Plant succession and soil development on the foreland of the Morteratsch glacier (Pontresina, Switzerland): straight forward or chaotic?

Abstract

As study area we selected the glacier foreland of Morteratsch (approx. 1900–2100 m a.s.l.) near Pontresina northwest of the Bernina pass, Upper Engadine, Grisons (Switzerland). The aim of this study is a multimethodological approach using floristic inventories, vegetation and soil mapping of the pro-glacial area in order to detect crucial parameters controlling plant resettlement in recently deglaciated areas as related to time, local microtopography and soil development. The following methodological approaches were included in this study: (i) floristic relevees along a chronosequence covering 134 years (1857–1990); (ii) dendrochronological data on tree establishment, collected on a grid with a mesh width of 40 m in the area, which became ice-free between 1857 and 1980; (iii) vegetation mapping; (iv) soil analyses including physical and chemical properties of 11 typical profiles; (v) soil mapping and (vi) data evaluation using GIS. Retreating glaciers successively expose mineral substrates that are colonised within a few years by vascular plants, mosses, lichens and soil biota. With increasing plant cover, also the abundance of soil organic matter increases. At first sight, the large-scale patterns of vegetation and soil seem to be driven by the time since deglaciation, whereas the small scale patterns may appear chaotic since they depend on local site conditions, which may change dramatically over short distances. The large-scale pattern seems to develop as follows. About 7 years after deglaciation the first pioneer plants establish themselves and form after an additional 20 years period the Epilobietum fleischeri community, which today dominates the recently deglaciated areas, but may be found in patches more or less on the whole pro-glacial area. By contrast, the first elements of the short living Oxyrietum digynae community appear approximately 10 years after deglaciation and persist for only about 30 years. Dendrochronology showed that the first European larch and Swiss stone pine trees established themselves 15 and 31 years, respectively, after deglaciation. Surprisingly, on the study area, Swiss stone pine is about twice as frequent as the typical pioneer species European larch (88 stems per ha vs. 45 stems per ha), despite the fact that larch starts earlier and grows faster than Swiss stone pine (annual height increment: 21 cm vs. 8 cm). Up-to-now, however, nowhere in the 150-year-old glacier foreland a near-to-mature larch-Swiss stone pine forest can be found. Besides large-scale factors such as time since deglaciation, topography and disturbance (floods, rockfalls, avalanches), also small-scale factors such as grain size and water content of the substrate, micro-relief and micro-climate seem to be crucial for the development of both vegetation and soil. Time since deglaciation and a straightforward single-pathway succession model are clearly not sufficient for understanding the small-scale patterns of succession. A non-linear succession model with different starting points and different pathways of potential primary successions for the different ecological niches is more promising for describing accurately the spatio-temporal vegetation dynamics of the pro-glacial area of Morteratsch.