The Swedish Oak Project

Research project

Short description

Biodiversity and ecology and management of closed-canopy mixed oakrich forest.

Oaks in many forests!

Mixed forests with large oaks (Quercus spp.) and high conservation values are scattered throughout the Holarctic temperate zone, from Asia to Europe to North America. In Sweden there are at least 30 000 such forests south of "Limes Norrlandicus", the biologically quite welldefined border separating the boreal forest from the temperate (or cold temperate) forest in our country. These forests are pleasant to stroll through and have a rich diversity of animals, plants and fungi – many of the species are relatively rare, since coniferous forests, and forestry, with Norway spruce and Scots pine dominate in Sweden. Some mixed forests with oaks are used for timber harvesting, some have been left to develop without any intervention by the forest owner (who may not need to harvest), many have been defined as "woodland key habitats" by the Swedish Forest Agency (more than 20 000 mixed oak forests), and some are protected in the form of national parks, nature reserves, biotope protection areas, or through conservation agreements or deals between the forest owner and the government. In Götaland in the south of Sweden, the Oak Project has studied 25 such "conservation forests" for more than 20 years (forests more or less protected for biodiversity purposes, and for research).

Aims and focus

The Swedish Oak Project at the University of Gothenburg started in the spring of 2000 and is a longterm research project aiming to improve our understanding of how oakrich mixed forests function and can be managed for (primarily) conservation purposes and biological diversity. Many oakrich mixed forests, and almost all our "noble" (hardwood) broadleaved forests developed from an older, more open, traditional agricultural landscape, where several special species thrived but are now declining. Active management measures to create more open forests are hence motivated, but need to be evaluated – a crucial point, which calls for an experimental approach. Oaks do well in more open habitats, and especially small oaks grow better under such circumstances. But "to manage" also includes what we call minimal intervention, in forests of high conservation value consider the oldgrowth forests, where nature and time has created living space for many species which currently cannot live in managed production forests (or whose populations are strongly reduced there).

Our concept conservation thinning is a management measure developed over 16 years and onwards an evaluation that requires a lot of planning, effort, and long time periods, as trees grow slowly and can grow much older than humans. A number of councils and foundations have supported the Swedish Oak Project over the years, including the Swedish Research Council (VR), FORMAS, and the Swedish Energy Agency (among other things we have evaluated careful harvesting of biofuel).

Experimental design

One approach for management of second-growth temperate stands with conservation values is to favour legacy trees that contain red-listed or otherwise valuable associated species. Old hollow oaks (Quercus spp.), threatened by other invading trees, is one example. Oak regeneration also requires that enough light reaches the oak seedlings and saplings, and conservationoriented partial cutting (conservation thinning) is one alternative for closed-canopy oak-rich forests. This is tested in the Swedish Oak Project, a BACI (Before-After-Control-Impact) field experiment that began in 2000 and is planned to be longterm. Because the forests contain many valuable trees and structures other than oaks, active management is not self-evident. Instead, we test non-traditional management (conservation thinning) versus minimal intervention – the latter alternative should not be neglected in research and management, and is sometimes called passive restoration.

Our 25 study sites are small nature reserves and woodland key habitats with large oaks and many other trees, essentially closed canopies, and basal areas of 20-38 square metres/hectare. About 60 years ago, canopy openness (% visible sky from the ground) was on average 50% and the sites have a history of agriculture (small fields and pasture woodland). The sites are spread over a large area and landscape factors are also analysed. At each site, we use one plot (1 hectare) for partial cutting, and one plot (1 hectare) nearby for minimal intervention, studying these both before and after cutting in the winter 2002/2003. We examine responses in vascular plants (herbs, shrubs and trees), bryophytes, lichens, saproxylic fungi, saproxylic and herbivorous beetles, fungus gnats (Mycetophilidae; Diptera and related families) and snails and slugs (terrestrial molluscs). 

These taxa were chosen to represent both light-demanding and shade-tolerant or dessication-sensitive organisms. We do not study birds and mammals, which are generally well-known in Sweden, and would have required large study sites. The region and landscape is a mosaic of many small scattered conservation forests. We cut about 2530% of the basal area at each site, harvesting mainly smaller, intermediate and some larger trees to create more open conditions, especially around larger oaks. Tops, branches, and two dead oaks were left in each plot, the rest was harvested (the project was funded for tests of careful biofuel cutting for biodiversity).

Result summary

The short-term (2-10 years) effects of conservation thinning are mainly positive or neutral for biodiversity. In several cases, responses in the taxa to cutting could not have been detected without our minimal intervention plots. In addition, species turnover was high also under minimal intervention. After 8-10 years, conservation thinning seems to favour regeneration of shrubs (e.g. Corylus, Rhamnus, Lonicera) rather than trees such as oaks. Planting of oak seedlings is one management possibility. While our results so far at least partly support non-traditional management, long-term data are needed to evaluate the management alternatives, including unpredictable events. We suggest that at least 30% of this kind of forest in the region should be reserved for minimal intervention.

A summary of the effect of conservation thinning on the different species groups studied in the Swedish Oak Project. Plus means that the species group benefitted, plus/minus means no discernible or significant effect, minus means that the species group was disfavoured (although the negative effects have turned out to be small, and disappear with time). Note that timber harvesting may reduce the amount of dead wood in a forest in the long run, which could disfavour some species groups.