Print PageThe ability to predict natural processes based on intrinsic biological factors of species has received much attention within ecologically-based management of biological communities. In this study I determined whether deterioration of mature canopy forests plays a role in influencing the probability of forest encroachment of grasslands. In particular I examined and tested the influence of aspen stem density on the probability of encroachment within 34 forest-grassland sites located in Prince Albert and Riding Mountain National Park, Canada. In addition I also examined the consequences of forest structure within encroaching and non-encroaching forest stands on the abundance and diversity of understorey species. I hypothesize that a reduction in stem density (mortality of mature stems, loss of apical dominance) facilitates both stand persistence and encroachment of aspen suckers into grasslands. I also hypothesize that the mortality-regeneration cycle of trembling aspen has predictable effects on the composition and diversity of savanna plant communities.
Results/Conclusions
My results indicate a significant inverse relationship between forest stem density and the probability of forest encroachment. Results are consistent with other studies that have found that during dieback aspen undergoes a secondary wave of recruitment; this process may be on going so long as the clone is alive and suggest that periodic dieback plays a role in episodic recruitment and subsequent clonal growth. When coupled with other regulatory factors the effects of aspen clonal expansion may be dampened or amplified. Canopy dieback not only influenced post-disturbance recruitment in aspen but also altered the assemblage and diversity of understorey species. Results from both principal coordinates and multivariate dispersion analysis reveal a significant increase in alpha and beta-diversity measures within the mature core forest plots for stands under going encroachment.
Differences in the degree of canopy dieback at the landscape-level may explain observed spatial variation in aspen encroachment in the western range of trembling aspen. In the absence biotic stressors such as fungal pathogen infection, the aspen mortality-regeneration cycle is an effective mechanism for ensuring long-term persistence of the species. Although the ecological consequences of aspen clonal biology are often ignored or downplayed, they are critical to achieving a conceptual and practical understanding of the persistence and dynamics of trembling aspen stands throughout the species’ range.
