Ungulates increase forest plant species richness to the benefit of non‐forest specialists

Large wild ungulates are a major biotic factor shaping plant communities. They influence species abundance and occurrence directly by herbivory and plant dispersal, or indirectly by modifying plant‐plant interactions and through soil disturbance. In forest ecosystems, researchers’ attention has been mainly focused on deer overabundance. Far less is known about the effects on understory plant dynamics and diversity of wild ungulates where their abundance is maintained at lower levels to mitigate impacts on tree regeneration. We used vegetation data collected over 10 years on 82 pairs of exclosure (excluding ungulates) and control plots located in a nation‐wide forest monitoring network (Renecofor). We report the effects of ungulate exclusion on (i) plant species richness and ecological characteristics, (ii) and cover percentage of herbaceous and shrub layers. We also analyzed the response of these variables along gradients of ungulate abundance, based on hunting statistics, for wild boar (Sus scrofa), red deer (Cervus elaphus) and roe deer (Capreolus capreolus). Outside the exclosures, forest ungulates maintained higher species richness in the herbaceous layer (+15%), while the shrub layer was 17% less rich, and the plant communities became more light‐demanding. Inside the exclosures, shrub cover increased, often to the benefit of bramble (Rubus fruticosus agg.). Ungulates tend to favour ruderal, hemerobic, epizoochorous and non‐forest species. Among plots, the magnitude of vegetation changes was proportional to deer abundance. We conclude that ungulates, through the control of the shrub layer, indirectly increase herbaceous plant species richness by increasing light reaching the ground. However, this increase is detrimental to the peculiarity of forest plant communities and contributes to a landscape‐level biotic homogenization. Even at population density levels considered to be harmless for overall plant species richness, ungulates remain a conservation issue for plant community composition.     

Beyond the browse line: complex cascade effects mediated by white‐tailed deer

In many ecosystems, browsing of large mammals can affect plant species compositions. However, much less is known about potential above‐ and below‐ground trophic interactions of large browsing mammals. This study focused on the direct and indirect effects of browsing on trophic and abiotic interactions within forest ecosystems. To quantify above‐ and below‐ground cascade effects, white‐tailed deer have been excluded for over 18 years from three 4‐ha plots, which were paired with same sized deer access plots. Our results demonstrate complex direct and indirect cascade effects on forest food webs. Deer exclusion directly altered woody species composition and significantly increased shrub and sapling density. Above‐ground cascade effects include greater leaf litter accumulation and higher arthropod density and biomass within the exclosures. Below‐ground indirect effects include significant decrease in soil nutrients, and higher arbuscular mycorrhizal fungal inoculum potential in the exclosures. Because ecosystems have finite resource availability, high deer density may imbalance the system by redirecting resources toward maintaining deer biomass at the expense of multiple trophic levels throughout the forest community. Both complex bottom up and top down trophic cascade effects demonstrated largely unidirectional negative responses suggesting that high deer density has reduced the biodiversity of the forest community.     

Trophic and non‐trophic interactions influence the mechanisms underlying biodiversity–ecosystem functioning relationships under different abiotic conditions

Plant diversity effects on ecosystem functioning usually have been studied from a plant perspective. However, the mechanisms underlying biodiversity–ecosystem functioning relationships may also depend on positive or negative interactions between plants and other biotic and abiotic factors, which remain poorly understood. Here we assessed whether plant–herbivore and/or plant–detritivore interactions modify the biodiversity–ecosystem functioning relationship and the mechanisms underlying biodiversity effects, including complementarity and selection effects, biomass allocation, vertical distribution of roots, and plant survival using a microcosm experiment. We also evaluated to what extent trophic and non‐trophic interactions are affected by abiotic conditions by studying drought effects. Our results show that biotic and abiotic conditions influence the shape of the biodiversity–ecosystem function relationship, varying from hump‐shaped to linear. For instance, total biomass increased linearly with plant richness in the presence of detritivores, but not in the absence of detritivores. Moreover, detritivore effects on belowground plant productivity were highly context dependent, varying in the presence of herbivores. Plant interactions with soil biota, especially with herbivores, influenced the mechanisms underlying diversity effects. Herbivores increased plant complementarity and modified biomass allocation and vertical distribution of roots. Furthermore, biotic–abiotic interactions influenced plant productivity differently across plant functional groups. Our findings emphasize the importance of complex biotic interactions underlying biodiversity effects, and that these biotic interactions may change with abiotic conditions. Despite minor changes in productivity in the short‐term, soil biota‐induced changes in plant–plant interactions and plant survival are likely to have significant long‐term consequences for ecosystem functioning. Considering the context‐dependency of multichannel interactions may contribute to reconciling differences among observed patterns in biodiversity studies. Further, abiotic conditions modified the effects of biotic interactions, suggesting that changes in environmental conditions may not only affect ecosystems directly, but also change the biotic composition of and dynamics within ecosystems.     

Interactions of herbivore exclusion with warming and N addition in a grass-dominated temperate old field

Field experiments used to explore the effects of global change drivers, such as warming and nitrogen deposition on plant productivity and species composition, have typically focused on bottom-up processes. However, both direct and indirect responses of herbivores to the treatments could result in important interactions between top-down and bottom-up effects. These interactions may be complicated by the simultaneous effects of multiple herbivore taxa. We used rodent and mollusc exclosures in the plots of a warming and N addition field experiment to examine how herbivore removal would influence plant biomass responses to the treatments. The effect of rodent exclusion on grass biomass more than doubled in response to nitrogen addition, but did not respond to warming, whereas the effect of mollusc exclusion on grass biomass increased in response to warming, but not nitrogen. In contrast, the effect of rodent exclusion on total biomass (grasses and forbs combined) increased in response to both nitrogen and warming, while the effect of mollusc exclusion on total biomass was insensitive to nitrogen and warming. In no cases were there interactions between nitrogen and warming with respect to their influence on exclosure effects. Overall, our results demonstrated substantial and variable effects of multiple herbivore taxa on plant biomass responses to warming and N addition, despite the absence of conspicuous damage to the plant canopy. These results therefore highlight the potential importance of interactions between top-down and bottom-up factors in global change field experiments.     

Quantifying Ecosystem Controlsand Their Contextual Interactionson Nutrient Export fromDeveloping Forest Mesocosms

The complexity of natural ecosystems makes it difficult to compare the relative importance of abiotic and biotic factors and to assess the effects of their interactions on ecosystem development. To improve our understanding of ecosystem complexity, we initiated an experiment designed to quantify the main effects and interactions of several factors that are thought to affect nutrient export from developing forest ecosystems. Using a replicated 2 × 2 × 4 factorial experiment, we quantified the main effects of these factors and the factor interactions on annual calcium, magnesium, and potassium export from field mesocosms over 4 years for two Vermont locations, two soils, and four different tree seedling communities. We found that the main effects explained 56%–97% of total variation in nutrient export. Abiotic factors (location and soil) accounted for a greater percentage of the total variation in nutrient export (47%–94%) than the biotic factor (plant community) (2%–15%). However, biotic control over nutrient export was significant, even when biomass was minimal. Factor interactions were often significant, but they explained less of the variation in nutrient export (1%–33%) than the main effects. Year-to-year fluctuations influenced the relative importance of the main effects in determining nutrient export and created factor interactions between most of the explanatory variables. Our study suggests that when research is focused on typically used main effects, such as location and soil, and interactions are aggregated into overall error terms, important information about the factors controlling ecosystem processes can be lost.     

Effects of drought, temperature, herbivory, and genotype on plant–insect interactions in soybean (Glycine max)

Climate change is predicted to cause continued increases in global temperatures, greater variability in precipitation and in some cases, more frequent insect pest outbreaks. Here we seek to understand how abiotic and biotic stresses associated with climate change can affect plant-herbivore interactions in a model crop species (soybean, Glycine max (L.) Merr.) by answering three questions: (1) Do the combined effects of abiotic and biotic stresses associated with climate change cause synergistic negative effects on plant biomass? (2) Can abiotic stress affect resistance of plants to insect herbivores? (3) Does genetic variation in plant traits modify a plant’s response to stress? We performed three experiments in controlled growth environments using up to 51 soybean genotypes selected to vary in numerous traits associated with drought and resistance against pests (e.g., insect herbivores, nematodes, and pathogenic fungi), and up to 3 generalist-feeding herbivorous noctuid moth species (Helicoverpa zea, Heliothis virescens, and Spodoptera exigua) that commonly feed on soybean in North America. Drought and herbivory had the largest and the most consistent negative effects on plant performance, reducing the above- and below-ground biomass by 10-45 %, whereas increased temperature had little to no effect on plants. Drought also increased susceptibility to generalist noctuid herbivores, but these results varied dramatically in magnitude and direction among plant genotypes. Our experiments show that the effects of abiotic and biotic stress on soybean biomass were largely due to the additive effects of these stresses, and there exists substantial genetic variation in the soybean germplasm pool we studied that could be used as a source of parental stock in breeding new crops that can more effectively tolerate and resist the combined negative effects of insect herbivory and drought.     

Seed limitation interacts with biotic and abiotic factors to constrain novel species' impact on community biomass and richness

Seed limitation can narrow down the number of coexisting plant species, limit plant community productivity, and also constrain community responses to changing environmental and biotic conditions. In a 10-year full-factorial experiment of seed addition, fertilisation, warming and herbivore exclusion, we tested how seed addition alters community richness and biomass, and how its effects depend on seed origin and biotic and abiotic context. We found that seed addition increased species richness in all treatments, and increased plant community biomass depending on nutrient addition and warming. Novel species, originally absent from the communities, increased biomass the most, especially in fertilised plots and in the absence of herbivores, while adding seeds of local species did not affect biomass. Our results show that seed limitation constrains both community richness and biomass, and highlight the importance of considering trophic interactions and soil nutrients when assessing novel species immigrations and their effects on community biomass.     

Complex effects of mammalian grazing on extramatrical mycelial biomass in the Scandes forest‐tundra ecotone

Mycorrhizal associations are widespread in high‐latitude ecosystems and are potentially of great importance for global carbon dynamics. Although large herbivores play a key part in shaping subarctic plant communities, their impact on mycorrhizal dynamics is largely unknown. We measured extramatrical mycelial (EMM) biomass during one growing season in 16‐year‐old herbivore exclosures and unenclosed control plots (ambient), at three mountain birch forests and two shrub heath sites, in the Scandes forest‐tundra ecotone. We also used high‐throughput amplicon sequencing for taxonomic identification to investigate differences in fungal species composition. At the birch forest sites, EMM biomass was significantly higher in exclosures (1.36 ± 0.43 g C/m²) than in ambient conditions (0.66 ± 0.17 g C/m²) and was positively influenced by soil thawing degree‐days. At the shrub heath sites, there was no significant effect on EMM biomass (exclosures: 0.72 ± 0.09 g C/m²; ambient plots: 1.43 ± 0.94). However, EMM biomass was negatively related to Betula nana abundance, which was greater in exclosures, suggesting that grazing affected EMM biomass positively. We found no significant treatment effects on fungal diversity but the most abundant ectomycorrhizal lineage/cortinarius, showed a near‐significant positive effect of herbivore exclusion (p = .08), indicating that herbivory also affects fungal community composition. These results suggest that herbivory can influence fungal biomass in highly context‐dependent ways in subarctic ecosystems. Considering the importance of root‐associated fungi for ecosystem carbon balance, these findings could have far‐reaching implications.     

Biotic Interactions Overrule Plant Responses to Climate,Depending on the Species' Biogeography

This study presents an experimental approach to assess the relative importance of climatic and biotic factors as determinants of species'' geographical distributions. We asked to what extent responses of grassland plant species to biotic interactions vary with climate, and to what degree this variation depends on the species'' biogeography. Using a gradient from oceanic to continental climate represented by nine common garden transplant sites in Germany, we experimentally tested whether congeneric grassland species of different geographic distribution (oceanic vs. continental plant range type) responded differently to combinations of climate, competition and mollusc herbivory. We found the relative importance of biotic interactions and climate to vary between the different components of plant performance. While survival and plant height increased with precipitation, temperature had no effect on plant performance. Additionally, species with continental plant range type increased their growth in more benign climatic conditions, while those with oceanic range type were largely unable to take a similar advantage of better climatic conditions. Competition generally caused strong reductions of aboveground biomass and growth. In contrast, herbivory had minor effects on survival and growth. Against expectation, these negative effects of competition and herbivory were not mitigated under more stressful continental climate conditions. In conclusion we suggest variation in relative importance of climate and biotic interactions on broader scales, mediated via species-specific sensitivities and factor-specific response patterns. Our results have important implications for species distribution models, as they emphasize the large-scale impact of biotic interactions on plant distribution patterns and the necessity to take plant range types into account.     

Waterbird impacts on widgeongrass Ruppia maritima in a Mediterranean wetland: comparing bird groups and seasonal effects

We studied the effect of waterbirds on the submerged macrophyte Ruppia maritima in eleven fish ponds within Doñana Natural Park (SW Spain). Separate exclosure designs allowed us to exclude flamingos or all waterbirds from 3×3 m plots within the ponds and compare them with control plots. Four experiments were conducted for three month periods at different points of the annual cycle with varying bird densities. Flamingos and wildfowl (ducks and coot) had significant negative additive effects on the presence of aboveground (leafs and shoots) or belowground (roots) parts of Ruppia at all times of the year. For plots where Ruppia was present, aboveground biomass was significantly higher in all-bird exclosures than in controls or flamingo exclosures. Presence and biomass of this annual plant varied significantly between seasons as did the density of seeds in sediments. Seasonal changes in seedbank densities were consistent with consumption by birds. There were no significant treatment×season interactions for Ruppia presence, aboveground biomass or seeds. This is the first exclosure study to compare the effects of waterbirds on submerged macrophytes at different times throughout the annual cycle, and the first to compare simultaneously the effects of different bird groups. Our findings refute previous suggestions that major effects of waterbirds are limited to temperate regions and to periods of early growth or when major concentrations of migratory wildfowl are formed in autumn. Flamingos are important in structuring shallow wetlands in the Mediterranean, and possibly many other regions.     

Insect herbivory in an experimental agroecosystem: the relative importance of habitat area, fragmentation, and the matrix

Habitat area, fragmentation, and the surrounding matrix influence levels of herbivory in various ecosystems, but the relative importance of these effects has rarely been assessed. We compared levels of herbivory and densities of dominant arthropod herbivores (the hemipteran insects Agallia constricta, Empoasca fabae, Therioaphis trifolii, Lygus lineolaris and Halticus bractatus ) among experimental plots that varied in the area and fragmentation of clover habitat and the composition of the matrix (bare ground or grass) surrounding clover habitat. To assess levels of herbivory, we compared clover biomass within herbivore exclosures to the biomass accessible to herbivores. The area and fragmentation of clover habitat, as well as matrix composition, significantly influenced the collective densities of herbivores, although each species exhibited unique responses to habitat structure. Herbivory was strongest in plots with large (64  m²) as compared to small (16  m²) amounts of clover habitat. The difference in clover biomass between the inside and outside of exclosures increased significantly with increasing density of Empoasca fabae but was unrelated to the densities of the other herbivores, suggesting that Empoasca fabae was an exceptionally important herbivore in this system. This study supports the view that herbivore densities and herbivory generally increase with increasing area of plant monocultures, but emphasizes that levels of herbivory may be driven primarily by one or a few key herbivore species.     

Effects of herbivory on competition intensity in two arctic-alpine tundra communities with different productivity

The effects of long-term (11 yr) exclusion of vertebrate herbivores on competition intensity and plant community structure were studied using manipulative field experiments in two arctic-alpine plant communities with contrasting productivity: an unproductive snowbed and a considerably more productive tall herb meadow. In the snowbed, the exclusion of herbivorous mammals resulted in a significant increase in the biomasses of vascular plants and cryptogams, whereas no corresponding response was observed on the tall herb meadow. The intensity of competition, measured with a neighbour removal experiment, did not differ significantly between three of the four habitat×treatment combinations – snowbed exclosures, meadow exclosures and open meadow plots – but was significantly lower on open snowbed plots. Our results thus suggest that the low competition intensity in the unproductive snowbed is caused by herbivorous mammals, which tend to depress plant biomass in relatively unproductive habitats. When herbivorous mammals have been excluded for a sufficiently long time to allow the build-up of plant biomass even in unproductive habitats, between-habitat differences in competition intensity disappear.     

Long-term effects of grazing on subalpine and alpine grasslands in the Central Alps,Austria

The effects of grazing exclusion on species diversity and functional diversity were analyzed along an elevation gradient from the subalpine (1960 m a.s.l.) to the lower and upper alpine zone (2275 m–2650 m a.s.l.) in the Austrian Central Alps for 15 years. Nine sites were chosen, including grasslands at different elevations, a bog and a glacier foreland site at the lower alpine zone and a snowbed at the upper alpine zone. Data were acquired by frequency counts in 1 m² permanent plots inside each fenced area (three plots per site) and outside in grazed areas (three plots). Diversity indices and functional diversity were analyzed by means of generalized linear mixed models (GLMMs). Exclosure, duration of exclusion and exclosure*years (interaction effect) were defined as predictor variables. Multivariate ordination techniques were used to (i) determine species responses to grazing exclusion (pRDA, partial redundancy analysis) and (ii) to create a distance matrix representing the changes between exclosures and control plots per year (NMDS, non-metric multidimensional scaling). At the subalpine grassland, first differences between exclosure and control plots occurred already only after three years, at the upper alpine zone after four and five years. Contrary to our expectation, dwarf shrubs did not increase within the exclosures of the subalpine grassland. Instead, mainly the tall forb Geranium sylvaticum increased. Species richness significantly decreased at the exclosures of the subalpine zone, the snowbed and at one upper alpine grassland sites. The communities of the glacier foreland and the bog were hardly affected by grazing exclusion.We conclude that plant species and communities react individually depending on elevation and grazing animals. Grazing exclusion studies at high elevations should definitely be carried out in the long-term.     

Soil legacy effects of plants and drought on aboveground insects in native and range-expanding plant communities

Soils contain biotic and abiotic legacies of previous conditions that may influence plant community biomass and associated aboveground biodiversity. However, little is known about the relative strengths and interactions of the various belowground legacies on aboveground plant–insect interactions. We used an outdoor mesocosm experiment to investigate the belowground legacy effects of range-expanding versus native plants, extreme drought and their interactions on plants, aphids and pollinators. We show that plant biomass was influenced more strongly by the previous plant community than by the previous summer drought. Plant communities consisted of four congeneric pairs of natives and range expanders, and their responses were not unanimous. Legacy effects affected the abundance of aphids more strongly than pollinators. We conclude that legacies can be contained as soil ‘memories’ that influence aboveground plant community interactions in the next growing season. These soil-borne ‘memories’ can be altered by climate warming-induced plant range shifts and extreme drought.     

Time‐dependent effects of fertilization on plant biomass in floating fens

Abstract. A cross‐over fertilization experiment was carried out in Dutch floating fens to investigate effects on biomass production in the same and the following years. In total 16 fertilizer treatments were applied, combining four treatments in 1999 with four treatments in 2000 (addition of 20 g.m^?2 N, 5 g.m^?2 P, both elements and unfertilized control). The above‐ground biomass production of vascular plants was co‐limited by N and P in both years. However, in plots that were only fertilized in 1999 the effects of individual nutrients differed between the two years: N‐fertilization slightly increased the amount of biomass produced in the same year (1999), whereas P‐fertilization did so in the following year (2000). Fertilizer applied in 1999 also influenced the effects of fertilizer applied in 2000. One year after N‐fertilization vascular plant growth was still co‐limited by N and P, but one year after P‐fertilization, vascular plant growth was only limited by N. Bryophyte biomass responded weakly to fertilization. Nutrient concentrations in plant biomass, nutrient standing crops and measurements of N and P availability in the soil indicated that one year after fertilization, the N‐fertilizer had mostly ‘disappeared’ from N‐fertilized plots, whereas the availability of P remained markedly enhanced in P‐fertilized plots. In addition, P‐fertilization enhanced the uptake of N by plants the following year. The time‐dependence of fertilizer effects was probably caused by (1) higher addition of P than of N relative to the requirements of plants; (2) longer retention of P than of N in the system; (3) positive effect of P‐fertilization on the availability of N; (4) contrasting effects of N‐ and P‐fertilization on nutrient losses by plants and/or on their responses to subsequent nutrient addition; (5) changing interactions between vascular plants and mosses (mainly Sphagnum spp.); (6) nutrient export through the repeated harvest of above‐ground biomass. To determine which nutrient limits plant growth fertilization experiments should be short, avoiding that indirect effects of a non‐limiting nutrient influence results. To indicate how changed nutrient supply will affect an ecosystem longer‐term experiments are needed, so that indirect effects have time to develop and be detected.     

Recovery of plant species composition and ecosystem function after cessation of grazing in a Mediterranean grassland

Short- and long-term changes in species composition, plant biomass production, and litter decomposition after cessation of grazing were examined in a Mediterranean grassland with high dominance of annual species and strong seasonality in biomass production. Short-term changes were assessed during three consecutive years in plots previously exposed to different grazing pressures and compared to plots in long-term (30–40 years) exclosures. Short-term cessation of grazing led in the short-term to an increase in relative biomass of annual crucifers and tall annual and perennial grasses, while biomass of annual legumes, annual thistles and short annual grasses decreased. Consequently, similarity increased between vegetation recently excluded from grazing and vegetation in long-term protected plots. Our research showed that in systems with high dominance of grasses and annual species, the rapid changes in plant species composition that occur after grazing cessation were associated with a fast recovery of the potential for biomass production to levels found in long-term protected plots, while litter decomposition rate did not change even after long-term cessation of grazing. Moreover, previous history of grazing did not affect plant litter decomposition, despite higher litter quality in grazed treatments. This study provides new insights about the processes involved in the diverse responses of ecosystem functions resulting from shifts in species composition associated with grazing cessation and land use change in Mediterranean grasslands.     

Urbanization alters plastic responses in the common dandelion Taraxacum officinale

Urban environments expose species to contrasting selection pressures relative to rural areas due to altered microclimatic conditions, habitat fragmentation, and changes in species interactions. To improve our understanding on how urbanization impacts selection through biotic interactions, we assessed differences in plant defense and tolerance, dispersal, and flowering phenology of a common plant species (Taraxacum officinale) along an urbanization gradient and their reaction norms in response to a biotic stressor (i.e., herbivory). We raised plants from 45 lines collected along an urbanization gradient under common garden conditions and assessed the impact of herbivory on plant growth (i.e., aboveground biomass), dispersal capacity (i.e., seed morphology), and plant phenology (i.e., early seed production) by exposing half of our plants to two events of herbivory (i.e., grazing by locusts). Independent from their genetic background, all plants consistently increased their resistance to herbivores by which the second exposure to locusts resulted in lower levels of damage suffered. Herbivory had consistent effects on seed pappus length, with seeds showing a longer pappus (and, hence, increased dispersal capacities) regardless of urbanization level. Aboveground plant biomass was neither affected by urbanization nor herbivore presence. In contrast to consistent responses in plant defenses and pappus length, plant fitness did vary between lines. Urban lines had a reduced early seed production following herbivory while rural and suburban lines did not show any plastic response. Our results show that herbivory affects plant phenotypes but more importantly that differences in herbivory reaction norms exist between urban and rural populations.     

Herbivores inhibit climate-driven shrub expansion on the tundra

Recent Pan-Arctic shrub expansion has been interpreted as a response to a warmer climate. However, herbivores can also influence the abundance of shrubs in arctic ecosystems. We addressed these alternative explanations by following the changes in plant community composition during the last 10 years in permanent plots inside and outside exclosures with different mesh sizes that exclude either only reindeer or all mammalian herbivores including voles and lemmings. The exclosures were replicated at three forest and tundra sites at four different locations along a climatic gradient (oceanic to continental) in northern Fennoscandia. Since the last 10 years have been exceptionally warm, we could study how warming has influenced the vegetation in different grazing treatments. Our results show that the abundance of the dominant shrub, Betula nana , has increased during the last decade, but that the increase was more pronounced when herbivores were excluded. Reindeer have the largest effect on shrubs in tundra, while voles and lemmings have a larger effect in the forest. The positive relationship between annual mean temperature and shrub growth in the absence of herbivores and the lack of relationships in grazed controls is another indication that shrub abundance is controlled by an interaction between herbivores and climate. In addition to their effects on taller shrubs (>0.3 m), reindeer reduced the abundance of lichens, whereas microtine rodents reduced the abundance of dwarf shrubs (<0.3 m) and mosses. In contrast to short-term responses, competitive interactions between dwarf shrubs and lichens were evident in the long term. These results show that herbivores have to be considered in order to understand how a changing climate will influence tundra ecosystems.     

Control of pathogenic PAC strains by non-pathogenic PAC strains in planta does not correlate with higher competitiveness of non-pathogenic PAC strains ex planta

Ascomycetes of the Phialocephala fortinii s.l.—Acephala applanata species complex (PAC) are frequent root endophytes of forest trees. Roots are colonized by multiple PAC genotypes that interact, and recent findings indicate that adverse effects on plant performance caused by pathogenic PAC strains are attenuated by non-pathogenic PAC strains. However, it was not known if this “self-control” works only in planta, or also ex planta, i.e., prior to infection during saprotrophic life of the PAC. Interactions between PAC strains were therefore studied in a plant-free system on malt extract agar. The mycelia of two pathogenic (A and T1) and two non-pathogenic (B and C) PAC strains were mixed pairwise 5:1, 1:1 and 1:5 (fresh weight ratios) and incubated at 15 and 25 °C. Mycelial biomass of each strain was measured after 2 and 8 weeks. The combination of strains and the mixture ratio had a significant effect on strain biomass, whereas temperature influenced only the biomass of pathogenic strain T1. Biomass production of strain T1 was inhibited by all other strains, whereas biomass production of the other pathogenic strain A was significantly stimulated by the two non-pathogenic strains. This contrasts strongly with results from a previous experiment in planta using strains A, B and C, because the two non-pathogenic PAC strains successfully inhibited the pathogenic strain, probably by space occupation or the induction of host resistance. Therefore, it is impossible to predict the outcome of PAC-PAC interactions in planta based on the results gained from interactions ex planta.     

A burst of plant NADPH oxidases

Reactive oxygen species (ROS) are highly reactive molecules able to damage cellular components but they also act as cell signalling elements. ROS are produced by many different enzymatic systems. Plant NADPH oxidases, also known as respiratory burst oxidase homologues (RBOHs), are the most thoroughly studied enzymatic ROS-generating systems and our understanding of their involvement in various plant processes has increased considerably in recent years. In this review we discuss their roles as ROS producers during cell growth, plant development and plant response to abiotic environmental constraints and biotic interactions, both pathogenic and symbiotic. This broad range of functions suggests that RBOHs may serve as important molecular 'hubs' during ROS-mediated signalling in plants.