SWB – Spruce-Willow-Birch Zone

SWB – Spruce-Willow-Birch zone

By: Derek Marcoux R.P.Bio., Selkirk College

Introduction

The Spruce-Willow-Birch (SWB) zone occupies subalpine environments in northern British Columbia. It is situated above the Boreal-White and Black Spruce Zone. This zone replaces the Engelmann-Spruce Subalpine fir zone in northern latitudes. Elevational ranges are between 1000-1600m in the southern part of its range and 600-1400m in the northern part of the range (Banner at. al. 1993). Generally the climate consists of dry, very cold winters and cool summers. Mean annual precipitation is between 460 and 700mm with a mean annual precipitation of about –0.7 to-0.3oC (Meidinger and Pojar 1991).

There have been no formal subzones described for the SWB. However, Banner et. al (1993) has tentatively identified a very wet, cold (SWBvk) subzone in the Tatshenshini- Alsek region. This region is reported to have very snowy winters and is primarily non- forested.

Banner et. al. (1993) reports that soils in this zone are generally young and weakly developed Regosols because of relative recent glaciation. In addition, they are subject to a wide range of geologic process such as solifluction and cryoturbation, which constantly disturbs the soil. Podzols, brunisols and Luvisols are fairly common soils on zonal forested ecosystems. In addition, cryosols can occur in valleys subject to extensive cold air drainage.

Ecosystems

Forested ecosystems occur in the lower elevation areas and are dominated by white spruce, subalpine-fir and lodgepole pine. Trembling aspen and black spruce occur less frequently on these sites. With increasing elevation, subalpine-fir becomes the dominant tree species. In broad valleys where extensive cold air drainage occurs, a characteristic “double tree-line” occurs where trees are restricted to mid slopes because it is too cold for their growth requirements in the valley bottoms. In these situations lower slopes are dominated by a mixture of willows (Salix spp) and scrub birch (Betula glandulosa) intermixed with a mosaic of altai fescue (Festuca altaica) grasslands.

Meidinger and Pojar (1991) recognize four representative plant communities in this zone.

White Spruce-Grey-leaved Willow-Scrub Birch can occupy dry to fresh moderately well- drained sites. White spruce and subalpine fir dominate the overstory and grey-leaved willow (Salix glauca) and scrub birch occur along with shrubby cinquefoil (Potentilla fruticosa) can dominate in the understory. Additional species found in the understory are crowberry (Empetrum nigrum) twinflower (Linnaea borealis) and lingonberry (Vaccinium vitis-idaea).

1 Dry, south-facing grasslands are dominated by an association of glaucous bluegrass (Poa glauca) and northern wormwood (Artemesia campestris). These sites are found on steep colluvial and glaciofluvial slopes. Additional species on these sites can include Altai fescue, junegrass (Koeleria macrantha) and sedges (Carex spp).

Thickets in lower slope and depression positions can be occupied by an association of Barclay’s willow (Salix barclyi) and Glowmoss (Aulocomnium palustre). Commonly, these sites are occupied by a variety of other willow species such as small-flowered woodrush (Luzula parviflorus), palmate colt’s-foot and tall bluebells (Mertensiana paniculata).

Barclay’s willow, scrub birch and water sedge (Carex aquatilis) are a common association on wetlands. These sites often develop organic soils. Other species associated on these sites include red bearberry (Arctostaphylos rubra), bog cranberry (Oxycoccus oxycoccos) and cloudberry (Rubus chamaemorus).

Resources

Hunting, trapping and guide-outfitting are the main resource uses of the SWB. Hiking and camping in provincial parks comprise a smaller resource use of this zone. Forestry operations are virtually non-existent in this zone due to the lack of productive forestland and remote access to many areas.

References

Meidinger, D. and Pojar, J. 1991. Ecosystems of British Columbia. British Columbia Ministry of Forests. 330 pp. http://www.for.gov.bc.ca/hfd/pubs/Docs/Srs/SRseries.htm

Banner, A., W. MacKenzie, S. Haeussler, S. Thomson, J. Pojar, and R. Trowbridge. 1993. A field guide to site identification and interpretation for the Prince Rupert Forest Region. B.C. Min. For., Res. Br., Victoria, B.C., Land Manage. Handb. No. 26. http://www.for.gov.bc.ca/hfd/pubs/docs/lmh/lmh26.htm