PAGES 601-612RANGES Improves Satellite-based Information and Land Cover Assessments in Southwest United States PAGES 601,[605][606] Because of its influence on hydrology climate, and global biogeochemical cycles, land cover change may be the most significant agent of global environmental change. Land degradation results not only from land cover conversion, but also land cover function. For example, human activities in the southwest U.S.,such as grazing regimes and fire frequency, are accel erating functional changes to fragile rangeland ecosystems, causing increased proportions of shrubs in grasslands, decreases in overall vege tation density and the introduction and spread of non-native invasive species.
[1] Vegetation species cover and photographic data have been collected at multiple grassand shrub-dominated sites in 1967, 1994, 1999, and 2005 at the U.S. Department of Agriculture Agricultural Research Service Walnut Gulch Experimental Watershed (WGEW) in southeastern Arizona. This study combines these measurements with meteorological and edaphic information, as well as historic repeat photography from the late 1880s onward and recent satellite imagery to assess vegetation change at WGEW. The results of classification and ordination of repeated transect data showed that WGEW had two main vegetation structural types, shrub dominated and grass dominated. Spatial distribution was closely linked to soil type and variations in annual and August precipitation. Other than the recent appearance of Eragrostis lehmanniana (Lehmann lovegrass) at limited sites in WGEW, little recruitment has taken place in either shrub or grass vegetation types. Effects of recent drought on both vegetation types were apparent in both transect data and enhanced vegetation index data derived from satellite imagery. Historic photos and a better understanding of WGEW geology and geomorphology supported the hypothesis that the shift from grass-to shrub-dominated vegetation occurred substantially before 1967, with considerable spatial variability. This work reaffirmed the value of maintaining long-term data sets for use in assessments of vegetation change.
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The semidesert grassland in southern Arizona has changed from a native grassland to a scattered Prosopis juliflora var. velutina (mesquite) woodland with an understory of African Eragrostis lehmanniana (Lehmann lovegrass) on many sites. To determine native grass restoration potential, seven species were direct seeded into E. lehmanniana stands that were left alive, burned, sprayed with an herbicide and then either left standing, or mowed. Initial native grass establishment was limited in the live standing treatment but was successful for all other treatments when either June or August sowing was followed by consistent summer precipitation and soil water availability. Four species, Bothriochloa barbinodis (cane beardgrass), Bouteloua curtipendula (sideoats grama), Digitaria californica (Arizona cottontop), and Leptochloa dubia (green spangletop) initially established most successfully, while only Muhlenbergia porteri (bush muhly) had consistently limited or no establishment. E. lehmanniana establishment from the seed bank was increased by canopy removal associated with burning. Densities of native grasses one year after successful initial establishment were much lower than that of E. lehmanniana. A possible revegetation strategy would be to spray emergent E. lehmanniana seedlings and surviving plants with an herbicide during the summer rainy season after spring burning. Native grasses could then be established by sowing in early August of that year or June and August of subsequent years until consistent precipitation produces a native grass stand.
We surveyed a group of rangeland managers in the Southwest about
vegetation monitoring needs on grassland. Based on their responses, the
objective of the RANGES (Rangeland Analysis Utilizing Geospatial Information
Science) project was defined to be the accurate conversion of remotely
sensed data (satellite imagery) to quantitative estimates of total (green
and senescent) standing cover and biomass on grasslands and semidesert
grasslands. Although remote sensing has been used to estimate green
vegetation cover, in arid grasslands herbaceous vegetation is senescent much
of the year and is not detected by current remote sensing techniques. We
developed a ground truth protocol compatible with both range management
requirements and Landsat’s 30 m resolution imagery. The resulting
ground-truth data were then used to develop image processing algorithms that
quantified total herbaceous vegetation cover, height, and biomass. Cover was
calculated based on a newly developed Soil Adjusted Total Vegetation Index
(SATVI), and height and biomass were estimated based on reflectance in the
near infrared (NIR) band. Comparison of the remotely sensed estimates with
independent ground measurements produced r2 values of 0.80, 0.85, and 0.77
and Nash Sutcliffe values of 0.78, 0.70, and 0.77 for the cover, plant
height, and biomass, respectively. The approach for estimating plant height
and biomass did not work for sites where forbs comprised more than 30% of
total vegetative cover. The ground reconnaissance protocol and image
processing techniques together offer land managers accurate and timely
methods for monitoring extensive grasslands. The time-consuming requirement
to collect concurrent data in the field for each image implies a need to
share the high fixed costs of processing an image across multiple users to
reduce the costs for individual rangeland managers.
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