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Biodiversity Guidebook Table of Contents]

Natural disturbance type 4:
ecosystems with frequent stand-maintaining fires

This NDT includes grassland, shrubland, and forested communities that normally experience frequent low-intensity fires. On grasslands, these fires limit encroachment by most woody trees and shrubs. Late seral and climax grasslands and shrublands are typically restricted to droughty sites that occur at low elevations or on steep south-facing slopes or fire-prone areas.

Less arid sites are characterized by forests of large, old trees with thick fire-resistant bark. Patches of less fire-resistant species develop in areas that have escaped low-intensity surface fires. The varied intensity and frequency of fires across the landscape has created a natural mosaic of mostly uneven-aged forests interspersed with grassy and shrubby openings.

Low elevation grasslands and open forests were more widespread throughout the Bunchgrass and Ponderosa Pine biogeoclimatic zones and drier elements of the Interior Douglas-fir biogeoclimatic zone before European settlement. Some of the open forests and grasslands were maintained in a “fire-climax” state by periodic lightning-caused fires and aboriginal use of fire. Recent human activities have altered fire regimes in much of this NDT, fostering litter accumulation and forest encroachment in some grasslands, and changing canopy composition and density in some forested areas. An increase in fire activity in the late 19th and early 20th centuries likely increased the extent of these ecosystems, but fire suppression during the last six decades has had the opposite effect.

The Ponderosa Pine and Interior Douglas-fir biogeoclimatic zones have a history of periodic surface fires that consumed woody fuels, rejuvenated most herb and shrub species while selecting against others, thinned the younger stands, and raised the height to the live tree crowns. Fire was historically responsible for maintaining the vegetative species composition and forest stand structure, and also for regulating coarse woody debris loading.

While this regime of fire maintenance would normally be easy to duplicate in a managed forest, the situation is complicated by several decades of fire exclusion, which has caused many Ponderosa pine and interior Douglas-fir stands to fill in with young conifers. This has resulted in fuel accumulations, increased probability of crown instead of surface fires, loss of understorey forage, and insect and disease damage.

Surface fire return intervals for the PP and IDF biogeoclimatic zones historically ranged from 4 to 50 years; stand-initiating crown fires were rare in the PP and occurred at intervals ranging from at least 150 to 250 years or more in the IDF.

Much of the area within this NDT is rangeland (forested and unforested land used for grazing). Grasslands and dry open forests provide permanent range (areas that produce substantial livestock forage throughout most or all successional stages) due to the abundance of forage. Areas that succeed to closed forests are used as transitory range, because there is negligible livestock forage except during a brief period following stand-initiating fires or timber harvesting. Many sites support livestock grazing and logging together. Recommendations for rangelands and recommendations for forested areas both apply in these areas.

The natural biodiversity in this NDT has been significantly affected by unregulated livestock grazing during the initial period of European settlement. Introduced weeds became established on many sites and spread to other areas after natural or human disturbances. Non-native forage plants have been deliberately established in some areas for a variety of reasons. As well, a number of human influences have eliminated shrubs and trees from some areas and reduced their abundance and size elsewhere to the detriment of wildlife species that rely on them for forage, protective cover, and breeding. Livestock and wildlife grazing have also reduced residual cover—living and dead vegetation that persists over-winter and provides protective and breeding cover during critical periods in the following spring before new growth takes over this function. Conventional range management practices in this NDT have not re-created all of the important attributes required to restore natural biodiversity.

Biogeoclimatic units in NDT4

The following biogeoclimatic subzones and variants make up this natural disturbance type:

Seral stage distribution (NDT4)

Permanent range

Most permanent rangelands should be managed so as to achieve appropriate desired plant communities within the climax or late seral stages. Properly regulated livestock use has encouraged the recovery of damaged rangelands in some areas. Some sites, however, cannot be restored using existing tools, even with the complete exclusion of livestock.

Range management objectives should be established according to the potential natural community (PNC) concept. A PNC would be established by allowing succession to be completed without further human interference. Natural disturbances are inherent in the development of a PNC, and acclimatized or naturalized non-native plant species are included where there are no known methods to control them.

At present, much of the rangeland in these ecosystems is in an earlier seral stage than climax, and therefore late-seral to PNC-climax communities will take some time to establish.

The seral stage of the current plant community is determined on the presence and abundance of species compared to their status in the PNC. Early-seral stages have 0–25% of the composition of the PNC, mid-seral stages have 25–50% of the PNC, late-seral stages have 50–75% of the PNC, and PNC-climax stages have 75–100% of the PNC.

Rangelands in early seral condition often require several decades before an upward trend to mid-seral condition is completed, even under the most favourable management regime. Therefore, sufficient areas of rangeland in early to mid-seral condition should be managed to achieve seral targets in the long term. Those targets are shown below:

Percent of landscape unit

Note: The mid-seral stage, between early and late seral, is not designated. The minimum requirement for the climax seral stage is included in the “late seral + climax” category.

Despite the descrepancy between seral stage objectives for forestry (low biodiversity option) and seral stage objectives for range, practices should be modified to help address both forestry and range requirement.

Once available, biogeoclimatic units that provide permanent forest range shall be listed and should be managed to achieve landscape level objectives for both range and forested areas.

Recommendations:

Forests

As a result of the more frequent and widespread stand-maintaining fires that occurred in these ecosystems before fire suppression became widespread, the landscape is characterized by mature and old mixed forests. To maintain the important aspects of these forests, selected partial cutting prescriptions should be used while considering forest health issues. Table 16 defines seral stages for each biogeoclimatic zone within this disturbance type; Table 17 recommends targets for seral stage distribution in the type.

Table 16. Seral stage definitions for biogeoclimatic zones in NDT4

Table 17. Recommended seral stage distribution for NDT4 (% of forest area within the landscape unit)

Recommendation:

Temporal and spatial distribution of patch sizes (NDT4)

Permanent range

An objective to restore and maintain at least 85% of rangeland in each landscape unit in late seral or climax potential natural communities will ensure that appropriate grazing regimes are applied over time. These targets will not be immediately attainable in areas currently dominated by early seral communities.

Recommendations:

Temporal and spatial distribution of the cut and leave areas (NDT4)

Forests

Partial cutting, combined with occasional smaller dispersed clearcuts, will approximate the pattern of the natural landscape. Each seral stage should be represented by the distribution of patch sizes (harvest units and leave areas) as shown in Table 18.

Table 18. Recommended distribution of patch sizes (harvest units and leave areas)[a] for NDT4

Recommendations:

Old seral stage rangeland retention (NDT4)

Recommendations:

Old seral stage forest retention and representativeness (NDT4)

The characteristics of old seral stage forests in this disturbance type are generally maintained by low-intensity fires. Ecosystems developing under fire suppression are generally atypical and not adequate substitutes for old seral stage, fire-maintained stands.

Recommendations:

Landscape connectivity (NDT4)

Wetland complexes and mature forests between them are important and interrelated functional units for biological diversity. In this disturbance type, mature forest and late seral or climax rangeland corridors and connections will maintain biological functions.

Before fire suppression and domestic grazing, much of the landscape and most of the grasslands in these ecosystems were likely in a fire-maintained late seral or climax state. At the same time, however, stand-initiating fires, intense use by large wildlife, and insect outbreaks probably ensured that some of the landscape was usually in earlier seral stages. Connectivity is assumed to have been high among most parts of the landscape in this disturbance type.

Connectivity can be maintained through the delineation of forest ecosystem networks (see “Designing forest ecosystem networks”). It can also be achieved at a broader scale within landscape units, according to the recommendations under “Temporal and spatial distribution of the cut and leave areas,” above. The methods selected should depend on the connectivity objectives of the landscape unit.

Management to reduce fragmentation and maintain connectivity in managed forest landscapes should be guided by the type and degree of connectivity found in each disturbance type. Connectivity can be maintained by a combination of the following methods:

Forests

Recommendations:

Table 19. The frequency with which connectivity characteristics of natural mature/old seral stage ecosystems occur for all biogeoclimatic subzones of NDT4

If an intermediate or higher biodiversity emphasis is chosen, the areas that are identified as old seral linkages may be incremental to the areas indicated in Table 17. If a lower biodiversity emphasis is chosen, linkages should not result in the areas of old seral stage exceeding the distribution objectives in Table 17.

Rangelands

Recommendations:

Stand structure (NDT4)

Maintaining a variety of canopy layers and gaps between and within stands is important for biodiversity in this NDT.

Recommendations:

Species composition (NDT4)

Natural forest succession provides for a mosaic of different successional stages in this NDT. Species composition within these successional stages varies from early seral communities to climax communities for both forests and grasslands. Maintaining that variety of species composition within seral stages is an important component of maintaining biodiversity.

Rare ecosystems within the landscape unit also contribute significantly to the richness of species composition and to the maintenance of diversity.

The section “Stand management to maintain biodiversity” recommends stand level practices for maintaining species composition.

Recommendations:


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