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Influence of fire, logging, and overstory composition on understory abundance, diversity, and composition in boreal forests, Ontario, Canada

Posted on:2007-09-20Degree:M.Sc.FType:Thesis
University:Lakehead University (Canada)Candidate:Hart, Stephen AFull Text:PDF
GTID:2453390005488552Subject:Forestry
Abstract/Summary:
Understory vegetation is the most diverse and least understood component of North American boreal forests and are important as they influence overstory succession and nutrient cycling. The objectives of this thesis were to (1) review the current understanding of boreal understory vegetation dynamics in the literature and (2) examine the effect of stand age, overstory composition, and logging versus fire on understory vegetation communities in northwestern Ontario, Canada.;Understory vegetation communities were sampled in 68 forest stands of conifer, mixedwood, and deciduous overstory types ranging from seven to 201 years since fire for fire origin stands, and 7 to 31 years old for logging origin stands. For fire origin stands, total species richness and cover followed similar trends for the three overstory types and were highest in the intermediate aged stands (72-90 years old). Trends in richness and cover, however, differed significantly for vascular and nonvascular plant groups. Vascular plant diversity and cover was generally higher under deciduous stands, and was lower on older stands, while nonvascular species diversity was highest under conifer stands and increased with time since fire. Neither alpha nor beta diversity was higher under mixedwood stands while mixedwood stands were compositionally intermediate to conifer and deciduous stands. Multivariate analysis using multiple response permutation procedures indicated that understory communities were compositionally distinct for all overstory types and showed no convergence with increasing time since fire. Compared with post-fire stands of similar ages, post-logged stands had similar total understory cover and diversity. Vascular cover and richness, however, were higher on post-logged stands, and nonvascular cover and diversity lower. Stands of logging and fire origin were compositionally distinct for all overstory types and ages. Compositional differences appeared to be driven by higher levels of preestablished rhizomatous species and the prevalence of pleurocarpous mosses on fire origin stands.;Understory vegetation communities in the central boreal shield appear to support the intermediate disturbance hypothesis. Understory richness, however, was not negatively affected by high cover values as predicted by the intermediate disturbance hypothesis. Moreover, richness appears to be highest on sites with high resource availability, suggesting that boreal understory communities are influenced more by plant tolerances for low resources, rather than competition.;Understory vegetation abundance and diversity increase rapidly after fire, in response to abundant resources and an influx of disturbance adapted species. The highest diversity occurs within the first 40 years after fire, and declines indefinitely thereafter as a result of decreasing productivity and increased dominance of a small number of late successional pleurocarpous mosses and woody plant species. Vascular plant and bryophyte/lichen communities undergo very different successional changes. Vascular plant communities are dynamic and change more dramatically with time since fire, whereas bryophyte and lichen communities are much slower to establish and change over time. Considerable variations exists depending on canopy composition, site condition, regional climate, and frequently occurring non-stand-replacing disturbances. Forest management practices represent a unique disturbance process and can result in different understory vegetation communities from those observed for natural processes, with potential implications for overstory succession and long-term productivity. Because of the importance of understory vegetation on nutrient cycling and overstory composition, post-harvest treatments emulating stand-replacing fire are required to maintain understory diversity, composition, and promote stand productivity in boreal forests.
Keywords/Search Tags:Understory, Boreal forests, Fire, Diversity, Composition, Overstory, Stands, Logging
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