Experimental evidence on biodiversity impacts of variable retention forestry,prescribed burning,and deadwood manipulation in Fennoscandia

Intensive forest management has been applied in most Fennoscandian forests for a period of almost one felling rotation.This paradigm has produced even-aged and even-structured forests of different successional stages that cover about 90%of forest land.At the same time,wildfires have been nearly eliminated in most of the Fennoscandian nature.Consequently,hundreds of species are red-listed because of forest management.To support these species,forest management requires improvements.Variable retention forestry and habitat restoration have been suggested to mitigate negative effects of forest management on biodiversity,and these have been practiced to some extent during the past few decades.Here,we review experimental results on the effects of variable retention and two restoration measures(prescribed burning and artificial addition of coarse woody debris)on different species groups in Fennoscandia.Our key findings are as follows:(i)Many species respond positively to felling within a few years,apparently due to released and often ephemeral resources,such as fresh residue and stumps.Species associated with shady conditions are negatively impacted,but any retention supports many of these,and their species composition remains almost unaffected with 50-70%retention of the initial tree volume.(ii)These effects remain detectable for at least 10-30 years or,according to some studies,nearly 100 years,e.g.,in polypore fungi.(iii)Initial effects of prescribed burning on most species groups(apart from pyrophiles)are negative,but within 10-15 years post-fire sites begin to support many rare and threatened deadwood-dependent species.Epiphytic lichens,however,remain negatively affected.(iv)Artificial addition of deadwood(mostly high stumps)supports a wide spectrum of deadwood-dependent species,but the species composition differs from that of naturally died trees.(v)Moisture and micro-habitat variation are crucial for forest species at harvested sites,at least in forests dominated by Norway spruce.We conclude that felling method as such is of little importance for threatened forest species,although retention mitigates many negative effects.These species require microclimatic continuity,and maintenance and active increase of legacies,such as deadwood of different qualities(species,downed/standing,snag/log/stump,decay stage),very old trees,and tree species mixtures.

Research on retention forestry in Northern Europe

Retention approaches in forest management are today common in several North European countries,integrated into the clearcutting practice as a way to promote biodiversity and maintain ecosystem functions.Individual green trees and retention patches(tree groups)are retained at final harvest,and deadwood is left at site or created.Here,we review research on retention in Sweden,Finland,Norway,the Baltic States,and NW Russia,with special focus on biodiversity.Following the first publication in 1994,about 180 peer-reviewed articles have been published.We present results from a systematic search of the retention literature,separated into the following topics:buffer zones,retention patches,high stumps,other types of deadwood,European aspen Populus tremula,and cost-efficiency.Russian literature is synthesized separately since studies from this region have so far almost exclusively been published in the Russian language.Furthermore,we describe six ongoing large-scale,replicated experiments with varying retention levels,five in Finland and one in Sweden,and summarize their main results.Among main conclusions for practice from the literature and experiments are that retention patches as large as 0.5 ha and 10-mwide buffers to watercourses are not enough to maintain pre-harvest species composition but survival of forest species is still larger than on conventional clearcuts.Deadwood on clearcuts may present important habitats to saproxylic species,including rare and red-listed ones and a prioritization of tree species per stand is recommended.We identify several important future research directions including switch of focus towards the landscape as well as the species population level.Surveys in parts of European Russia where retention has been unintentionally implemented already for a century would indicate possible future trajectories of biodiversity and their drivers in other regions of Northern Europe.A stronger link to ecological theory would help in study designs and in the formulation of predicted outcomes.