British Columbia has tremendous ecological variation. Although some general forest and range management practices can broadly accommodate the needs of all ecosystems, more often a variety of practices is needed to respond to the different natural disturbance regimes under which ecosystems have evolved.
For the purpose of setting biodiversity objectives, five natural disturbance types (NDTs) are recognized as occurring in British Columbia. They are:
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 ecosystems
These disturbance types characterize areas with different natural disturbance regimes. Stand-initiating disturbances are those processes that largely terminate the existing forest stand and initiate secondary succession in order to produce a new stand. The disturbance agents are mostly wildfires, windstorms and, to a lesser extent, insects and landslides. The stand-maintaining disturbances—such as the understorey surface fires that occur in the Interior Douglas-fir and Ponderosa Pine biogeoclimatic zones—serve to keep successional processes stable.
The following descriptions of the five NDTs include the biogeoclimatic zones, subzones, and variants that fall under each disturbance regime. As additional information becomes available, the number of NDTs and the biogeoclimatic units within them may be revised and refined. Appendix 3 provides an alphabetically listed cross reference of biogeoclimatic units to NDTs and forest region.
The geographic distribution of NDTs is presented in Figure 5.
Figure 5. The distribution of NDTs across British Columbia. (Prepared by Research Branch, Ministry of Forests. Based on Provincial Biogeoclimatic Subzone/Variant Mapping (Version 1.0).
Recommended procedure for establishing landscape unit biodiversity objectives
The composition of plant and animal communities changes as forest stands develop through time after a disturbance. Various organisms find their habitat needs during different stages of forest development (known as seral stages, or forest ages) and most specialist species are associated with either the early herb/shrub stage or the mature to old seral stages.
Different natural disturbance regimes have created forests with greatly differing seral stage distributions. Portions of the province with less frequent stand-initiating disturbance have more older forest, and a greater abundance of species adapted to landscapes of older forests, than do areas with more frequent disturbance.
Forest harvesting generally increases the amount of young forest and decreases the amount of older forest, because commercial forest rotations are generally shorter than natural disturbance return periods. This effect is most pronounced in forest types that have the lowest frequency of natural stand-initiating disturbance. Therefore, the more that managed forests diverge from natural disturbance regimes, the greater the risk of loss of biodiversity.
The seral stage guidelines provided by NDT are intended to help foresters maintain the diversity of seral stages and disturbance regimes found within various biogeoclimatic zones, subzones and variants. The method used for establishing seral stage definitions and distributions is described in Appendix 4.
It is unclear to what extent management can deviate from natural seral stage distributions without losing elements of biodiversity. Even at the scale of landscape units, natural patterns often vary from the average. This guidebook, while acknowledging the uncertainties, presents the minimum requirements considered to have a good probability of maintaining biodiversity within the landscape unit. It assumes that greater change from natural seral stage distributions would increase the risk to biodiversity, and less change would decrease the risk.
The maximum cutblock size that may be proposed is either 40 or 60 ha, depending on which forest region the plan is in (OPR 21 (2)). However, smaller (OPR 21 (3) (a)) or larger (OPR 21 (3) (b)) maximum cutblock sizes may be allowed by the district manager for specific reasons. Where larger cutblocks are specified, the cutblock design must be consistent with the structural characteristics and the temporal and spatial distribution of natural openings (see recommendations in the section on “Stand management to maintain biodiversity”).
Linked to the importance of maintaining the temporal and spatial distribution of cutblocks and natural openings is the importance of maintaining landscape level connectivity. Connectivity recommendations in general are provided in Chapter 3; and forest ecosystem networks (FENs), one way to achieve landscape connectivity, are described in detail in the section “Designing forest ecosystem networks.” Where locations of FENs are known, they must be identified and described on forest development plans (OPR 15 (2) (g)).
Historically, these forest ecosystems were usually uneven-aged or multi-storied even-aged, with regeneration occurring in gaps created by the death of individual trees or small patches of trees. When disturbances such as wind, fire, and landslides occurred, they were generally small and resulted in irregular edge configurations and landscape patterns.
The mean return interval for these disturbances is generally 250 years for the CWH and ICH, and 350 years for the ESSF and MH biogeoclimatic zones.
Occasionally, major windthrow events have occurred as a result of hurricane-force winds on certain exposed parts of coastal British Columbia. The average return interval for these has been approximately 100 years. Those portions of the CWHvm1, CWHvm2, CWHvh1 and CWHvh2 variants where these wind events occurred should therefore be considered to fall within NDT3, ecosystems with frequent stand-initiating events.
While stand-initiating fire disturbances also occur—though rarely—in the Interior Douglas-fir and Ponderosa Pine biogeoclimatic zones, these zones are not included in NDT1 because the stand-maintaining surface fires are the dominant ecological influence.
Table 2 defines seral stages for each biogeoclimatic zone within this disturbance type; Table 3 recommends targets for seral stage distribution in the type.
Table 2. Seral stage definitions by biogeoclimatic zones in NDT1
Table 3. Recommended seral stage distribution for NDT1 (% of forest area within the landscape unit)
Partial cutting and some small clearcuts could be used in this NDT. Complete reliance on small, dispersed clearcuts and small leave areas, however, 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 within the operable forest should be distributed in the landscape unit as shown in Table 4.
Table 4. Recommended distribution of patch sizes (harvest units and leave areas)[a] for NDT1
– 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 3 are in place.
Connectivity can be maintained through the delineation of 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:
– 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.
Table 5. The frequency with which connectivity characteristics of natural mature/old seral stage ecosystems occur for all biogeoclimatic subzones of NDT1
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 3. If a lower biodiversity emphasis is chosen, linkages should not result in the areas of old seral stage exceeding the distribution objectives in Table 3.
Maintaining a variety of canopy layers (vertical structure) and spatial patchiness (horizontal structure) is important for maintaining biodiversity in this NDT.