Changes in the understory vegetation following a water level rise resulting in extensive tree mortality were examined in two zones in Bryant’s Bog. Tree mortality was especially severe in the outer zone which was located on a grounded mat and had soil water levels very close to the surface. Conditions were more moderate in the inner zone which was located on a free-floating mat. In a five-year period following the bulk of the tree mortality, the understory vascular plants and bryophytes increased substantially in total cover and frequency throughout the bog. Much of the increase in vascular cover was due to leather-leaf which increased considerably in cover and reached a frequency of 80% or more in both zones. Most other vascular plants, however, remained unchanged, whereas water-horehound decreased. Much of the change in total bryophyte cover was due to Sphagnum spp. which increased substantially in cover and reached a frequency of 100% in both zones. The results indicate that communities dominated by leather-leaf and Sphagnum spp. can develop rapidly from treed bogs, following rises in water levels, tree mortality caused by reduced substrate aeration, and subsequent increased light.