Effects of ecological restoration to promote structural variability in boreal forests

Abstract

Ecological restoration of boreal forests aims at re-instating the structure and dynamics of forests that have been degraded by silvicultural management. Forest restoration mimics natural disturbance dynamics to diversity forest stand structure and brings back fire as a natural part of forests successional development. Restoration has been shown to enhance populations of fire-dependent insects, benefit various deadwood dependent organisms and enhance the establishment of pioneering species. Using a 12-year large-scale field experiment conducted in Natura 2000 protection areas along eastern Finland, we assessed how two alternative restoration methods (partial felling to add dead wood and felling combined with subsequent burning) affect structural diversity of boreal forests stands in comparison to untreated control (“passive restoration”). The measurements of trees, tree seedlings and dead wood have been conducted before and three times after restoration treatments during 2005-2017. In ten years, restoration by felling+burning increased both the volume and diversity of deadwood, whereas felling only increased the volume of deadwood, diversity was not greatly affected. Instead, the development of the diversity of living trees strongly depended on initial stand structure and was usually lower on restored stands in comparison to untreated control stands. In the most severely burned stands, all trees died within two years after treatments, imperiling the continuity of deadwood in the future. Within initially mixed-wood stands burning enhanced the establishment of new generation of deciduous trees, but in pine-dominated stands hardly any deciduous establishment was seen in ten years. To conclude, restoration clearly speeds up the development of deadwood volumes and diversity needed to host large portions of biodiversity, but in landscape scale, we suggest several restoration methods to be used simultaneously within the landscape, to ensure high variability in tree stand structure and deadwood continuity in short term and in the future.