Biodiversity Guidebook Table of Contents]

Natural disturbance type 2:
ecosystems with infrequent stand-initiating events

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.

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)


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


Old seral stage retention and representativeness (NDT2)

The target for old seral stage retention in this disturbance type is described in the recommendations below.


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:


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.


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.


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