Mycorrhizal fungi influence plant growth, local biodiversity and ecosystem function. Effects of the symbiosis on plants span the continuum from mutualism to parasitism. We sought to understand this variation in symbiotic function using meta-analysis with information theory-based model selection to assess the relative importance of factors in five categories: (1) identity of the host plant and its functional characteristics, (2) identity and type of mycorrhizal fungi (arbuscular mycorrhizal vs. ectomycorrhizal), (3) soil fertility, (4) biotic complexity of the soil and (5) experimental location (laboratory vs. field). Across most subsets of the data, host plant functional group and N-fertilization were surprisingly much more important in predicting plant responses to mycorrhizal inoculation (Ôplant responseÕ) than other factors. Non-N-fixing forbs and woody plants and C 4 grasses responded more positively to mycorrhizal inoculation than plants with N-fixing bacterial symbionts and C 3 grasses. In laboratory studies of the arbuscular mycorrhizal symbiosis, plant response was more positive when the soil community was more complex. Univariate analyses supported the hypothesis that plant response is most positive when plants are P-limited rather than N-limited. These results emphasize that mycorrhizal function depends on both abiotic and biotic context, and have implications for plant community theory and restoration ecology.

A commonly cited mechanism for invasion resistance is more complete resource use by diverse plant assemblages with maximum niche complementarity. We investigated the invasion resistance of several plant functional groups against the nonindigenous forb Spotted knapweed (Centaurea maculosa). The study consisted of a factorial combination of seven functional group removals (groups singularly or in combination) and two C. maculosa treatments (addition vs. no addition) applied in a randomized complete block design replicated four times at each of two sites. We quantified aboveground plant material nutrient concentration and uptake (concentration 3 biomass) by indigenous functional groups: grasses, shallow-rooted forbs, deep-rooted forbs, spikemoss, and the nonindigenous invader C. maculosa. In 2001, C. maculosa density depended upon which functional groups were removed. The highest C. maculosa densities occurred where all vegetation or all forbs were removed. Centaurea maculosa densities were the lowest in plots where nothing, shallowrooted forbs, deep-rooted forbs, grasses, or spikemoss were removed. Functional group biomass was also collected and analyzed for nitrogen, phosphorus, potassium, and sulphur. Based on covariate analyses, postremoval indigenous plot biomass did not relate to invasion by C. maculosa. Analysis of variance indicated that C. maculosa tissue nutrient percentage and net nutrient uptake were most similar to indigenous forb functional groups. Our study suggests that establishing and maintaining a diversity of plant functional groups within the plant community enhances resistance to invasion. Indigenous plants of functionally similar groups as an invader may be particularly important in invasion resistance.

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. Mycorrhizae are important mediators of plant competition, but little is known about the role of mycorrhizae in the intense competitive effects that exotic plants can have on native species. In the greenhouse, we tested the effect of arbuscular mycorrhizal (AM) fungi on interspecific competition between Centaurea maculosa and Festuca idahoensis, on intraspecific competition between individuals of both species, and the growth of C. maculosa with either inorganic or organic phosphorus. Mycorrhizae had no direct effect on either species, but mycorrhizae increased C. maculosa's negative effect on F. idahoensis. When competing with C. maculosa, nonmycorrhizal F. idahoensis were 171% larger than they were when mycorrhizae were present. In a second experiment, C. maculosa grown with larger F. idahoensis were 66% larger, in the presence of AM fungi, than when AM fungi were absent. Centaurea maculosa biomass was not affected by AM fungi, in either phosphorus treatment, in the absence of F. idahoensis. Root: shoot ratios differed between phosphorus treatments, but this difference seemed to be a result of slower growth in the organic phosphorus treatment. Our results were unusual in that the direct effects of mycorrhizae on both species were weak, but the indirect effect of AM fungi on the interactions between C. maculosa and F. idahoensis was strong. Our results suggest that AM fungi strongly enhance the ability of C. maculosa to invade native grasslands of western North America.

BackgroundLocal adaptation, the differential success of genotypes in their native versus foreign environment, arises from various evolutionary processes, but the importance of concurrent abiotic and biotic factors as drivers of local adaptation has only recently been investigated. Local adaptation to biotic interactions may be particularly important for plants, as they associate with microbial symbionts that can significantly affect their fitness and may enable rapid evolution. The arbuscular mycorrhizal (AM) symbiosis is ideal for investigations of local adaptation because it is globally widespread among most plant taxa and can significantly affect plant growth and fitness. Using meta-analysis on 1170 studies (from 139 papers), we investigated the potential for local adaptation to shape plant growth responses to arbuscular mycorrhizal inoculation.ResultsThe magnitude and direction for mean effect size of mycorrhizal inoculation on host biomass depended on the geographic origin of the soil and symbiotic partners. Sympatric combinations of plants, AM fungi, and soil yielded large increases in host biomass compared to when all three components were allopatric. The origin of either the fungi or the plant relative to the soil was important for explaining the effect of AM inoculation on plant biomass. If plant and soil were sympatric but allopatric to the fungus, the positive effect of AM inoculation was much greater than when all three components were allopatric, suggesting potential local adaptation of the plant to the soil; however, if fungus and soil were sympatric (but allopatric to the plant) the effect of AM inoculation was indistinct from that of any allopatric combinations, indicating maladaptation of the fungus to the soil.ConclusionsThis study underscores the potential to detect local adaptation for mycorrhizal relationships across a broad swath of the literature. Geographic origin of plants relative to the origin of AM fungal communities and soil is important for describing the effect of mycorrhizal inoculation on plant biomass, suggesting that local adaptation represents a powerful factor for the establishment of novel combinations of fungi, plants, and soils. These results highlight the need for subsequent investigations of local adaptation in the mycorrhizal symbiosis and emphasize the importance of routinely considering the origin of plant, soil, and fungal components.Electronic supplementary materialThe online version of this article (doi:10.1186/s12862-016-0698-9) contains supplementary material, which is available to authorized users.

Mycorrhizae are important mediators of plant competition, but little is known about the role of mycorrhizae in the intense competitive effects that exotic plants can have on native species. In the greenhouse, we tested the effect of arbuscular mycorrhizal (AM) fungi on interspecific competition between Centaurea maculosa and Festuca idahoensis, on intraspecific competition between individuals of both species, and the growth of C. maculosa with either inorganic or organic phosphorus. Mycorrhizae had no direct effect on either species, but mycorrhizae increased C. maculosa’s negative effect on F. idahoensis. When competing with C. maculosa, nonmycorrhizal F. idahoensis were 171% larger than they were when mycorrhizae were present. In a second experiment, C. maculosa grown with larger F. idahoensis were 66% larger, in the presence of AM fungi, than when AM fungi were absent. Centaurea maculosa biomass was not affected by AM fungi, in either phosphorus treatment, in the absence of F. idahoensis. Root:shoot ratios differed between phosphorus treatments, but this difference seemed to be a result of slower growth in the organic phosphorus treatment. Our results were unusual in that the direct effects of mycorrhizae on both species were weak, but the indirect effect of AM fungi on the interactions between C. maculosa and F. idahoensis was strong. Our results suggest that AM fungi strongly enhance the ability of C. maculosa to invade native grasslands of western North America.