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 tolerant to shading under drier conditions. One observation that, at first sight, seems incompatible with this idea is the fact that the establishment of new plants in dry areas is often restricted to shady sites under the canopy of other plants, called ''nurse plants.'' We use a graphical model to resolve this paradox. The model visualizes how facilitative patterns can be understood from the simultaneous effects of plant 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 under dry conditions, whereas in less dry situations, competition rather than facilitation is observed. The model shows how changes in water availability may shift interactions from competitive to facilitative and vice versa, as observed in some field patterns. It is argued that other environmental factors explaining facilitative patterns can be understood in the same context.