Nutrients cause grassland biomass to outpace herbivory.

Human activities are transforming grassland biomass via changing climate, elemental nutrients, and herbivory. Theory predicts that food-limited herbivores will consume any additional biomass stimulated by nutrient inputs ('consumer-controlled'). Alternatively, nutrient supply is predicted to increase biomass where herbivores alter community composition or are limited by factors other than food ('resource-controlled'). Using an experiment replicated in 58 grasslands spanning six continents, we show that nutrient addition and vertebrate herbivore exclusion each caused sustained increases in aboveground live biomass over a decade, but consumer control was weak. However, at sites with high vertebrate grazing intensity or domestic livestock, herbivores consumed the additional fertilization-induced biomass, supporting the consumer-controlled prediction. Herbivores most effectively reduced the additional live biomass at sites with low precipitation or high ambient soil nitrogen. Overall, these experimental results suggest that grassland biomass will outstrip wild herbivore control as human activities increase elemental nutrient supply, with widespread consequences for grazing and fire risk.

Soil net nitrogen mineralisation across global grasslands.

Soil nitrogen mineralisation (Nmin), the conversion of organic into inorganic N, is important for productivity and nutrient cycling. The balance between mineralisation and immobilisation (net Nmin) varies with soil properties and climate. However, because most global-scale assessments of net Nmin are laboratory-based, its regulation under field-conditions and implications for real-world soil functioning remain uncertain. Here, we explore the drivers of realised (field) and potential (laboratory) soil net Nmin across 30 grasslands worldwide. We find that realised Nmin is largely explained by temperature of the wettest quarter, microbial biomass, clay content and bulk density. Potential Nmin only weakly correlates with realised Nmin, but contributes to explain realised net Nmin when combined with soil and climatic variables. We provide novel insights of global realised soil net Nmin and show that potential soil net Nmin data available in the literature could be parameterised with soil and climate data to better predict realised Nmin.

Two's company, three's a crowd: co-occurring pollinators and parasite species in Breynia oblongifolia (Phyllanthaceae).

BACKGROUND: Obligate pollination mutualisms (OPMs) are specialized interactions in which female pollinators transport pollen between the male and female flowers of a single plant species and then lay eggs into those same flowers. The pollinator offspring hatch and feed upon some or all of the developing ovules pollinated by their mothers. Strong trait matching between plants and their pollinators in OPMs is expected to result in reciprocal partner specificity i.e., a single pollinator species using a single plant species and vice versa, and strict co-speciation. These issues have been studied extensively in figs and fig wasps, but little in the more recently discovered co-diversification of Epicephala moths and their Phyllanthaceae hosts. OPMs involving Epicephala moths are believed occur in approximately 500 species of Phyllanthaceae, making it the second largest OPM group after the Ficus radiation (> 750 species). In this study, we used a mixture of DNA barcoding, genital morphology and behavioral observations to determine the number of Epicephala moth species inhabiting the fruits of Breynia oblongifolia, their geographic distribution, pollinating behavior and phylogenetic relationships. RESULTS: We found that B. oblongifolia hosts two species of pollinator that co-occurred at all study sites, violating the assumption of reciprocal specificity. Male and female genital morphologies both differed considerably between the two moth species. In particular, females differed in the shape of their ovipositors, eggs and oviposition sites. Phylogenetic analyses indicated that the two Epicephala spp. on B. oblongifolia likely co-exist due to a host switch. In addition, we discovered that Breynia fruits are also often inhabited by a third moth, an undescribed species of Herpystis, which is a non-pollinating seed parasite. CONCLUSIONS: Our study reveals new complexity in interactions between Phyllantheae and Epicephala pollinators and highlights that host switching, co-speciation and non-pollinating seed parasites can shape species interactions in OPMs. Our finding that co-occurring Epicephala species have contrasting oviposition modes parallels other studies and suggests that such traits are important in Epicephala species coexistence.

Effects of competition and herbivory over woody seedling growth in a temperate woodland trump the effects of elevated CO2.

A trend of increasing woody plant density, or woody thickening, has been observed across grassland and woodland ecosystems globally. It has been proposed that increasing atmospheric [CO2] is a major driver of broad scale woody thickening, though few field-based experiments have tested this hypothesis. Our study utilises a Free Air CO2 Enrichment experiment to examine the effect of elevated [CO2] (eCO2) on three mechanisms that can cause woody thickening, namely (i) woody plant recruitment, (ii) seedling growth, and (iii) post-disturbance resprouting. The study took place in a eucalypt-dominated temperate grassy woodland. Annual assessments show that juvenile woody plant recruitment occurred over the first 3 years of CO2 fumigation, though eCO2 did not affect rates of recruitment. Manipulative experiments were established to examine the effect of eCO2 on above-ground seedling growth using transplanted Eucalyptus tereticornis (Myrtaceae) and Hakea sericea (Proteaceae) seedlings. There was no positive effect of eCO2 on biomass of either species following 12 months of exposure to treatments. Lignotubers (i.e., resprouting organs) of harvested E. tereticornis seedlings that were retained in situ for an additional year were used to examine resprouting response. The likelihood of resprouting and biomass of resprouts increased with lignotuber volume, which was not itself affected by eCO2. The presence of herbaceous competitors and defoliation by invertebrates and pathogens were found to greatly reduce growth and/or resprouting response of seedlings. Our findings do not support the hypothesis that future increases in atmospheric [CO2] will, by itself, promote woody plant recruitment in eucalypt-dominated temperate grassy woodlands.

Effects of vehicle exhaust emissions on urban wild plant species.

Very few investigations have examined the direct impacts of vehicle exhausts on plants and attempted to separate out the key pollutants responsible for observed effects. This paper describes a multi-phase investigation into this topic, using 12 herbaceous species typical of urban areas and representing different functional groups. Fumigations were conducted in solardomes with diesel exhaust pollutants at concentrations designed to simulate those close to a major highway in inner London. A wide range of effects were detected, including growth stimulation and inhibition, changes in gas exchange and premature leaf senescence. This was complemented by controlled fumigations with NO, NO(2) and their mixture, as well as a transect study away from a busy inner London road. All evidence suggested that NO(x) was the key phytotoxic component of exhaust emissions, and highlights the potential for detrimental effects of vehicle emissions on urban ecosystems.

Open Air Laboratories (OPAL): a community-driven research programme.

OPAL is an English national programme that takes scientists into the community to investigate environmental issues. Biological monitoring plays a pivotal role covering topics of: i) soil and earthworms; ii) air, lichens and tar spot on sycamore; iii) water and aquatic invertebrates; iv) biodiversity and hedgerows; v) climate, clouds and thermal comfort. Each survey has been developed by an inter-disciplinary team and tested by voluntary, statutory and community sectors. Data are submitted via the web and instantly mapped. Preliminary results are presented, together with a discussion on data quality and uncertainty. Communities also investigate local pollution issues, ranging from nitrogen deposition on heathlands to traffic emissions on roadside vegetation. Over 200,000 people have participated so far, including over 1000 schools and 1000 voluntary groups. Benefits include a substantial, growing database on biodiversity and habitat condition, much from previously unsampled sites particularly in urban areas, and a more engaged public.

Effects of host plant exposure to cadmium on mycorrhizal infection and soluble carbohydrate levels of Pinus sylvestris seedlings.

In a Cd-contaminated environment, not only mature trees but also their seeds and young seedlings can be exposed to Cd. Cadmium taken up by young seedlings may influence mycorrhizal infection, which might in turn influence resistance to Cd toxicity. In order to eliminate soil-mediated responses of mycorrhizal infection to Cd, Pinus sylvestris seedlings were exposed to Cd prior to fungal inoculation and replanted to clean substrates with fungal inoculum. Cadmium pretreatment reduced the proportion of living mycorrhizal short roots of seedlings inoculated with Paxillus involutus. However, no such effect was observed for seedlings inoculated with Suillus bovinus and Rhizopogon subcaerulescens. Therefore, infection by P. involutus appeared to be affected by Cd taken up by seedlings. Overall stem length and biomass of the seedlings were reduced by Cd pretreatment. Infection by S. bovinus and R. subcaerulescens increased stem length and biomass of the seedlings. Root soluble carbohydrate concentrations were lower in mycorrhizal seedlings than non-mycorrhizal seedlings.

Evolution of a complex coevolved trait: active pollination in a genus of fig wasps.

Only three insect lineages have evolved complex active pollination behaviour and only fig wasps (Agaonidae) have also reverted from active to passive pollination. Previously, it was assumed that there was a single origin of active pollination in fig wasps, followed by one independent loss in each of five genera. We show here that there have been three to six changes in pollination behaviour within just one genus (Pleistodontes). The results suggest multiple gains of active pollination in fig wasps, but are sensitive to assumptions about the relative costs of gaining and losing this complex behaviour. In addition, previous comparative studies at higher taxonomic levels have reported correlated evolution between active pollination in wasps and low anther/ovule ratios in figs. We report that changes in pollination behaviour between congeneric species correlate perfectly with changes in anther/ovule ratios in the host figs, showing no phylogenetic inertia in coadaptation at the species level.

Effects of cadmium on growth and glucose utilisation of ectomycorrhizal fungi in vitro.

Effects of Cd on growth and glucose utilisation of Paxillus involutus, Rhizopogon subcaerulescens and Suillus bovinus were investigated in vitro in liquid culture. S. bovinus was the species most sensitive to Cd in terms of dry matter production and P. involutus was less sensitive than R. subcaerulescens. Greater production of hyphae of P. involutus than the other fungi appeared to confer some degree of Cd resistance, possibly by binding Cd onto cell walls. Growth of the three fungi was increased by glucose addition. While Cd significantly reduced dry matter production of the fungi, there were no significant differences in glucose consumption caused by Cd treatment. This suggests that the use of glucose might have been diverted to detoxification and/or repair mechanisms. Further studies on respiration rates and energy metabolites of these fungi under Cd exposure are needed in order to clarify the results of the present study.

Responses of fen and fen-meadow communities to ozone.

• Species of fen and fen-meadow communities, well supplied with water and nutrients, are characterised by high rates of growth, stomatal conductance values and specific leaf areas, all factors which have been associated with high sensitivity to ozone. We therefore examined the effects of ozone on 12 characteristic fen and fen-meadow species. • Plants received either filtered air or ozone; AOT40 exposures ranged from 9200 to 14 300 ppb h. Eight of the 12 species exhibited foliar injury in response to ozone exposure, with the first signs of injury on Vicia cracca, following an AOT40 exposure of only 1950 pbb h. • Ozone exposure significantly reduced plant photosynthetic rate, stomatal conductance and biomass production in four species. Cirsium arvense exhibited the greatest biomass response to ozone (32% and 58% reduction in above- and below-ground weight, respectively). Species with higher levels of visible injury tended to show greater reductions in biomass. There was a significant positive association between stomatal conductance and the magnitude of ozone effects on root biomass. • The widespread occurrence of either visible injury or growth reductions amongst the species screened, and the magnitude of effects on the most sensitive species, indicate that species of fens and fen-meadows may be more sensitive to ozone than other seminatural ecosystems which have been the focus of recent ozone studies.

Habitat management: a tool to modify ecosystem impacts of nitrogen deposition?

Atmospheric nitrogen deposition has been shown to affect both the structure and the function of heathland ecosystems. Heathlands are semi-natural habitats and, as such, undergo regular management by mowing or burning. Different forms of management remove more or less nutrients from the system, so habitat management has the potential to mitigate some of the effects of atmospheric deposition. Data from a dynamic vegetation model and two field experiments are presented. The first involves nitrogen addition following different forms of habitat management. The second tests the use of habitat management to promote heathland recovery after a reduction in nitrogen deposition. Both modelling and experimental approaches suggest that plant and microbial response to nitrogen is affected by management. Shoot growth and rates of decomposition were lowest in plots managed using more intensive techniques, including mowing with litter removal and a high temperature burn. Field data also indicate that ecosystem recovery from prolonged elevated inputs of nitrogen may take many years, or even decades, even after the removal of plant and litter nitrogen stores which accompanies the more intensive forms of habitat management.