Forestry patch retention can assist re-establishment of biodiversity into adjacent harvested areas

Background/Question/Methods

Retention forestry practices were initially developed in the PNW, and are now applied globally in place of clearcutting and other traditional silvicultural systems. A number of research studies have shown the value of small retained patches (aggregates) within harvesting units for maintaining species and structures associated with older forest. In Tasmania, we are extending this research to focus on the role of aggregates in facilitating re-establishment of biodiversity into the harvested areas, a concept known as ‘forest influence’. In most parts of the world where retention forestry is practiced, retention targets guide the amount and types of biological legacies that are retained. Additionally, forest influence targets are used in both Tasmania and British Columbia to limit the distance of harvest areas from retained areas of unlogged forest, and prevent large gap areas akin to clearcuts. As a result, roughly equal focus is given to re-establishment of biodiversity as to retention per se. However, there has been almost no research to test the effectiveness of retention in assisting re-colonisation, either as a source of dispersing individuals and propagules, or by buffering the microclimate of the harvested areas. To address this knowledge gap, we are researching the spatial and temporal components of forest influence on vascular and non-vascular plants, beetles, birds, small ground mammals and bats.

Results/Conclusions

The responses and mechanisms vary among biodiversity groups. Retaining aggregates results in shaded conditions in a higher proportion of harvested areas. Aggregates provide seed sources for vascular plants, with late-successional species abundance declining with distance from mature forest in sites ranging in age from 5 to 46 years post-harvest. The majority of mature-forest bryophyte species declined in abundance with increasing distance from mature forest. Bats preferentially forage in harvested areas close to aggregates and edges, even though they rarely forage in the aggregates themselves. This response related to preferred structural conditions rather than dispersal limitation. By contrast, proximity to unlogged forest appears not to benefit small ground mammals or birds, even though they utilize the aggregates themselves. Our results therefore demonstrate that harvest layouts emphasising forest influence are beneficial for some, but not all, forest biodiversity. Results from this research will be used by forest managers to design spatial configurations of harvest unit layouts to maximize benefits for biodiversity.