Abstract
In modern agriculture, only the cultivation of highly productive and easily accessible slopes remains profitable. As a consequence, inaccessible and steep areas are being increasingly abandoned. In this paper, the mechanisms of natural reforestation of abandoned areas are examined on three levels as a prelude to determine natural reforestation rates. The study sites selected on the municipality level (1:25,000) are situated in four agrarian structure regions in the Alps: ‘Innsbruck Land’, ‘Südtiroler Berggebiet’, ‘Unterland/Überetsch’ and ‘Carnia’. For more detailed data, studies on the level of a case study area (1:5000) and plot level (permanent plots) were carried out in the municipality of St. Leonhard in Passeier (‘Südtiroler Berggebiet’, South Tyrol, Italy). Data were collected through analysis of historic photographic material as well as by detailed field work. Significant explanatory variables of natural reforestation were determined using count data models. A zero-inflated negative binomial model was employed as analysing tool. Key findings are: (1) in the past 150 years, areas were abandoned in all analysed agrarian structure regions. Between 8 and 67% of the formerly used areas are now abandoned, and in the majority of these abandoned areas, forest re-growth is observed. (2) Seed dispersal and agricultural use are the most important variables influencing natural reforestation. The nearer an area is situated to old trees, the higher the reforestation rate. Grazing and mowing reduce reforestation. (3) The less intensively the land was formerly used and the longer the area was abandoned, the higher the tree density is. Whereby, the average sapling density is significantly higher in mixture communities than in grasslands or dwarf shrub communities. However, snow gliding and avalanches cause damage to trees that retard or hinder tree growth.