Studies of changes in plant species composition have been underway since 1974 in three large Connecticut River oxbows of differing ages in western Massachusetts. Here we compare vegetation development in the youngest and oldest oxbows. Marshes at Hatfield in the most recently formed of the three oxbows occur in zones that parallel much of the old river bank, while marshes at Whately in the oldest oxbow are confined to the vicinity of a single pond. The major purpose of the present study was to examine the marshes of the oxbows and to compare the dominance (contribution to cover) and distribution of the vascular plant species present in 1974 with their dominance and distribution three decades later. Vegetation was sampled in three zones (high, mid, and low marsh) along permanent transect lines running perpendicular to the old river channel.
In both oxbows in 1974, high marsh was dominated by Onoclea sensibilis and low marsh by Lemna minor and Nuphar variegatum. While Onoclea sensibilis continues to dominate high marsh in Hatfield, and is still important in Whately high marsh, Impatiens capensis now dominates Whately high marsh. In the Hatfield low marsh Lemna minor and Nuphar variegatum continue to be important, but in the Whately low marsh, Leersia oryzoides and Typha latifolia have become prominent, suggesting that Whately low marsh is becoming more terrestrial. During the 30 year sampling period, species richness has increased in all three marsh zones in Hatfield, while richness has increased only in the low marsh zones in Whately.