Our Living Rc.\<mrci'.\ — Hcihitat Assessments 469 



produced by the U.S. Geological Survey 

 (USGS; Loveland et al. 1991). The USGS land- 

 cover data were inteipreted from 1990 satellite 

 imagery from the Advanced Very High 

 Resolution Radiometer (AVHRR) sensor 

 aboard the National Oceanic and Atmospheric 

 Administration's polar-orbiting meteorological 

 satellites. The USGS map of land cover is lim- 

 ited in its use for local applications because of 

 thf coarse ground resolution of AVHRR data 

 and its subsequent inability to distinguish vege- 

 tation structure, serai stages, and exotic versus 

 natural vegetation. It does, though, provide a 

 picture of vegetation and land-cover patterns at 

 the national level. For example, in the lower 48 

 states, about 38% of the land is forested, 29% 

 is rangeland or grassland, and 23% is agricul- 

 tural land. While the USGS land-cover study 

 did not identify urban lands, information from 

 the Defense Mapping Agency's Digital Chart of 

 the World shows that at Teast 14.500 km- (3,655 

 mi-) or 1 .0% of the conterminous United States 

 is urbanized (Danko 1992). 



It must be noted that a comprehensive 

 assessment of accuracy of the 1990 land-cover 

 map has not been completed, although an inde- 

 pendent study shows that the classification of 

 forest lands is within 4%- of the estimate of the 

 U.S. Forest Service (Turner et al. 1993). 

 Comparisons with selected state land-cover 

 maps and U.S. Department of Agriculture crop 

 area statistics have also shown general corre- 

 spondence between land-cover estimates at the 

 national level (Merchant et al. 1995). 



Change in Natural Vegetation 



The estimated extent of change in the natur- 

 al vegetation since European settlement is 

 derived by comparing Ktichler's potential natur- 

 al vegetation (Kiichler 1964) with the 1990 

 land-cover data set produced by the USGS 

 (Loveland et al. 1991). Both potential natural 

 vegetation (Fig. 1 ) and 1990 land cover (Fig. 2) 

 have been generalized to show six vegetation 

 groups: needleleaf forest, broadleaf forest, 

 mixed forest, grassland, shrubland, and grass- 

 land-shrubland. Note that the 1990 land-cover 

 classification does not distinguish between nat- 

 ural and altered vegetation (e.g., an even-age 

 tree plantation is mapped as forest even though 

 it does not have the ecological value or function 

 of a natural forest). The 1990 land-cover map 

 (Fig. 2) also includes four additional categories: 

 urban areas, cropland, cropland-woodland 

 mosaics, and cropland-grassland mosaics. 



A representation of the percentage of land 

 modified from its natural state by either cultiva- 

 tion or urban development was produced by cal- 

 culating the percentage of 1990 agriculture and 

 urban lands found within each Kiichler 



i ^ Needleleaf forest 

 I J Broadleaf forest 

 C3 Mixed forest 

 I Shrubland 

 Grassland 

 L; Grassland/stirubland mix 



Kiy. I. (InHipciK.iic'jiiiics ol pnicmi.ii n.itui.il XL-yciation aggregated from Kiichler (1964). 



^ Urban 



[; Cropland 



en Cropland/woodland mosa c 



^ Cropland/grassland mosaic 



tm Needleleaf forest ;._ Slirubland 



CD Broadleaf forest ; r Grassland 



1=1 H/lixed forest cd Grassland/stirubland mix 



Fi);. 2. (iniiipod categories ol I'Wd land ciuer depicting 1990 conterminous U.S. land cover that 

 was developed from 1990 AVHRR imagery. 



I 0-10% en 51-60" 



111-20 c 61-70 



121-30 --71-80 



s 31-40 r_ 81-90 



c:341-50 c;i91 too 



Fig. 3. Percentage of Ktichler's potential natural vegetation types (Kiichler 1964) that have been 

 converted to agricultural and urban land cover. The lighter tones represent the higher levels of 

 human modification. Percentages of modification are displayed as deca-percentiles. 



