Decay of Picea abies snag stands on steep mountain slopes

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Abstract
In a 30-ha Picea abies mountain forest in the Swiss Alps, almost all trees were killed by bark beetles (Ips typographus) between 1992 and 1996. Snag decay was studied using full-callipering within transects, and the height of lying logs above ground level was studied using the line intersect method. None of the dead trees had been uprooted, but 75% were found broken in 2000. The probability of snag breakage was independent of both tree diameter and time since stand death, but 28% of the snags broke close to the ground during a storm in December 1999. The log sections that were not in direct contact with the ground (73% of the log length sampled) were on average 85 cm above the soil surface in 2001. The orientation of the logs could be explained with the prevailing wind direction even on this steep slope. Leaving snag stands unharvested in P. abies forests on such slopes is likely to result in effective protection against rockfall and avalanche release for about 30 years. Key words: Norway spruce, bark beetle, decay process, rockfall, avalanche, mountain forest, Switzerland, decomposition, coarse woody debris