Climate and vegetation patterns in surface samples from arid western U.S.A. Application to Holocene climatic reconstructions

Nearly 1,400 samples from over 50 sources have been assembled and analyzed to characterize the contemporary pollen rain of the arid western U.S.A. Of the nearly 300 pollen types recorded, Pinus, Quercus, and Cupressaceae are the most common arboreal types; Chenopodiaceae-Amaranthus. Gramineae, Artemisia, Ambrosia, and “Other Compositae”; are the most frequent non-arboreal pollen types. Forest vegetation is represented by 661 samples, steppe by 450 samples, and desert by 116 samples. Ambrosia, Cactaceae, Cruciferae, Leguminosae, Larrea, Malvaceae, Nyctaginaceae, and Prosopis achieve maximum percentages in hot - dry climate (>20°C mean annual temperature and <250 mm mean annual precipitation); Artemisia, Juniperus, Sarcobatus, Caryophyllaceae, Liguliflorae, Other Compositae, and Polygonaceae have minor peaks in cold dry climate (<15°C and <250 mm); Arceuthobium, Abies, Picea, Pinus, Pseudotsuga, Sequoiadendron, Tsuga heterophylla, and T. mertensiana reach maxima in cool moist climate (10-15°C, 500 - 2000 mm); and Cyperaceae, Umbelliferae, and Salix reach maxima in cold wet climate (<5°C and >2000 mm). The frequency distribution of dissimilarity values among vegetation types is similar to that of other surface sample studies, with squared-chord-distances <0.15, five times more likely to be same-type comparisons than different-type. Holocene temperature and precipitation curves for three western U.S.A. sites, based on the closest analogs in the contemporary surface samples, exhibit the “early Holocene Xerothermic”; of the Pacific Northwest and early Holocene moistness of the Southwest.