Positive-feedback Switches in Plant Communities

Abstract

This chapter discusses the positive-feedback switches in plant communities. A vegetation positive-feedback switch is a process in which a community modifies the environment, making it more suitable for that community. Positive-feedback switches operate by modifying any of several features of the environment, including water, pH, soil elements, light, temperature, wind, fire, or allelopathic toxins. The four types of switch can be distinguished as: (1) one-sided switch, where a single community modifies the environment of the patches it occupies, (2) reaction switch, where the community additionally modifies the patches it is not in, but in the opposite direction, (3) symmetric switch, where communities of both alternative states modify the same factor of their environment, but in opposite directions, and (4) two-factor switch, where the two communities both modify their environments, but in different factors. The positive-feedback switches producing four major vegetational effects (A–D): a stable vegetational mosaic may be produced in a previously uniform environment(situation A), or a vegetational gradient caused by environmental change can be intensified to give a sharp boundary (situation B). These mosaics and boundaries can occur at a wide variety of spatial scales, from landscape-scale to individual plant-scale. Switches can also sharpen or displace temporal boundaries: succession can be accelerated (situation C) or delayed (situation D). Not all of these effects can be produced by all types of switch; in particular, a one-sided (type 1) switch cannot produce a stable mosaic. © 1992, Academic Press Limited