Windthrow in streamside key habitats: Effects of buffer strip width and selective logging

Abstract Streamside forests are preserved from clear-cut logging in production forests and protected with uncut buffer strips in many countries. However, buffer strips often remain narrow due to economic reasons and, therefore, provide weak protection against adverse edge effects of clear-cuts and are vulnerable to windthrow. Selective logging of buffer strips is sometimes allowed to reduce their costs, but the decreased tree density may expose the buffer to higher occurrence of windthrow. We used a replicated two-factor experiment to assess the effects of buffer width (15 m or 30 m) and selective logging (0% or 30% of the basal area removed) on the risk of windthrow in boreal streamside forests in Finland. We examined the windthrown trees 12 years after experimental logging at 29 sites and at seven unlogged control sites. In addition, we studied the influence of topography and the extent of clear-cut logging in the surrounding forests on windthrow risk. The proportion of windthrown spruces at sites with 15 m buffer strips was, on the average, six times higher than at control sites and 2.5 times higher than at sites with 30 m buffer strips. In contrast, the proportion of windthrown spruces did not differ between sites with 30 m buffer strips and control sites. Selective logging did not increase the risk of windthrow strongly. However, sites with selectively logged 30 m buffers were slightly more prone to windthrow than control sites. The proportion of windthrown trees tended to increase with the extent of the adjacent clear-cut areas on both sides of the stream. We conclude that a 15 m buffer strip is not wide enough to protect streamside forests from substantial windthrow, while a 30 m buffer strip is sufficient in most cases. Selective logging of 30 m buffers may be undertaken at sites that are not under a high risk of windthrow. If selective logging enables a wider buffer strip, it may be a better option for protecting the streamside habitat from substantial windthrow than leaving a narrow buffer strip. Moreover, clear-cut harvesting on both sides of the stream should be avoided if the aim is to prevent excessive windthrow.