HABITAT CHARACTERISTICS AND MORPHOLOGICAL DIFFERENCES OF TRIFOLIUM-THOMPSONII POPULATIONS

Trifolium thompsonii (Morton) is a rare and threatened legume of the E astern Cascades previously known only in Swakane Canyon of Chelan Coun ty, Washington. Recent wildfire disturbances and new population sighti ngs prompted investigation of the habitat characteristics of this taxo n. Transect sampling and cluster analysis identified T. thompsonii occ urrence in four plant associations: Pseudotsuga menziesii/Calmagrostis rubescens (PSME/CARU), Pinus ponderosa/Agropyron spicatum (PIPO/AGSP) , Artemisia tridentata/Agropyron spicatum (ARTR/AGSP), and Artemisia t ridentata-vaseyana/Agropyron spicatum (ARVA/AGSP). Thompson's clover o ccurs in a mosaic pattern characteristic of fire disturbed and grazed communities with 24-69 percent cover of forbs and grasses and 0-36 per cent cover of trees and shrubs. Morphological expression of T. thomson ii ranged from tall (32 cm) multifloral stands in mesic PSME to relati vely small (25 cm) individuals with only one flowerhead in the xeric A RTR. In contrast, plant density was low in PSME st 0.6 plants-m(-2) bu t high in ARTR at 6.4 plants-m(-2). This study confirms T. thompsonii to be a vigorous and dominant fort, component of early seral communiti es in the ponderosa pine-shrub steppe ecotone.