The migration of vegetation north during deglaciation does not occur in a smooth fashion. There is a "lag" period which starts to occur after the initial pioneer plant is established on the newly deglaciated land (M.J. crawley, 1990). As the climate continues to warm, the new migrating species must now compete with the already well established pioneer plants for space. The migrating species must take advantage of gaps in the existing vegetation. Unfortunately, it is unlikely that a migrating species would be able to move into an area dominated by another species. Plants have a strong will to persist in an area even if the climate has become unfavorable. As a result, plants may persist in an area until some natural disaster, such as a forest fire, destroys large areas of the species (E.C. Pielou).
Due to the delay in response time, shorter-lived climate changes may not actually result in a significant change in vegetation. This may cause a "damped response" where the vegetation looks as thought it has changed steadily over time, not indicating a response to the numerous minor glacial reversals which have been documented.