Identifying Critical Thresholds for Acute Response
of Plants and Ecosystems to Water Stress (TARP)
Biogeography
models and dynamic ecosystem simulation models used in the assessment of
climatic change routinely predict species displacement and population migration
following mortality. Although such predictions are based on reasonable
hypotheses regarding plant response to warming and associated inter-specific
competition, predicted mortality rates driving current models and mechanisms
remain largely untested. We propose a manipulative field study to provide
data on the impact of acute drought on mechanisms responsible for growth
and mortality of deciduous forest canopy trees representative of common
plant functional types (Liriodendron and Quercus). We propose
to deploy replicated understory canopies for the removal of 100 percent
throughfall and stem flow around mature trees to allow the artificial introduction
of a spring drought. Such droughts overlap the period of optimum
stem growth and have a greater potential to disrupt plant function.
Key measurements include weekly dendrometer band observations, periodic
evaluation of plant nonstructural carbohydrates status, automated measurements
of sapflow, periodic observations of foliar photosynthesis and conductance,
and observations of soil moisture status by depth and horizontal extent
of the tree root zone. A primary goal of this research will be to translate
such data into mechanistic expressions of the threshold levels of moisture
stress responsible for limiting plant function and growth and the inclusion
of mechanistic expressions in biogeochemical and biogeography models to
enhance their usefulness for assessments of climatic change. While the
proposed field experiments and observations will be limited to acute precipitation
manipulations, interactions between acute drought, future warming, elevated
CO2 and increasing tropospheric ozone will be addressed with stand-level
ecosystem models to evaluate the potential for mitigating (CO2) or exacerbating
(temperature, ozone) impacts.
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The TARP study strives to evaluate growth and physiological responses of
large trees to severe soil water deficits.
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Critical data quantifying growth reductions and mortality leading to forest
failure are needed to test extant ecosystem models.
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Driver: Global environmental change prediction.
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Unique contribution: Replicated study of acute water stress on large
trees that will provide key data for testing the validity of model outputs.
Objectives
1) Demonstrate that a late spring/early summer drought will
have a direct impact on current year growth due to reductions in development
of functional sapwood.
2) Identify the level and extent of drought needed to induce reduce
stored carbohydrate and nutrient reserves, accelerate canopy senescence,
and lead to reduced canopy production in subsequent years for two common
plant functional types.
3) Translate these observations into appropriate algorithms for inclusion
in biogeochemical and biogeography models being used for assessments of
climatic change impacts.
Project Details
Photo album
Experimental System
Instrumentation (to be added)
The TARP Experiment is supported by the U.S. Department of Energy's
Office
of Science (BER) through the Program
for Ecosystem Research (PER) in the DOE Office of Science, Office
of Biological and Environmental Research (BER).
For additional information, please contact:
Paul J. Hanson
Walker Branch Project Coordinator
Environmental Sciences Division
Oak Ridge National Laboratory
P.O. Box 2008, Building 1059
Oak Ridge, TN 37831-6422 USA
hansonpj@ornl.gov
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