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Gap Analysis of the Vegetation of the Intermountain Semi-Desert Ecoregion

David Stoms
University of California-Santa Barbara

The nation’s first formal Gap Analysis of a multistate ecoregion has been conducted for the Intermountain Semi-Desert ecoregion (Bailey 1995). The Intermountain Semi-Desert ecoregion encompasses approximately 412,000 km2 in portions of Washington, Oregon, Idaho, Nevada, California, Utah, Wyoming, Colorado, and Montana. Two geographically disjunct subregions make up the ecoregion, the Columbia Plateau in the west, and the Wyoming Basin in the east. The Intermountain Semi-Desert boundary corresponds closely to the limits of Küchler’s sagebrush steppe potential natural vegetation type. The southern boundary of the Intermountain Semi-Desert ecoregion grades into the Intermountain Semi-Desert and Desert Province, which tends to be warmer, drier, and with greater topographic relief than the Intermountain Semi-Desert ecoregion. The Cascade and Sierra Nevada mountain ranges bound the ecoregion on the west and the northern Rocky Mountains bound it on the north and east.

This ecoregion was selected for the prototype regional gap analysis for both practical and conservation reasons. From a practical standpoint, the Intermountain Semi-Desert ecoregion was among the first for which the requisite land cover and land management mapping were completed by the individual state-level GAP projects. Additionally, the area provides a suitable testing ground for demonstrating whether GAP can overcome the technical challenges associated with large-area regional mapping that have concerned some program reviewers. Very little land in the Intermountain Semi-Desert ecoregion has been designated for maintenance of biodiversity, while potentially conflicting land uses such as grazing and cultivation are extensive. Enough undeveloped habitat remains, however, for pro-active conservation action to be effective. Thus the Intermountain Semi-Desert ecoregion makes a representative case study that could be applied to other regions throughout the western U. S. Planning for conservation and ecosystem management within this ecoregion is under way by The Nature Conservancy, the Oregon Biodiversity Project, and the Interior Columbia Basin Ecosystem Management Project (a joint effort by the U. S. Forest Service and Bureau of Land Management). Also, BLM is considering wilderness proposals in Wyoming, and proposals for other new wilderness areas and national parks in Idaho and Wyoming are being discussed. A regional Gap Analysis could add valuable information for all of these planning programs.

Land cover was originally mapped independently for each of the states in the Intermountain Semi-Desert ecoregion. Although most state GAP projects used 1990 (+/- 2 yrs) satellite imagery from the Landsat Thematic Mapper sensor, combined with field inventories and existing maps of vegetation in compiling their land cover data, they differed in methods and products. Maps for Idaho and Oregon used photointerpretation techniques with older, lower-resolution Multispectral Scanner images and had larger minimum mapping units than the other states. In contrast, land cover mapping in Nevada and Utah was done with digital image processing of TM image mosaics. This digital classification approach generally achieved greater spatial resolution at some expense in classification detail. The other state projects fall somewhere in between these methods, using manual photointerpretation of higher resolution TM data.

Experienced GAP staffers from states across the ecoregion collaborated to compile and standardize the database and to conduct the analyses. A workshop was held at the University of California, Santa Barbara (UCSB), in June 1996 for the members of this ecoregional team, led by Frank Davis and David Stoms at UCSB. The group first cross-walked the state land cover types to a standardized set of alliances—or to a higher level of classification when necessary. A preliminary regional map was generated by mosaicking the cross-walked state maps together. Then the CA-GAP staff developed an innovative mapping technique to produce a regional land cover map with greater spatial and thematic consistency. Multitemporal satellite imagery from the NOAA Advanced Very High Resolution Radiometer (AVHRR) was used to refine the preliminary map by providing a more consistent spatial resolution (1 km2 or 100 ha pixel size) across the entire Intermountain Semi-Desert ecoregion while retaining its basic floristic information. This mapping technique is described in Stoms et al. (in review). The team also assisted in compiling a consistent regional land management status map, which required some standardizing of definitions and attributes.

The total amount of land permanently protected in the ecoregion is less than 4%, and most types characteristic of the region have less than 10% of their area represented in conservation lands. Of 48 land cover types, twenty were found to be particularly vulnerable to potential loss or degradation because of the low level of representation in biodiversity management areas and the likely impact of land use activities. The gap analysis data and findings (described in Stoms et al. in press) will be useful in providing a regional perspective in project impact assessment and future conservation planning within this ecoregion.

Literature Cited

Bailey, R.G. 1995. Description of the ecoregions of the United States, 2nd ed., revised and expanded. USDA Forest Service, Miscellaneous Publ. 1391. Washington, D.C. 180 pp with separate map.

Stoms, D.M., M.J. Bueno, F.W. Davis, K.M. Cassidy, K.L. Driese, and J.S. Kagan. In review. Map-guided classification of regional land cover with multitemporal AVHRR data. Submitted to Photogrammetric Engineering and Remote Sensing.

Stoms, D.M., F.W. Davis, K.L. Driese, K.M. Cassidy, and M.P. Murray. In press. Gap analysis of the vegetation of the Intermountain Semi-Desert Ecoregion. Great Basin Naturalist.

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