Natural disturbance type 3:
ecosystems with frequent stand-initiating events

Natural burns usually contained unburned patches of mature forest that were missed by fire. Consequently, these forests produced a landscape mosaic of even-aged regenerating stands ranging in size from a few to thousands of hectares and usually containing mature forest remnants.

There were also frequent outbreaks of defoliating insects and an extensive presence of root diseases caused by Armillaria and Phellinus (especially in the ICH biogeoclimatic subzones). The impact of these infections on tree survival and stand structure ranged from low to severe. Tree mortality within mature forest remnants and regenerating stands resulted in dead trees, decaying logs, and canopy gaps. Riparian areas within the forest landscape provided special habitat characteristics not found in the upland areas.

Mean return interval for disturbances is about 100 years for the wind-dominated CWH and the fire-dominated SBPS and BWBS with deciduous species prominent. For the SBS and BWBS with coniferous species prominent, the mean fire return interval is about 125 years. The ESSF, ICH and MS units in this NDT experience a mean disturbance return interval of about 150 years.

The presence or absence of Douglas-fir does not influence the disturbance frequency, but determines the number and size of mature remnant stands that survive extensive crown fires to provide structural diversity. Douglas-fir is the most fire-resistant tree species in this NDT.

Biogeoclimatic units in NDT3

Seral stage distribution (NDT3)

Table 10. Seral stage definition for biogeoclimatic zones in NDT3

Table 11. Recommended seral stage distribution for NDT3 (% of forest area within the landscape unit)

Recommendation:

• Seral stages should occur in a variety of patch sizes within the landscape unit.

Temporal and spatial distribution of the cut and leave areas (NDT3)

A clustered harvest pattern, using large aggregated harvest units, most closely simulates the natural pattern of large fires and large unburned areas or, in certain coastal variants of the CWH, simulates windthrow disturbance. It also results in less fragmentation of the landscape. Retention of patches of forest or single trees within aggregated harvest units simulates the island remnants left within areas of large burns. These remnants are vital to maintain biological diversity, especially when large cuts are used.

A harvest strategy such as this provides numerous ecological benefits. Concentrating harvesting activity in one area allows other large areas of older forest to be left intact and unfragmented for extended periods. As well, the combination of seral stage distribution and harvest unit size recommendations are designed to ensure that some large, unfragmented mature forests are always present on the landscape.

Harvest units and the remaining mature forest stands within the operable forest should be distributed in the landscape unit as shown in Tables 12 to 14.

Table 12. Recommended distribution of patch sizes (harvest units and leave areas)[a] for biogeoclimatic subzones with Douglas-fir throughout stands in NDT3

Table 13. Recommended distribution of patch sizes (harvest units and leave areas)[a] for biogeoclimatic subzones with Douglas-fir restricted or absent in NDT3

Table 14. Recommended distribution of patch sizes (harvest units and leave areas)[a] for alluvial ecosystems in the BWBS biogeoclimatic zone in NDT3

Recommendations:

• Patch sizes greater than 40 or 60 ha can be created by harvesting the entire larger patch at one time or by aggregating small cutblocks over time. In either case, structural attributes (i.e., live and dead trees) consistent with the natural disturbance type are to be retained within the patch. If smaller cutblocks are aggregated over time, the district manager may waive or reduce green-up requirements to accomplish this. When approved cutblocks exceeding 40 or 60 ha are advertised, and appear in the Gazette, the fact that they meet biodiversity retention objectives should be indicated.

• The size range of leave areas should be the same as that for openings.

Old seral stage retention and representativeness (NDT3)

Recommendations:

• The total area that should be retained within the landscape unit in old seral stage condition is shown in Table 11. Rare site series should be retained in this condition in greater proportion than is their occurrence in the landscape unit; other site series should generally be retained in proportion to their occurrence in the landscape unit. Where site series mapping is not available, a combination of forest cover and site productivity or site index information should be used to determine representativeness.

• The old seral stage retention objective should include patches designed to provide some forest interior conditions across a landscape unit. Where a lower biodiversity emphasis is chosen, the target for forest interior conditions may be as low as 10% of the old seral area indicated in Table 11. If an intermediate or higher biodiversity emphasis is chosen, the target should be 25% the area indicated in Table 11. Objectives for obtaining forest interior conditions can sometimes be accomplished most efficiently by increasing old seral stage retention around wildlife habitat areas, riparian management areas, or other suitable areas. (For a description of forest interior, see Appendix 1.)

• Old seral retention objectives set in Table 11 sometimes cannot be met because of previous harvesting or natural disturbance history.

  – For a higher or intermediate biodiversity emphasis area, all the existing old seral forest should be retained and additional areas designated to be left to become old seral forest and make up the shortfall in the future.

  – For a lower biodiversity emphasis area, the economic and social consequences of halting the timber harvest of old seral forest may be politically unacceptable. If so, some additional harvesting of old seral stands may proceed, and the area equivalent to the shortfall in old seral area must be recruited over time, according to an approved long-term recruitment plan. The old seral retention objective must be in place by the end of three rotations. In this situation a much higher risk to biodiversity exists until the old seral requirements in Table 11 are in place.

Landscape connectivity (NDT3)

Connectivity can be maintained through the delineation of forest ecosystem networks (see the section “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:

• Maintenance of late successional forest linkages as part of forest ecosystem networks (see “Designing forest ecosystem networks”). These linkages should mimic the type and degree of connectivity found in each disturbance type.

• Management of the forest matrix (the forest between the defined linkages) to maintain additional connectivity through the use of:

  – seral stage and patch size guidelines
  – stand remnants left during harvesting operations (such as small unharvested patches or advanced regeneration)
  – stand management to maintain some of the structural attributes of older forests.

Recommendation:

• Connectivity should be maintained or provided for – especially in those areas identified as “high” in Table 15 – using variable width linkages as part of forest ecosystem networks. The full spectrum of biogeoclimatic subzones and variants within a landscape unit should be represented.

• Tailor the application of seral stage, patch size, and stand structure recommendations to manage the area outside of defined linkages to meet the specific connectivity objectives of the landscape unit.

Table 15. The frequency with which connectivity characteristics of natural mature/old seral stage ecosystems occur for all biogeoclimatic subzones of NDT3

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 11. If a lower biodiversity emphasis is chosen, linkages should not result in the areas of old seral stage exceeding objectives in Table 11.

Stand structure (NDT3)

Recommendations:

• Even-aged management systems with wildlife tree patches most closely simulate natural disturbances in much of this NDT. Note, however, that these wildlife tree patches should not be additive to those recommended in the section “Stand management to maintain biodiversity.”

• Partial cutting systems should be used in Douglas-fir and larch stands.

• Partial cutting systems should be used to maintain mature forest attributes in some spruce and true fir stands.

• Some mature Douglas-fir and larch should be retained in stands where they constitute a minor component of the stand.

• Where Douglas-fir or larch is a component of a stand, it should also form a component of the regenerating stand.

• Practices for maintaining stand structure should be considered for all stands and applied as outlined in “Stand management to maintain biodiversity.”

Species composition (NDT3)

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:

• A significant component of the landscape unit should be maintained in communities with plant species composition similar to that in communities that have developed through natural succession.

• Extensive conversion from climax to young seral species, or from young seral to climax or non-native species, should be avoided.

• Rare forest stand types within the landscape unit (that is, those accounting for less than 2% of the area, such as birch, cottonwood, aspen, alder, and maple) should be maintained over the rotation.

• The proportion and distribution of the deciduous broadleaf components of stands should be maintained—in both mixed wood and pure stands—within the range found in unmanaged stands in the landscape unit.