摘要
Introduction:Primary succession on glacial forelands is increasingly relevant as rapid glacial retreat is exposing growing land areas to plant colonization.We investigated temporal trends,controls,and outcomes in floral succession on a subarctic glacial foreland.Specifically,we examined changes in community composition(mosses,low shrubs,forbs,trees,and graminoids)over long-term(decadal)and short-term(<10 years)scales and attempted to identify the underlying processes responsible for the observed successional patterns.Methods:The study area was the foreland of the Skaftafellsjӧkull,located in Vatnajӧkull National Park near the south coast of Iceland.We established nine transect lines at varying distances from the ice front representing surfaces of age ranging from less than one decade to over 100 years.Each transect consisted of five measurement stations of 1 m2 where we measured vegetative cover(VC),species richness(SR),and species density(SD)and calculated species evenness(SE).Measurements were made initially in 2007 and repeated at the same geographic coordinates in 2014.Results:VC increased with distance from the ice front from 16%to over 90%.SR and SD increased from the youngest pioneer community through a mid-successional stage corresponding to an age of over 60 but less than 100 years.Increased VC but declining SR,SD,and SE characterized the oldest(over 100 years)bryophyte-dominated surfaces.Species turnover,which involved forbs almost exclusively,increased moderately from early through midsuccessional sites and declined on older sites.Comparison of the measurements made in 2014 to those made in 2007 demonstrates increased SR at mid-successional sites while SD remained relatively constant.Conclusion:At a small scale,colonization is controlled by local factors such as microtopography and aspect,particularly in proximity to the glacier.At the landscape level,changes in VC and community structure are controlled by time and nutrient availability.Low nutrient levels and limited site availability favor bryophyte dominance on the oldest surfaces.The greatest community-level changes observed over the 7-year interval were increases in surface cover by mosses and low shrubs,particularly in mid-successional and older sites.These changes suggest that the community on the oldest surfaces has not yet reached equilibrium.
