Smooth goosefoot (Chenopodium subglabrum) COSEWIC assessment and status report: chapter 8
Limiting Factors and Threats
The Grassland Natural Region is one of the most threatened natural regions in Canada. Over two-thirds of the Mixed Grassland has been lost to cultivation or other development (Wallis 1987). Although some sandy plains have been cultivated, much of the sand hills in the prairies is still intact. The main limiting factors affecting Chenopodium subglabrum are its natural narrow preference for unstabilized sites within dune fields and loss of natural habitat through succession. Invasive species, oil and gas exploration and extraction, and to a lesser extent recreation are also potential threats (Wallis 1987; Robson 1997a; Lamont and Gerry 1998).
Conversion to cropland
Although at present most C. subglabrum populations occur on active sand dunes and blowouts, in the past this species may have been common on sand plains or low sand hills that were periodically denuded of vegetation by migrating bison and prairie dogs (Fahnestock and Detling 2002). A population of C. subglabrum was found on a sandy plain southeast of Swift Current near the town of McMahon prior to 1940. Most of this area has since been cultivated representing a loss of habitat for this species.
Dune stabilization
Rare, threatened and endangered plants have been studied to map their distribution and assess the degree to which dune stabilization was occurring and how this was affecting native plants. While the exact mechanisms are unclear, a combination of drought conditions and land use appear to be influential (Hugenholtz and Wolfe in press, Wolfe et al. 2001, Wolfe et al. 2002). Large areas of once active sand have become stabilized over the last fifty years. If the current trends continue, rare native plants that now have dangerously low populations could be eliminated entirely.
In Alberta, sand hills comprise 19,080 km² but only 1,652 km² occur south of the 52nd parallel where C. subglabrum has been observed before (Wolfe 2001). The active sand surface of some dunes in the Milk River Sand Dunes has been reduced by 50% to 75% over 40 years (Wallis 1988). Continued stabilization of the dunes at this site would likely be detrimental to the long-term survival of C. subglabrum (Wallis and Wershler 1988).
From the 1950 to 1987, there has been a 30 to 40% reduction in active sand at Dune Point with invasion by Salsola kali into the gravelly sands. A series of active dunes stretched virtually unbroken for 2-km along the South Saskatchewan River in 1950 – today all these dunes are stabilized and there are only minor active blowouts. All 16 sand blowouts at Remount Community Pasture, which were active in 1950, are now stabilized. Of 51 blowouts active in 1950 in the Middle Sand Hills, only 20 are still active and, of these, 10 are partly stabilized and 7 are mostly stabilized. In total, 90% of the sand that was active in the Middle Sand Hills in 1950 is now stabilized (Wallis 1988).
In Saskatchewan sand dunes cover 13,010 km² but only 3,418 km² of them occur south of the 52nd parallel (Wolfe 2001). Of the 18 sand hills in the southern portion of the province, 10 have active dunes (Wolfe 2001). In addition to the sand hills, there are also several undifferentiated eolian deposits such as minor dunes, sand sheets, loess and deflation plains (Wolfe 2001). These areas, if some sand is exposed, also represent potential habitat for C. subglabrum.
The Dundurn and adjacent Pike Lake Sand Hills near Saskatoon, Saskatchewan have mostly been stabilized by vegetation (Wolfe et al. 2002). Small areas still exist where wind erosion and deposition are altering landforms, particularly under disturbed conditions such as those incurred under heavy grazing in times of drought. Earlier aerial photographs (1944) reveal areas of active dune complexes more extensive in the past (Pylypec 1989).
The Great Sand Hills of Saskatchewan are located west of Swift Current near the Alberta border. Tall, stabilized dunes cover the largest area – 50% of the dune field – and are the most sensitive to disturbance (Epp 1980). Active complexes are the least extensive comprising less than 1% (Wolfe et al. 2001). Hugenholtz and Wolfe (in press) estimate that stabilization rates in the Great Sand Hills have ranged from 7.2 to 10.5 ha/yr in the northern portion and 1.3-1.4 ha/yr in the southern portion since 1946.
Smaller sand hills in Saskatchewan where active dunes are still present include: Bigstick-Crane Lakes, Birsay, Burstall, Cramersburg, Elbow, Piapot, Seward, and Tunstall Sand Hills. These areas contain active dunes and dune blowouts. Stabilization rates from 1944 to 1996 in the Elbow Sand Hills have ranged from 0.4 to 1.9 ha/yr (Hugenholtz and Wolfe in press). In the Seward Sand Hills stabilization rates from 1970-1991 were estimated to be 1.2 to 3.8 ha/yr (Hugenholtz and Wolfe in press). The Tunstall Sand Hills stabilization rates from 1969-1991 were estimated to be 0.6 to 3.6 ha/yr (Hugenholtz and Wolfe in press).
In Manitoba sand hills cover 1,224 km² (Wolfe 2001). Only one of the six sand hills has active dunes (Wolfe 2001). The other sites have exposed sand as part of dune ridges or blowouts. In the Brandon Sand Hills 50% of the southern part was active in 1947 but only 25% in 1990. Brandon Sand Hills’ stabilization rate from 1928-1990 has ranged from 1.8 to 17.7 ha/yr (Hugenholtz and Wolfe in press; Wolfe et al. 2000).
Encroachment of vegetation on active blowouts could eliminate major and minor populations of C. subglabrum. The Turin site in Alberta is the only one with some active dunes present. Sites in Saskatchewan where the amount of open sand is extremely small due to stabilization include: Beaver Creek, Broderick, Caron and McMahon. At these sites the populations of C. subglabrum are very small and in danger of disappearing altogether. Populations in the Aspen Parkland (i.e. Routledge and Dundurn Sand Hills) are particularly imperiled as the amount of open sand covers a small area and woody vegetation continues to encroach on the habitat. Wolfe and Thorpe (2005) speculate that climate change may result in a potential increase in the susceptibility of sand hills to erosion, possibly reversing this stabilization trend.
Grazing and fire control
The large tracts of uncultivated grassland east of the mountains, mostly community pasture or crown land leased by ranchers, are grazed by livestock (Bird 1988). Recent research suggests that fire control and grazing may have an impact on dune stabilization (Hugenholtz and Wolfe in press). A combination of fire and grazing during appropriate seasons helps keeps blowouts active. Dunes have been stabilizing in the Middle Sand Hills where there have been repeated fires but little grazing and in other areas where there has been grazing but few fires (Wallis 1988). The positive or negative impacts of grazing at different seasons are unknown (Wallis and Wershler 1988). A current hypothesis is that late summer or fall fires formerly created lush green areas the following spring. These green patches attracted large herds of grazing animals like bison and resulted in reactivation of the sand dunes. The sand hills were also used for shelter by bison during the winter and this could have been significant in keeping dunes active. Fire control and changes in grazing patterns have completely changed the factors that shape sand dune environments (Wallis 1988).
Recent research suggests that native annual forbs benefit from intensive grazing activities by cattle (Hayes and Holl 2003), bison and prairie dogs (Fahnestock and Detling 2002). Grazing reduces grass cover, providing more habitat for forbs, particularly rare annual ones.
There is some evidence that animals graze on C. subglabrum. The single plant found at Lonesome Lake, Alberta in 1988 was severely browsed. Browsed plants were observed at two of the Seward Sand Hills sites and one of the Great Sand Hills sites in Saskatchewan. Trampling by cattle may dislodge some C. subglabrum plants. Cattle and wildlife tracks, and some trampled plants were observed at most sites visited. A dugout (i.e. an excavated pit to catch water for livestock) placed next to an active dune at the Turin site attracted numerous cattle to the dune. Heavy summer use by cattle at this site may cause some trampling damage (Wallis and Wershler 1988).
Invasive weeds
Exotic invasive weeds are becoming more common in dune areas and have the potential to usurp C. subglabrum habitat (Robson 1997a). The dune slack at the Barnwell, Alberta site is being invaded by Agropyron cristatum and Melilotus spp. (Wallis and Wershler 1988). At Lost River, the exotics Agropyron cristatum, Chenopodium album and Salsola kali are encroaching on C. subglabrum habitat (Elchuk pers. comm. 2004). Lamont and Gerry (1998) observed the exotics Salsola kali and Bromus tectorum at several sites during their surveys but not in the exact same spot where C. subglabrum was growing. In the Routledge Sand Hills in Manitoba and the Pelican Lake Sand Hills in Saskatchewan (COSEWIC 2000), Euphorbia esula was growing in the dunes. Schmoller (2002) noted that the largest populations of C. subglabrum in North Dakota were also threatened by invading Euphorbia esula. Loss of habitat to exotics may lead to the loss of certain sites over time.
Oil and gas exploration and extraction
Since 1992, oil and gas exploration and extraction activities have expanded greatly in Saskatchewan, particularly in the Great, Cramersburg and Seward Sand Hills. Oil extraction infrastructure occurs immediately adjacent to the Routledge Sand Hills in Manitoba as well. Although rare plants surveys are typically conducted before any proposed developments, they may be conducted at times when C. subglabrum is not normally in flower. Further, annual plants like C. subglabrum may not grow every year, which would also result in the species not being detected (Robson 1997c). A gas well is now located in the same ¼ section as two of the dune blowouts containing C. subglabrum plants in the Seward Sand Hills. Oil or gas spills potentially endanger the plants at this location.
Ironically, oil exploration has created some habitat; several C. subglabrum plants were growing in soil that was cleared of vegetation to create access roads and oil well pads in the Seward Sand Hills. However, these plants may be driven over by trucks accessing the wells or subjected to damage from oil and gas leaks.
Tourism and recreation
Hikers and “dune surfers” use the dunes at Douglas Provincial Park, Spruce Woods Provincial Park and on the Sandhill Stockman’s Association land. Although All-terrain Vehicles (ATVs) are not supposed to be used on Sandhill Stockman’s Association land, some tracks were observed anyway. ATV use in sand hills may directly disturb C. subglabrum plants or alter the habitat (Schmoller 2002). Hiking is less of a concern since feet do not churn the soil the way a vehicle can. Chenopodium subglabrum is unlikely to be picked by tourists since it isn’t particularly attractive due to its small flowers and stature, but it may be trampled.
At the Caron site in Saskatchewan the Besant Campground lies within the sand hills area upon which C. subglabrum is known to occur. Given the small area of appropriate habitat at this site any intrusion would likely have a substantial impact (COSEWIC 2000).