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Biodiversity Guidebook Table of Contents]

Natural disturbance type 5:
alpine tundra and subalpine parkland

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.

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.

Recommendations:

Temporal and spatial distribution of the cut and leave areas (NDT5)

Recommendation:

Old-seral stage retention and representativeness (NDT5)

Recommendation:

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.

Recommendations:

The characteristics of natural mature/old-growth ecosystem connectivity described in other disturbance types do not apply to this disturbance type.

Stand structure (NDT5)

Recommendation:


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