Kurt O. Reinhart

Effects of soil biota from different ranges on Robinia invasion: acquiring mutualists and escaping pathogens

The net effects of soil biota on exotic invaders can be variable, in part, because net effects are produced by many interacting mutualists and antagonists. Here we compared mutualistic and antagonistic biota in soils collected in the native, expanded, and invasive range of the black locust tree, Robinia pseudoacacia. Robinia formed nodules in all soils with a broad phylogenetic range of N-fixing bacteria, and leaf N did not differ among the different sources of soil. This suggests that the global expansion of Robinia was not limited by the lack of appropriate mutualistic N-fixers. Arbuscular mycorrhizal fungi (AMF) from the native range stimulated stronger positive feedbacks than AMF from the expanded or invasive ranges, a biogeographic difference not described previously for invasive plants. Pythium taxa collected from soil in the native range were not more pathogenic than those from other ranges; however, feedbacks produced by the total soil biota were more negative from soils from the native range than from the other ranges, overriding the effects of AMF. This suggests that escape from other pathogens in the soil or the net negative effects of the whole soil community may contribute to superior performance in invaded regions. Our results suggest that important regional evolutionary relationships may occur among plants and soil biota, and that net effects of soil biota may affect invasion, but in ways that are not easily explained by studying isolated components of the soil biota.

Plant-soil feedbacks and mycorrhizal type influence temperate forest population dynamics

Soil biota and plant diversity Soil biota, including symbionts such as mycorrhizal fungi and nitrogen-fixing bacteria, as well as fungal and bacterial pathogens, affect terrestrial plant diversity and growth patterns (see the Perspective by van der Putten). Teste et al. monitored growth and survival in Australian shrubland plant species paired with soil biota from plants of the same species and from other plants that use different nutrient acquisition strategies. Plant-soil feedbacks appear to drive local plant diversity through interactions between the different types of plants and their associated soil biota. Bennett et al. studied plant-soil feedbacks in soil and seeds from 550 populations of 55 species of North American trees. Feedbacks ranged from positive to negative, depending on the type of mycorrhizal association, and were related to how densely the same species occurred in natural populations. Science, this issue p. 134, p. 173; see also p. 181 A large-scale study of North American trees reveals how different soil-associated fungi can either help or hinder tree growth. Feedback with soil biota is an important determinant of terrestrial plant diversity. However, the factors regulating plant-soil feedback, which varies from positive to negative among plant species, remain uncertain. In a large-scale study involving 55 species and 550 populations of North American trees, the type of mycorrhizal association explained much of the variation in plant-soil feedbacks. In soil collected beneath conspecifics, arbuscular mycorrhizal trees experienced negative feedback, whereas ectomycorrhizal trees displayed positive feedback. Additionally, arbuscular mycorrhizal trees exhibited strong conspecific inhibition at multiple spatial scales, whereas ectomycorrhizal trees exhibited conspecific facilitation locally and less severe conspecific inhibition regionally. These results suggest that mycorrhizal type, through effects on plant-soil feedbacks, could be an important contributor to population regulation and community structure in temperate forests.

Predicting plant responses to mycorrhizae: integrating evolutionary history and plant traits.

We assessed whether (1) arbuscular mycorrhizal colonization of roots (RC) and/or plant responses to arbuscular mycorrhizae (MR) vary with plant phylogeny and (2) MR and RC can be more accurately predicted with a phylogenetic predictor relative to a null model and models with plant trait and taxonomic predictors. In a previous study, MR and RC of 95 grassland species were measured. We constructed a phylogeny for these species and found it explained variation in MR and RC. Next, we used multiple regressions to identify the models that most accurately predicted plant MR. Models including either phylogenetic or phenotypic and taxonomic information similarly improved our ability to predict MR relative to a null model. Our study illustrates the complex evolutionary associations among species and constraints of using phylogenetic information, relative to plant traits, to predict how a plant species will interact with AMF.

Virulence of soil‐borne pathogens and invasion by Prunus serotina

*Globally, exotic invaders threaten biodiversity and ecosystem function. Studies often report that invading plants are less affected by enemies in their invaded vs home ranges, but few studies have investigated the underlying mechanisms. *Here, we investigated the variation in prevalence, species composition and virulence of soil-borne Pythium pathogens associated with the tree Prunus serotina in its native US and non-native European ranges by culturing, DNA sequencing and controlled pathogenicity trials. *Two controlled pathogenicity experiments showed that Pythium pathogens from the native range caused 38-462% more root rot and 80-583% more seedling mortality, and 19-45% less biomass production than Pythium from the non-native range. DNA sequencing indicated that the most virulent Pythium taxa were sampled only from the native range. The greater virulence of Pythium sampled from the native range therefore corresponded to shifts in species composition across ranges rather than variation within a common Pythium species. *Prunus serotina still encounters Pythium in its non-native range but encounters less virulent taxa. Elucidating patterns of enemy virulence in native and nonnative ranges adds to our understanding of how invasive plants escape disease. Moreover, this strategy may identify resident enemies in the non-native range that could be used to manage invasive plants.

The organization of plant communities: negative plant-soil feedbacks and semiarid grasslands.

Understanding how plant communities are organized requires uncovering the mechanism(s) regulating plant species coexistence and relative abundance. Negative soil feedbacks may affect plant communities by suppressing dominant species, causing rarity of most plants, or reducing the competitive abilities of all species. Here, three soil feedback experiments were used to differentiate the effects of soil feedbacks on mid- to late-successional and semiarid grasslands. Then I tested whether the direction and degree of soil feedback accounts for variation in relative abundance among species that coexist within each plant community. Negative soil feedbacks predominated across all species and sites and were individually discernible for 40% of plant species. Negative soil feedbacks affected rare to dominant plant species. Negative soil feedbacks, capable of having negative frequency-dependent effects, have the potential to act as a fundamental driver of species coexistence.

A common soil handling technique can generate incorrect estimates of soil biota effects on plants

An active area of research seeks to understand how soil biota effects on plants vary across experimental factors (i.e. regions, treatments). The study biotas are obtained by gathering soil sample(s) from randomly selected location(s) within each experimental unit, with an experimental unit being a site within a study region or a field plot of a manipulative experiment. Then, plant growth is measured in glasshouse containers housing soil and biota from the various sites or plots. Results of these glasshouse bioassays are sensitive to a common soil handling decision. In particular, it is common to either: (1) fill each container with soil from one experimental unit (e.g. Callaway et al., 2004; Hood et al., 2004; Kardol et al., 2006; Wardle et al., 2012), or (2) fill each container with a mixture of soils from multiple experimental units (i.e. all sites within a region, all plots that received the same treatment) (e.g. Van der Putten et al., 1993; Nijjer et al., 2007; Felker-Quinn et al., 2011; Pendergast et al., 2013; Rodr ıguezEcheverr ıa et al., 2013; Yang et al., 2013; Gundale et al., 2014; Pizano et al., 2014; Hilbig & Allen, 2015; Larios & Suding, 2015) (Fig. 1). We define samples generated from these two approaches as ‘individual soil samples’ (ISS) and ‘mixed soil samples’ (MSS). The term ‘individual soil sample’ is slightly misleading, as ISS are often formed by mixing multiple samples gathered from the same experimental unit (i.e. pooling subsamples). Combining subsamples from individual experimental units is a perfectly acceptable approach. Conversely, the express purpose of this paper is to illustrate that, without exception, the MSS approach of mixing together soils from multiple experimental units (Fig. 1) is fatally flawed. Hypotheses regarding differences among regions or treatments cannot be legitimately tested by the MSS approach of mixing together soils from multiple sites within regions or multiple plots receiving the same treatment. The importance of this point is clearly underappreciated: We estimate, 52% of published studies use MSS in place of the correct ISS methodology (of 76 evaluated studies using ISS or MSS, 40 used MSS) (K. O. Reinhart & M. J. Rinella, unpublished, 2015). Estimating treatment (e.g. region, nutrient) differences entails computing residual variance. Residual variance describes variation not explained by treatments, and it is needed to compute relevant statistics (i.e. P-values, confidence intervals). In experiments considered here, there are two contributors to residual variance in plant growth: (1) spatial variation in soil biotas (i.e. site-to-site variation not explained by region, plot-to-plot variation not explained by treatment) and (2) glasshouse variation owing to environmental gradients (e.g. temperature) and plant genetics. With the ISS approach (Fig. 1), having two contributors to residual variance poses no unique analytic challenges, and standard regression and analysis of variance (ANOVA) approaches give correct inferences. With the MSS approach, all information regarding residual variation in soil mutualists and pathogens is lost, and if this variation is nonzero, MSS and ISS are guaranteed to give different inferences, with the MSS inferences being incorrect. More specifically, if residual variation in mutualists and/or pathogens is nonzero, statistical estimates from MSS will be falsely precise and evidence for differences among treatments (e.g. regions, nutrients) will be weaker than reported. Assuming only factors being studied cause soil mutualists/ pathogens to vary spatially is highly unrealistic, particularly given that plant disease expression (e.g. Martin & Loper, 1999) and soil microbe compositions (e.g. Ettema & Wardle, 2002; Ritz et al., 2004) are known to vary widely across even small spatial scales (i. e. < 1.0 m).

More closely related plants have more distinct mycorrhizal communities

We observed that mycorrhizal communities were more divergent among closely related plant species than among distantly related plant species. This was counter to the observation that plant mutualists (e.g. pollinators, seed dispersers) are often shared among closely related host plant species. Since mycorrhizae may affect nutrient competition among neighboring plants, closely related plant neighbors that associate with unique mycorrhizae may have greater functional complementarity and a greater capacity to coexist.

Soil aggregate stability was an uncertain predictor of ecosystem functioning in a temperate and semiarid grassland

We estimate rangeland managers assessing ecosystem health have measured soil stability >800,000 times. Our aim was to use quantitative data from a site in the Northern Great Plains, USA and a semi-quantitative literature search to demonstrate the robustness of soil stability as an indicator of ecosystem functioning. Empirical data included measurements of plant and soil properties along a local livestock grazing gradient to determine whether soil stability (e.g., % water-stable aggregates) explained primary productivity and soil water transport for a mixed-grass prairie site in the Northern Great Plains. We measured: annual net primary productivity (ANPP), elevation, % soil moisture, measures of soil stability, and soil water transport (field-saturated infiltrability and sorptivity) across points spanning a local gradient in livestock grazing intensity (none vs. light to moderate stocking rates; mean distance separating points = 39.9 m [range = 5.2–71.3 m]). Across the sampled gradient, variation in ANPP ...

Factors affecting host range in a generalist seed pathogen of semi-arid shrublands

Generalist pathogens can exhibit differential success on different hosts, resulting in complex host range patterns. Several factors operate to reduce realized host range relative to potential host range, particularly under field conditions. We explored factors influencing host range of the naturally occurring generalist ascomycete grass seed pathogen Pyrenophora semeniperda. We measured potential host range in laboratory experiments at high inoculum loads with 26 grass species, including the primary host Bromus tectorum, and developed models to predict susceptibility and tolerance based on host traits, including germination speed, seed hardness, seed size, and phylogenetic relations. We also examined pathogen and host density effects on infection and mortality. All species tested were at least somewhat susceptible to the pathogen at high inoculum loads, but both infection and mortality varied widely. Species more closely related to the original host (B. tectorum) were more susceptible to infection, whereas species with slower germination were less tolerant and therefore more likely to suffer mortality. Infection and mortality were sharply reduced as inoculum load was reduced. Intermediate loads had major negative impacts on dormant B. tectorum seeds but generally minimal effects on native species. In addition, field seed bank studies determined that P. semeniperda rarely exploits native grass species as hosts. This marked reduction in realized host range relative to potential host range indicates that laboratory host range studies are potentially a poor predictor of either the current or possible future realized host range for wildland plant pathogens.

Comparing susceptibility of eastern and western US grasslands to competition and allelopathy from spotted knapweed [ Centaurea stoebe L. subsp. micranthos (Gugler) Hayek]

Centaurea stoebe L. subsp. micranthos is native to Eurasia and is invasive in the western portion of the US. Negative impacts of C. stoebe micranthos present in the eastern US have not been recorded. In this study, we examine the effects of C. stoebe micranthos on species diversity on an eastern grassy bald, compare the competitive abilities of plant species from eastern and western grasslands against C. stoebe micranthos, and assess the production of allelopathic compounds in an eastern population of C. stoebe micranthos. Field observations indicated that increasing C. stoebe micranthos abundance was not associated with decreasing abundance or diversity of species. In a greenhouse experiment, C. stoebe micranthos growing with plant species from an eastern grassland were smaller than C. stoebe micranthos growing with species from western grasslands, suggesting that species from the eastern grassland are more competitive against C. stoebe micranthos. We found no evidence that the eastern population of C. stoebe micranthos has allelopathic effects. While the invasion dynamics may change over time, the possibility that C. stoebe micranthos may never become invasive in the studied grassy bald should be weighed when considering control measures here and throughout the eastern US. This study illustrates that invasion dynamics can vary geographically and that land managers need relevant information to gauge an appropriate and economical response.