Neighbor influences and seasonal growth patterns for Vallisneria americana in a mesotrophic lake.

Biomass of the submersed macrophyte Vallisneria americana rose steadily from early June to 50 g dry wt·m in ealry August. Leaves dominated the biomass of this perennial from mid-June through August, after which plants senesced to leave only winter buds as perennating organs. Only 24% of plants sampled flowered during the 1978 growing season, yielding a population mean of less than 0.6% of dry weight allocated to sexual reproduction, with a maximum value of 7.8% for one plant. In contrast, allocation to winter buds accounted for 11% of total dry weight at the close of the growing season.Total dry weight, leaf length, stolon length, rosette number, and winter bud production of Vallisneria americana at the close of the growing season had significantly higher mean values for individual plants in a monospecific stand of Vallisneria than at sites dominated by Potamogeton amplifolius or Chara vulgaris. The basis of site differences in growth by V. americana is unknown, although we demonstrate significantly higher interstitial soluble reactive phosphorus concentration and organic content of sediment at a Vallisneria site.Removal of neighbors of some Vallisneria plants at the Vallisneria, Potamogeton, and Chara sites resulted in longer leaves, but fewer rosettes for plants with neighbors than plants without neighbors. It appears that this changing growth pattern permitted Vallisneria plants to compensate for the presence of neighbors, and thereby attain similar biomass and sustain similar winter bud production in the presence and absence of neighbors.