Overabundant deer populations are a major factor affecting forest ecosystem dynamics in many parts of North America. Sitka black-tailed deer (Odocoileus hemionus sitkensis) were introduced to Haida Gwaii, a remote island archipelago in British Columbia, in the late 19th century. The mild climate, abundant vegetation, absence of predators, low hunting pressure and lack of competing herbivores on Haida Gwaii allowed the deer to flourish and the population exploded. This long-term monitoring experiment studies the effects of Sitka black-tailed deer on the diversity, abundance and reproduction of understory vegetation. In this study 20 deer exclosures were monitored over a period of 12 years from 1997 to 2009 in old growth forests on Graham Island, the largest island of the Haida Gwaii archipelago. The results showed that protection from deer did not have an effect on species richness. However, deer are having a significant impact on the abundance of understory vegetation in the medium height stratum (0.5 m - 1.5 m) and they are shifting the community composition towards less palatable species. The plant species most affected by deer browsing were Bunchberry (Cornus unalaschkensis) and Fern-leaved goldthread (Coptis aspleniifolia), and these are important forage species for Sitka black-tailed deer in their natural habitat in Alaska. Deer are also having an impact on plant reproduction, as a significantly higher proportion of flowers and fruits were found inside the exclosures. More research is needed to understand the impacts of deer on understory vegetation and therefore this study provides practical recommendations for improving this long-term monitoring experiment as well as recommendations for future research.