[Biodiversity Guidebook Table of Contents]
Natural disturbance type 2:
Historically, these forest ecosystems were usually even-aged, but extended post-fire regeneration periods produced stands with uneven-aged tendencies, notably in the ESSF and SWB biogeoclimatic zones where multi-storied forest canopies result.
ecosystems with infrequent stand-initiating events
Wildfires were often of moderate size (20 to 1000 ha), with unburned areas resulting from sheltering terrain features, higher site moisture or chance. Many larger fires occurred after periods of extended drought, but the landscape was dominated by extensive areas of mature forest surrounding patches of younger forest.
The mean return interval for these disturbances is about 200 years for the CDF, CWH, ICH, SBS, ESSF and SWB biogeoclimatic zones.
Biogeoclimatic units in NDT2
The following biogeoclimatic subzones and variants make up this disturbance type:
Seral stage distribution (NDT2)
As a result of the major fires that occurred in this disturbance type, the landscape would have consisted of extensive areas of even-aged stands with snags and veteran trees that had survived previous fires. Small areas were missed by the burns. Table 6 defines seral stages for each biogeoclimatic zone within this disturbance type; Table 7 recommends targets for seral stage distribution in the type.
Table 6. Seral stage definitions by biogeoclimatic zones in NDT2
Table 7. Recommended seral stage distribution for NDT2 (% of forest area within the landscape unit)
- Seral stages should occur in a variety of patch sizes within the landscape unit.
Temporal and spatial distribution of the cut and leave areas (NDT2)
The pattern and timing of forest harvesting are the dominant factors that determine the size and spatial distribution of similarly aged forest patches in managed landscapes. The objective in this disturbance type is to maintain a range of small to medium-sized (up to 250 ha) similarly aged forest patches on the landscape. The forest patch size distribution applies to both harvest unit and the leave area between harvest units.
Clearcutting with wildlife tree patches and some small clearcuts can be used to simulate the small-scale disturbances that naturally occurred in this disturbance type. However, complete reliance on small, dispersed clearcuts and small leave areas would lead to excessive forest fragmentation. Therefore, some larger patches should be cut to form larger openings; others should be identified as leave areas.
Harvest units and the remaining mature forest stands of various sizes within the operable forest should be distributed in the landscape unit as shown in Table 8.
Table 8. Recommended distribution of patch sizes (harvest units and leave areas)[a] for NDT2
- 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 (NDT2)
The target for old seral stage retention in this disturbance type is described in the recommendations below.
- The total area that should be retained within the landscape unit in old seral stage condition is shown in Table 7. 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 option is chosen, the target for forest interior conditions may be as low as 10% of the old seral area indicated in Table 7. If an intermediate or higher biodiversity emphasis option is chosen, the target should be 25% of the area indicated in Table 7. 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 7 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 7 are in place.
Landscape connectivity (NDT2)
Historically, these forests existed as contiguous tracts of old seral stage forest in which stand structure was complex because major stand-initiating events were rare. Current forest practices greatly alter these forests compared to historical conditions.
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.
A combination of smaller dispersed clearcuts, some dispersed partial cuts, a few large aggregated harvest units, and some mature and old seral stage forests, maintained in a connected network, should more closely approximate the historic pattern of this landscape type.
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:
- Connectivity should be maintained or provided for—especially in those areas identified as “high” in Table 9—using variable width linkages as part of forest ecosystem networks.
- 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 9. The frequency with which connectivity characteristics of natural mature/old seral stage ecosystems occur for all biogeoclimatic subzones of NDT2
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 7. If a lower biodiversity emphasis is chosen, linkages should not result in the areas of old seral stage exceeding the objectives in Table 7.
Stand structure (NDT2)
Maintaining snags, veteran trees, and coarse woody debris within predominantly even-aged stands is important for biodiversity in this NDT. Reserving a component of the old seral stages that historically have not burned is also important.
- Even-aged silvicultural systems with wildlife tree patches are appropriate to mimic the natural stand-initiating events. Note, however, that these wildlife tree patches should not be additive to those recommended in “Stand management to maintain biodiversity.”
- Partial cutting, including commercial thinning, can be used to maintain a number of mature forest characteristics in some stands.
- Where possible, existing veteran trees should be maintained when second-growth stands are harvested.
- Practices for maintaining stand structure should be considered for all stands and applied as outlined in “Stand management to maintain biodiversity.”
Species composition (NDT2)
Natural forest succession in this NDT created a mosaic of different successional stages. Species composition within these successional stages varies from early seral communities to climax communities. Maintaining that variety of species composition within seral stages is an important component of maintaining biodiversity in this NDT.
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.
- 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 within the range found in unmanaged stands within the landscape unit.
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