United States is captured in the map available at this link. Results Photosynthesis, conductance, and canopy water use were reduced from 30 to 50% under soil water deficits, and dormant season carbohydrate storage pools dropped below 10% following severe droughts. Evidence that such physiological responses to water deficit affect tree basal area growth is lacking for trees greater than 20 cm diameter, however. An evaluation of all trees >10 cm diameter indicates that long term reductions and increases in individual tree basal area growth are being driven by chronic changes in forest biogeochemical cycles.
A surprising resilience of large canopy trees to drought on Walker Branch Watershed was observed [e.g. Hanson et al. (2001)]. The explanation for the lack of correlation between tree growth and drought in this forest is that stem growth and drought are temporally disconnected (or unsynchronized); tree growth is a spring and early summer phenomenon, but droughts in east Tennessee (and much of the eastern United States) typically develop only in the late summer and fall. Total stand basal area growth through 2004 was, however, apparently negatively affected by drying. Tree seedlings and saplings have shown sensitivity to drought, including higher rates of mortality. Such changes might lead to long-term species composition changes in a drier climate as older trees would be gradually replaced by alternate understory species.
Annual leaf production (dry mass growth) for the dry treatment exceeded that of the wet and ambient treatment plots by 20% or more starting in 1998. This greater growth on the dry plot relative to the wet and ambient plots was contrary to an original TDE hypothesis that reduced precipitation would limit leaf growth in the canopy. The greater leaf growth in the dry plot is likely the result of excessive leaching of beneficial plant elements in the presence of more abundant throughfall on the ambient an dwet plots. This conclusion is supported by measured increases in cation concentrations in the soil solution leaching from plots with elevated acidic precipitation [Johnson et al. 2002]. A comparison between ecosystem model simulations and field data collected in and near the TDE indicated that current ecosystem models are ill equipped to capture (mimic) observed effects of interannual and seasonal changes in throughfall on the growth and physiology of this forest, and must be improved before they can be used to reliably predict effects of changes in precipitation on forest ecosystem functioning [Hanson et al. (2004)]. A recognition that large trees have access to deep soil moisture sources not commonly included in ecosystem models may be key to such improvements. More details about the study are at the TDE website. Why this is important The TDE (and collaborative research) provided ecosystem models with response mechanisms that are essential for making reliable forecasts of effects of potential changes in precipitation on deciduous forest structure and functioning, including net primary production, tree species composition, and carbon, water, and nutrient cycling. Several a priori hypotheses about likely effects of long-term changes in precipitation amount on eastern decisuous forest functioning have been invalidated by the TDE, to date. Methods Between 1993 and 2006 a catchment-scale (1.9 ha) manipulation of precipitation reaching the forest floor (i.e., throughfall) of an upland oak forest was conducted (throughfall manipulation wa used as a surrogate for precipitation change). Manipulations of throughfall were made by passively transferring a fraction of the throughfall from one experimental plot to another (+ or - 33%). Three side-by-side experimental plots were used, each 80 x 80 m. One is designatied wet (133% of ambient throughfall), one is designated ambient (no change to throughfall), and one is designated dry (67% of ambient throughfall). Each plot is divided into 100 subplots, each 8 x 8 m, that serve as locations for repetitive, nondestructive measurements of soil and plant characteristics. A variety of measurements are being carried out including: plant growth (seedlings, saplings, trees), leaf and stem physiology, soil respiration and organic matter pool dynamics, and the cycling of key elements (e.g., N, Ca, Mg). Collaborative studies have also looked at treatment impacts on patterns of herbivory, distribution of spider populations and pathogenic fungi. Long-term patterns of mycorrhizae, ground beetles, and microarthropods were characterized by treatment.
Key publications Wullschleger SD, Hanson PJ (2006) Sensitivity of canopy transpiration to altered precipitation in an upland oak forest: evidence from a long-term field manipulation study. Global Change Biology 12:97-109 Hanson PJ, Wullschleger SD, Norby RJ, Tschaplinski TJ, Gunderson CA (2005) Importance of changing CO2, temperature, precipitation, and ozone on carbon and water cycles of an upland-oak forest: incorporating experimental results into model simulations. Global Change Biology 11:1402-1423 Hanson PJ, Amthor JS, Wullschleger SD, Wilson KB, Grant RF, Hartley A, Hui D, Hunt ER Jr, Johnson DW, Kimball JS, King AW, Luo Y, McNulty SG, Sun G, Thornton PE, Wang SS, Williams M, Baldocchi DD, Cushman RM (2004) Carbon and water cycle simulations for an upland oak forest using 13 stand-level models: intermodel comparisons and evaluations against independent measurements. Ecological Monographs 74:443-489 Hanson PJ, Wullschleger SD (editors) (2003) North American Temperate Deciduous Forest Responses to Changing Precipitation Regimes. Ecological Studies, Vol. 166. Springer, New York, 472 p Johnson DW, Hanson PJ, Todd DE Jr (2002) The effects of throughfall manipulation on soil leaching in a deciduous forest. Journal of Environmental Quality 31:204-216 Hanson PJ, Todd DE Jr, Amthor JS (2001) A six-year study of sapling and large-tree growth and mortality responses to natural and induced variability in precipitation and throughfall. Tree Physiology 21:345-358 Personnel Paul J. Hanson, Oak Ridge National Laboratory Carla A. Gunderson, Oak Ridge National Laboratory Dale W. Johnson, University of Nevada, Reno Stan D. Wullschleger, Oak Ridge National Laboratory Funding period: 1993-2007 |
