Range-Level Lodgepole Pine (Pinus contorta var. latifolia Engelmann ex S. Watson) Understory Types in Western North America

Western North American lodgepole pine (Pinus contorta var. latifolia Engelmann ex S. Watson) releves (n = 6,918) were classified to identify regional difference in understory vegetation types. Seventy-nine types with > 5 releves were recognized, but only 34 included > 40 releves. Among the latter, shrubs dominated 15 types, excluding consideration of feathermosses. Buffaloberry (Shepherdia canadensis (L.) Nutt.), twinflower (Linnaea borealis L.), and bearberry (Arctostaphylos uva-ursi (L.) Spreng.) understory types spanned most of the latitudinal range. Eleven types occurred only north and four only south of similar to 50 degrees N latitude. Latitude explained similar to 82% (P < 0.001, n = 5,759) of the elevation variance among releves, with a northward decline decreasing from 81 to 21 m/degree of latitude. A discontinuity in elevations at similar to 50 degrees N is thought to represent a northward shift from a cordilleran-prairie to a cordilleran-boreal ecoclimatic regime. A comparison of releve and regression-predicted elevations identified 18 understory types that differed (P <= 0.02) from the regional trend. May August solar insolation, which was used as a proxy for latitudinal differences in slope gradient and orientation, indicated that similar to 77% of P. contorta releves were associated with more intensive solar insolation sites. When cross-referenced with elevation, 13 types differed from the norm in terms of both elevation and insolation. The results provide a framework for more detailed forest classification.