Predicting the effects of forest encroachment for Sedum lanceolatum and Rhodiola integrifolia at a subalpine ecotone

Abstract

The elevation of treeline is rising in many areas throughout the world. Encroachment of forest into alpine regions fragments habitats and causes changes in microhabitat potentially altering the density and distribution of species. Our study investigates two species of alpine Crassulaceae, Sedum lanceolatum and Rhodiola integrifolia, as part of a larger project determining the effects of forest encroachment in the front ranges of the Rocky Mountains. Previous studies of S. lanceolatum showed a decrease in its density, but an increase in herbivory by larvae of the butterfly Parnassius smintheus with increasing distance from the treeline. To better understand the potential impact of forest encroachment on these species and the microhabitat associations of the plant species, we counted their abundance and determined soil depth, soil type, and dominant rock type in 8417 quadrats of 1 m2 area spaced at 10 m intervals throughout 17 alpine meadows along Jumpingpound Ridge, AB, Canada. Using a geographic information system we extracted slope, aspect, elevation, and distance from treeline for each quadrat. S. lanceolatum was abundant in these meadows and its density was influenced by all factors examined, predominantly through effects on soil moisture. Its positive association with treeline was primarily at high elevation in meadows with west-facing aspects. R. integrifolia had a more restricted distribution and relatively low density; it was most abundant in gravelly soils with north-facing aspects and little slope. Comparing the distribution of each species to a model predicting future forest encroachment indicates that both will lose habitat to forest encroachment. R. integrifolia is predicted to lose 31 % of its current habitat area. S. lanceolatum, which occurs across a broader range of microhabitats, is predicted to lose 19 % of its current habitat area to forest encroachment. These losses will negatively affect P. smintheus, but the overall impacts of forest encroachment on the butterfly are predicted to be minimal.