Impacts of retention felling on coarse woody debris (CWD) in mature boreal spruce forests in Finland

We studied the immediate changes in pre-treatment coarse woody debris (CWD) after retention felling and mechanical site preparation (scarification) in mature Picea abies-dominated boreal forest. Retention felling and scarification were hypothetised to affect the amount of CWD. The disturbance caused to CWD was assumed to depend on species and decay class. Logs were inventoried before fellings, after fellings, and after scarification, estimating the damage percentage for each log. After felling, 7.8% of the total pre-treatment volume of CWD was destroyed in the felled area. After scarification, the decline from pre-treatment volume was 67.6% in the felled area. The amount of CWD decreased also inside the retention tree groups; in the 1st post-treatment season, 4.6% was destroyed of the pre-treatment volume and 20% in the 2nd season. Of the retained trees, 40% were uprooted by the end of the 2nd season. If the majority of the initial downed CWD is destroyed by scarification, as our results show, we can estimate that since scarification became a widely used regeneration method in the 1960s, at least from 4 to 6 million m3 of CWD has already been destroyed in Finland. The role of CWD as a key element for biodiversity in boreal forest is generally accepted, which has led to retention of trees in fellings instead of clear felling. We suggest that at least as important as leaving trees in order to maintain continuum in CWD and species diversity is to preserve existing CWD in fellings over the regeneration phase. This can be done using less destructive harvesting methods, reduced use of scarification and placing retention tree groups in patches with high amounts of CWD.