SR6 How is biodiversity influenced by the management of forest set-asides?

Burning of forest stands can benefit their biodiversity. Photo: Trons/TT bild.

Should protected forests be left completely untouched, or is some kind of active management required if their biodiversity is to be preserved? That question is hotly debated among conservationists today. EviEM has now assessed what evidence exists on the topic.

Non-intervention or active management

Large-scale forestry has had an increasing impact on forest biodiversity over the past century. In order to preserve at least a part of the threatened diversity, however, some forest areas have been excluded from harvesting, either through statutory protection in reserves or on a voluntary basis. Most of the forests thus set aside for conservation have been left more or less untouched – a policy of non-intervention has prevailed.

In some protected forests, however, biodiversity and other natural values are largely a result of past disturbances or interventions, such as wildfires, livestock grazing or small-scale harvesting. If such forests are left entirely to their own devices, they will gradually change. In most cases they will become denser and darker, to the detriment of many of the species that actually were to be protected. Some kind of active management – e.g. burning, thinning, resumption of forest grazing or removal of spruce – may therefore be required if forests of these kinds are to remain as diverse as they used to be.

Interest in such measures has grown in recent years, but opinions differ among conservationists on how active management should be balanced against non-intervention. A choice between these two alternatives also has to be made when areas formerly used for industrial-scale forestry are to be restored, i.e. allowed to return to a more natural state.

In order to compile and analyse what science has to say on the matter, EviEM initiated a review of the impacts of active management vs. non-intervention on biodiversity in forests set aside for conservation or restoration. The review team was chaired by Bengt Gunnar (Bege) Jonsson, professor of plant ecology at Mid-Sweden University in Sundsvall, and Claes Bernes, EviEM, acted as a project manager.

Mapping the evidence

As a first step towards a more complete synthesis, we compiled a systematic map. Such a map gives an overview of the evidence base by providing a database with descriptions of relevant studies, but it does not synthesise reported results.

We identified studies on a variety of interventions that could be useful for conserving or restoring any aspect of forest biodiversity. We searched not only for studies of interventions in actual forest set-asides, but also for appropriate evidence from commercially managed forests, since some practices applied there may be useful for conservation or restoration purposes too.

Since the review is based on Swedish initiatives, we focused on the boreal and temperate forest types that are represented in Sweden, but these forests are parts of vegetation zones that extend over many parts of the world. We looked for relevant studies performed throughout these zones, which means that the evidence covered by our review should be relevant for managers of forest set-asides not only in Sweden but in many other regions as well.

More than 800 relevant studies found

Initially, our searches for literature identified almost 17,000 articles as possibly relevant to the subject of the review. Through several stages of screening, most of these articles were subsequently excluded, but after close reading, 812 studies have been assessed as useful.

Most of the studies included in the systematic map were conducted in North America or Europe.

Almost two thirds of the included studies were conducted in North America, whereas most of the rest were performed in Europe. Of the European studies, more than half were conducted in Sweden or Finland (69 and 68 studies, respectively). The interventions most commonly studied were partial harvesting, burning, thinning, and grazing or exclusion from grazing/browsing. The outcomes most frequently reported were effects of interventions on trees, other vascular plants, dead wood, vertical stand structure and birds.

Data available here!

Data on all of the 812 studies are available in an Excel file, and also in an interactive Geographical Information System (GIS) application. The GIS application shows the locations of the included studies on a map of the world, it displays all information that we have recorded on the studies, and it allows the user to select studies of any special kind that may be of interest. A brief explanation of how the GIS application can be used is available here.

Links to our report on the systematic map can be found in the margin at right.

Full systematic reviews of dead wood and forest grazing

Our systematic map identifies a wealth of evidence on the impact of active management practices that could be utilised to conserve or restore biodiversity in forest set-asides. As such it should be of value to conservation managers, researchers and policymakers. Moreover, since the map also highlights important knowledge gaps, it could inspire new primary research. Finally, it provides a foundation for more complete synthesis of specific subtopics.

Based on our map of the evidence, we identified the following four subtopics as sufficiently covered by existing studies to allow such synthesis:

1. Effects of thinning, partial harvesting and understorey removal on the diversity of ground vegetation.
2. Impacts of various interventions on dead wood and deadwood-dependent species.
3. Effects of prescribed burning on the diversity of species other than those dependent on fire and dead wood.
4. Impacts of grazing and browsing by livestock, deer etc. on the diversity of forest plants and invertebrates.

We have since continued with full systematic review of subtopics 2, 3 and 4 (see links below).

Review team

Bengt Gunnar (Bege) Jonsson (Chair), Dept. of Natural Sciences, Mid-Sweden University, Sundsvall, Sweden
Kaisa Junninen, Natural Heritage Services, Metsähallitus, Joensuu, Finland
Asko Lõhmus, Inst. of Ecology and Earth Sciences, Tartu University, Estonia
Ellen Macdonald, University of Alberta, Edmonton, Canada
Jörg Müller, Bavarian Forest National Park, Grafenau, Germany
Claes Bernes (Project manager), Stockholm Environment Institute, Stockholm, Sweden
Jennie Sandström (Project assistant), Dept. of Natural Sciences, Mid-Sweden University, Sundsvall, Sweden

First meeting of the review team, April 2014. Standing, from left: Jörg Müller, Bege Jonsson, Asko Lõhmus, Claes Bernes, Ellen Macdonald, Kaisa Junninen. In front: Jennie Sandström and Miriam Matheis (master’s student). Photo: Sif Johansson.