Evaluating ecoregions for sampling and mapping land-cover patterns

Ecoregional stratification has been proposed for sampling and mapping land-cover composition and pattern over time. Using a wall-to-wall land-cover map of the United States, we evaluated geographic scales of variance for nine landscape-level and eight forest pattern indices, and compared stratification by ecoregions, administrative units, and watersheds. Ecoregions accounted for 65 percent to 75 percent of the total variance of percent agriculture and percent forest because dominant land-cover is included in ecoregional definitions. In contrast, ecoregions explained only 13 percent to 34 percent of the variance of the other seven landscape-level pattern indices. After accounting for differences in amount of forest, ecoregions explained less than 5 percent of the variance of the eight forest pattern indices. None of the stratifications tested would be effective mapping units for land-cover pattern because within-unit variance of land-cover pattern is typically two to four times larger than between-unit variance.