General Biodiversity Management Considerations (module 2, p. 2)

Underlying assumptions about biodiversity

One of the key underlying assumptions about biodiversity management is that native species and ecological processes are most likely to be maintained if managed forests resemble those forests created by natural disturbances such as fire, wind, insects and disease. In other words, it is assumed that the habitat needs of most species will be provided by:

  • Maintaining a variety of ecologically valuable structure within and adjacent to cutblock boundaries in perpetuity
  • Maintaining a variety of cutblock sizes, seral stages, and forest stand attributes across a variety of landscapes
  • Maintaining the ability of species to disperse across and within a landscape though connected habitat links (biological refuges and dispersal points)-within and outside of harvested cutblocks
  • Providing an appropriate balance of interior to edge forest habitat

The biodiversity management recommendations in this course are primarily aimed at managed forests. The recommendations are designed to promote long-term stand level maintenance and recruitment of important structural attributes such as:

  1. Wildlife trees
  2. Coarse, woody debris
  3. Horizontal and vertical structural diversity
  4. Diversity of vegetation species
  5. Special or unique habitats
  6. Riparian areas and wetlands

What do you know about these six attributes? Where can you go to find more information about them? Are there others? If so, what are they?

It is not possible to maintain all elements of biodioversity in all places, all the time, in large-scale commercial forest operations. However, by maintaining all species within their historic range in perpetuity, we will enable dispersal and recolonization of biodiversity elements across the vast majority of their naturally occurring areas.

Given the high degree of ecological variability in our forests and the multiple resource objectives that are often being managed, managers need to be flexible and creative. Forest managers need to consider on a site-specific basis, the most effective way to provide for biodiversity.

This major heading is divided into three subheadings:

  • Forest edge and interior habitat (patch size)
  • Forest management practices to manage for edge and interior habitat
  • Natural disturbances and cutblock size

Forest edge and interior habitat (patch size)

Forest edge is the area where one ecosystem type meets another (e.g., where a forest meets a meadow, boundary between a harvested area and old-growth forest, or a gap in the forest canopy). Often, there is a difference in microclimate conditions (such as temperature, humidity, light, snow/moisture accumulation, and wind) that can penetrate 100-200 metres from an edge into the intact forest. 

In addition to environmental impacts, forest edge also produces a biotic effect. Biotic edge effect occurs when plant and animal species associated with open areas and forest edges move into the intact forest. These species impact the forest-dwelling species in various ways, such as predation, habitat competition/displacement, and parasitism. Biotic influences can extend 400 metres or more into the forest due to the mobility of animals and invasive plant species.

Forest interior is the area of the forest not influenced by microclimate or biotic edge effect. Some forest stands form an edge where they border natural features, such as a rock outcrop, meadow, or lake, or a human-caused opening such as a cutblock. As forests regenerate and grow in disturbed areas, edge contrast diminishes and the amount of interior habitat increases. Thus, interior stand conditions and forest structure become relatively new to that stand, providing a certain microclimate (solar radiation, wind speed, relative humidity) and stand structural elements (e.g., tree species, canopy layers). Some species are thought only to prosper in interior, mature or climax forest conditions. They are old-growth dependent.

Smaller areas of forest are generally dominated by edge, with little or no interior habitat. Furthermore, circular patch shapes have less edge than highly irregular patches. In a landscape with many irregular shaped patches of forest, the amount of interior habitat may be much less, than it appears due to the amount of forest functioning as edge. On the other hand, some irregularity in patch shape simulates natural canopy disturbances and openings. Shorter distances from interior habitat can enhance species dispersal to regenerating patches and increase proximity to escape cover from predators.

It is generally recognized that increased edge results in a shift toward early seral (open habitat) species along the edge. The edge between patches may be highly contrasting (e.g., trees next to lakeshore, or tall older trees next to regenerating seedlings). This is sometimes called a hard edge. As forests regenerate in disturbed areas, stand area increases and edge contrast diminishes. Under these conditions, or along lesser contrasting habitats such as riparian/hardwood interfaces, a soft edge exists.

In managed forests, generally as opening sizes decrease the proportion of edge increases. In a landscape with many irregularly shaped patches of forest, the amount of interior habitat may be much less than it appears, because the amount of forest that actually functions as edge. On the other hand, some irregularity in patch shape simulates natural canopy disturbances and opening, resulting in curvilinear boundaries. This shape of patch boundary can enhance species dispersal to other patches or increase proximity to escape cover from predators.

Figure 6 (left) illustrates the relationship between patch shape and the amount of edge and forest interior habitat. Figure 6a depicts a circular patch with less edge and increased interior habitat area. However, in real terms of scale, it is important to understand that a small, irregular shaped 2 ha patch of forest may have no forest interior conditions. However, the same irregular-shaped patch of 100 ha will have areas of forest interior condition.


Forest management practices to manage for edge and interior habitat

When designing cutblock size, shape and arrangement, consider the habitat needs of local species-some species benefit from edge, others are affected negatively. Forest managers can greatly influence the amount of forest edge, and therefore wildlife habitat, with their planning and activities.  This strategy should be determined at the landscape level, while designing reserves and managing for identified wildlife, not when designing cutblocks. It is implemented at the cutblock planning stage, if deemed an objective in the higher-level plan.

Natural disturbances and cutblock size

Natural disturbances can affect relatively small areas such as singletree or affect very large areas such as a landscape by catastrophic wildfires and forest pest outbreaks. When biodiversity/habitat objectives (or other objectives including VQO, hydrology, fuel management, wildlife habitat needs) support larger opening sizes, it may be appropriate. (This will require district manager approval). Where large or contiguous (aggregated) cutblocks occur for these reasons, more emphasis should be placed on stand level retention practices.

Overall, it is important to have a range in cutblock sizes from small to large, with a corresponding similar range of leave areas, as a means of maintaining biodiversity across landscapes and reducing habitat fragmentation. The scale of sizes to be variable depending upon NDT.