ECOLOGY OF A STEPPE-TUNDRA GRADIENT IN INTERIOR ALASKA

Subarctic steppe is currently restricted in interior Alaska and the Yu kon Territory to steep, south-facing river bluffs. Paleoecolo,oical an d biogeographic evidence suggests that some steppe taxa may have been more widespread during the Full-Glacial. We examined factors controlli ng the distribution of steppe taxa on an elevation gradient across a s teppe tundra ecotone; such analyses may help define potential Full-Gla cial distributions of these taxa. Multivariate analyses suggest that s pecies can be divided into four spatially distinct groups, but individ ualistic species distributions create considerable overlap among these groups. The steppe-tundra ecotone comprises a broad zone of mixing be tween steppe taxa and drought-tolerant alpine tundra taxa, followed by an abrupt shift to alpine shrub tundra. The transition from low stepp e to tundra vegetation is primarily associated with a gradient of decr easing soil temperature. The more abrupt transition from mixed steppe- tundra to alpine shrub tundra vegetation is primarily associated with changes in soil depth and soil moisture. Variation in vegetation withi n steppe is associated with gradients in soil phosphorus and moisture. Greenhouse experiments on drought tolerance of two steppe and two tun dra taxa suggest that the individualistic distribution of species alon g the ecotone is partly a function of physiological differences among species. Our analyses of vegetation-environment relationships support the hypothesis that some components of the steppe community could have been more widespread during the colder Full-Glacial.