Reconstruction of a long-term recovery process from pasture to forest

Abstract

We used space-for-time substitution to obtain a directed successional sequence for subalpine meadow vegetation in the Swiss National Park. Since human impacts (e.g., domestic animal grazing) ceased in 1914, the successional processes documented are assumed to be autogenic in nature. The data consist of 59 permanent plots spanning almost 90 years, and include many spatial replications. An initial inspection of the individual time series revealed the existence of a variety of response patterns, which are described in the literature as representing different successional types. However, a closer inspection suggested that many of these series can be superimposed, as they are part of a much longer deterministic series. Linking the individual time series proved to be challenging. A heuristic approach produced results that differed depending on initial starting conditions. We therefore derived a deterministic algorithm to produce a unique solution. The resulting sequence largely confirmed the heuristic interpretation, suggesting a trend from early successional (post-grazing) grassland to pine invasion spanning about 400 years. This timespan is valid only for the climatic conditions near the treeline, and for plant species specific to the study site. Our results suggest that the various species temporal response models described in the literature may be artifactual, representing portions of underlying Gaussian responses. The data also indicate that species assemblages may persist for several decades with only minor fluctuations, only to change suddenly for no apparent reason