Richard J. Reader

Plant coexistence mediated by arbuscular mycorrhizal fungi

Recent research has indicated the importance of arbuscular mycorrhizal fungi (AMF) in mediating plant coexistence. Coarse-scale studies compare the effects of the presence versus absence of AMF on plant coexistence, a phenomenon that is most relevant in early successional ecosystems where AMF are patchily distributed. By contrast, fine-scale studies investigate interactions that might occur once AMF have developed more fully within ecosystems, and most plants come into contact with AMF. Whereas coarse-scale effects are well understood, our understanding of fine-scale factors is just developing, as a result of investigations into AMF‐ plant specificity, AMF species richness, shared mycelial networks, and plant‐AMF feedback effects. Further research into these areas will provide a better understanding of factors that mediate plant species co-existence and, ultimately, the maintenance of biodiversity within plant communities. The term ‘coexistence’ has been used by ecologists to describe a balanced mixture of species in a biotic community. Such coexistence is a biological riddle, because the tendency towards competitive exclusion should favour a monoculture. Theories attempting to explain plant coexistence have focused on either interactions among species, such as competitive balance [1–3], or the avoidance of interaction among species [4,5]. Non-interaction theories have traditionally examined the role of spatial segregation and disturbance in promoting or suppressing plant coexistence. Agent-mediated coexistence is a non-interaction theory proposed as a mechanism for maintaining multi-species assemblages in plant communities [6,7]. When the ‘agent’ is a pathogen or a predator, it can reduce the ability of a plant to compete for resources if the tissues affected are involved in resource gathering (root or leaf). Often, the effects of pathogens and predators can be density dependent, in that the most abundant host plant species loses more tissue than do less abundant plant species. As a result, less abundant plant species experience reduced competition, lessening their chance of competitive exclusion, and thus promoting species coexistence within plant communities.

Does percent root length colonization and soil hyphal length reflect the extent of colonization for all AMF

Abstract. Percent root length colonization may not be an appropriate measure of root colonization by arbuscular mycorrhizal fungi (AMF) in all cases. We suggest that AMF will differ in how well percent root length colonization measures the amount of AMF colonization in the root due to differences among AMF in hyphal structure and hyphal aggregation. Although soil hyphal length accounts for hyphal density, we suggest that it does not consider differences in hyphal structure in measurements of external colonization and thus might also misrepresent the true amount of AMF in the soil. To test these suggestions, we measured and compared percent root length colonization and soil hyphal length with root ergosterol and soil ergosterol, respectively, for 21 different species of AMF from three families in a greenhouse experiment. Percent root length colonization predicted intra-radical colonization best for Glomaceae and Acaulosporaceae isolates, while soil hyphal length best represented soil ergosterol for Gigasporaceae isolates. The results show that conventional methods for estimating AMF colonization are not universal for all AMF. Caution is advised when drawing inferences for different groups of AMF.

Scale-dependent inconsistencies in the effects of trampling on a forest understory community

The response of forest understory vegetation to trampling applied at different temporal and spatial scales was determined in a cliff-edge forest in Ontario, Canada. Three frequencies (0, 50, 500 passes per year) of short-term trampling (one year) were applied to plots previously undisturbed. Existing trails that had received three frequencies (approx. 100, 500, 25,000 passes per year) of long-term trampling (18 years) were also studied. Community composition, species richness, and individual species frequency were recorded in plots within 4 m and (or) 1 m of the patch centerline. The quantitative and qualitative form of plant response to increased trampling was compared for short-term and long-term treatments, both within 4 m and within 1 m of the path centerline, to judge the consistency of trampling effects at different temporal and spatial scales.As trampling frequency increased, community composition changed progressively, but consistently, in plots both within 4 m and 1 m of the path centerline. Species richness was less affected by trampling and only decreased within 1 m of the path centerline at the highest level of trampling (25,000 passes per season for 18 years). Effects of trampling on individual species frequency were much less consistent at different temporal and spatial scales of trampling. The scale-dependence results suggest that field workers and resource managers both should try explicitly to include and define multiple scale components when trying to ascertain the response of vegetation to human disturbance factors.

Interactive effects of herbivory and competition on blue vervain (Verbena hastata L.: Verbenaceae)

We conducted an herbivore-exclusion plus neighbor-removal experiment to determine whether herbivory and competition had independent or interactive effects on a wetland plant, blue vervain (Verbena hastata). Eight-week-old plants were added to 1.5 m2 experimental plots with neighboring reed canary grass (Phalaris arundinacea) either left intact or removed and herbivores either not excluded or excluded. Neighbors were removed with herbicide and hand-weeding while herbivores were excluded by surrounding a plant with 6-mm wire mesh and spraying the plant weekly with a systemic insecticide. Treatment effects were determined by monitoring plant survival over two growing seasons and by measuring inflorescence mass in the second season. The two treatments had a synergistic, interactive effect on plant survival (G=25.7, P<0.001). Four of the five plants per treatment survived when exposed to herbivory alone or to competition alone, but no plants survived when exposed to both herbivory and competition. The two treatments had a substitutive, interactive effect on inflorescence mass (F=11.26, P<0.0005). Mean inflorescence mass was only 0–1 g plant−1 when plants were exposed to herbivory alone or to competition alone or to both herbivory and competition. Plants not exposed to either herbivory or competition had a mean inflorescence mass of 60 g plant−1. Since effects of herbivory and competition were interactive rather than independent, future studies measuring either herbivory or competition effects on wetland plants should include both herbivore-exclusion and neighbor-removal treatments.

Using the guild concept in the assessment of tree harvesting effects on understory herbs: A cautionary note

To help determine whether plants should be grouped into guilds for environmental impact assessment, the responses of six members of a guild of deciduous forest herbs to selective tree harvesting were compared. Harvesting operations themselves (tree cutting and skidding) had little effect on five of the six species but the sixth species decreased more in cut plots than in uncut plots. Subsequent microclimatic changes, resulting from tree cutting, also affected guild members differently. In the first year after cutting, two species increased more in cut plots than in uncut plots, while three other species did not change in frequency of occurrence and the sixth species decreased more in cut plots than in uncut plots. This inconsistent response of guild members to tree harvesting suggests that caution should be exercised in using guilds to assess plant response to environmental change.