Small-scale plant species composition and diversity in relation to environmental and disturbance factors in fescue grasslands of the aspen parkland of Alberta

A comprehensive study was conducted in the summer of 1996 in remnant grasslands across the aspen parkland in order to associate plant species composition with environmental conditions and small-scale disturbances, and to relate plant species diversity to the levels of disturbance. A total of 490 1m{dollar}sp2{dollar} quadrats were established in 11 grassland remnants. For each quadrat, percentage cover of plant species, litter height, percentage cover of exposed mineral soil, slope and aspect were recorded, and a soil core was collected for soil analyses. Each quadrat was assigned to one of ten levels of disturbance. Direct gradient analyses related species composition to environmental and disturbance variables. Different diversity indices were calculated and related to the disturbance levels; polynomial regressions were used to test the Intermediate Disturbance Hypothesis. Direct gradient analyses indicated that disturbance is a major factor influencing small-scale plant species composition in remnant grasslands. Festuca hallii dominates undisturbed areas, while species composition in disturbed areas depends on the magnitude of the disturbance and environmental conditions, such as moisture, sodicity, and nitrogen levels. It was demonstrated that the majority of exotic species prefer disturbed and moist areas. The Intermediate Disturbance Hypothesis was supported regardless of the measure of diversity used. Richness of exotic (not native to North American continent) vascular plant species showed a significant but weak positive relationship with the magnitude of the disturbance; native species richness predominated over exotic species richness at all levels of disturbance. It is also suggested that the assembly of plant species in fescue grasslands may follow the "centrifugal organization model" previously discussed only for forest and wetland communities. Festuca hallii dominates the relatively undisturbed ("core") habitat, while the composition of plants in the "peripheral" habitats depends on the magnitude of the disturbance and environmental conditions. Finally, recommendations regarding management of remnant grasslands are given.