THE INTERPLAY OF FACILITATION AND COMPETITION IN PLANT-COMMUNITIES

If plants cannot simultaneously acclimate to shade and drought because of physiological trade-offs, then plants are expected to be less tole rant to shading under drier conditions. One observation that, at first sight, seems incompatible with this idea is the fact that the establi shment of new plants in dry areas is often restricted to shady sites u nder the canopy of other plants, called ''nurse plants.'' We use a gra phical model to resolve this paradox. The model visualizes how facilit ative patterns can be understood from the simultaneous effects of plan t canopies on microsite light and moisture, and the growth response of establishing seedlings to those factors. The approach emphasizes the fact that positive and negative effects of plant canopies always occur simultaneously. In the presented light-water model, facilitation only occurs when the improvement of plant water relations under the canopy exceeds the costs caused by lower light levels. This may be true unde r dry conditions, whereas in less dry situations, competition rather t han facilitation is observed. The model shows how changes in water ava ilability may shift interactions from competitive to facilitative and vice versa, as observed in some field patterns. It is argued that othe r environmental factors explaining facilitative patterns can be unders tood in the same context.