JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. Ecological Society of America is collaborating with JSTOR to digitize, preserve and extend access to Ecology. Abstract. Interactions among organisms take place within a complex milieu of abiotic and biotic processes, but we generally study them as solitary phenomena. Complex combinations of negative and positive interactions have been identified in a number of plant communities. The importance of these two processes in structuring plant communities can best be understood by comparing them along gradients of abiotic stress, consumer pressure, and among different life stages, sizes, and densities of the interacting species. Here, we discuss the roles of life stage, physiology, indirect interactions, and the physical environment on the balance of competition and facilitation in plant communities. Bertness, M. D., and S. W. Shumway. 1993. Competition and facilitation in marsh plants. American Naturalist 142: 718-724. Bertness, M. D., and S. M. Yeh. 1994. Cooperative and competitive interactions in the recruitment of marsh elders. Ecology 75:2416-2429. Billick, I., and T. J. Case. 1994. Higher order interactions in ecological communities: what are they and can they be detected? Ecology 75:1529-1543. Callaway, R. M. 1992. Effect of shrubs on recruitment of Quercus douglasii and Quercus lobata in California. Ecology 73:2118-2128. . 1994. Facilitative and interfering effects of Arthrocnemum subterminale on winter annuals in a California salt marsh. Ecology 75:681-686. . 1995. Positive interactions among plants. Botanical Review 61:306-349. Callaway, R. M., and M. D. Bertness. 1994. Gradients of physical stress and the relative importance of facilitation and interference. ISEM Symposium: facilitation, nurse plants, and nucleation: vegetation cluster processes driving plant succession. E. J. Rykiel Jr. (organizer). Program and Abstracts, 79th Annual ESA Meeting, Knoxville, Tennessee, USA. Callaway, R. M., E. H. DeLucia, D. Moore, R. Nowak, and W. H. Schlesinger. 1996. Competition and facilitation: contrasting effects of Artemisia tridentata on Pinus ponderosa and P. monophylla. Ecology 77:2130-2141. Callaway, R. M., and L. King. 1996. Oxygenation of the soil rhizosphere by Typha latifolia and its facilitative effects on other species. Ecology 77:1189-1195. Callaway, R. M., N. M. Nadkarni, B. E. Mahall. 1991. Facilitation and interference of Quercus douglasii on understory productivity in central California.

During succession, ecosystem development occurs; but in the long-term absence of catastrophic disturbance, a decline phase eventually follows. We studied six long-term chronosequences, in Australia, Sweden, Alaska, Hawaii, and New Zealand; for each, the decline phase was associated with a reduction in tree basal area and an increase in the substrate nitrogen-to-phosphorus ratio, indicating increasing phosphorus limitation over time. These changes were often associated with reductions in litter decomposition rates, phosphorus release from litter, and biomass and activity of decomposer microbes. Our findings suggest that the maximal biomass phase reached during succession cannot be maintained in the long-term absence of major disturbance, and that similar patterns of decline occur in forested ecosystems spanning the tropical, temperate, and boreal zones.