Bioassay experiments were used to assess the allelopathic potential of nodding thistle seeds on cumulative germination, speed of germination, and radicle elongation of five test species i.e., cocksfoot, perennial ryegrass, white clover, subterranean clover, and nodding thistle. Nodding thistle seeds inhibited germination of all species except subterranean clover in at least one of the bioassays used. Radicle elongation of all species was substantially inhibited in the presence of nodding thistle seeds both on filter paper and on soil. In soil systems, nodding thistle seeds inhibited radicle elongation of seeds over distances of up to 4-7 mm. A probability analysis demonstrated that a high density of nodding thistle seeds has the potential to exert a substantial negative effect on establishment of newly sown pasture. Nodding thistle seeds were either stimulated or largely unaffected by other seeds of their own species (except when seeds were touching) suggesting that the allelopathic potential of nodding thistle seeds may be an evolved mechanism to encourage establishment of thistle seedlings.
The side effects of glyphosate on the soil microflora were monitored by applying a range of glyphosate concentrations (0, 2, 20, and 200 μg g−1
Following the creation of new land surfaces, there is an initial build-up phase of ecosystem development, but after a prolonged absence of major disturbance a retrogressive (decline) phase often follows due to reduced nutrient availability over time. Although many studies have considered how the soil community changes during the build-up phase, the response of this community to the retrogressive phase is poorly known. We measured litter and soil communities of microfauna and macrofauna along the Franz Josef Glacier chronosequence in New Zealand that spans ca. 120,000 years, and includes well-established build-up and retrogressive stages. We aimed to assess whether the abundances, community structure and diversity of these groups show the same pattern across the sequence as that for vegetation. With regard to microfaunal abundances, litter-dwelling microbe-feeding nematodes were most abundant in the first stage of the chronosequence, but several other groups of microfauna in both the soil and litter increased sharply during the first few stages and declined sharply during the last (retrogressive) stages. The ratios of bacterial- to fungal-feeding nematodes in both soil and litter were lowest for the final stages of the chronosequence, and (in the case of soil) for some of the early stages, pointing to domination by the fungal-based energy channel at those stages for which soil organic matter content or quality were lowest. This is consistent with the fungal-based energy channel being better adapted than the bacterial-based channel for resource-poor conditions. The main groups of macroinvertebrates typically had their lowest abundances at the very early and late stages of the chronosequence, although the relative abundances of different taxa differed during the intermediate stages. Taxonomic diversity of nematodes and macroinvertebrates in both litter and soil varied strongly with chronosequence stage but differed among taxa; diversity of only one group (macroinvertebrates in litter) declined significantly during retrogression. Diversity of nematodes and macroinvertebrates along the sequence did not closely match tree diversity or soil chemical properties, but community composition of these groups was often related to tree community composition and ratios of soil C to N, C to P and N to P. Different groups of soil invertebrates show contrasting responses to chronosequence stage, probably because they differ in their relative response to bottom-up and top-down controls. However, the abundance of most groups increased during the build-up phase and declined during retrogression. As such, the build-up and decline phases observed for plant communities and ecosystem processes across long-term chronosequences also apply to soil communities, pointing to the importance of resource availability as a major driver of soil biota during long-term ecosystem change.
A study reveals that increasing aridity alters the balance of carbon, nitrogen and phosphorus in dryland soils, providing insight into how global climate change will affect soil fertility and ecosystem services. See Letter p.672 It is thought likely that climatic change, such as the increased aridity predicted for many drylands, could disrupt the biogeochemical cycles of carbon, nitrogen and phosphorus during the twenty-first century. These elements are essential nutrients for biomass production in terrestrial ecosystems. This study finds that aridity has a negative effect on the concentration of global dryland soil organic carbon and nitrogen, but a positive effect on the concentration of inorganic phosphorus. This suggests a decoupling of nutrient cycles in response to an increase in aridity that could have a negative impact on biogeochemical reactions that control key ecosystem functions such as primary productivity.
1 The development of general principles regarding biotic interactions involving plants, or plant species effects in ecosystems, is best achieved through simultaneous evaluation of several species. We utilized a comparative approach involving 20 dicotyledonous herbaceous species, to explore possible relationships between several plant ecophysiological traits and plant litter decomposition, interactions involving competition and herbivory, and plant species effects on soil properties. 2 Decomposition rates of plant stem and leaf litter were negatively related to plant mass, time until flowering and vegetative growth rate, and positively related to stem nitrogen content. Root decomposition was also related to several traits. Multiple regression relationships showed that 74% and 84% of the variation across species for stem and root litter decomposition, respectively, could be predicted by plant traits; this suggests that plant traits may be powerful predictors of decomposition and have potential as alternative predictors to the litter quality characteristics that previous studies have concentrated on. 3 Palatability of both seedlings and leaf discs by the invertebrate herbivores Deroceras reticulatum and Listronotus bonariensis were frequently related to plant traits. Those traits that showed the strongest relationships with the palatability data included various vegetative growth characteristics and (for the leaf disc data) nitrogen concentrations of flowering plant stems. 4 Competitive effects of the dicotyledonous species against a phytometer species, the grass Lolium perenne , were negatively related to leaf nitrogen concentration, and multiple regression relationships involving this trait in combination with others explained over 50% of the variation across species. The competitive response of both plant mass and total seed production to L. perenne was poorly related to plant traits. 5 The effects of plant species on soil properties including microbial biomass and activity, pH, nitrate concentration and total nitrogen were often closely related to various plant traits. Multiple regression relationships revealed that combinations of several traits were often important in determining these effects; the strongest relationships found were for effects of senescent plants on soil respiration and for the effects of flowering plants on soil nitrate. Plant traits were therefore clearly important in determining plant species effects on soils. 6 Our study emphasizes the importance of plant traits in understanding (and predicting) species interactions and effects in communities and ecosystems, and shows that properties considered at the whole plant level have the potential to manifest themselves over much larger scales. We therefore conclude that there are clear linkages between plant ecophysiological traits, biotic interactions involving plants, and ecosystem level properties and processes.
1. A study on the interactions between Carduus nutans L. (nodding thistle), and pasture grasses and legumes was conducted in a New Zealand perennial pasture. 3.8 × 3.8 m plots were sown with each of 10 forage species (six grasses and four legumes), or maintained free of pasture (bare ground plots). A 2 × 1 m subplot was established in each plot in which total thistle seedling emergence and pasture species composition were assessed over a 26-month period; in the remainder of each plot 10 thistles were labelled and monitored for subsequent growth rate, development and survivorship. 2. Thistle seedling emergence was greater in some of the legume-sown treatments than in the grass-sown treatments for the first few months of the study. Seedling emergence was negatively related to sown grass cover and the cover of volunteer Poa annua L. in the plots. However, seedling emergence in the bare ground plots was often substantially less than for the other treatments, probably because at least some pasture cover was necessary to provide an adequate microclimate for establishment. 3. The preflowering mortality of thistles was lowest in the bare-ground treatment, intermediate in the legume-sown treatments and highest in the grass-sown treatments. All thistles which survived to flowering in the bare-ground and legume plots behaved as annuals, while between 3 and 16% of thistles in the various grass-sown plots continued into their second year as rosettes, and many of these flowered as biennials. Preflowering thistle mortality was strongly related to the grass (including P. annua) content of the plots. 4. Flowering thistles in the bare ground plots grew larger than in the sown plots and produced substantially more capitula. Generally, the thistles in the legumesown plots also grew larger than those in the grass-sown plots. 5. Grass-sown plots were likely to be invaded by P. annua in the first few months following sowing, while the legume-sown plots tended to be invaded by broadleaved weeds. The sown grass ― P. annua associations appear to be substantially more effective than the sown legume ― broadleaved weed associations in reducing the problem of nodding thistle invasions
Forest dwelling browsing mammals, notably feral goats and deer, have been introduced to New Zealand over the past 220 years; prior to this such mammals were absent from New Zealand. The New Zealand forested landscape, therefore, presents an almost unique opportunity to determine the impacts of introduction of an entire functional group of alien animals to a habitat from which that group was previously absent. We sampled 30 long-term fenced exclosure plots in indigenous forests throughout New Zealand to evaluate community- and ecosystem-level impacts of introduced browsing mammals, emphasizing the decomposer subsystem. Browsing mammals often significantly altered plant community composition, reducing palatable broad-leaved species and promoting other less palatable types. Vegetation density in the browse layer was also usually reduced. Although there were some small but statistically significant effects of browsing on some measures of soil quality across the 30 locations, there were no consistent effects on components of the soil microfood web (comprising microflora and nematodes, and spanning three consumer trophic levels); while there were clear multitrophic effects of browsing on this food web for several locations, comparable numbers of locations showed stimulation and inhibition of biomasses or populations of food web components. In contrast, all microarthropod and macrofaunal groups were consistently adversely affected by browsing, irrespective of trophic position. Across the 30 locations, the magnitude of response of the dominant soil biotic groups to browsing mammals (and hence their resistance to browsers) was not correlated with the magnitude of vegetation response to browsing but was often strongly related to a range of other variables, including macroclimatic, soil nutrient, and tree stand properties. There were often strong significant effects of browsing mammals on species composition of the plant community, species composition of leaf litter in the litter layer, and composition of various litter-dwelling faunal groups. Across the 30 locations, the magnitude of browsing mammal effects on faunal community composition was often correlated with browser effects on litter layer leaf species composition but never with browser effects on plant community composition. Browsing mammals usually reduced browse layer plant diversity and often also altered habitat diversity in the litter layer and diversity of various soil faunal groups. Across the 30 locations, the magnitude of browser effects on diversity of only one faunal group, humus-dwelling nematodes, was correlated with browser effects on plant diversity. However, browser effects on diversity of diplopods and gastropods were correlated with browser effects on habitat diversity of the litter layer. Reasons for the lack of unidirectional relationships across locations between effects of browsers on vegetation community attributes and on soil invertebrate community attributes are discussed. Browsing mammals generally did not have strong effects on C mineralization but did significantly influence soil C and N storage on an areal basis for several locations. However the direction of these effects was idiosyncratic and presumably reflects different mechanisms by which browsers affect soil processes. While our study did not support hypotheses predicting consistent negative effects of browsing mammals on the decomposer subsystem through promotion of plant species with poorer litter quality, our results still show that the introduction of these mammals to New Zealand has caused far-ranging effects at both the community and ecosystem levels of resolution, with particularly adverse effects for indigenous plant communities and populations of most groups of litter-dwelling mesofauna and macrofauna.
Changes in plant communities can have large effects on ecosystem carbon (C) dynamics and long-term C stocks. However, how these effects are mediated by environmental context or vary among ecosystems is not well understood. To study this, we used a long-term plant removal experiment set up across 30 forested lake islands in northern Sweden that collectively represent a strong gradient of soil fertility and ecosystem productivity. We measured forest floor CO2 exchange and aboveground and belowground C stocks for a 22-yr experiment involving factorial removal of the two dominant functional groups of the boreal forest understory, namely ericaceous dwarf shrubs and feather mosses, on each of the 30 islands. We found that long-term shrub and moss removal increased forest floor net CO2 loss and decreased belowground C stocks consistently across the islands irrespective of their productivity or soil fertility. However, we did see context-dependent responses of respiration to shrub removals because removals only increased respiration on islands of intermediate productivity. Both CO2 exchange and C stocks responded more strongly to shrub removal than to moss removal. Shrub removal reduced gross primary productivity of the forest floor consistently across the island gradient, but it had no effect on respiration, which suggests that loss of belowground C caused by the removals was driven by reduced litter inputs. Across the island gradient, shrub removal consistently depleted C stocks in the soil organic horizon by 0.8 kg C/m2 . Our results show that the effect of plant functional group diversity on C dynamics can be relatively consistent across contrasting ecosystems that vary greatly in productivity and soil fertility. These findings underline the key role of understory vegetation in forest C cycling, and suggest that global change leading to changes in the relative abundance of both shrubs and mosses could impact on the capacity of boreal forests to store C.
An enormous recent research effort focused on how plant biodiversity (notably species richness) influences ecosystem functioning, usually through experiments in which diversity is varied through random draws of species from a species pool. Such experiments are increasingly used to predict how species losses influence ecosystem functioning in ‘real’ ecosystems. However, this assumes that comparisons of experimental communities with low vs high species richness are analogous to comparisons of natural communities from which species either have or have not been lost. I explore the validity of this assumption, and highlight difficulties in using such experiments to draw conclusions about the ecosystem consequences of biodiversity loss in natural systems. Notably, these experiments do not mimic what happens in real ecosystems either when local extinctions occur or when species losses are offset by gains of new species. Despite limitations, this single experimental approach for studying how biodiversity loss affects ecosystems has often been advocated and implemented at the expense of other approaches; this limits understanding of how natural ecosystems respond to biodiversity loss. I conclude that a broader spectrum of approaches, and more explicit consideration of how species losses and gains operate in concert to influence ecosystems, will help progress this field.
Summary 1. Chronosequences and associated space‐for‐time substitutions are an important and often necessary tool for studying temporal dynamics of plant communities and soil development across multiple time‐scales. However, they are often used inappropriately, leading to false conclusions about ecological patterns and processes, which has prompted recent strong criticism of the approach. Here, we evaluate when chronosequences may or may not be appropriate for studying community and ecosystem development. 2. Chronosequences are appropriate to study plant succession at decadal to millennial time‐scales when there is evidence that sites of different ages are following the same trajectory. They can also be reliably used to study aspects of soil development that occur between temporally linked sites over time‐scales of centuries to millennia, sometimes independently of their application to shorter‐term plant and soil biological communities. 3. Some characteristics of changing plant and soil biological communities (e.g. species richness, plant cover, vegetation structure, soil organic matter accumulation) are more likely to be related in a predictable and temporally linear manner than are other characteristics (e.g. species composition and abundance) and are therefore more reliably studied using a chronosequence approach. 4. Chronosequences are most appropriate for studying communities that are following convergent successional trajectories and have low biodiversity, rapid species turnover and low frequency and severity of disturbance. Chronosequences are least suitable for studying successional trajectories that are divergent, species‐rich, highly disturbed or arrested in time because then there are often major difficulties in determining temporal linkages between stages. 5. Synthesis. We conclude that, when successional trajectories exceed the life span of investigators and the experimental and observational studies that they perform, temporal change can be successfully explored through the judicious use of chronosequences.
No abstract is provided for this article.
The coupling between a resonant excitation and a nonresonant parametric process in a nonlinear system is studied experimentally under non-phase-matched conditions. Our study performed in the context of anti-Stokes stimulated Raman scattering provides a clear observation of the self-induced phase matching of a parametric process. A close agreement with theoretical predictions is observed.
