Ecology of rare vascular plants in Soutwestern Saskatchewan
Robson, Diana Bizecki
Rare plant species have small populations and limited distributions but make significant contributions to the richness of biodiversity in the grasslands of western Canada. Ecology of rare vascular plants was examined as part of the Prairie Ecosystem Study (PECOS), a multidisciplinary study on the sustainability of biophysical resources and human communities in southwest Saskatchewan. Land use practices can have significant impacts on rare plant populations in the PECOS study area (a 15,000 km2 area in the Agricultural Census region 3BN). Assessment of the extent of these impacts requires better information on the numbers, distribution and ecology of rare plant species. This study was designed to collect basic population data and characterize habitats of rare plants in the PECOS area. Rare plant sites were selected by examining specimens in the W.P. Fraser Herbarium at the University of Saskatchewan. In total 42 rare plants were discovered within the boundaries of the PECOS area in the last century. During two summers of field research, 22 of the rare plants were found. Of the 113 reported rare plant populations in the study area, 63 were re-located. These 63 populations were found at 50 different sites. Some rare plant populations could not be relocated. The original herbarium record may have been poor or, the plant may have been inconspicuous or temporarily in the seed bank, making the population difficult to find. In several cases, the plant disappeared from the original site due to destruction or alteration of habitat. Sometimes the habitat was intact but the plant was not present. In four cases, the population was not found because landowners would not allow access to their property. At each site where a rare plant was found, the size of the plant population was determined, the slope/aspect were measured, the position and habitat were described, and the soil and vegetation were sampled. Latitude and longitude were recorded using a Global Positioning System. The soil samples were used to determine bulk density, texture, pH and conductivity. Soils, vegetation and habitat data were used in Two-way Indicator Species Analysis (TWINSP AN) and Canonical Correspondence Analysis (CCA). Landscape and soil maps were used to characterize the generalized landscape type and habitat at each site. Rare plant communities in slough and sand dune habitats were unique resulting in their early separation in both TWINSP AN and CCA analysis. Sites in riparian areas and hill lands shared many plant species. Sites in these areas were grouped closely together in statistical analysis. Environmental variables that most influenced the CCA ordination were sand content of soil, position of the plant on a slope and aspect. Conductivity, silt content of soil and slope also influenced the CCA, but to a lesser degree. Rare plants were found in six landscapes: wetlands, drylands, plains, hill lands, bedrock uplands and valleys. Within the six landscapes, rare plants were found in ten habitats: aquatic habitats, slough edges, moist draws, valley breaks, moist valley flats, mobile dunes, dune blowouts, morainal hills, dissected uplands and disturbed areas. Some rare species were found in the same habitats, suggesting that they may be rare for similar reasons. Rare plants were found in eight different soil associations: Haverhill, Chaplain, Weyburn, Dune Sand, Exposure, Alluvium, Hillwash and Runway. Rare plants were most often found in valleys, active dune complexes, wetlands and hilly areas with regosolic or miscellaneous soils. Any area that contains these features is likely to contain rare plants. Ruderal plants that colonize disturbed areas are the exception; they can occur in many different landscape types. Rare plants are currently threatened by a variety of direct and indirect human activities. Direct activities are those that destroy habitat while indirect activities alter the habitat in some manner. Conservation of these species will require habitat protection, land management, and continued research and monitoring.