Heritage oaks in a changing landscape : managing biodiversity in southern Sweden

Abstract: In Sweden the landscape has been subject to pronounced changes over the past hundred years. In south Sweden, heritage oaks (Quercus robur and Quercus petraea) of large dimensions are scattered in the landscape as remnants of a former land-use. Heritage oaks constitute important habitats for associated species and are therefore related to Sweden’s own and international responsibilities regarding the protection of biodiversity. One conservation strategy is the action plan for protection of trees with high conservation value (THCV). The action plan has resulted in a registration of all heritage trees with a DBH above 80 centimetres. The THCV registration holds measures of spatial distribution and structural features considered relevant to biodiversity. The information on 982 heritage oaks from the THCV register forms the basis for this study. It aims to gain significant insights regarding the current range of threats and the effectiveness of current actions taken to secure the long-term future of heritage oaks and their associated biodiversity in southern Sweden. GIS analysis are assessing the spatial distribution of heritage oaks within nature reserves, 20-meter buffer zones around different land-use categories and those areas subject to commercial timber harvesting. The field component of the study contains a selection of 32 of the largest heritage oaks. During the field work in May 2012 the areas of living and dead crown were measured in order to examine first the influence of the present land-use categories on the area of living crown. Second, the crown areas were compared to the forest density south of the heritage oak, to evaluate competition between the heritage oak and adjacent forest. To compliment these results a visual assessment of aerial photos and orthophotos (georeferenced satellite images) is used to investigate the land-use changes that heritage oaks have experienced over the last 70 years. The assessment revealed that recommendations given in the THCV register for the treatment of 333 heritage oaks planned for within the first 2 years after termination of the inventory have not been fulfilled. The largest oaks are not included in the nature reserves which should provide the optimal protective status. The adjacent landscape appears to be influencing the crown development of the heritage oaks. We find that competition from high and dense vegetation, as found within coniferous forest, is contributing to a decrease in the area of living crown. Regarding associated biodiversity it is found that the density of heritage oaks within Häckeberga nature conservation area is consistent with densities associated with the maintenance of high species richness. Another structural feature associated with biodiversity is the presence of tree hollows and nearly half of the 982 heritage oaks in Häckeberga nature conservation area are known from the THCV register to possess them. We find that one third of those are within the protective category of nature reserves and discuss the implementation of this result. In addition we find that the vegetation adjacent to the heritage oaks has changed during the last 70 years from open land (pastures and fields) to broadleaved-dominated forest, with the amount of oaks growing within pastures and fields having decreased by 40 percent from the 1940s to 2012. We discuss these results in terms of the insights they provide regarding the current range of threats and the effectiveness of current actions taken to secure the long-term future of heritage oaks and their associated biodiversity in southern Sweden.