study area : northwestern ontariounderstory 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 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.