spacer graphic
spacer graphic

View or download the Acrobat PDF file
spacer graphic
The Importance of Natural Disturbance Processes

Natural ecosystems go through processes of establishment, aging, disturbance, and renewal. Renewal can be initiated by large wildfires, or by
the toppling of a single old tree. While these natural disturbances and their subsequent effects are sometimes actively suppressed by humans due to the perception that they are harmful and destructive, many organisms and ecosystems depend on disturbance for survival. For instance, the black cottonwood, a riparian tree, times the release of seed to coincide with peak spring flows of the adjacent river. In years when the river floods and spills over its banks, cottonwood seed gets widely distributed downstream (Gayton 2001). In other examples, the unique
Natural Disturbances

Insect outbreaks, fire, snow press, diseases, ice, storms, windstorms, avalanches, volcanic eruptions, floods
high-elevation shrub communities created by repeated snow avalanches are crucial for foraging bears. Many species of forest birds rely on dead standing trees for nesting habitat, and seeds of the shrub ceanothus and lodgepole pine both germinate in response to fire.

Tanis Douglas
Thick stands of lodgepole pine on the site of a 1985 wildfire. Lodgepole pine germinates and thrives in response to fire.

Natural Disturbance as an Agent of Ecosystem Health - Forest Disease
The role of disease-causing tree fungi (e.g. Armillaria spp. and Fomes spp.) in creating un-even aged forest stands and valuable wildlife trees is only now beginning to be appreciated. Where forest pathogens and pests are traditionally viewed as negative and costly, disease agents are now sometimes acknowledged to be an integral part of a healthy ecosystem. The role of some of these agents can have a profound effect on large areas of anthropogenically-impacted forests. For example, mountain pine beetle (Dendroctonus ponderosae Hopkins) attacks older, even-aged lodgepole pine that have been allowed to establish over extensive areas due to wildfire suppression. The damage caused by the beetle kills most of the pine overstory, and allows different successional understory species to establish. While these beetle-killed forests usually represent serious economic losses, this disease-causing beetle can actually restore more natural and stable conditions to the dense forests created by decades of fire control.

Using Natural Disturbance Regimes to set Restoration Goals

An understanding of the local natural disturbance regime will help a restoration practitioner understand the types, patterns, and ages of ecosystems that would have been present prior to European influence. Natural Disturbance Types (NDT) are a useful tool, developed for British Columbia as part of the Forest Practices Code Biodiversity Guidebook (Province of BC 1995). These Types categorize the Province into zones based on the frequency and severity of pre-European disturbance events. It is important to note that this definition of "natural" disturbance includes aboriginal land management activities such as burning as they were conducted before European contact (Gayton 2001).

Natural Disturbance Types, as defined in the
Forest Practices Code Biodiversity Guidebook:
NDT1: ecosystems with rare stand-initiating events
NDT2: ecosystems with infrequent stand-initiating events
NDT3: ecosystems with frequent stand-initiating events
NDT4: ecosystems with frequent stand-maintaining fires
NDT5: alpine tundra and subalpine parkland

The Biodiversity Guidebook assigns groups of biogeoclimatic subzones and variants to each NDT, and also provides general guidelines for forest stand age-class distribution in each of the five categories (i.e., it provides guidance on landscape-level ecosystem patterns). The NDT concepts are not specific to the local, stand level at which most restorations take place; however, the guidebook, together with biogeoclimatic maps, form valuable starting points for terrestrial restoration planning (Gayton 2001).

Identifying local-level natural disturbance regimes can be tricky; for example, the average fire-return interval in a fire-maintained (NDT4) landscape depends on slope, aspect, elevation, and topography. However, a variety of techniques can be used to understand your local disturbance regime (see 'Historic Reference Ecosystems'), and more information is available all the time. In general, methods of investigating the former disturbance regime and disturbance pattern on a site might include coring older trees, examining fire-scarred trees, assessing soil pits, looking at historic photos and records, and investigating local knowledge.

Mimicking Natural Disturbance - Ecosystem Management in the East Kootenays
In recent decades, the effects of fire suppression have become a cause for concern for many residents of the East Kootenays, BC. Historically, before widespread settlement and fire suppression efforts, ground fires would have burned relatively frequently due to lightning strikes and due to intentional ignitions by First Nations to increase hunting opportunities. Now, the amount of grassland and open forest is in serious decline, causing concern to various sectors of society. Hunters, government managers and ranchers are concerned about the loss of grassland and open forest habitat, formerly available to big game species, currently rare and endangered species, and livestock. Forest managers are also critically concerned about the risk of cataclysmic fire due to the increase in dense forests and fuel build-up. Hence, under the Kootenay-Boundary Land Use Plan (1990), there is wide agreement to manage the area to mimic the former disturbance regime.

Under the Land Use Plan, the Rocky Mountain Trench is zoned into the three main types of ecosystems desired: open forest, grasslands, and closed forest, based on interpretations of old air photos and site capabilities. It will take decades of selective logging, in-growth 'slashing', and ground fires to restore the area closer to how it was when fire was the main disturbance agent. However, all segments of society are in agreement that the alternative is not acceptable. The alternative to current management plans is an ecosystem far removed from its natural range of variability, with serious impacts to the plants, animals and humans that depend on it.

Go to Previous SectionGo to Previous Section spacer Go to Next SectionGo to Next Section
footer graphic
spacer graphic spacer graphic spacer graphic spacer graphic