Isolation and edge effects among woodland key habitats in Sweden: Is forest policy promoting fragmentation?

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Abstract

Fragmentation of natural forests is a major threat to forest biodiversity. In areas with a long history of forestry, the remaining patches of old forests constitute only a minor part of the landscape. In such situations small stands may be valuable and important for conservation. However, as they may suffer from strong edge effects and isolation, their value may be lower than anticipated. In Sweden a national inventory of woodland key habitats (WKHs) has identified about 1% of the forest landscape as sites where red-listed species occur or may occur. Most are small (national median 1.4 ha) and isolated stands within an intensively managed landscape. The present analyses calculate WKH core area based on a range of depths of edge influence, and isolation based on both distance to nearest WKH and a weighted isolation measure that includes all neighboring WKHs and protected forest. These analyses are done on the WKHs in Norrbotten County in northern Sweden and include almost 5000 stands. The actual core area in the WKHs is about 30% given a 50 m edge influence. The degree of isolation is species dependent but the results indicate that only species with high dispersal abilities may effectively utilize the network of WKHs. For species with effective dispersal distances of less than 2 km the network is probably insufficient. The results emphasize the need to create buffer zones, to increase reserve areas and to manage the matrix so that species dispersal is promoted. This likely includes a necessity to aggregate biodiversity efforts on the landscape scale.

Introduction

Fragmentation due to land use is a major reason for the declining biodiversity in forest ecosystems (Harris, 1984). Besides decreasing habitat area, fragmentation results in increasing isolation of suitable habitats and increasing exposure to negative edge effects (Saunders et al., 1991, Haila, 1999). Fragmentation effects are caused by a multitude of both biotic and abiotic factors. Given the fragmented situation in most forest ecosystems the survival of many forest species relies upon repeated colonization of habitats to compensate for local extinctions. This implies that dispersal ability, colonization conditions as well as local survival drive the dynamics of species occurrences on a landscape scale (Hanski, 1999). The spatial configuration of habitats and their quality are thus crucial factors that directly relate to the degree of isolation and influence of edge effects.

The Swedish forest landscape has, during the last centuries, been transformed from natural conditions to a situation where the vast majority is dominated by stands shaped by forestry (Östlund et al., 1997, Esseen et al., 1997). While this transformation has increased productivity in terms of commercial yields, the consequences for many species restricted to natural forests have been severe. At present, roughly 1000 forest species are considered as threatened (Gärdenfors, 2000). The situation has made imprints into forest legislation and currently environmental concern and timber production has equal legal status (Anon, 1993).

A cornerstone in the conservation efforts in Swedish forests during the last decade has been to identify so called woodland key habitats (WKHs) (Nitare and Norén, 1992). These are small areas that host or potentially host red-listed species. They are situated in the managed landscape and still retain some of the characteristics of natural stands. A nation-wide inventory of such habitats is being conducted and today the vast majority of Swedish forests have been inventoried. This has substantially increased our knowledge of the occurrence and distribution of remaining valuable sites. Although the inventory may have missed many potential WKHs, the identified network will strongly influence future forest management.

However, the WKH inventory has also emphasized the fragmented habitats experienced by species confined to natural stands. It is thus a challenge to optimize the “use” of the WKHs to ensure the long-term viability of many red-listed forest species. Given the small size and rarity of the WKHs, they may potentially experience strong edge effects and many may be too isolated from other natural stands. Sensitivity to isolation is different from species to species due to their ability of dispersal. Edges will affect the environment in forest patches by changes in both biotic and abiotic conditions (Laurance and Yensen, 1991, Saunders et al., 1991). Edge effects are also complex, and the extent to which edges influence the occurrences of species is species-specific (Murcia, 1995). In boreal forests a major component of the biodiversity is composed of non-vascular plants. For these groups there is emerging knowledge on their relation to edge environments (Esseen and Renhorn, 1998, Snäll and Jonsson, 2001, Moen and Jonsson, 2003). These studies indicate that edge effects may vary among species groups but that they generally extend at least 25 m into the forest and probably greater than 50 m for some groups. This corresponds fairly well with the “rule-of-thumb” estimate that edge effects extend 2–3 tree lengths into forests.

The aim of the present paper is to explore both the problem associated with edge effects in WKHs and their isolation. This is done based on data from the county of Norrbotten, the northernmost province in Sweden. Although not all existing WKHs has been identified and not all forest owners have finished their inventories, the available data is extensive and an analysis is motivated considering the weight given to WKHs in conserving biodiversity in Swedish forests. The central question is whether the network of WKHs will contribute to the viability of old-growth dependent species, or whether they are too small, too few and too isolated. The present analysis will not fully answer this question but provide important insights to the reality that old-growth dependent species experience.

Section snippets

Data sources

Data on WKHs was obtained from the Swedish Forestry Board (for small non-industrial landowners) and from forest companies (SCA, AssiDomän, SveaSkog). As estimates of isolation need to consider all potential habitats, established nature reserves and national parks had to be included in the analysis. This data was made available by the County Administration Board in Norrbotten. In total the data represent 94% of the 3.9 million ha of forests in Norrbotten County below the mountain region. The

Results

The 4976 analyzed WKHs are distributed throughout all of Norrbotten County, with a tendency for aggregation in the central parts (Fig. 1). The sizes of the WKHs range from 0.1 to 470 ha, with a strong skew towards small sizes (Fig. 2). Their average size is 8.6 ha while the median is only 4.6 ha as a consequence of the skewed size distribution. The majority of the area is located in a fairly low number of large WKHs: habitats larger than 10 ha constitute only 23% of the total number but include 65%

Discussion

Current forest legislation in Sweden states that the environmental goal has equal weight as the economical (Anon, 1993). This means that the biodiversity of Swedish forests should be maintained. In practice, this implies changes in the management of forests, which include both concerns taken during active management and the creation of areas set-aside from forestry. The latter includes both creation of larger state forest reserves and the retention of smaller areas with high nature values, i.e.

Conclusions

The analysis exemplifies the fragmented nature of the managed Swedish forest landscape. It points at a severe situation for species restricted to natural forests. At the same time the establishment of a network of WKHs may be the first step towards a more successful management of forest biodiversity. They may constitute the focal areas, together with nature reserves, around which management should be concentrated. These management activities include creating buffer zones to minimize edge

Acknowledgements

Information on WKHs was kindly provided by the Swedish Forestry Board, SCA Forest and Timber, AssiDomän AB Forestry and SveaSkog while the County Administration Board in Norrbotten provided data on nature reserves. The paper has benefited from discussion with Mattias Edman and Nicholas Kruys.

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