Western spiderwort (Tradescantia occidentalis) COSEWIC assessment and status report: chapter 5

Habitat

Habitat Requirements

Western spiderwort sites are located on partly stabilized sand dune ridges, usually on steepsouth-facing slopes but also on blow-out areas (Figures 4 and Figure 5). There is usually some degree of active drifting sand associated with the sites. In Manitoba, the sand dune ridge systems can be fairly extensive. In Alberta and Saskatchewan, western spiderwort is restricted to small areas within larger partly stabilized dune complexes. Common species associated with western spiderwort sites include: sand grass (Calamovilfa longifolia), chokecherry (Prunus virginiana), poison ivy (Rhus radicans), rose (Rosa woodsii), pasture sagewort (Artemisia frigida), prairie sagebrush (Artemisia cana), ground juniper (Juniperus horizontalis), bluebell (Campanula rotundifolia), buckbrush (Symphoricarpos occidentalis), silverberry (Elaeagnus commutata), June grass (Koeleria macrantha), Indian rice grass (Oryzopsis hymenoides), needle-and-thread (Stipa comata), golden bean (Thermopsis rhombifolia), cushion cactus (Coryphantha vivipara), spear grass (Stipa viridula), sand bluestem (Andropogon scoparius), blue grama (Bouteloua gracilis), and brittle prickly-pear (Opuntia fragilis). Leafy spurge (Euphorbia esula) can be a problem introduced species as it flourishes on sandy habitat restricting native vegetation (Smith 2001, Goulet and Kenkel 1997).

Figure 4. Dune ridge habitat at Routledge Sand Hills, Manitoba (photo by B. Smith, July 1990).

Figure 4. Dune ridge habitat at Routledge Sand Hills, Manitoba (photo by B. Smith, July 1990).

Figure 5. Sand hills habitat at Routledge Sand Hills, Manitoba (photo by B. Smith, July 1990).

Figure 5. Sand hills habitat at Routledge Sand Hills, Manitoba (photo by B. Smith, July 1990).

In Manitoba, species that were found mainly in the spring include June grass, hairy golden-aster (Heterotheca villosa), yellow flax (Linum rigidum) and low townsendia (Townsendia exscapa). Dead grass cover was high in the spring. Species that were more abundant in the late summer include sand bluestem, spear grass and sand dropseed (Sporobolus cryptandrus). Typical species of sand dune habitat in Manitoba include the following: spear grass, sand bluestem, blue grama, chokecherry, roses, and bearberry (Arctostaphylos uva-ursi). Skeltonweed (Lygodesmia juncea) is characteristic of more open dune areas. Creeping or ground juniper can cover extensive dune areas. Open, stabilized and ungrazed dunes (Hellman ESA site) are characterized by high cover of the moss Tortula ruralis, which form a stabilizing 'crust' on sand. The moss is uncommon on grazed areas since trampling by cattle easily breaks up the moss 'crust'. Other characteristic species of Manitoba sand dunes include wolf willow, purple prairie clover (Petalostemon purpureum) and goldenrod (Solidago missouriensis). Manitoba populations of western spiderwort are found on coarse, sandy soils containing very little silt and clay (Goulet and Kenkel 1997).

Vegetation composition at the Loutit site in the Lauder Sand Hills differs from the other two Manitoba sites. Grass cover is lower, and tree-shrub encroachment onto the dunes is widespread. Destabilized open sand 'bowls' are common in the northern and southern sections of the property (Goulet and Kenkel1997).

The dune habitat differs among the three Manitoba sites. The Hellman site, in the Routledge Sand Hills, is divided by natural shrub and forest breaks between the sand dunes, and by property boundaries. The main dune ridge on the Hellman property runs southeast to northwest. The sand dunes at the MHHC site in the Lauder Sand Hills are generally smaller and broader than those of the Hellman site. The dunes run from north to south, curving at both ends to form an enclosed circle. The sand dunes at the Loutit site, also in the Lauder Sand Hills, run continuously from north to south on the property, over two quarter sections. Many of these dunes are high (12 m or more) and steep (60% slopes) (Goulet and Kenkel 1997).

Climatic factors are probably critical to the distribution of western spiderwort at the northern limit of its distribution. Above average moisture availability in the summer months increases both population size and length of the flowering period. Plants in moist, well-drained sites are generally more robust than those found in excessively drained habitats (Smith and Bradley 1990, Hohn 1994, Goulet and Kenkel 1997).

Field observations indicate that western spiderwort is most commonly encountered in areas where there is considerable snow accumulation in winter, and where warm and sunny conditions prevail in summer (Goulet and Kenkel 1997).Sufficient and continuous winter snow cover is probably essential to the survival of western spiderwort populations. The overwintering of above-ground shoots undoubtedly require a protective layer of snow to survive the severe Manitoba winters. Since the prevailing winter winds in Manitoba are from the north, the south-facing dune slopes collect and accumulate a considerable amount of blowing snow in winter. This deep snow layer is a particularly effective insulator. In the spring, snow on the south-facing slopes melts earlier and more quickly than that on north-facing slopes, thus extending the microenvironmental 'growing season' for western spiderwort

Trends

Alberta habitat appears to be disappearing or substantially diminishing as a result of dune stabilization by shrubby vegetation (Smith 2001B). Saskatchewan habitat is stable as is Manitoba habitat due to the continued formation of blow-out areas required for colonization.

Protection/ownership

The Alberta population occurs on Crown land that is presently under a grazing lease (Smith and Bradley 1990). The Saskatchewan population occurs on a managed community pasture near the boundaries of Douglas Provincial Park. Douglas Provincial Park allows cattle grazing (Godwin and Thorpe 1991).

The Ecologically Significant Area (ESA) at the Routledge Sand Hills affords a degree of protection to Manitoba western spiderwort populations and habitats. The ESA is fenced off, and permission is required to enter the area. Unfortunately, enforcement is difficult as the owners do not reside on the property. Most of the rest of the Routledge Sand Hills population is subject to cattle grazing and/or human disturbance (mainly motorized vehicles). The MHHC (Manitoba Habitat Heritage Corporation) (Lauder Sand Hills) is managed by an absentee landowner. Permission is required to gain access to the land, but this regulation is not enforced. The site is very accessible as it is adjacent to Provincial Road 345. The eastern population in the Lauder Sand Hills is used as pasture land for cattle (Goulet and Kenkel 1997).

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