[Biodiversity Guidebook Table of Contents]
Natural disturbance type 5:
The ecosystems in this natural disturbance type occur above or immediately below the alpine treeline, and are characterized by short, harsh growing seasons. The vegetation is strongly patterned by variations in local topography. Fire can have a dramatic effect in this disturbance type, weakening or killing plants and causing long-term shifts in the position of the tree line. The harsh climate and short growing season restrict the rate of plant growth that can take place following a stand-initiating disturbance.
alpine tundra and subalpine parkland
Windward slopes and exposed ridge crests remain free of snow for extensive periods during the winter. They also tend to be dry during the growing season and have low fertility, which limits plant growth. These dry conditions favour a high proportion of deep-rooted cushion and rosette plants. Snow often forms deep drifts on lee slopes, remaining there well into the growing season. This limits vegetation growth, but because moisture is less of a limiting factor, forbs, bunchgrasses and dwarf evergreens generally dominate the vegetation. Level areas and depressions, often collecting meltwater, are free of late-lying snow and less prone to desiccation. These areas tend to support the most productive vegetation, dominated by sedges, grasses, forbs, and lush forbs, often along with deciduous shrubs. Because parkland variants at the upper limit of the ESSF and MH biogeoclimatic zones often have extensive areas of unforested vegetation similar to that in plant communities within the AT, they are included in this disturbance type.
Grazing by wildlife and non-native species drives ecosystem change in many areas within NDT5. The most noteworthy large grazers include bighorn sheep, mule deer, elk, moose, and cattle. Mountain sheep often calve and winter on exposed slopes and favour more productive sites for summer range. Mule deer, elk, and moose generally make lighter use of the disturbance type, primarily for summer range. Cattle are often grazed here in late summer.
Given the short, harsh growing season and generally infertile soils of this disturbance type, vegetation can be rapidly altered by grazing animals. Plants with large reserves of nutrients and energy below ground may leaf out quickly after being grazed, but it may take years to replenish their root reserves. Repeated grazing, or a mixture of grazing and other stresses, can therefore exhaust plant reserves and cause plants to die out several years after the onset of intensified grazing. Species with small reserves may die out quickly in response to increased grazing pressure. Late-season grazing generally has the least effect on plant reserves, but because many species form their flower buds in the season before flowering, it may have a negative effect on seed production.
Trampling by large herbivores can also greatly affect some ecosystems. Sites that remain waterlogged well into the growing season are susceptible to trampling damage during early-season grazing; dry sites are susceptible to such damage during fall grazing, after the first snows have moistened soils. Some areas used to graze domestic sheep still show substantial impacts decades after sheep use was discontinued.
Biogeoclimatic units in NDT5
The following biogeoclimatic subzones and variants make up this disturbance type:
Seral stage distribution (potential natural community) (NDT5)
Despite fires, landslides, and wildlife grazing, the vast majority of areas within this NDT were climax communities prior to early settlement.
- Livestock should be managed such that no more than 15% of the landscape is maintained in early to mid-seral condition.
- Range tenure plans for areas within this disturbance type should identify standards for determining seral stage and trend, and specify stocking rates and range readiness indicators that can be used to meet seral stage distribution criteria. Until this can be accomplished, livestock grazing in this disturbance type should only take place after the majority of flowering has occurred.
Temporal and spatial distribution of the cut and leave areas (NDT5)
- Livestock grazing should be managed in a manner that limits patches in early to mid-seral condition to no greater than 5 ha.
Old-seral stage retention and representativeness (NDT5)
- To retain old seral stages, 85% of each ecosystem in this disturbance type should be maintained in late seral or climax condition. This late seral/climax component should be well dispersed throughout the landscape unit, across the range of ecosystem types that occur in the biogeoclimatic subzone variant.
Landscape connectivity (NDT5)
Ecosystems in this disturbance type continue to exist as contiguous tracts of late seral to climax vegetation. Major stand-initiating events are still infrequent. Livestock grazing can reduce large portions to early or mid-seral states, but most areas within this type are not currently used to support livestock.
- No new areas should be approved for livestock grazing unless adequate measures of range condition and trend are established for the ecosystems present, and the impacts of the proposed livestock use are consistent with the biodiversity objectives for this NDT. If livestock use is very low and transitory, range condition and trend measures are not necessary.
- No new areas should be approved for livestock grazing unless approval is obtained from the Ministry of Environment, Lands and Parks.
The characteristics of natural mature/old-growth ecosystem connectivity described in other disturbance types do not apply to this disturbance type.
Stand structure (NDT5)
- Livestock grazing should be managed in a manner that does not lead to extensive trail creation or browse damage to shrubs and trees.
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