CLADONIA BIOGEOGRAPHY IN PACIFIC AREAS OF THE UNITED STATES

Abstract

  The biogeography of 58 taxa in the lichen genus Cladonia reflects two major distributional trends in Pacific areas of the western United States. One trend includes species that are oceanic. Many of these species are found primarily or exclusively west of the Cascade Mountains, and are endemic to the western United States. This species assemblage, particularly south of the California-Oregon border, is isolated from more northerly populations. Like the vascular plant flora, it has experienced increased drought through the Holocene. A second trend includes taxa that are found in interior areas, primarily east of the Cascade Mountains. Most of these taxa have holarctic affinities, and many of them are found in the eastern United States as well as in Europe and Japan. The species assemblages on either side of the mountains are statistically distinct. Species from both groups are found in wet maritime localities in northwest Washington State, and to some extent in coastal Oregon and extreme northwestern California. These sites may represent an ancient assemblage which once covered more of the region.