Since the end of World War II, especially between 1960 and 1980, the natural reforestation of the Swiss Alps accelerated, whereas a decline in forest stands and density characterized other regions in the world. The reforestation history of abandoned lands of the Inn Valley can be precisely reconstructed by dendrochronological methods.

We measured tree ring width and determined the ontogenetic ages of more than 450 trees and shrubs at eight different South-exposed stands in an inner-Alpine dry-valley near Ramosch, Lower Engadine Switzerland. Comparison with actual species distribution patterns, air photographs and interviews with residents revealed the mechanisms and causes of the reforestation. Initially, pioneer copses (Juniperus sp.) grew adjacent to rocks. Protected by juniper shrubs, Scots pine (Pinus sylvestris) established. Larch (Larix decidua) settled in open meadows. Grass and shrubs disappeared as pioneer tree stands expanded and provided new germination sites. The patches of forest merged with each other and formed extended forested areas. Today fast growing pine dominates over spruce (Picea abies) and larch on pioneer tree stands, but slow growing spruce dominates on stands reforested more than 80 years ago. Although browsing of wild and domesticated animals may have slowed the natural reforestation process, rates of reforestation continued to accelerate due to disruption of the grass cover by human and animal activity. Predominant causes are connected to socio-economic changes, population shift to urban areas, abandonment of limited yield stands and changes in land management methods.

This study demonstrates the usefulness of dendrochronological techniques for describing natural landscape development processes and reconstructing terrestrial re-colonization patterns and causes.

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