472 



Habital Asxessniviilx — Our Livirifi Rt'saiines 



Table 1. Kiichler vegetation types 

 least modified by urbanization and 

 agricultural developments. 



Table 2. Kiichler vegetation types 

 most affected by urbanization, 

 their locales, and associated urban 

 areas. 



Table 3. Selected grassland types 

 arranged by percentage cultivation. 



Fescue oalgrass (western slopes ol northern coast ranges, 



California, San Francisco) 



Subtropical pine forest (southern Flonda, Miami) 21.07 



Coastal sagebrush (coastal regions of southern California, Los . ^ „_ 



Angeles) 



Pine-cypress forest (coastal California) 6.10 



Northeastern oak-pine forest (coastal New England to New 



Jersey New York, Newark. Philadelphia) 



2400 



5 86 



Grassland type and location 



Cultivation 



% 



90,28 



82.43 



76.24 



Bluestem praine (North Dakota and Minnesota southward to 



Oklahoma) 



Wheatgrass-bluesfem-needlegrass (North 



Dakota. South Dakota, Nebraska) 



Bluestem-grama prairie (Kansas, Nebraska, Colorado, 



Oklahoma) 



Nebraska sandhills prairie (Nebraska, South Dakota) 73.32 



Wheatgrass-needlegrass (North Dakota. South Dakota. -„ _. 



Montana. Wyoming, Colorado) 



Grama-buflalograss (New Mexico, Colorado, Wyoming, 



Nebraska, Kansas, Oklahoma, Texas) 



Whealgrass-grama-buffalograss (South Dakota) 10.21 



Grama -needlegrass-wheatgrass (Wyoming, Montana) 7.39 



forest types. The exception is the elm-ash forest 

 south and west of Lake Eiie (91.03% cropland). 

 This vegetation type covers a relatively small 

 area (23',103 km-; 9,010 mi"). The principal 

 vegetation type that is now more than 90% 

 cropland or inixed cropland is Kiichler's 

 bluestem prairie, which covers 271,990 km- 

 (106, 076 mi-), 3.5% of the conterminous 

 United States. The 1990 land-cover data indi- 

 cate that 90.28% is predominately cropland. 



The least cultivated of Kuchler's types are 

 grama-tobosa prairie (0.18%), trans-pecos 

 shrub savanna (0.28%), creosote bush (0.60%), 

 and blackbrush (0.66%). These four types are 

 all part of the western shrub and woodland 

 group. In the eastern United States, the most 

 "uncultivated" of Kiichler's types is the mixed- 

 mesophytic forest (5.07%), which covers an 

 area of 496,790 km^ (193.748 mi^) and has 

 been noted as having the highest species diver- 

 sity of all the easteiTi broadleaf forests (Braun 

 1950). 



There are several other ways to evaluate the 



differences in the 1990 landscape versus the 

 potential natural state. For example, certain 

 Kiichler types retain the highest percentages of 

 areas not covered by agriculture or urban land 

 cover (Table 1), although these areas may be 

 presently highly disturbed by logging, road 

 building, strip mining, grazing, or other activi- 

 ties. With the exception of the mixed-mesophyt- 

 ic forest and the relatively small southeastern 

 spruce-fir forest, these are all in the western part 

 of the country. Vegetation types from the 

 Kiichler map that have the highest percentage of 

 urbanization on the USGS map (Table 2) are 

 relatively small and are all coastal. Some, like 

 coastal sagebrush, are types that are considered 

 threatened (see Stoms and Davis, this section). 

 For selected grassland types of the Great Plains 

 and central lowlands, there is a decrease in per- 

 centage of cultivation from east to west (Table 

 3), reflecting the role of annual precipitation in 

 conversion of grassland areas to cultivation. 



Comparing forested areas from the USGS 

 map with the Kiichler map would indicate that 

 about 57% of the potential forested area is cur- 

 rently covered by tree species (TuiTier et al. 

 1993). The potential impacts of the.se changes 

 are significant. For exainple, the loss of forest 

 cover since before European settlement (43%) 

 has increased both albedo and carbon dioxide 

 levels. A rise in albedo has been shown to cause 

 a decrease in mesoscale rainfall (Charney et al. 

 1975). Increases in irrigated agriculture can 

 result in a decrease in albedo, which can cause 

 an increase in mesoscale rainfall (Bamston and 

 Shickedanz 1984). Also, a shift from forest to 

 grasses results in a decrease in piimary produc- 

 tivity by a factor of two, thus reducing the rate 

 of atmospheric carbon fixation. 



Continuing Transformations 



The comparison of 1990 land cover with 

 potential natural vegetation illustrates the mag- 

 nitude of change that has possibly occurred in 

 the past 250 years. Changes in the landscape are 

 not exclusive to that period, however; in fact, 

 the 1990 view of United States land cover is 

 already becoming outdated in some regions as 

 natural and human forces continue to transform 

 the landscape. For example, a comparison of 

 1970"s and 1980"s satellite images from the 

 Landsat Multispectral Scanner (MSS; see box) 

 shows that significant changes in some areas 

 selected for examination are taking place. 

 Landsat MSS images have been acquired over 

 most of the United States since July 1972. With 

 approximately 80 m x 80 m (260 ft x 260 ft) 

 resolution, they provide a means to map in more 

 detail the changes that have occurred in the past 

 22 years. 



