[Biodiversity Guidebook Table of Contents]

Appendix 1.

Landscape level biodiversity concepts

Forest habitats can be grouped into three categories at the landscape level: early seral stage, mid-seral stage and late seral stage (old growth). Species diversity is generally greatest in early and late seral stages.

Vertebrates tend to be associated with the structural attributes typical of a seral stage at a stand scale, rather than with stand age per se. It is therefore possible to maintain populations of most vertebrate species by providing the necessary structural attributes. What is less clear is how much habitat is required to do this.

Other species are believed to be tied to the interior microclimate conditions of late seral forests. These species include some plants, especially epiphytes, and invertebrates. Thus, to maintain populations of these species, it may be necessary to preserve blocks of old growth sufficiently large to provide suitable interior microclimate conditions.

Fragmentation

Fragmentation is the process of transforming large contiguous forest patches into one or more smaller patches surrounded by disturbed areas. This occurs naturally through such agents as fire, landslides, windthrow and insect attack. In the managed coastal forest, timber harvesting and related activities have been the dominant agent of disturbance during this century.

Fragmentation leads to declines in biodiversity in three ways. One is through the actual loss of habitat in the conversion of forests from natural to managed stands. The second is through the increase in microclimatic and biotic edge effects (see the following section) as the size of forest patches is reduced. The third is through the increasing isolation of the remaining forest patches, which can impose barriers to gene flow and dispersal.

Rate of cut, cutblock size and distribution, and silvicultural system all influence fragmentation. In British Columbia, the overall rate of cut is the major factor determining the amount of fragmentation that will occur. The higher the rate of cut, the faster the proportion of older seral stands will decline. If a landscape is dominated initially by older forest, some conversion to younger forest will increase the abundance of species which thrive in edge environments (see the following section). At some threshold, however, species requiring an older forest will begin to be lost. At what level of cut the threshold will occur is uncertain, though the higher the cut, the greater the risk to species requiring mature forest.

Edge effects and forest interior

At the boundary between older forest and newly harvested areas an edge environment is created which affects conditions within the forest. Along a transect from the open cutblock environment, through the edge, and into the forest there are strong gradients of light intensity, temperature, wind, relative humidity, and patterns of snow accumulation and melt. These microclimatic edge effects penetrate into the forest to varying distances depending on the variable measured, but very little edge influence penetrates farther than 100–200 m (2–4 tree heights in coastal British Columbia) into the forest. The magnitude of an edge effect is influenced by the surrounding vegetation and topography. The boundary between well-established second-growth and old-growth forest is less distinct than the boundary between a recent clearcut and the same old-growth. Similarly, an edge that is protected by topographic features such as bluffs or the margins of a gully is less influenced by the surrounding open environment than an edge that is fully exposed.

Biotic edge effects occur when species associated with open areas or edges penetrate the intact forest. These species affect forest interior dwellers through the processes of predation, competition, and parasitism. Biotic edge effects are less well understood than microclimatic edge effects, but can exert influence farther into the forest because of the mobility of animals and the invasive plants they disperse. As natural forests are converted to managed stands, remnant patches of forest become smaller. As patch size decreases, the edge becomes a greater proportion of the total forest area. Forest interior is the area uninfluenced by microclimatic or biotic edge effects. Edges have been traditionally viewed as good habitat for wildlife, and it is true that some important game species thrive in edge habitats during certain seasons.

However, forest edges have a negative effect on other species, notably some songbirds that experience increased levels of nest predation. Past attempts to increase edges in managed forests have increased some species at the expense of others.

A patch of forest 400 m wide will generally contain little, if any, forest interior. We recommend targeting 600 m as a minimum width when providing forest interior is a management objective. This should give a core of 200 m in the centre, which is buffered from most microclimatic edge effects. Managers wishing to protect the interior of a FEN from possible biotic edge effects should consider minimum sizes substantially greater than those required for microclimatic edge effects.

Reserves

Forest reserves are fragments of what were once much larger biotic communities. They may be areas requiring special management or even total protection from harvesting. Reserves are necessary within managed landscapes to maintain natural ecological conditions for those species that depend on them. They can be established, for example, to conserve tracts of old growth that are sufficiently large to maintain forest interior microclimate conditions, or to protect special areas such as wetlands, calcareous and serpentine bedrock exposures, and riparian ecosystems.

Biodiversity is not distributed evenly across the landscape. Certain areas are especially rich in the numbers of species or unique habitats they support. These areas may play critical roles in the maintenance of biodiversity at the landscape level and they therefore merit special attention. Estuaries and riparian ecosystems are important examples of such areas. Not only do they influence aquatic habitat, but they also offer a unique interface between terrestrial and aquatic habitats.

Forest managers should also strive to preserve representatives of the different types of forest ecosystems in British Columbia, because different types of forest harbour different species. Some of these ecosystems may harbour endangered or valuable species that have yet to be discovered, such as insects or fungi. Recent work on southern Vancouver Island by entomologists from the Royal British Columbia Museum and the University of Victoria suggests that there are hundreds of species of forest insects that are unknown to science.

Linkages among reserves

Unless they are extremely large, reserves will not be able to maintain viable populations or ecological and evolutionary processes the way whole ecosystems can. For example, while some species disperse well between forest fragments, others found in old-growth forests (such as flightless invertebrates) are poor dispersers and are thus vulnerable to isolation impacts. Small populations of animals and plants that are restricted to forest fragments and isolated by roads and other barriers may not survive in the long term.

To avoid this problem, reserves should be linked as much as possible. These linkages can provide important seasonal and annual movement corridors for some species, and critical habitat for the dispersal of other species among isolated habitat fragments.

Threatened and endangered species

The intent of these guidelines is to maintain habitat diversity at the landscape and stand levels so that viable populations of all native species can be maintained. Species known to be threatened or endangered may require specific habitat management strategies to address their needs if the general landscape level approach is inadequate. Where they are present, it is important that threatened and endangered species receive special emphasis. Actions needed to protect and enhance populations of threatened and endangered species are specified in the Managing Identified Wildlife Guidebook.
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