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Precipitation and carbon storage with woody plant encroachment into grasslands
Project Abstract: Shifting dominance among herbaceous and woody vegetation alters primary production, plant allocation, and rooting depth, potentially meters underground. This project addresses three hypotheses related to the carbon balance of the southern and southwestern U.S.: 1) The effect of woody plant encroachment on carbon storage varies predictably with precipitation, with plant biomass gains increasingly offset by decreased soil organic carbon (SOC) at wetter sites; 2) SOC losses with the conversion of grasslands to woody vegetation occur primarily in the top meter of soil, with minimal losses in deeper layers. 3) Belowground productivity in mesic grasslands will be higher than in the adjacent woody stands, contributing to the changes SOC distributions observed. In the first six months of our project, we have examined different soil carbon fractions to understand where the changes occur (active, passive, or intermediate pools), established measurements of plant productivity to determine the causes of altered soil carbon stocks with woody encroachment, and located six additional field sites for sampling. In summary, the proposed research will evaluate the net effects of woody plant encroachment for sustained grassland productivity and regional carbon budgets. Summary, in "layman's terms," of what is being done and why: The expansion of woody plants into deserts, grasslands, and savannas contributes the largest source of uncertainty in current analyses of North American terrestrial carbon sinks, with increased plant biomass C in woody vegetation either enhanced or offset to an unknown degree by carbon storage in the soil. The research proposed here will help address this uncertainty along a precipitation gradient (200 to 1100 mm) in the southwestern U.S. In addition to complete carbon balances above- and belowground in adjacent grasslands and former grasslands invaded by woody species, "C and ^C dynamics will be used to constrain the scope and age of the changes in soil organic carbon. The research also provides insight into primary productivity of southwestern U.S. rangelands.
How will the project contribute to one or more of the critical
questions relevant to society, to the Regional Center's strategic vision
and goals, and to the global change community? Currently, the
number one uncertainty in analyses of North American carbon sinks is the
effect of woody plant encroachment into deserts, grasslands, and savannas.
The degree to which increased plant biomass C in woody vegetation will
be enhanced or offset by changes in soil organic carbon is unknown. The
research proposed here will address this uncertainty along a precipitation
gradient (200 to 1100 mm) in the southwestern U.S. and contributes to
two NIGEC initiatives: 1) The Regional Climate Impact Analysis Program
(RCIAP) for regional scientific & technical analysis of the consequences
of climatic & atmospheric changes, and 2) The Ecological Effects of
Climate Change, to help identify and reduce uncertainties about ecological
responses to environmental change. |