TREE-TUNDRA COMPETITIVE HIERARCHIES, SOIL FERTILITY GRADIENTS, AND TREELINE ELEVATION IN GLACIER NATIONAL PARK, MONTANA

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Abstract
The position of treeline has been attributed to climatic factors affecting trees, and some studies mention soils. No studies, however, examine treeline as a limit for tundra vegetation; the treeline is also the tundra-line. The position of treeline on an elevation gradient is examined in relation to soil fertility. Soils were sampled at 25 treeline sites and analyzed for organic fraction, total nitrogen fraction, available phosphorus, and exchangeable potassium. Mean values of N, P, K, organic fraction, and soil depth all are greatest in forest interior samples and least at the tundra samples. Regressions of site elevation on the soil variables show that elevation is negatively related to all variables except phosphorus, but significantly related only to potassium. We applied the centrifugal model of plant community organization and propose that for this ecotone the gradient in biomass is discontinuous and the abruptness of change varies with elevation, moisture, mineral nutrients, nitrogen, disturbance, and slope steepness. The tundra-line may be lower where more nutrients are available because tundra species may more readily alter their competitive ability than do the tree species in this area. [Key words: alpine-treeline ecotone, subalpine forest, tundra, soil fertility, Glacier National Park.]