Plant Strategies in Relation to Elevation and Light in Floodplain Herbs

Abstract

The distribution of herbaceous species in southern Wisconsin [USA] floodplain forests was examined in relation to 2 major environmental factors: elevation and light. Most species had definite elevation optima and could be categorized as either light specialists for high or low light, or light generalists. To test Grime''s ideas regarding the existence of 3 primary plant strategies (competitive, stress tolerant and ruderal), simple autecological indices were used to classify species into guilds. Higher elevation areas were dominated by tall competitive perennial forbs. As flooding frequency increased, smaller perennial forbs (stress-tolerant competitors) and tall annuals (competitive ruderals) increased in importance. In areas of greatest flooding frequency, the dichotomy in plant strategies was most extreme: slow-growing, flood-tolerant sedges and grasses (stress tolerators) shared the ground with small, fast-maturing, annual forbs (ruderals). Tolerance to flooding in sedges and grasses may be related to the position of their shoot apex, which affords protection from mechanical damage caused by turbulent flooding. Light was important in differentiating guilds (high light specialists, light generalists, small spring-flowering forbs) only when flooding was infrequent. Grime''s triangle of plant strategies maps onto axes of disturbance frequency and physiological tolerance, emphasizing the interaction of environment and adaptation in determining strategy. One environmental event, such as flood, fire, or drought, may be seen as a disturbance by short-lived ruderal species and as a stress by tolerant, long-lived perennials. Combinations > 1 important stress or disturbance further complicate Grime''s model.