PLANT DISTRIBUTIONS ALONG ALPINE MOISTURE GRADIENTS: THE ROLES OF COMPETITION AND PHYSIOLOGICAL TOLERANCES (EXPERIMENT)

In the alpine tundra, plant species often grow only in certain habitats along complex moisture and exposure gradients. The mechanisms responsible for the restriction of plants to particular habitats were studied at two low alpine tundra sites in central Colorado. Field experiments, including transplants of seedlings and adult shoots, were supplemented by laboratory studies of competitive relationships between seedlings. Although the environmental gradient studied is also a productivity gradient, little evidence was found that a competitive hierarchy model explains species distributions.

In the field, transplants of shoots generally do best in the site at which each species is most abundant. The responses of most species were similar in repeated experiments, but the response of two ridge species differed between Cumberland Pass and Pennsylvania Mtn., possibly reflecting different ecotypes or different environments. Seedling transplants do best in moist sites, but no evidence was found that under field conditions, seedlings of moist-site species are more tolerant of competition.

In the greenhouse, seedlings established and grew better when planted into blocks of moist-site sod than when planted into dry-site sod. In de Wit replacement series, seedlings of species from dry sites are less competitive than seedlings from moist sites, but in greenhouse conditions, the canopy generated by moist-site vegetation could not be shown to depress the growth of seedlings.

Although competition may contribute to limiting the distributions of a few species at one site, it does not appear to play a major role in determining species distributions along this moisture and exposure gradient. Instead, plant distribution appears to be a consequence of each species's individual physiological tolerances.