The effect of gap openings on soil reinforcement in two conifer stands in northern Italy

Bischetti G.B., Bassanelli C., Chiaradia E.A., Minotta G., Vergani C.

Gap-oriented forestry is a subject that is gaining interest worldwide as a compromise between economic and operational requirements on the one hand and environmental and safety concerns on the other. In principle, the approach mimics the effects of fine-scale natural disturbance while avoiding the threats of soil degradation and instability associated with large clearcuts, but at the same time, it does not limit the productivity associated with single-tree selection.

Despite this interest, studies of the real effects of gap-oriented forestry are still limited, particularly those dealing with its impact on slope stability. This paper provides quantitative results about the slope stability of two mixed-conifer stands subjected to gap management in the Italian Central Alps that were monitored for five years after felling.

The results showed that root decay, both in strength and density, further reduces additional root cohesion by approximately 60% in the first two years after felling and by another 20% in the third year; it completely vanishes by the fifth year. The observed reduction in root reinforcement values has dramatic consequences for slope stability. In fact:

• In undisturbed conditions, the contribution of forest root systems guarantees a very low probability of instability (approximately 10% for a factor of safety of less than 1 as well as for steep slopes and high levels of saturation).
• After only two years, the probability of instability dramatically increases for gentle slopes and low levels of saturation. We estimated a 50% probability of a factor of safety less than 1 when steepness was greater than 36–38° and soil saturation was 25% and when steepness was greater than 24–26° and soil saturation was 75%.
• The probability of instability is highest five years after felling when the contribution of the root system has vanished. The probability of failure exceeded 50% when steepness was greater than 25–28° and saturation was 25%, and the slopes were practically unstable when saturation was 75%.
• At the point when the contribution of the root system to slope stability was null, the regeneration process was still too undeveloped to contribute to soil reinforcement in any way.

These results suggest that although the gap-oriented approach is, in principle, more conservative than large clearcuts, the pace of natural regeneration could be slower than the process of root decay, so instability could occur, especially where local slopes exceed 25° and soils are frequently saturated.