We present a quantitative literature review to assess the extent to which field experiments with plants have addressed questions about patterns of competition over time and space, consequences of competition for community structure, and comparisons of competitive ability among species. We outline the necessary treatment comparisons and statistical analyses to answer each question and then describe the number of experiments that meet these criteria and their results. Although we found a total of 101 experiments in 89 studies, 63% of these experiments only addressed whether competition significantly affected some component of individual fitness of a single species at a single time and site. Despite the limited data base to address more complex questions about competitive interactions, we did find consistent results for a few of the questions we reviewed. Where tested, competition always had significant effects on distribution patterns (five experiments), on relative abundances (two experiments), and on diversity (four experiments), consistent with the notion that competition has strong effects on community structure. On the other hand, intraspecific competition was not usually stronger than interspecific competition for either competitive effect (four experiments) or response (three experiments), which suggests that resource partitioning may not be an important mechanism of coexistence in plants.

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. I investigated variation in the effect of ants at extrafloral nectaries on plant reproductive success among three populations of Cassia fasciculata (Leguminosae) in northern Florida. Results show that the possession of extrafloral nectaries can lead to increased reproduction. Density of ants and removal of experimentally placed Drosophila larvae were far higher on C. fasciculata than on nonnectariferous background plants. Ants preyed on eggs and small larvae of lepidopteran herbivores. C. fasciculata plants from which ants were excluded had more larvae of the major seed pod predator Phoebis sennae (Pieridae; Lepidoptera), suffered higher pod predation (at one site), and had lower reproductive output (at two sites) than did control plants to which ants had access. Effects of extrafloral nectaries on reproduction of C. fasciculata varied among the three populations, due apparently to differences in density of ants and potential herbivores. At the site with the lowest density of ants and herbivores, pod set was not significantly different between control and ant exclusion groups. Where they were more numerous, ants had a small but significant positive effect. Only where the abundance of both ants and potential herbivores was high did ants visiting extrafloral nectaries strongly increase the reproductive success of individuals of C. fasciculata. The species of ants present and differences among herbivore species in susceptibility to ant attack may have also influenced the effect of extrafloral nectaries at any one site. Although the average effect of extrafloral nectaries throughout a plant species may be positive, the strength of this effect was found to vary widely among local populations.

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. This content downloaded from 139.184.Abstract. Recent models suggest that a trade-off in plants between tolerance of water limitation vs. tolerance of light limitation results in changes in dominant species over productivity gradients of increasing soil moisture and decreasing forest-floor light. With increasing elevation (1568-2296 m) in the Chiricahua Mountains in southeastern Arizona, soil moisture and plant cover increased and, as a result, mean forest-floor light levels decreased, in accordance with the models. The light-moisture trade-off hypothesis predicts that, over this gradient, (1) shade tolerance and drought resistance should be negatively correlated, (2) decreasing light and lack of shade tolerance (i.e., tolerance of light competition) should control upper elevational limits of species distributions, and (3) low soil moisture availability and lack of drought resistance should control lower elevational limits. With increasing elevation, however, fire frequency and litter depth also increased and soil temperature decreased. I tested the trade-off hypothesis and the role of these three additional factors in controlling upper elevational limits of three pine species distributed along this gradient.Consistent with the trade-off hypothesis, results suggested that water stress controlled lower elevational limits of all three species. Seeds of each species germinated with the summer rains in experimental plots below their respective lower elevational limits, but all seedlings died by the end of the following May-June drought, apparently from water stress. In contrast, seedlings were still alive in experimental plots within each species' range after 2 yr. Furthermore, with decreasing elevation, seedlings of the three species increasingly occurred in microsites with relatively low light, low soil temperature, and deep litter, all reflecting high soil moisture compared to random microsites. From the middle to the lower portion of each species' range, recruitment, seedling survival, and seedling abundance decreased but height growth increased. Thus, dry season water stress appeared to control lower elevational limits by causing high mortality of young seedlings, rather than by curtailing seed germination or the performance of older seedlings.Inconsistent with the trade-off hypothesis, upper elevational limits were not controlled uniformly across species by light limitation. In Pinus leiophylla, the middle elevation species, low light and deep litter appeared to control the upper elevational limits. In a field experiment, P. leiophylla emergence and survival were significantly lower above its upper elevational limit than in plots within its range, removal of litter increased em...

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