The influences of site conditions, age and disturbance by wildfire or winter logging on the species composition of naturally regenerating boreal plant communities and some implications for community resilience
Ehnes, James W.
Few boreal studies have assessed the sustainability of logging by directly comparing post-fire and post-logging vegetation dynamics beyond the first 15 years. This study compared the species composition of naturally regenerating plant communities grouped by site type, age (13, 37 and 65 years) and disturbance type (wildfire or winter logging). Differences in species composition were used to assess the relative influence of these grouping variables and community resilience to wildfire and logging. Replicate burns or cutovers were randomly selected in eastern Manitoba and plots located within them using probabilistic methods. Data relating to understory cover, trees, soils and topography were collected. Each plot was classified into a site type category. Criteria were developed to identify species which had a substantial site type, age or disturbance type difference in performance. Community resilience was measured by calculating the percent dissimilarity of the species composition of 13 and 37 year old post-fire and post-logging communities with that of mature (65 year old) post-fire communities. The strongest influence on species composition was site type followed by fire. Communities exhibited greater short-term resilience to logging than to fire but this situation reversed itself over the medium-term. Post-fire recovery between 13 and 37 years was rapid, while in post logging communities several species tolerant of moderate to high light intensity and low nutrients maintained or expanded their cover. By 37 years most of the species which performed more poorly in one disturbance type did so in post-logging communities. Most of the poorer performers were herbs or shade intolerants which typically require a moderate to rich nutrient regime. The poorer performance of herbs in 37 year old post-logging communities could not be attributed to differences in the initial direct effects of fire and logging. Based on the species which had performance differences, it appears that logging may have had negative medium-term impacts on site fertility. Also, a number of species seem to require some of the fire's effects to maintain their abundance and distribution on the landscape. This has important implications for the conservation of ecosystem diversity.