The ecosystem and evolutionary contexts of allelopathy

Plants can release chemicals into the environment that suppress the growth and establishment of other plants in their vicinity, a process known as ‘allelopathy’. However, chemicals with allelopathic functions have other ecological roles, such as plant defense, nutrient chelation, and regulation of soil biota in ways that affect decomposition and soil fertility. These ecosystem-scale roles of allelopathic chemicals can augment, attenuate or modify their community-scale functions. In this review we explore allelopathy in the context of ecosystem properties, and through its role in exotic invasions consider how evolution might affect the intensity and importance of allelopathic interactions

Ecological Facilitation May Drive Major Evolutionary Transitions

There is a growing consensus among ecologists that ecological facilitation comprises a historically overlooked but crucial suite of biotic interactions. Awareness of such positive interactions has recently led to substantial modifications in ecological theory. In this article we suggest how facilitation may be included in evolutionary theory. Natural selection based on competition provides a conceptually complete paradigm for speciation, but not for major evolutionary transitions-the emergence of new and more complex biological structures such as cells, organisms, and eusocial populations. We find that the successful theories developed to solve these specific problematic transitions show a consistent pattern: they focus on positive interactions. We argue that facilitation between individuals at different levels of biological organization can act as a cohesive force that generates a new level of organization with higher complexity and thus allows for major evolutionary transitions at all levels of biological hierarchy

Facilitation by Pinus Flexilis During Succession: a Hierarchy of Mechanisms Benefits Other Plant Species

Studies of facilitation have primarily been limited to single mechanisms, species, or environments. We examined interacting mechanisms governing the facilitative effects of Pinus flexilis on two later successional understory species, Pseudotsuga menziesii and Ribes cereum, in different microhabitats and seasons at the ecotone between the Rocky Mountain forests and Great Plains grasslands in Montana, USA. In field surveys, 69% of Pseudotsuga and 91% of Ribes were located beneath P. flexilis even though P. flexilis subcrowns accounted for a small proportion of available habitat. For three years, we monitored the survival of Pseudotsuga and Ribes seedlings experimentally planted beneath P. flexilis and in the open at a windward and a leeward site. Survival of both species was highest beneath P. flexilis at a site topographically protected from strong unidirectional winds (38% for Pseudotsuga and 63% for Ribes), and lowest at a windward site and in the open where tree crowns did not provide shelter from winds (2% and 6%, respectively). These results suggest that wind amelioration contributed to the facilitative effect of P. flexilis. However, even at the leeward site, where wind speed was low, survival of Pseudotsuga and Ribes was higher beneath P. flexilis, suggesting the importance of shade. To explore the relative importance of different mechanisms, we designed an experiment with six treatments: “shade,” “shade + wind,” “shade + drift,” “wind,” “drift,” and a “control.” After two years, we found shade to be of overwhelming importance for the survival of Pseudotsuga and Ribes. Without shade, no other treatments were significant, but once shade was provided, wind amelioration and snow pack accumulation increased survival of Pseudotsuga, suggesting that these different facilitative mechanisms functioned in a nested hierarchical manner: some mechanisms were important only when others were already functioning. Many studies have demonstrated multiple interacting mechanisms in the way that plants interact, but to our knowledge hierarchical interactive processes have not been previously documented. If the effects of positive or competitive mechanisms are often hierarchical, then studies of isolated mechanisms may not accurately assess their importance in nature. Read More: http://www.esajournals.org/doi/abs/10.1890/0012-9658(2006)87%5B1816%3AFBPFDS%5D2.0.CO%3B

Temperature-Driven Variation in Substrate Oxygenation and the Balance of Competition and Facilitation

Emergent wetland plants often alleviate the effects of anaerobic soils on root respiration by passively transporting oxygen belowground through continuous air spaces (aerenchyma) in leaves and roots. Oxygen leaked from the roots into the rhizosphere may oxidize minerals in the soil or become available to other plants. Some spatial patterns in marsh plant communities suggest interspecific facilitative interactions, but there is little experimental evidence for interplant facilitation via soil oxygenation. We investigated the capability of the widespread, highly aerenchymous wetland plant, Typa latifolia, to aerate sediments and affect the growth of two non—aerenchymous neighbors, Salix exigua and Myosotis laxa, both in greenhouse experiments and in a natural pothole pond in western Montana. At soil temperatures of 11°—12°C in the greenhouse, mean dissolved oxygen (DO) ranged from 2.75±0.41 to 4.43±1.11 mg/L (mean ± 1 SE) in pots with Typha, whereas in pots without Typha DO content ranged from 0.40 ± 0.06 to 0.65 ± 0.06 mg/L. At soil temperatures of 18°—20°C, DO in the Typha treatment was lower than in the low—temperature experiment and did not differ significantly from DO in the pots without Typha. At substrate temperatures 11°—12°C, all rooted Salix cuttings survived when planted with Typha, whereas non survived when planted alone. At the same substrate temperature, Myosotis transplants grew significantly larger when planted with Typha than when alone, also indicating that facilitation occurred. When grown with Typha at soil temperatures of 18°—20°C, however, Myosotis root mass was significantly less than when grown alone, suggesting that competition occurred. At the pond margins where Typha and Myosotis coexist in water—saturated soils, naturally occurring Myosotis plants adjacent to transplanted Typha had significanlty larger shoot and fruit mass than did control Myosotis without nearby Typha. In this experiment DO levels in soil water and available soil nitrogen did not differ among treatments. These experiments indicate that aerenchymous wetland plants have the potential to facilitate neighbors via soil oxygenation, and that facilitaton and competition may shift in importance with changes in the physical environment

Novel Weapons: Invasive Success and the Evolution of Increased Competitive Ability

When introduced to new habitats by humans, some plant species become much more dominant. This is primarily attributed to escape from specialist consumers. Release from these specialist enemies is also thought by some to lead to the evolution of increased competitive ability, driven by a decrease in the plant\u27s resource allocation to consumer defense and an increase in allocation to size or fecundity. Here, we discuss a new theory for invasive success – the “novel weapons hypothesis”. We propose that some invaders transform because they possess novel biochemical weapons that function as unusually powerful allelopathic agents, or as mediators of new plant–soil microbial interactions. Root exudates that are relatively ineffective against their natural neighbors because of adaptation, may be highly inhibitory to newly encountered plants in invaded communities. In other words, the novel weapons of some plant invaders provide them with an advantage that may arise from differences in the regional coevolutionary trajectories of plant communities. Furthermore, the selective advantage of possessing a novel weapon may result in rapid evolution of that weapon – for example, the production of greater quantities of allelopathic or antimicrobial root exudates. Direct selection of competitive traits provides an alternative to the “grow versus defend” trade-offs that underpin the theory of the evolution of increased competitive ability. Read More: http://www.esajournals.org/doi/abs/10.1890/1540-9295(2004)002%5B0436%3ANWISAT%5D2.0.CO%3B

Competition and Facilitation: a Synthetic Approach to Interactions in Plant 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

Soil Biota Facilitate Exotic Acer Invasions in Europe and North America

The primary hypothesis for successful exotic plant invasions is that the invaders have escaped the specialist consumers that control them (Enemy Release Hypothesis). However, few studies have rigorously tested this assertion with biogeographical experiments or considered the effects of soil organisms. We tested the Enemy Release Hypothesis and the enhanced role of mutualisms by comparing density patterns of the North American Acer negundo and European A. platanoides trees in their native and nonnative ranges. Invaders that have escaped their natural enemies are predicted to attain greater densities in nonnative than native ranges. To determine whether interactions with soil biota could explain the population distributions observed in the field, we compared the effects of sterilized and nonsterilized soil associated with Acer and non-Acer trees in native and nonnative ranges on the growth of seedlings in the greenhouse. In the field study, distances from focal trees to the nearest Acer conspecifics were 56– 77% less in their nonnative ranges than in their native ranges. In the greenhouse experiment, the effect of soil biota also differed between native and nonnative ranges of Acers. Relative to sterilized controls, soil associated with conspecifics and heterospecifics from the native ranges decreased the total biomass and relative change in height of Acer seedlings by 35% and 40%, respectively. Soil associated with conspecifics in the nonnative ranges decreased the biomass and relative change in height of Acer seedlings by 112% and 64%, respectively; but the soil associated with heterospecifics in the nonnative ranges increased biomass and relative change in height of Acer seedlings by 13% and 37%, respectively. Our results suggest that invasion of Acers is enhanced by soil biota associated with dominant native species and that the soil biota effect becomes more inhibitory as the Acers establish. The relative difference in soil biota effects between ranges supports the Enemy Release Hypothesis but also suggests that mutualists are relatively more beneficial to Acers in their nonnative ranges than in their native ranges. Mutualisms may be relatively more beneficial in nonnative ranges because the invader has escaped from the negative effect of natural enemies that may attenuate the positive effect of mutualists

The Advantages of Clonal Integration Under Different Ecological Conditions: a Community-Wide Test

The connectedness of clonal plants has been shown to promote survival and growth in a variety of single-species, single-factor studies, but experiments comparing the relative advantages of clonality across multiple habitats and species are rare, raising the concern that generalizations about the benefits of clonality might be biased by the particular species or habitat studied. We studied the importance of clonal integration in southeastern USA salt marsh plants, using all six of the common clonal species in the community, by following the success of intact and severed clonal fragments invading three habitat treatments. Clonal integration was most important for growth of clonal fragments invading hypersaline salt pans, likely because parent clones supplied salt-stressed fragments with water; of moderate importance for fragments invading the neighbors-clipped treatment, likely because parent clones supplied fragments with resources enabling rapid exploitation of unused patches; and least important when neighbors were present, consistent with suggestions that size-based asymmetrical competition is relatively unimportant in clonal plants. Our results indicate that the importance of clonal integration can differ between habitats and species within a community. We encourage explicit consideration of these potential sources of variability to best understand the importance of clonal integration in the field