This explanatory note is intended to accompany the map included in this issue. The map is on a scale of 1:30,000,000 (1 cm = 300 km) and shows regional-scale ‘ecosystem’ units, or ecoregions, differentiated according to a scheme modified from J.M. Crowley (1967), and using climate and vegetation as indicators of the extent of each unit. The units are similar in concept to ‘ecobiomes’ as proposed by Polunin (1984). The map was developed following a proposal by Bailey & Hogg (1986) to supplement the Udvardy (1975) system of biogeographical provinces with a treatment of higher resolution. Three levels or categories of this hierarchy are shown. Of these the broadest, domains, and within them divisions, are based largely on the broad ecological climate zones following the Koppen system as modified by Trewartha (1968), and summarized in our Tables I and II. Climate is emphasized at the broadest levels because of its overriding effect on the composition and productivity of ecosystems etc. from region to region.
p1 Program Manager, Land Management Planning Staff, USDA Forest Service, Washington, DC 20013, USA; currently Land Management Planning Systems, USDA Forest Service, 3825 East Mulberry Street, Fort Collins, Colorado 80524, USA.