A model of alpine species distribution in relation to snowmelt time and altitude

The focus of this study is the response of species to time of snowmelt and altitude in alpine areas and an examination of changes in species response to snowmelt as attitude increases and temperature decreases. Transects (n = 43) were placed evenly along an altitudinal gradient at Finse, Hardanger-vidda, western Norway, from ridges to late snowbeds. These gradients were systematically sampled ('Repeated Gradient Analysis, RGA') and an adjusted F-test was used to determine repeated trends in species distribution along the transects. Of the 41 taxa analysed 22 showed a significant change in expected occurrence in response to time of snowmelt (when a site becomes free of snow) as altitude increased. Three types of response were observed: (1) no change in response: (2) increased occurrence as altitude increases, i.e. the taxon invades snow-free sites as altitude increases, and (3) decreased occurrence as altitude increases, i.e. the taxon retreats from snow covered areas. It is suggested that the changes in response are due to both environmental factors (temperature related) and biological interactions. Decreases in expected occurrence are probably due to increased environmental severity as altitude increases (temperature related decreases). These species are represented by taxa preferring intermediate cover of snow. The invasion of earlier snow-free sites is probably due to reduced competition from lee-side taxa as altitude increases. A predictive model based on the species-environmental relationships suggests that a 1 degreesK temperature increase changes the limits of occurrence in response to time of snowmelt from 3 to 20 days for the different taxa.