Long-term changes in abundance-occupancy relationships and trembling aspen cover and density with prescribed burning in the Fescue Grasslands of Prince Albert National Park

Abstract

Loss of Plains Rough Fescue (Festuca hallii (Vasey) Piper) Grassland to woody species encroachment is a significant conservation concern that threatens remaining Fescue Prairie. Outlying patches of Fescue Prairie exist among predominantly trembling aspen (Populus tremuloides Michx.) forest in Prince Albert National Park (PANP), Saskatchewan. These Fescue Grassland patches have shrunk considerably over time through periods of fire suppression and prescribed burning; however, the effects of different season, number, and time since burning treatments used to control trembling aspen encroachment in the Fescue Grasslands of PANP have not been studied. The overall objective of this research was to determine if succession after burning shapes the relationship between the density and distribution of species (abundance-occupancy relationship) within and between plant communities, and to determine if prescribed burning is effective in controlling trembling aspen encroachment in the Fescue Grasslands of PANP. The relationship between species abundance and occupancy, and the density and cover of trembling aspen in grassland and forest transition (ecotone between Fescue Grassland and forest where trembling aspen encroachment was evident) plant communities was assessed using a 35-year dataset collected in PANP. Abundance-occupancy relationships with varying time since burning were examined to determine if they changed through time. Relationships between trembling aspen density and cover with varying season, number of burns, and years after burning were examined to determine if trembling aspen encroachment was suppressed with the prescribed burning. An increase in species abundance relative to occupancy was observed in 1983 and in 2010 in the grassland community, while abundance-occupancy relationships in the forest transition community did not change through time. Variability in abundance-occupancy relationships within and between plant communities suggests succession may play a role in shaping abundance-occupancy relationships. None of the burn treatments were effective in controlling trembling aspen; rather they promoted increased density through vegetative reproduction. Trembling aspen density increased after burning, and was likely caused by consumption of aboveground structures, removing apical dominance that stimulates intense vegetive suckering. Through time, the high density of trembling aspen suckers that initiated after burning likely self-thinned as cover increased. Spring burning increased density of trembling aspen more than fall burning; however, trembling aspen cover was greater with fall burning than spring burning over time. Cover of trembling aspen increased through time in both the grassland and forest transition communities, and was greater with more burns in the forest-transition community. It appears the forest transition and grassland communities in the Fescue Grasslands of PANP have changed considerably over time, and require prompt management adjustments if trembling aspen encroachment and loss of Fescue Grassland is to be controlled. Prescribed burning for the conservation of these Fescue Grasslands is ideal, but should not be practiced in isolation without secondary control of trembling aspen suckering. Modeling management practices from historic grazing and fire interactions may improve the control of trembling aspen encroachment in the Fescue Grasslands of PANP.