Pathogens promote plant diversity through a compensatory response

Linking Anthropogenic Landscape Perturbation to Herbivory and Pathogen Leaf Damage in Tropical Tree Communities

Anthropogenic disturbance of tropical humid forests leads to habitat loss, biodiversity decline, landscape fragmentation, altered nutrient cycling and carbon sequestration, soil erosion, pest/pathogen outbreaks, among others. Nevertheless, the impact of these alterations in multitrophic interactions, including host-pathogen and vector-pathogen dynamics, is still not well understood in wild plants. This study aimed to provide insights into the main drivers for the incidence of herbivory and plant pathogen damage, specifically, into how vegetation traits at the local and landscape scale modulate such interactions. For this purpose, in the tropical forest of Calakmul (Campeche, Mexico), we characterised the foliar damage caused by herbivores and pathogens in woody vegetation of 13 sampling sites representing a gradient of forest disturbance and fragmentation in an anthropogenic landscape from well preserved to highly disturbed and fragmented areas. We also evaluated how the incidence of such damage was modulated by the vegetation and landscape attributes. We found that the incidence of damage caused by larger, mobile, generalist herbivores, was more sensitive to changes in landscape configuration, while the incidence of damage caused by small and specialised herbivores with low dispersal capacity was more influenced by vegetation and landscape composition. In relation to pathogen symptoms, the herbivore-induced foliar damage seems to be the main factor related to their incidence, indicating the enormous importance of herbivorous insects in the modulation of disease dynamics across tropical vegetation, as they could be acting as vectors and/or facilitating the entry of pathogens by breaking the foliar tissue and the plant defensive barriers. The incidence of pathogen damage also responded to vegetation structure and landscape configuration; the incidence of anthracnose, black spot, and chlorosis, for example, were favoured in sites surrounded by smaller patches and a higher edge density, as well as those with a greater aggregation of semi-evergreen forest patches. Fungal pathogens were shown to be an important cause of foliar damage for many woody species. Our results indicate that an increasing transformation and fragmentation of the tropical forest of southern Mexico could reduce the degree of specialisation in plant-herbivore interactions and enhance the proliferation of generalist herbivores (chewers and scrapers) and of mobile leaf suckers, and consequently, the proliferation of some symptoms associated with fungal pathogens such as fungus black spots and anthracnose. The symptoms associated with viral and bacterial diseases and to nutrient deficiency, such as chlorosis, could also increase in the vegetation in fragmented landscapes with important consequences in the health and productivity of wild and cultivated plant species. This is a pioneering study evaluating the effect of disturbances on multitrophic interactions, offering key insights on the main drivers of the changes in herbivory interactions and incidence of plant pathogens in tropical forests.

Parasites as ecosystem modulators: foliar pathogens suppress top-down effects of large herbivores

Parasites can catalyze or inhibit interactions between their hosts and other species, but the ecosystem-level effects of such interaction modifications are poorly understood. We conducted a large-scale field experiment in temperate grasslands of China to understand how foliar fungal pathogens influenced top-down effects of cattle on plant diversity and productivity. When foliar pathogens were suppressed, cattle grazing strongly reduced biomass of the dominant grass, Leymus chinensis, generating competitive release that significantly increased community-level species richness and evenness. In the absence of grazing, pathogen attack on L. chinensis had no measurable effect on host biomass. However, pathogens disrupted top-down effects of herbivory by inhibiting grazing effects on plant biomass and species richness. Mechanistically, fungal pathogens were linked to increased alkaloid and reduced nitrogen levels in leaf tissue, which appeared to deter cattle grazing on L. chinensis. In conclusion, foliar pathogens can suppress top-down effects of large herbivores on grassland community composition and ecosystem function by modifying the strength of their host's interactions with dominant consumers. Parasites may act as modulators of ecosystem function when their direct effects on host abundance are overshadowed by powerful influences on host traits that modify their interactions with competitors, herbivores, or predators.

Effects of nutrient heterogeneity on root foraging and plant growth at the individual and community level

Plants enhance nutrient uptake in heterogeneous nutrient environments through selective root placement. Many studies have documented that plants grow better under heterogeneous than under homogeneous nutrient distribution, but comprehensive syntheses are relatively few. In a meta-analysis, we examined the effects of patch scale and contrast on plant responses by synthesizing the effects of nutrient heterogeneity on root foraging and plant growth in 131 comparative studies. Plant responses to nutrient heterogeneity were phylogenetically conserved, and the response in shoot biomass was significantly correlated with the response in root biomass but not with root foraging precision. Root precision depended on the competition regime, and plants had lower precision in interspecific than in conspecific competition. Community-level growth was significantly promoted by nutrient heterogeneity and was less variable than individual-level responses. Along with increasing patch scale, overall shoot and root responses of individuals increased but root foraging precision declined. In addition, moderate patch contrast induced the highest root responses. Our results indicate that plants optimize nutrient acquisition from heterogeneous patches mainly through increasing root growth, and plant communities exploit heterogeneous nutrients more effectively than individuals. Understanding the roles of patch attributes in nutrient-heterogeneity effects may help in designing fertilization practices to promote productivity and conserve biodiversity.

Climate warming dominates over plant genotype in shaping the seasonal trajectory of foliar fungal communities on oak

Leaves interact with a wealth of microorganisms. Among these, fungi are highly diverse and are known to contribute to plant health, leaf senescence and early decomposition. However, patterns and drivers of the seasonal dynamics of foliar fungal communities are poorly understood. We used a multifactorial experiment to investigate the influence of warming and tree genotype on the foliar fungal community on the pedunculate oak Quercus robur across one growing season. Fungal species richness increased, evenness tended to decrease, and community composition strongly shifted during the growing season. Yeasts increased in relative abundance as the season progressed, while putative fungal pathogens decreased. Warming decreased species richness, reduced evenness and changed community composition, especially at the end of the growing season. Warming also negatively affected putative fungal pathogens. We only detected a minor imprint of tree genotype and warming × genotype interactions on species richness and community composition. Overall, our findings demonstrate that warming plays a larger role than plant genotype in shaping the seasonal dynamics of the foliar fungal community on oak. These warming-induced shifts in the foliar fungal community may have a pronounced impact on plant health, plant-fungal interactions and ecosystem functions.

Emerging fungal pathogen of an invasive grass: Implications for competition with native plant species

Infectious diseases and invasive species can be strong drivers of biological systems that may interact to shift plant community composition. For example, disease can modify resource competition between invasive and native species. Invasive species tend to interact with a diversity of native species, and it is unclear how native species differ in response to disease-mediated competition with invasive species. Here, we quantified the biomass responses of three native North American grass species (Dichanthelium clandestinum, Elymus virginicus, and Eragrostis spectabilis) to disease-mediated competition with the non-native invasive grass Microstegium vimineum. The foliar fungal pathogen Bipolaris gigantea has recently emerged in Microstegium populations, causing a leaf spot disease that reduces Microstegium biomass and seed production. In a greenhouse experiment, we examined the effects of B. gigantea inoculation on two components of competitive ability for each native species: growth in the absence of competition and biomass responses to increasing densities of Microstegium. Bipolaris gigantea inoculation affected each of the three native species in unique ways, by increasing (Dichanthelium), decreasing (Elymus), or not changing (Eragrostis) their growth in the absence of competition relative to mock inoculation. Bipolaris gigantea inoculation did not, however, affect Microstegium biomass or mediate the effect of Microstegium density on native plant biomass. Thus, B. gigantea had species-specific effects on native plant competition with Microstegium through species-specific biomass responses to B. gigantea inoculation, but not through modified responses to Microstegium density. Our results suggest that disease may uniquely modify competitive interactions between invasive and native plants for different native plant species.

Is Variation in Conspecific Negative Density Dependence Driving Tree Diversity Patterns at Large Scales?

Half a century ago, Janzen and Connell hypothesized that the high tree species diversity in tropical forests is maintained by specialized natural enemies. Along with other mechanisms, these can cause conspecific negative density dependence (CNDD) and thus maintain species diversity. Numerous studies have measured proxies of CNDD worldwide, but doubt about its relative importance remains. We find ample evidence for CNDD in local populations, but methodological limitations make it difficult to assess if CNDD scales up to control community diversity and thereby local and global biodiversity patterns. A combination of more robust statistical methods, new study designs, and eco-evolutionary models are needed to provide a more definite evaluation of the importance of CNDD for geographic variation in plant species diversity.

Neighboring trees regulate the root-associated pathogenic fungi on the host plant in a subtropical forest

Root-associated fungi and host-specific pathogens are major determinants of species coexistence in forests. Phylogenetically related neighboring trees can strongly affect the fungal community structure of the host plant, which, in turn, will affect the ecological processes. Unfortunately, our understanding of the factors influencing fungal community composition in forests is still limited. In particular, investigation of the relationship between the phytopathogenic fungal community and neighboring trees is incomplete. In the current study, we tested the host specificity of members of the root-associated fungal community collected from seven tree species and determined the influence of neighboring trees and habitat variation on the composition of the phytopathogenic fungal community of the focal plant in a subtropical evergreen forest. Using high-throughput sequencing data with respect to the internal transcribed spacer (ITS) region, we characterized the community composition of the root-associated fungi and found significant differences with respect to fungal groups among the seven tree species. The density of conspecific neighboring trees had a significantly positive influence on the relative abundance of phytopathogens, especially host-specific pathogens, while the heterospecific neighbor density had a significant negative impact on the species richness of host-specific pathogens, as well as phytopathogens. Our work provides evidence that the root-associated phytopathogenic fungi of a host plant depend greatly on the tree neighbors of the host plant.

Inoculum handling alters the strength and direction of plant-microbe interactions

The pooling of soil samples in plant-microbe interaction studies is commonly employed, but the impact of sample handling has rarely been explored experimentally. Concerns have been raised that sample pooling may reduce biological variation leading to inflated type I errors or may alter the magnitude of microbial effects observed, invalidating the results achieved. To assess the impact of inocula pooling on plant-microbe interactions, we examined the reciprocal influence of unpooled and pooled soil microbial inocula on growth of Solidago altissima and Schizachyrium scoparium, with and without inoculum sterilization. Soil pooling had no effect on the variance among replicates in either plant species. However, pooling dramatically altered the magnitude and direction of microbial impacts on plant performance. Pooling of Solidago altissima soil increased the antagonistic effects on growth of both target species. In contrast, pooling of Schizachyrium scoparium soil shifted impacts on Solidago altissima from effectively neutral to slightly positive. Pooling in this system altered both the strength and direction of plant-microbe interactions relative to unpooled soils. Therefore soil mixing should be avoided when the research goal is to determine naturally occurring interaction strengths, even within a single habitat.

Soil-borne fungi influence seed germination and mortality, with implications for coexistence of desert winter annual plants

Soil-borne fungi influence coexistence of plant species in mesic environments, but much less is known about their effects on demographic processes relevant to coexistence in arid and semi-arid systems. We isolated 43 fungal strains that naturally colonize seeds of an invasive winter annual (Brassica tournefortii) in the Sonoran Desert, and evaluated the impact of 18 of them on seed germination and mortality of B. tournefortii and a co-occurring native annual (Plantago ovata) under simulated summer and winter temperatures. Fungi isolated from B. tournefortii seeds impacted germination and mortality of seeds of both plant species in vitro. Seed responses reflected host-specific effects by fungi, the degree of which differed significantly between the strains, and depended on the temperature. In the winter temperature, ten fungal strains increased or reduced seed germination, but substantial seed mortality due to fungi was not observed. Two strains increased germination of P. ovata more strongly than B. tournefortii. In the summer temperature, fungi induced both substantial seed germination and mortality, with ten strains demonstrating host-specificity. Under natural conditions, host-specific effects of fungi on seed germination may further differentiate plant species niche in germination response, with a potential of promoting coexistence. Both host-specific and non-host-specific effects of fungi on seed loss may induce polarizing effects on plant coexistence depending on the ecological context. The coexistence theory provides a clear framework to interpret these polarizing effects. Moreover, fungi pathogenic to both plant species could induce host-specific germination, which challenges the theoretical assumption of density-independent germination response. These implications from an in vitro study underscore the need to weave theoretical modeling, reductive empirical experiments, and natural observations to illuminate effects of soil-borne fungi on coexistence of annual plant species in variable desert environments.

A highly resolved food web for insect seed predators in a species-rich tropical forest

The top-down and indirect effects of insects on plant communities depend on patterns of host use, which are often poorly documented, particularly in species-rich tropical forests. At Barro Colorado Island, Panama, we compiled the first food web quantifying trophic interactions between the majority of co-occurring woody plant species and their internally feeding insect seed predators. Our study is based on more than 200 000 fruits representing 478 plant species, associated with 369 insect species. Insect host-specificity was remarkably high: only 20% of seed predator species were associated with more than one plant species, while each tree species experienced seed predation from a median of two insect species. Phylogeny, but not plant traits, explained patterns of seed predator attack. These data suggest that seed predators are unlikely to mediate indirect interactions such as apparent competition between plant species, but are consistent with their proposed contribution to maintaining plant diversity via the Janzen-Connell mechanism.

Variations in phyllosphere microbial community along with the development of angular leaf-spot of cucumber

The phyllosphere is colonized by a wide variety of microorganisms including epiphytes, plant-pathogenic fungus, bacteria, as well as human or animal pathogens. However, little is known about how microbial community composition changes with the development of angular leaf-spot of cucumber. Here, 18 mixed samples were collected based on the lesion coverage rate (LCR) of angular leaf-spot of cucumber from three disease severity groups (DM1: symptomatic-mild, DM2: symptomatic-moderate, DM3: symptomatic-severe). In our study, the microbial community structure and diversity were examined by Illumina MiSeq sequencing. A significant differences was observed in α diversity and community structure among three disease severity groups. The phyllosphere microbiota was observed to be dominated by bacterial populations from Proteobacteria, Actinobacteria, and Firmicutes, as well as fungal species from Ascomycota and Basidiomycota. In addition, some plant-specific microbe such as Sphingomonas, Methylobacterium, Pseudomonas, and Alternaria showed significant changes in their relative abundance of population. The LCR was correlated negatively with Sphingomonas, Methylobacterium, Quadrisphaera, and Lactobacillus, whereas correlated positively with Pseudomonas and Kineococcus (p < 0.05). The LCR was negatively correlated with Alternaria and Arthrinium of the fungal communities (p < 0.05). Molecular ecological networks of the microbial communities were constructed to show the interactions among the OTUs. Our current results indicated that the competitive relationships among species were broken with the development of angular leaf-spot of cucumber. The microbial community composition changed over the development of angular leaf-spot of cucumber. The result of molecular ecological networks indicated that the overall bacterial community tends toward mutualism from the competition. The development of angular leaf-spot of cucumber affected the ecosystem functioning by disrupting the stability of the microbial community network. This work will help us to understand the host plant-specific microbial community structures and shows how these communities change throughout the development of angular leaf-spot of cucumber.

Tolerance and overcompensation to infection by Phytophthora infestans in the wild perennial climber Solanum dulcamara

Studies of infection by Phytophthora infestans-the causal agent of potato late blight-in wild species can provide novel insights into plant defense responses, and indicate how wild plants might be influenced by recurrent epidemics in agricultural fields. In the present study, our aim was to investigate if different clones of Solanum dulcamara (a relative of potato) collected in the wild differ in resistance and tolerance to infection by a common European isolate of P. infestans. We performed infection experiments with six S. dulcamara genotypes (clones) both in the laboratory and in the field and measured the degree of infection and plant performance traits. In the laboratory, the six evaluated genotypes varied from resistant to susceptible, as measured by degree of infection 20 days post infection. Two of the four genotypes susceptible to infection showed a quadratic (concave downward) relationship between the degree of infection and shoot length, with maximum shoot length at intermediate values of infection. This result suggests overcompensation, that is, an increase in growth in infected individuals. The number of leaves decreased with increasing degree of infection, but at different rates in the four susceptible genotypes, indicating genetic variation for tolerance. In the field, the inoculated genotypes did not show any disease symptoms, but plant biomass at the end of the growing season was higher for inoculated plants than for controls, in-line with the overcompensation detected in the laboratory. We conclude that in S. dulcamara there are indications of genetic variation for both resistance and tolerance to P. infestans infection. Moreover, some genotypes displayed overcompensation. Learning about plant tolerance and overcompensation to infection by pathogens can help broaden our understanding of plant defense in natural populations and help develop more sustainable plant protection strategies for economically important crop diseases.

Deeply conserved susceptibility in a multi-host, multi-parasite system

Variation in susceptibility is ubiquitous in multi-host, multi-parasite assemblages, and can have profound implications for ecology and evolution in these systems. The extent to which susceptibility to parasites is phylogenetically conserved among hosts can be revealed by analysing diverse regional communities. We screened for haemosporidian parasites in 3983 birds representing 40 families and 523 species, spanning ~ 4500 m elevation in the tropical Andes. To quantify the influence of host phylogeny on infection status, we applied Bayesian phylogenetic multilevel models that included a suite of environmental, spatial, temporal, life history and ecological predictors. We found evidence of deeply conserved susceptibility across the avian tree; host phylogeny explained substantial variation in infection status, and results were robust to phylogenetic uncertainty. Our study suggests that susceptibility is governed, in part, by conserved, latent aspects of anti-parasite defence. This demonstrates the importance of deep phylogeny for understanding present-day ecological interactions.

Heterospecific plant-soil feedback and its relationship to plant traits, species relatedness, and co-occurrence in natural communities

Plant-soil feedback is one of the mechanisms affecting co-existence of species, ecological succession, and species invasiveness. However, in contrast to conspecific plant-soil feedback, general patterns in heterospecific feedback are mostly unknown. We used a meta-analysis to search for correlations between heterospecific feedback and species relatedness, functional traits, and field co-occurrence patterns. We searched published literature and compiled a data set of 618 PSF interactions. We gathered data on species traits reflecting plant size and growth rate (height, specific leaf area, and life span), co-occurrence in habitats and phylogenetic distance between species pairs. We found that species grew better in soil conditioned by (i) close relatives than in conspecific soil, whereas there was no relationship with phylogeny for distantly related species, (ii) species of greater plant height (but there was no relationship with species SLA or life span), and (iii) species more frequently co-occurring in the field. The results show that heterospecific plant-soil feedback can be explained by plant traits (height) and is reflected in co-occurrence patterns. Phylogeny was a significant predictor of feedbacks over short phylogenetic distance, suggesting fast evolution of traits related to feedback. The low variability explained by the models, however, indicates that other factors such as environmental conditions possibly alter plant-soil feedback responses.

Adult trees cause density-dependent mortality in conspecific seedlings by regulating the frequency of pathogenic soil fungi

Negative density-dependent seedling mortality has been widely detected in tropical, subtropical and temperate forests, with soil pathogens as a major driver. Here we investigated how host density affects the composition of soil pathogen communities and consequently influences the strength of plant-soil feedbacks. In field censuses of six 1-ha permanent plots, we found that survival was much lower for newly germinated seedlings that were surrounded by more conspecific adults. The relative abundance of pathogenic fungi in soil increased with increasing conspecific tree density for five of nine tree species; more soil pathogens accumulated around roots where adult tree density was higher, and this greater pathogen frequency was associated with lower seedling survival. Our findings show how tree density influences populations of soil pathogens, which creates plant-soil feedbacks that contribute to community-level and population-level compensatory trends in seedling survival.

Tree phyllosphere bacterial communities: exploring the magnitude of intra- and inter-individual variation among host species

Background

The diversity and composition of the microbial community of tree leaves (the phyllosphere) varies among trees and host species and along spatial, temporal, and environmental gradients. Phyllosphere community variation within the canopy of an individual tree exists but the importance of this variation relative to among-tree and among-species variation is poorly understood. Sampling techniques employed for phyllosphere studies include picking leaves from one canopy location to mixing randomly selected leaves from throughout the canopy. In this context, our goal was to characterize the relative importance of intra-individual variation in phyllosphere communities across multiple species, and compare this variation to inter-individual and interspecific variation of phyllosphere epiphytic bacterial communities in a natural temperate forest in Quebec, Canada.

Methods

We targeted five dominant temperate forest tree species including angiosperms and gymnosperms: Acer saccharum, Acer rubrum, Betula papyrifera, Abies balsamea and Picea glauca. For one randomly selected tree of each species, we sampled microbial communities at six distinct canopy locations: bottom-canopy (1–2 m height), the four cardinal points of mid-canopy (2–4 m height), and the top-canopy (4–6 m height). We also collected bottom-canopy leaves from five additional trees from each species.

Results

Based on an analysis of bacterial community structure measured via Illumina sequencing of the bacterial 16S gene, we demonstrate that 65% of the intra-individual variation in leaf bacterial community structure could be attributed to the effect of inter-individual and inter-specific differences while the effect of canopy location was not significant. In comparison, host species identity explains 47% of inter-individual and inter-specific variation in leaf bacterial community structure followed by individual identity (32%) and canopy location (6%).

Discussion

Our results suggest that individual samples from consistent positions within the tree canopy from multiple individuals per species can be used to accurately quantify variation in phyllosphere bacterial community structure. However, the considerable amount of intra-individual variation within a tree canopy ask for a better understanding of how changes in leaf characteristics and local abiotic conditions drive spatial variation in the phyllosphere microbiome.

Impact of disease on diversity and productivity of plant populations

Experiments were conducted on the role of intra- and inter-genotypic competition in ecological processes operating at the population scale in diseased plant populations.Combinations of Arabidopsis thaliana genotypes showing variation for phenotypic traits relating to competitive ability and pathogen compatibility were infected with the oomycete Hyaloperonospora arabidopsidis and Turnip yellows virus in separate experiments. Plant fitness and competitive ability were estimated from phenotypic measurements.Pathogen-induced reduction in competitive ability for susceptible genotypes increased the competitive ability of resistant genotypes, resulting in maintenance of yield via competitive release. The two diseases had different effects on competitive interactions between plants. In experiments involving the oomycete, the highest yields were produced by mixtures of two weakly competing genotypes.The Arabidopsis model system has elucidated the ecological processes by which compensatory competitive interactions can increase the buffering capacity of plant populations under pathogen attack. Highly competitive genotypes may not maximize the productivity of the population as a whole, as they may over-yield at the expense of less competitive, more productive genotypes. The specific outcomes of competitive interactions cannot be generalized because they depend on the disease and the host genotypes.

Parasite and nutrient enrichment effects on Daphnia interspecific competition

Increased productivity due to nutrient enrichment is hypothesized to affect density-dependent processes, such as transmission success of horizontally transmitting parasites. Changes in nutrient availability can also modify the stoichiometry and condition of individual hosts, which may affect their susceptibility for parasites as well as the growth conditions for parasites within the host. Consequently, if not balanced by increased host immuno-competence or life history responses, changes in the magnitude of parasite effects with increasing nutrient availability are expected. If these parasite effects are host-species specific, this may lead to shifts in the host community structure. We here used the Daphnia- parasite model system to study the effect of nutrient enrichment on parasite-mediated competition in experimental mesocosms. In the absence of parasites, D. magna was competitively dominant to D. pulex at both low and high nutrient levels. Introduction of parasites resulted in infections of D. magna, but not of D. pulex and, as such, reversed the competitive hierarchy between these two species. Nutrient addition resulted in an increased prevalence and infection intensity of some of the parasites on D. magna. However, there was no evidence that high nutrient levels enhanced negative effects of parasites on the hosts. Costs associated with parasite infections may have been compensated by better growth conditions for D. magna in the presence of high nutrient levels.

Linking winter conditions to regional disease dynamics in a wild plant-pathogen metapopulation

Pathogens are considered to drive ecological and evolutionary dynamics of plant populations, but we lack data measuring the population-level consequences of infection in wild plant-pathogen interactions. Moreover, while it is often assumed that offseason environmental conditions drive seasonal declines in pathogen population size, little is known about how offseason environmental conditions impact the survival of pathogen resting stages, and how critical the offseason is for the next season's epidemic. The fungal pathogen Podosphaera plantaginis persists as a dynamic metapopulation in the large network of Plantago lanceolata host populations. Here, we analyze long-term data to measure the spatial synchrony of epidemics and consequences of infection for over 4000 host populations. Using a theoretical model, we study whether large-scale environmental change could synchronize disease occurrence across the metapopulation. During 2001-2013 exposure to freezing decreased, while pathogen extinction-colonization-persistence rates became more synchronized. Simulations of a theoretical model suggest that increasingly favorable winter conditions for pathogen survival could drive such synchronization. Our data also show that infection decreases host population growth. These results confirm that mild winter conditions increase pathogen overwintering success and thus increase disease prevalence across the metapopulation. Further, we conclude that the pathogen can drive host population growth in the Plantago-Podosphaera system.

Tree diversity and the role of non-host neighbour tree species in reducing fungal pathogen infestation

The degree to which plant pathogen infestation occurs in a host plant is expected to be strongly influenced by the level of species diversity among neighbouring host and non-host plant species. Since pathogen infestation can negatively affect host plant performance, it can mediate the effects of local biodiversity on ecosystem functioning.We tested the effects of tree diversity and the proportion of neighbouring host and non-host species with respect to the foliar fungal pathogens of Tilia cordata and Quercus petraea in the Kreinitz tree diversity experiment in Germany. We hypothesized that fungal pathogen richness increases while infestation decreases with increasing local tree diversity. In addition, we tested whether fungal pathogen richness and infestation are dependent on the proportion of host plant species present or on the proportion of particular non-host neighbouring tree species.Leaves of the two target species were sampled across three consecutive years with visible foliar fungal pathogens on the leaf surface being identified macro- and microscopically. Effects of diversity among neighbouring trees were analysed: (i) for total fungal species richness and fungal infestation on host trees and (ii) for infestation by individual fungal species.We detected four and five fungal species on T. cordata and Q. petraea, respectively. High local tree diversity reduced (i) total fungal species richness and infestation of T. cordata and fungal infestation of Q. petraea and (ii) infestation by three host-specialized fungal pathogen species. These effects were brought about by local tree diversity and were independent of host species proportion. In general, host species proportion had almost no effect on fungal species richness and infestation. Strong effects associated with the proportion of particular non-host neighbouring tree species on fungal species richness and infestation were, however, recorded.Synthesis. For the first time, we experimentally demonstrated that for two common forestry tree species, foliar fungal pathogen richness and infestation depend on local biodiversity. Thus, local tree diversity can have positive impacts on ecosystem functioning in managed forests by decreasing the level of fungal pathogen infestation.

Effects of virus on plant fecundity and population dynamics

Microorganisms are ubiquitous and thought to regulate host populations. Although microorganisms can be pathogenic and affect components of fitness, few studies have examined their effects on wild plant populations. As individual traits might not contribute equally to changes in population growth rate, it is essential to examine the entire life cycle to determine how microorganisms affect host population dynamics. In this study, we used data from common garden experiments with plants from three Cucurbita pepo populations exposed to three virus treatments. These data were used to parameterize a deterministic matrix model, which allowed us to estimate the effect of virus on components of fitness and population growth rate. Virus did not reduce fruit number, but population growth rates varied among virus treatments and wild C. pepo populations. The effect of virus on population growth rate depended on virus species and wild C. pepo population. Contributions of life-history transitions and life-history traits to population growth rates varied among populations and virus treatments. However, this population-virus interaction was not evident when examining individual components of fitness. Thus, caution must be used when interpreting the effects of changes in individual traits, as single traits do not always predict population-level change accurately.

Pathogen regulation of plant diversity via effective specialization

The Janzen-Connell (JC) hypothesis, one of the most influential hypotheses explaining forest diversity, is inconsistent with evidence that tree species share the same natural enemies. Through the discussion of seedling diseases from a pathogen-centered perspective, we expand the JC hypothesis to tie in host-pathogen-environment interactions at three levels: local adaptation, host specificity of the combined effect of multiple infections, and environmental modulation of disease. We present evidence from plant pathology, disease ecology, and host-parasite evolution relevant to (but not commonly associated with) forest species diversity maintenance. This expanded view of the JC hypothesis suggests ways to direct new experiments to integrate research on pathogen local adaptation, co-infection, and environmental effects on infection by using high-throughput molecular techniques and statistical models.

Migrate or evolve: options for plant pathogens under climate change

Findings on climate change influence on plant pathogens are often inconsistent and context dependent. Knowledge of pathogens affecting agricultural crops and natural plant communities remains fragmented along disciplinary lines. By broadening the perspective beyond agriculture, this review integrates cross-disciplinary knowledge to show that at scales relevant to climate change, accelerated evolution and changing geographic distribution will be the main implications for pathogens. New races may evolve rapidly under elevated temperature and CO2 , as evolutionary forces act on massive pathogen populations boosted by a combination of increased fecundity and infection cycles under favourable microclimate within enlarged canopy. Changing geographic distribution will bring together diverse lineages/genotypes that do not share common ecological niche, potentially increasing pathogen diversity. However, the uncertainty of model predictions and a lack of synthesis of fragmented knowledge remain as major deficiencies in knowledge. The review contends that the failure to consider scale and human intervention through new technology are major sources of uncertainty. Recognizing that improved biophysical models alone will not reduce uncertainty, it proposes a generic framework to increase focus and outlines ways to integrate biophysical elements and technology change with human intervention scenarios to minimize uncertainty. To synthesize knowledge of pathogen biology and life history, the review borrows the concept of 'fitness' from population biology as a comprehensive measure of pathogen strengths and vulnerabilities, and explores the implications of pathogen mode of nutrition to fitness and its interactions with plants suffering chronic abiotic stress under climate change. Current and future disease management options can then be judged for their ability to impair pathogenic and saprophytic fitness. The review pinpoints improving confidence in model prediction by minimizing uncertainty, developing management strategies to reduce overall pathogen fitness, and finding new sources of data to trawl for climate signatures on pathogens as important challenges for future research.

Multihost saprobes are facultative pathogens of bullfrog Lithobates catesbeianus eggs

Pathogens that infect more than one host species create complex linkages in ecological communities. We tested whether saprobes that grow on multiple host species in aquatic systems can be facultative pathogens of amphibian eggs. We isolated oomycetes from dead arthropods, vertebrates, plant leaves, and frog eggs that coexisted in a small pond. Analysis of internal transcribed spacer regions of rDNA (ITS1-5.8S-ITS2) indicated that several of the strains colonized more than one substrate, including bullfrog Lithobates catesbeianus eggs. In a controlled experiment, isolates from 7 different host species were pathogenic to L. catesbeianus eggs. These results demonstrate that dead organisms can serve as reservoirs for facultative pathogens.

The composition of phyllosphere fungal assemblages of European beech (Fagus sylvatica) varies significantly along an elevation gradient

Little is known about the potential effect of climate warming on phyllosphere fungi, despite their important impact on the dynamics and diversity of plant communities. The structure of phyllosphere fungal assemblages along elevation gradients may provide information about this potential effect, because elevation gradients correspond to temperature gradients over short geographic distances. We thus investigated variations in the composition of fungal assemblages inhabiting the phyllosphere of European beech (Fagus sylvatica) at four sites over a gradient of 1000 m of elevation in the French Pyrénées Mountains, by using tag-encoded 454 pyrosequencing. Our results show that the composition of fungal assemblages varied significantly between elevation sites, in terms of both the relative abundance and the presence-absence of species, and that the variations in assemblage composition were well correlated with variations in the average temperatures. Our results therefore suggest that climate warming might alter both the incidence and the abundance of phyllosphere fungal species, including potential pathogens. For example, Mycosphaerella punctiformis, a causal agent of leaf spots, showed decreasing abundance with elevation and might therefore shift to higher elevations in response to warming.

Community compensatory trend prevails from tropical to temperate forest

Community compensatory trend (CCT) is thought to facilitate persistence of rare species and thus stabilize species composition in tropical forests. However, whether CCT acts over broad geographical ranges is still in question. In this study, we tested for the presence of negative density dependence (NDD) and CCT in three forests along a tropical-temperate gradient. Inventory data were collected from forest communities located in three different latitudinal zones in China. Two widely used methods were used to test for NDD at the community level. The first method considered relationships between the relative abundance ratio and adult abundance. The second method emphasized the effect of adult abundance on abundance of established younger trees. Evidence for NDD acting on different growth forms was tested by using the first method, and the presence of CCT was tested by checking whether adult abundance of rare species affected that of established younger trees less than did abundance of common species. Both analyses indicated that NDD existed in seedling, sapling and pole stages in all three plant communities and that this effect increased with latitude. However, the extent of NDD varied among understory, midstory and canopy trees in the three communities along the gradient. Additionally, despite evidence of NDD for almost all common species, only a portion of rare species showed NDD, supporting the action of CCT in all three communities. So, we conclude that NDD and CCT prevail in the three recruitment stages of the tree communities studied; rare species achieve relative advantage through CCT and thus persist in these communities; CCT clearly facilitates newly established species and maintains tree diversity within communities across our latitudinal gradient.

Geographical ecology of the palms (Arecaceae): determinants of diversity and distributions across spatial scales

BACKGROUND

The palm family occurs in all tropical and sub-tropical regions of the world. Palms are of high ecological and economical importance, and display complex spatial patterns of species distributions and diversity.

SCOPE

This review summarizes empirical evidence for factors that determine palm species distributions, community composition and species richness such as the abiotic environment (climate, soil chemistry, hydrology and topography), the biotic environment (vegetation structure and species interactions) and dispersal. The importance of contemporary vs. historical impacts of these factors and the scale at which they function is discussed. Finally a hierarchical scale framework is developed to guide predictor selection for future studies.

CONCLUSIONS

Determinants of palm distributions, composition and richness vary with spatial scale. For species distributions, climate appears to be important at landscape and broader scales, soil, topography and vegetation at landscape and local scales, hydrology at local scales, and dispersal at all scales. For community composition, soil appears important at regional and finer scales, hydrology, topography and vegetation at landscape and local scales, and dispersal again at all scales. For species richness, climate and dispersal appear to be important at continental to global scales, soil at landscape and broader scales, and topography at landscape and finer scales. Some scale-predictor combinations have not been studied or deserve further attention, e.g. climate on regional to finer scales, and hydrology and topography on landscape and broader scales. The importance of biotic interactions - apart from general vegetation structure effects - for the geographic ecology of palms is generally underexplored. Future studies should target scale-predictor combinations and geographic domains not studied yet. To avoid biased inference, one should ideally include at least all predictors previously found important at the spatial scale of investigation.

Indigenous arbuscular mycorrhizal fungal assemblages protect grassland host plants from pathogens

Plant roots can establish associations with neutral, beneficial and pathogenic groups of soil organisms. Although it has been recognized from the study of individual isolates that these associations are individually important for plant growth, little is known about interactions of whole assemblages of beneficial and pathogenic microorganisms associating with plants.We investigated the influence of an interaction between local arbuscular mycorrhizal (AM) fungal and pathogenic/saprobic microbial assemblages on the growth of two different plant species from semi-arid grasslands in NE Germany (Mallnow near Berlin). In a greenhouse experiment each plant species was grown for six months in either sterile soil or in sterile soil with one of three different treatments: 1) an AM fungal spore fraction isolated from field soil from Mallnow; 2) a soil pathogen/saprobe fraction consisting of a microbial community prepared with field soil from Mallnow and; 3) the combined AM fungal and pathogen/saprobe fractions. While both plant species grew significantly larger in the presence of AM fungi, they responded negatively to the pathogen/saprobe treatment. For both plant species, we found evidence of pathogen protection effects provided by the AM fungal assemblages. These results indicate that interactions between assemblages of beneficial and pathogenic microorganisms can influence the growth of host plants, but that the magnitude of these effects is plant species-specific.

The potential of plant viruses to promote genotypic diversity via genotype x environment interactions

BACKGROUND AND AIMS

Genotype by environment (G × E) interactions are important for the long-term persistence of plant species in heterogeneous environments. It has often been suggested that disease is a key factor for the maintenance of genotypic diversity in plant populations. However, empirical evidence for this contention is scarce. Here virus infection is proposed as a possible candidate for maintaining genotypic diversity in their host plants.

METHODS

The effects of White clover mosaic virus (WClMV) on the performance and development of different Trifolium repens genotypes were analysed and the G × E interactions were examined with respect to genotype-specific plant responses to WClMV infection. Thus, the environment is defined as the presence or absence of the virus.

KEY RESULTS

WClMV had a negative effect on plant performance as shown by a decrease in biomass and number of ramets. These effects of virus infection differ greatly among host genotypes, representing a strong G × E interaction. Moreover, the relative fitness and associated ranking of genotypes changed significantly between control and virus treatments. This shift in relative fitness among genotypes suggests the potential for WClMV to provoke differential selection on T. repens genotypes, which may lead to negative frequency-dependent selection in host populations.

CONCLUSIONS

The apparent G × E interaction and evident repercussions for relative fitness reported in this study stress the importance of viruses for ecological and evolutionary processes and suggest an important role for viruses in shaping population dynamics and micro-evolutionary processes.

Foliar fungal pathogens and grassland biodiversity

By attacking plants, herbivorous mammals, insects, and belowground pathogens are known to play an important role in maintaining biodiversity in grasslands. Foliar fungal pathogens are ubiquitous in grassland ecosystems, but little is known about their role as drivers of community composition and diversity. Here we excluded foliar fungal pathogens from perennial grassland by using fungicide to determine the effect of natural levels of disease on an otherwise undisturbed plant community. Importantly, we excluded foliar fungal pathogens along with rabbits, insects, and mollusks in a full factorial design, which allowed a comparison of pathogen effects along with those of better studied plant enemies. This revealed that fungal pathogens substantially reduced aboveground plant biomass and promoted plant diversity and that this especially benefited legumes. The scale of pathogen effects on productivity and biodiversity was similar to that of rabbits and insects, but different plant species responded to the exclusion of the three plant enemies. These results suggest that theories of plant coexistence and management of biodiversity in grasslands should consider foliar fungal pathogens as potentially important drivers of community composition.

Herbivore and fungal pathogen exclusion affects the seed production of four common grassland species

Insect herbivores and fungal pathogens can independently affect plant fitness, and may have interactive effects. However, few studies have experimentally quantified the joint effects of insects and fungal pathogens on seed production in non-agricultural populations. We examined the factorial effects of insect herbivore exclusion (via insecticide) and fungal pathogen exclusion (via fungicide) on the population-level seed production of four common graminoid species (Andropogon gerardii, Schizachyrium scoparium, Poa pratensis, and Carex siccata) over two growing seasons in Minnesota, USA. We detected no interactive effects of herbivores and pathogens on seed production. However, the seed production of all four species was affected by either insecticide or fungicide in at least one year of the study. Insecticide consistently doubled the seed production of the historically most common species in the North American tallgrass prairie, A. gerardii (big bluestem). This is the first report of insect removal increasing seed production in this species. Insecticide increased A. gerardii number of seeds per seed head in one year, and mass per seed in both years, suggesting that consumption of flowers and seed embryos contributed to the effect on seed production. One of the primary insect species consuming A. gerardii flowers and seed embryos was likely the Cecidomyiid midge, Contarinia wattsi. Effects on all other plant species varied among years. Herbivores and pathogens likely reduce the dispersal and colonization ability of plants when they reduce seed output. Therefore, impacts on seed production of competitive dominant species may help to explain their relatively poor colonization abilities. Reduced seed output by dominant graminoids may thereby promote coexistence with subdominant species through competition-colonization tradeoffs.

Do fungal pathogens drive density-dependent mortality in established seedlings of two dominant African rain-forest trees?

Where one or a few tree species reach local high abundance, different ecological factors may variously facilitate or hinder their regeneration. Plant pathogens are thought to be one of those possible agents which drive intraspecific density-dependent mortality of tree seedlings in tropical forests. Experimental evidence for this is scarce, however. In an African rain forest at Korup, we manipulated the density of recently established seedlings (~5–8 wk old; low vs. high-density) of two dominant species of contrasting recruitment potential, and altered their exposure to pathogens using a broad-spectrum fungicide. Seedling mortality of the abundantly recruiting subcanopy tree Oubanguia alata was strongly density-dependent after 7 mo, yet fungicide-treated seedlings had slightly higher mortality than controls. By contrast, seedling mortality of the poorly recruiting large canopy-emergent tree Microberlinia bisulcata was unaffected by density or fungicide. Ectomycorrhizal colonization of M. bisulcata was not affected by density or fungicide either. For O. alata, adverse effects of fungicide on its vesicular arbuscular mycorrhizas may have offset any possible benefit of pathogen removal. We tentatively conclude that fungal pathogens are not a likely major cause of density dependence in O. alata, or of early post-establishment mortality in M. bisulcata. They do not explain the latter's currently very low recruitment rate at Korup.

Continua of specificity and virulence in plant host-pathogen interactions: causes and consequences

Ecological, evolutionary and molecular models of interactions between plant hosts and microbial pathogens are largely based around a concept of tightly coupled interactions between species pairs. However, highly pathogenic and obligate associations between host and pathogen species represent only a fraction of the diversity encountered in natural and managed systems. Instead, many pathogens can infect a wide range of hosts, and most hosts are exposed to more than one pathogen species, often simultaneously. Furthermore, outcomes of pathogen infection vary widely because host plants vary in resistance and tolerance to infection, while pathogens are also variable in their ability to grow on or within hosts. Environmental heterogeneity further increases the potential for variation in plant host-pathogen interactions by influencing the degree and fitness consequences of infection. Here, we describe these continua of specificity and virulence inherent within plant host-pathogen interactions. Using this framework, we describe and contrast the genetic and environmental mechanisms that underlie this variation, outline consequences for epidemiology and community structure, explore likely ecological and evolutionary drivers, and highlight several key areas for future research.