Saproxylic beetles in pre-commercially thinned Norway spruce stands and woodland key habitats: How do conservation values differ?

Abstract

In even-aged clear-cutting forestry, the biodiversity effects of treatments must be well understood for proper forest management and conservation. While biodiversity on clear-cuts has been much studied, other common forestry stages such as thinning have received less attention. We studied saproxylic beetles, a highly diverse group with many red-listed species, in young spruce stands 2–5 years after pre-commercial thinning. This intervention often creates sun-exposed dead wood, also of deciduous trees such as birch, potentially favoring many of these beetle species. To help evaluate this forestry stage, we compared the saproxylic beetle fauna of 10 thinned spruce stands with that of 10 geographically matched semi-natural woodland key habitats (WKHs) in Sweden. Local and overall species numbers of saproxylic beetles were similar in the two forest types, but they differed in species composition. Compared with WKHs, thinned stands had an overall lower number of species on deciduous wood, fungivorous species, and red-listed or previously red-listed species. Between-stand diversity was also lower for thinned than for WKH stands. Thus, while pre-commercially thinned spruce forests hosted many saproxylic beetle species, including some red-listed ones, WKHs provide habitat for a more variable fauna, especially for beetle groups of conservation concern. Woodland key habitats, although small, are a valuable complement to young production forests of spruce for the conservation of saproxylic beetle species.