Autecology of Kobresia bellardii: Why Winter Snow Accumulation Limits Local Distribution

In the Colorado Front Range Kobresia bellardii distribution is restricted to alpine meadows which are neither extremely wind exposed nor buried under snow in winter. This study examined reasons for such a limited distribution by comparing the behavior of undisturbed Kobresia with that of transplants into alpine habitats with less and more winter snow accumulation. Kobresia's success in snow—free meadows was related to rapid summer growth and to its use of an extended period for development, from about 1 April to 20 October. New leaves began elongation in the autumn and completed growth the following summer. Most carbohydrates were stored as oligosaccharides in the shoots. During periods of relatively warm weather (>—4°C) in winter, when soil water potential (ψ s ) rose above —2.0 MPa, leaves elongated slightly; they died back when cold, windy weather resumed. Kobresia did not survive in a fellfield primarily because of mechanical damage by windblown snow and sand. Low ψ s led to leaf wilting in summer and winter. Shallow snow cover (°15 cm) that melted early permitted extensive April leaf elongation in saturated soils. The prematurely developed leaves apparently lost frost hardiness and were injured in late spring freezes. In sites of moderate (°75 cm) and deep (°140 cm) snow accumulations, autumn dieback was incomplete when drifts first formed in September and October. Winter freezing destroyed the apparently unhardened leaf tissues and meristems, resulting in loss of carbohydrate reserves. In the moderate snow accumulation sites, leaf elongation was delayed until snowmelt was complete in late June or early July. Interspecific competition appeared unimportant in determining Kobresia distribution. Flower and seed production were reduced up to 99% by habitat alteration. Kobresia may not be reproducing by seed in the Front Range at the present time.