Influence and Mechanisms of Plant-plant Facilitation

Introduction

Understanding and quantifying the mechanisms which influence the observed organization in plant communities is a central tenet in ecology. The common usage of the term ‘community’ in the describes an assemblage of two or more species coexisting within a defined geographical area and timeframe (Lidicker 2008). Members of an ecological community constantly interact with one another in a variety of complex combinations, such that the presence of an individual may directly or indirectly influence the success or failure of another individual’s survival within the community. The relative importance and intensity of these constant interactions are widely considered to be a fundamental driving force in shaping community structure and diversity (Holmgren et al. 1997, Bertness et al. 1999, Menge 2000, Mougi and Kondoh 2012).

Ecological interactions between individuals can be categorized in several different ways; Intra-specific, and inter-specific interactions occur between members of the same species, and members of two or more different species, respectively; Competitive interactions where one individual has a negative association with another individual, and facilitative interactions where the association is positive.

Plant-plant facilitation is defined as the positive association between two or more plants, where the presence of one plant is beneficial to the growth, reproduction, and survival of another plant in its proximity (Callaway 2007). Whereas plant-plant competition is its antithesis, with the presence of the plant being detrimental to the growth, survival, and reproduction of the other plant.

Because of the short time of observation and the young age of the juvenile plant species,… In order to fully comprehend the scope of this thesis it is necessary to define certain terms to avoid confusion

However, both competitive and facilitative interactions occur between individuals simultaneously, with the net effect defining the eventual outcome. This is an important distinction

CITATION.

Mechanisms of interaction.

Plant-interactions are mediated by abiotic and biotic mechanisms. Abiotic mechanisms reduce physiological stress by altering environmental conditions, such as soil moisture (Callaway and King 1996, Caldwell et al. 1998), soil nutrients (Vetaas 1992), temperature (Callaway and King 1996), light (Flores and Jurado 2003), wind (Gerdol et al. 2000), and oxygen levels in soils (Drew 1983). Whereas, biotic mechanisms do so through the attraction of pollinators and seed dispersers (Sieber et al. 2011), granting protection from herbivores CITATION, root grafting (Tarroux and DesRochers 2011) Graham & Boorman, and altering the composition of soil micro-organism  and mycorrhizal communities (Johnson et al. 1992) (Rodriguez-Echeverria et al. 2016) join.

The modes by which plants interact with one another are generally well understood mechanistically, and empirical evidence of their facilitative affects are common within the literature. For example, Sieber et al. (2011) investigated whether the overall net effect of aggregations of plants on pollination in an alpine environment was positive (i.e. Facilitative) or negative (i.e. Competitive). They varied the relative frequency of two species of cushion plants E. nanum and Saxifraga within 60 plots located in the Swiss Alps. The authors found that the plots which contained a higher proportion of Saxifraga were visited by insect pollinators more often than those that contained a greater amount of E. nanum. However, within plots that contained both Saxfraga and E. nanum the visitation frequency of pollinators to each of the species was not significantly different. This would suggest that Saxifraga is attracting pollinators at a greater rate than E. nanum at a distance, but once the pollinator arrives E. nanum’s proximity to Saxifraga is facilitating the rate at which pollinators will visit it and hence increasing its reproductive output.

Tarroux and DesRochers (2011) looked at how radial growth rates in Pinus banksiana were affected by the formation of natural root grafts. They found that during the formation of root grafts, radial growth rates were supressed compared to trees which were not forming root grafts. However, after graft formation was completed, radial growth rates were greater than those trees which were still undergoing root graft formation, and those that had not root grafts. They concluded that that while root grafts are initially energetically costly, once completed there is a significant benefit to partner trees.

Rodriguez-Echeverria et al. (2013) investigated the effects of soil biota on the growth, reproduction and abundance of 14 different species grown in soils either inoculated with the surrounding soil of a nurse plant, or from soil within an open space. They found that inoculation with soil from beneath nurse plant species containing soil microbes had a significant positive effect on the response of seedlings. Different soil biota were also observed to have formed associations with distinct plant species, suggesting that there may be a degree of specificity between associations. One implication of this is that nurse species may be able to affect the composition of soil micro-organisms within the surrounding soil

Studies which have focused on researching either competitive or facilitative interactions as separate effects are far more prevalent within the literature, yet the two are not mutually exclusive and both forces act simultaneously within plant communities (Agrawal et al. 2007, Callaway 2007). However, what is important when qualifying plant-plant interaction is the overall net effect of competitive and facilitative interactions. Franco and Nobel (1989) observed that 89% of Carnegiea gigantea cactus seedlings found at two separate locations in the Sonoran Desert were established under the canopy of two species of shrub. Under the shrub canopies available light for photosynthesis was reduced by 77% resulting in a predicted loss in photosynthetic efficiency by ~90% compared to seedlings which had direct light. However, soil nitrogen content beneath the shrubs was up to 60% higher than in open areas. Nitrogen is a limiting resource for C. gigantean seedlings with just a 0.03-0.05% increase in concentration providing a dry-weight gain of ~60%. Therefore, the increased access to nitrogen under the shrub canopies outweighed the negative effect of a decrease in available light, resulting in a net benefit to C. gigantea seedlings and hence their establishment under shrub canopies.

Nutrient transfer pathways

Mycorrhizal networks

Stress Gradient Hypothesis

Many of the early pioneers of plant successional theory acknowledged that both facilitative and competitive interactions mediated plant community structure (Clements 1916, Gleason 1926, Whittaker 1956). Yet historically, competitive interactions have dominated the scientific literature, driven by popular models on community organization which highlighted the principles of competition (Grime 1973, Tilman 1982). Evidence for the importance of strong facilitative interactions organizing plant communities began to accumulate from studies in environments which were considered to be physiologically stressful for plants, such as arid, alpine, and intertidal zones (Franco and Nobel 1989, Bertness and Shumway 1993, Callaway 2007). In order to provide a theoretical background for the observed prevalence of facilitative interactions in stressful environments, Bertness and Callaway (1994) developed a model, in which the frequency of competitive and facilitative interactions observed changed along gradients of abiotic stress. The theoretical underpinning of the model is derived from the theories of competitive strategies; in which species must pre-empt limiting resources by expending energy in order to gain them (Grime 1977). In a relative benign environment species can devote more energy towards competing for resources, and less energy in maintaining physiological homeostasis. However, under harsh environmental conditions where physiological stress is increased, species cannot devote as much energy to competing for resources. Hence there is less competitive interactions occurring and neighbour effects are more likely to be facilitative (Bertness and Shumway 1993). In essence, the model postulates that along an increasingly physiologically stressful gradient, or gradients of increasing consumer pressure, there will be a change from primarily competitive interactions to being more facilitative. The predictions generated by the model have been formally termed the “Stress Gradient Hypothesis” (SGH), and is in general well supported by empirical evidence from experiments conducted along gradients of stress (Berkowitz et al. 1995, Callaway et al. 2002, Gomez-Aparicio et al. 2004).

However, the SGH model is not without criticism. Results from several studies reveal that even within the same study system there are differences in how the frequency of facilitative and competitive interactions change (Choler et al. 2001, Michalet et al. 2006, Butterfield et al. 2016). For instance, Tielbörger and Kadmon (2000) found that the interplay of facilitation and competition between a perennial nurse shrub and annual beneficiary species changed from predominantly negative to neutral, or from neutral to positive interactions when environmental conditions became less stressful, contrary to the predictions of SGH. Maestre et al. (2009) argues that the SGH model needs to be refined to account for the relative strength of interaction, rather than their frequency of occurrence. This is due to concerns that empirical support for the original SGH model was collected from studies which investigated only a single pair of species (a nurse species and its beneficiary or beneficiaries), or a few pairs of different species. Moreover, many studies examined gradients in stress over large spatial scales rather than experimentally manipulating stress at a local level. They considered experiments conducted in this way to have a high degree of risk in being confounded by local variations within species. As a refinement to SGH, Maestre et al. (2009) propose a new model where facilitative intensity is predicted to be strongest at intermediate to high levels of stress. Whereas, competitive intensity is greatest at low and extreme levels of stress.

Current focus of the literature

Evidently, the literature is full of examples where the results from empirical studies have not conformed to the specific predictions of the SGH, revealing a high degree of variability in the response of plant communities to stress. Recent research on facilitation has tended to be directed towards decoupling the ultimate causes determining interaction intensity. Many facilitative interactions have a high degree of specificity between a nurse species and a small number of beneficiary species, whereas only a few interactions occur randomly between all species (Callaway 1998). In other words, the strength and direction of plant-plant interactions is often dependent on species identity. Mesquita et al. (2001) investigated secondary succession of abandoned pastures and cut forest, using two different pioneer nurse species, of the genus Cecropia and Vismia. They found that nurse species identity was a strong determinant of the identity and composition of beneficiaries. A similar specificity was observed in an examination of the interaction between the identity of a nurse species and its beneficiary, and their relative ontogeny. (Paterno et al. 2016). They reported that beneficiary identity changed both with nurse species identity as well as with ontogenetic shifts. During germination of beneficiary species, all nurse species had a positive effect on growth.

The relative importance of interactions in structuring plant communities are also being reported as varying temporally and spatially, and may depend on factors such as the age and traits possessed by species, as well as the distribution of resources within the habitat and variation in environmental conditions (Butterfield 2009). For example, Johnson et al. (1997) found that the interaction between a plant and its symbiotic relationship with a mycorrhizal fungi varied depending on environmental conditions and the developmental stage of the plant. Interactions changed from being overall facilitative in nutrient-poor soils, to overall parasitic in nutrient rich soils. Likewise, mycorrhizae had negative effects on the germination and early growth of seedlings by sequestering carbon from seed reserves normally assigned for use in seedling development. There is also evidence to suggest that the strength of facilitative interactions varies depending on the age and traits of nurse plants and their beneficiaries. Stands of Olneya tesota trees promoted species diversity depending on the size of the O. tesota canopy, whether beneficiary plants were perennials or ephermal, as well as the degree of environmental stress (Tewksbury and Lloyd 2001).

Recent empirical evidence has also revealed that competitive and facilitative interactions may not be symmetrical both above and below ground (Zhang et al. 2013). For example, in an experimental test of the SGH using a ‘foundation’ tree Pinus edulis and the shrub Fallugia paradoxia, Sthultz et al. (2007) found that the intensity of interactions depended on whether or not they were partitioned into above or below ground, or grouped as above and below ground. They noted that interaction intensity did conform to the predictions of SGH when both above and below interactions were grouped, however, when analysed individually they found that both above and below ground interactions were competitive, but facilitative interaction only occurred above ground. In another experimental test of how the relative intensity and direction of above and below ground interactions affected communities, Lamb et al. (2009) reported the intensity of root completion was greater than shoot competition, yet root competition had negligible effect on community structure and shoot competition did.

Wetland habitats

To date most of the studies investigating facilitative interactions have been conducted in arid environments where water is scarce or inaccessible, or intertidal zones where the salinity of the water is a major stressor. Yet, wetlands have received very little significant investigation despite flooding stress being well documented. In comparison to arid environments, wetland ecosystems are characterized by the movement, distribution, and quality of large quantities of water either on the surface or within the rhizosphere (Mitsch and Gosselink 2007). The effects of standing water and waterlogged soils on plants is well established. In well drained soils oxygen moves via diffusion, caused by the respiration of roots and soil micro-organisms, from the atmosphere into pores within the soil substrate at a rate of 17Lm^-2 per day (Drew 1983). However, when flooding occurs, the water on the surface forms a barrier between the atmosphere and the soil reducing the rate of diffusion by a magnitude of four (Gambrell and Patrick Jr 1978). The rate of biological respiration then depletes the amount of available oxygen faster than the rate of diffusion through the water to the soil. These conditions can cause physiological stress to plants by creating anaerobic conditions which prevent the root system from accessing oxygen in the quantities it needs to perform respiration. If these conditions persist without amelioration, fermentation rather than respiration occurs resulting in a reduced growth rate, less ATP produced, and the formation of ethanol as a by-product causing cell damage. Complete submergence of plants will also prevent transpiration and reduce photosynthesis by reducing the available light and preventing access to atmospheric CO₂ (Blom and Voesenek 1996)_.

Conclusion.

The influence and mechanisms of plant-plant facilitation have been clearly established in the literature as having an influential role in the organization of plant communities.  The development of the SGH provided a theoretical framework and set of predictions which could be empirically tested. What is evident from the research conducted is that the intensity and direction of interactions are dependent on a wide array of variables, which need to be investigated carefully and in more detail. The strength of interactions needs to be investigated by incorporating both above and below ground interactions while also account for temporal and spatial variability in stressors. The literature is also predominantly composed of arid environments where water is a limited resource, which could perhaps enhance competitive effects. However, experiments on facilitation in wetland habitat where water is potentially a major stressor are extremely scarce despite our knowledge that strong physiological stressors. Future investigations into wetland environment could provide useful knowledge and progress our understanding of facilitative interactions between plants.

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