Project Funding: 2005 - 2007

NRA: 2004 NASA: Carbon Cycle Science   

Funded by NASA

Abstract:
Arid and semi-arid ecosystems cover about 3.4 million square kilometers of North America. The spatial patterns and abundance of herbaceous and woody plants throughout these regions are determined by bio-climatic conditions, topography, soil properties, and disturbance regimes. During the past century, the balance between woody and herbaceous plants has shifted in many U.S. dryland ecosystems to favor trees and shrubs. Recent syntheses suggest that woody plant encroachment contributes significantly to a North American carbon sink. However, current estimates of gross or net rates of woody cover change in the western U.S. are crude and have not been linked to changes in C storage, thus the potential contribution of woody encroachment to the U.S. carbon budget remains elusive. While our regional knowledge of current woody cover distributions, woody vegetation changes over time, and ecosystem C responses is very crude, what we do know has largely come from studies in “lowland” arid and semi-arid regions. Pinyon-juniper (P-J) woodlands are among the least understood systems in terms of woody encroachment and thickening. However, stand age structure data from P-J woodlands in Utah show that more than half of the stands are in the 40-120 yr age range, with only 20% of stands dating to > 200 years, suggesting that recruitment has dramatically increased over the last century. At maximum stand densities, P-J systems can maintain biomass and NPP levels 2-4 times that of more heavily studied mesquite and oak savannas and 10 times that of creosote shrublands. We do not know: (1) the current distribution, cover and carbon stocks of P-J ecosystems in a ~ 500,000 km2 portion of the Southwest; (2) regional rates of P-J cover and carbon change in relation to soils and grazing history; (3) soil organic carbon responses to changes in P-J cover; and (4) how to model current and future distributions of carbon stores in the P-J region. The broad goal of this project is to quantify and understand regional effects woody encroachment on carbon storage in pinyon-juniper ecosystems of the Southwest U.S. We will quantify the contribution of woody encroachment in these systems to a proposed U.S. carbon sink and the interaction of aboveground changes in carbon with direct grazing impacts on soil carbon. We will combine multi-platform remote sensing, field biogeochemical and dendrochronology studies, and spatio-temporal carbon modeling to achieve this goal. Using AVIRIS imaging spectroscopy data acquired in 2003, we will quantify differences in woody and herbaceous cover in paired ungrazed mesa/grazed mainland regions of southern Utah and northern Arizona. This approach isolates the effect of grazing, while controlling for soil type and mean climate conditions. We will also quantify differences in woody cover along a grazing-release gradient that controls for climate and soil type. Combining field allometric and dendrochronology data, and historical aerial photography with the AVIRIS results, we will develop carbon budgets for grazed and ungrazed pinyon-juniper systems, allowing a quantitative comparison of long-term changes in carbon storage resulting from grazing practices. These budgets will encompass an area of approximately 840 km2, and will be representative of the grazing, soils, and climate variations found throughout P-J systems in a 500,000 km2 region of the Southwest. Using the high spatial/spectral resolution AVIRIS imagery, we will establish a spectral/temporal mixture modeling approach for remote sensing of P-J fractional cover from Terra and Aqua MODIS satellite observations.

Publications:

Huang, C., Asner, G. P., Martin, R. E., Barger, N. N., Neff, J. C. 2009. Multiscale analysis of tree cover and aboveground carbon stocks in pinyon-juniper woodlands. Ecological Applications. 19(3), 668-681. DOI: 10.1890/07-2103.1

More details may be found in the following project profile(s):

• NASA Terrestrial Ecology (TE) Project Profile
• North American Carbon Program (NACP) Project Profile