Current concepts of the role of interspecific interactions in communities have been shaped by a profusion of experimental studies of interspecific competition over the past few decades. Evidence for the importance of positive interactions — facilitations — in community organization and dynamics has accrued to the point where it warrants formal inclusion into community ecology theory, as it has been in evolutionary biology.

https://www.cell.com/trends/ecology-evolution/pdf/0169-5347(94)90088-4.pdf

Positive interactions in communities.

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.

/pdf/competition-and-facilitation-a-synthetic-approach-to-51azsyo86c.pdf

Competition and facilitation: a synthetic approach to interactions in plant communities

Experimental evidence for positive interactions, or facilitation, among plants has increased markedly during the last 10 years. Experiments documenting facilitation have been conducted in many diverse ecological systems, which suggests that positive interactions may be fundamental processes in plant communities. Here, I review the evidence for facilitation, the mechanisms by which facilitation operates, and the effects facilitation has on community structure. Facilitative mechanisms may act simultaneously with resource competition or allelopathy, and the overall effect of one species on another may be the product of multiple, complex interactions. Positive interactions may also determine community spatial patterns, permit coexistence, enhance diversity and productivity, and drive community dynamics. Once viewed as anecdotal and idiosyncratic, facilitation is now contributing to a more complete understanding of community structure and dynamics.

Positive interactions among plants

What determines the number and size of the seeds produced by a plant? How often should it reproduce them? How often should a plant produce them? Why and how are seeds dispersed, and what are the implications for the diversity and composition of vegetation? These are just some of the questions tackled in this wide-ranging review of the role of seeds in the ecology of plants. The authors bring together information on the ecological aspects of seed biology, starting with a consideration of reproductive strategies in seed plants and progressing through the life cycle, covering seed maturation, dispersal, storage in the soil, dormancy, germination, seedling establishment, and regeneration in the field. The text encompasses a wide range of concepts of general relevance to plant ecology, reflecting the central role that the study of seed ecology has played in elucidating many fundamental aspects of plant community function.

The ecology of seeds

1 Although its importance for plant mineral nutrition and nutrient cycling has long been recognized, the soil community has rarely been integrated into dynamical frameworks of plant populations, in spite of abundant evidence for its involvement. The concept of feedback may provide theoretical and experimental tools for investigating the importance of the soil community in the population ecology and evolution of plants. 2 A mathematical model demonstrates the potential for two divergent dynamics, with positive feedback leading to the loss of diversity at a local scale and negative feedback leading to its maintenance. A linear contrast of the growth of plants in association with their own soil communities compared to the growth of plants in association with each others' soil communities can be used to differentiate between these possibilities in empirical studies. 3 Spatially explicit computer simulations demonstrate that the dynamics of a spatially structured community, as the soil community is likely to be, can differ from those predicted for a well-mixed population. Specifically, diversity can be maintained between locally homogeneous patches when positive feedback and dispersal occur at local scales. 4 Using a simple experimental protocol, we have found substantial negative feedback on plant growth through the soil community, suggesting that it may be involved in the maintenance of plant species diversity. 5 We discuss the importance of the soil community in other areas of plant ecology and evolution, including the suggestion that interactions with the soil community may be involved in the maintenance of sexual or asexual reproductive systems.

Incorporating the soil community into plant population dynamics: the utility of the feedback approach.

Historically facilitative interactions were widely believed to be important structuring forces in nature (Clements et al. 1928; Allee et al. 1949; Odum 1969). This point of view, however, has received little recent support (see Connell and Slayter 1977 and Boucher et al. 1982 for reviews of the evidence). In contrast, contemporary ecologists have devoted tremendous effort o documenting the role of competition in nature. Studies of the role of competition in natural communities have found it to be generally stronger and more important in communities under benign than harsh physical conditions (see, e.g., Connell 1972; Keddy 1989). Many ecologists, however, have cautioned that the relationship between interspecific interactions and environmental stress has not been given adequate attention (Connell 1975; Fowler 1986; Goldberg 1990; Dunson and Travis 1991). Moreover, a number of ecologists have suggested that facilitative or positive interactions among species may be more characteristic of harsh physical environments where neighbors can potentially buffer one another from physical stress (Allee et al. 1949; Connell and Slayter 1977; Bertness 1989). The role of physical stress in mediating the relative importance of facilitative interactions in natural communities, however, has received little direct attention. Here we present the results of a field experiment hat examines the relationship between competitive and facilitative interactions in salt marsh habitats. Specifically, we test the hypothesis that facilitation is common in secondary succession under harsh physical conditions but that competition dominates under benign physical conditions. The seaward border of high marsh habitats in New England is dominated by the grass Spartina patens, whereas dense standsof the rush Juncus gerardi characterize the terrestrial border (Miller and Egler 1950; Nixon 1982). Competition determines this zonation. Juncus competitively excludes Spartina from the terrestrial border, while both Juncus and Spartina restrict a third perennial, Distichlis spicata, to low densities and disturbed habitats (Bertness 1991). Natural disturbance in these habitats frequently occurs when tidally transported plant debris smothers underlying plants (Reidenbaugh and Banta 1980; Bertness and Ellison 1987). Secondary succession in the resulting bare patches is very predictable. Bare patches are initially colonized by the succulent annual Salicornia europaea (fig. 1), which is competitively displaced by perennial turfs within 2-3 yr (Ellison 1987). Distichlis by vegetative growth rapidly invades disturbed patches, but within 3-4 yr it too is competitively displaced by the zonal dominants (Bertness and Ellison 1987). Because bare patches in marshes become hypersaline (30%o100%o) without plant

Competition and facilitation in marsh plants.

Fugitive species often live under harsh physical conditions, yet their ability to cope with environmental extremes has received little attention. We studied the association of fugitive plants and stressful physical conditions using salt marsh plants that live in hypersaline bare patches. In New England salt marshes, primary space is dominated by dense monospecific stands of perennial turfs, but disturbances frequently leave bare space that rapidly becomes hypersaline. Elevated salinities of bare patches result form direct exposure of soil to solar radiation and evaporative water loss. A number of fugitive plants that are rare in undisturbed salt marsh vegetation are commonly associated with bare patches before being displaced during secondary succession. Greenhouse studies show that plants that dominate undisturbed vegetation are stunted by high substrate salinities, whereas the photosynthesis and growth of patch—dependent fugitives are relatively independent of typical salinity variation. Moreover, in the field, watering more than doubled the survivorship and growth of seedlings in patches and more strongly affected dominants than patch—dependent fugitives. Our results show that physical conditions in salt marsh bare patches can limit plant colonization, and suggest that high salt tolerances may permit fugitive plants to utilize bare patches as refugia from competitors. Since fugitive plants are commonly restricted to physically stressful habitats, the ability to withstand harsh physical conditions is likely often an important aspect of their biology.

https://www.jstor.org/stable/info/1940035

Salt Tolerances and The Distribution of Fugitive Salt Marsh Plants

Only recently has the importance of positive interactions among plant species in structuring natural communities been supported by experimental evidence. Most studies have focused on interactions between a pair of species at a single life-history stage. In this study positive interactions between a woody nitrogen-fixing shrub (Myrica pensylvanica) and two herbaceous sand dune species (Solidago sempervirens, Ammophila breviligulata) which frequently grow beneath shrub canopies are examined throughout the life cycles of the herbaceous species. Comparisons of S. sempervirens and A. breviligulata growing beneath and outside M. pensylvanica shrubs show that plants growing in association with shrubs are larger, are more likely to flower, produce greater numbers of flowers and seeds, have higher midday xylem water potentials, have higher tissue nitrogen concentrations, and have higher photosynthetic efficiencies. Measurements of environmental conditions show that areas beneath shrubs are more shaded, have lower soil temperatures, and have higher soil nitrogen levels. The results from experimental manipulations designed to test the effects of Myrica shrubs on understory species suggest that the observed differences in plant performance are strongly influenced by canopy shading and soil nutrient enrichment associated with the shrubs. The results demonstrate that M. pensylvanica facilitates growth, reproduction, and recruitment of S. sempervirens and A. breviligulata growing beneath it. This study, one of the few to examine positive interactions at different life-history stages, supports previous predictions that positive interactions may be particularly important in plant communities characterized by physiologically stressful conditions.

Facilitative effects of a sand dune shrub on species growing beneath the shrub canopy

Seedling recruitment in salt marsh plant communities is generally precluded in dense vegetation by competition from adults, but is also relatively rare in disturbance-generated bare space. We examined the constraints on seedling recruitment in New England salt marsh bare patches. Under typical bare patch conditions seed germination is severely limited by high substrate salinities. We examined the germination requirements of common high marsh plants and found that except for one notably patch-dependent fugitive species, the germination of high marsh plants is strongly inhibited by the high soil salinities routinely encountered in natural bare patches. Watering high marsh soil in the greenhouse to alleviate salt stress resulted in the emergence of up to 600 seedlings/225 cm2. The vast majority of this seed bank consisted of Juncus gerardi, the only common high marsh plant with high seed set. We tested the hypothesis that salt stress limits seedling contributions to marsh patch secondary succession in the field. Watering bare patches with fresh water partially alleviated patch soil salinities and dramatically increased both the emergence and survival of seedlings. Our results show that seedling recruitment by high marsh perennial turfs is limited by high soil salinities and that consequently their population dynamics are determined primarily by clonal growth processes. In contrast, populations of patch-dependent fugitive marsh plants which cannot colonize vegetatively are likely governed by spatially and temporally unpredictable windows of low salinities in bare patches.

Scott W. Shumway

Papers

Competition and facilitation in marsh plants.

Salt Tolerances and The Distribution of Fugitive Salt Marsh Plants

Facilitative effects of a sand dune shrub on species growing beneath the shrub canopy

Salt stress limitation of seedling recruitment in a salt marsh plant community.

Physiological Integration among Clonal Ramets during Invasion of Disturbance Patches in a New England Salt Marsh