Low-diversity Mixtures Research Articles

Are bees attracted by flower richness? Implications for ecosystem service-based policy

Agri-environmental policies aim to reverse the degradation of ecosystem services in rural landscapes by implementing biodiversity-based land-use solutions. One such agri-environmental measure is the contract for bee-forage fields in Estonia. We developed a multi-site experiment to estimate the effect of plant diversity on the quality properties of flower-based services, such as flowering duration (indicating functional stability) and the foraging activity of insect pollinators (indicating functional intensity). Each site consisted of eight randomly ordered strip-segments forming a flower diversity gradient.The period of abundant flowering was longer in all kinds of species mixture than in monocultures. However, the foraging activity of honey bees and bumblebees was greatest in monocultures and in a low-diversity mixture, while a balanced high-diversity mixture was least attractive. Foraging activity was lowered when an abundant melliferous plant species was flowering within a high-diversity mixture (i.e. high species richness, but low evenness). The extended flowering of species mixtures did not compensate for the lower daily visitation rate.We challenge the largely biodiversity-oriented agricultural policy designs. This case study provides evidence that plant species richness is not a comprehensive indicator of the service provision quality of an ecosystem. Specifically, low-diversity flower areas are the best foraging sites for bees and other flower visitors. A field mosaic of various monocultures and low-diversity mixtures seems to be the ecologically most efficient rural landscape design to support bees and other potential pollinators. Suggested and marketed pollinator-oriented seed mixtures should be quantitatively tested for ecological efficiency.

Soil microbiome mediates positive plant diversity‐productivity relationships in late successional grassland species

Which processes drive the productivity benefits of biodiversity remain a critical, but unanswered question in ecology. We tested whether the soil microbiome mediates the diversity-productivity relationships among late successional plant species. We found that productivity increased with plant richness in diverse soil communities, but not with low-diversity mixtures of arbuscular mycorrhizal fungi or in pasteurised soils. Diversity-interaction modelling revealed that pairwise interactions among species best explained the positive diversity-productivity relationships, and that transgressive overyielding resulting from positive complementarity was only observed with the late successional soil microbiome, which was both the most diverse and exhibited the strongest community differentiation among plant species. We found evidence that both dilution/suppression from host-specific pathogens and microbiome-mediated resource partitioning contributed to positive diversity-productivity relationships and overyielding. Our results suggest that re-establishment of a diverse, late successional soil microbiome may be critical to the restoration of the functional benefits of plant diversity following anthropogenic disturbance.

The legacy of initial sowing after 20years of ex-arable land colonisation.

Priority effects provide an advantage to early establishing species and are thought to significantly affect the course of succession. We conducted a 20-year long experiment sowing high- and low-diversity mixtures in an ex-arable field. We ask how long the effect of sowing persists and which sown species affect the course of succession. The experiment was established in the Czech Republic in five replicate blocks, each containing three random 10 × 10m plots with three treatments: natural colonisation, sowing low- and high-diversity seed mixtures. The species cover was annually estimated in 12 permanent 1m2 quadrates within each plot. To identify the effects of sowing, we used an innovative method analysing the data separately for each year using Redundancy analysis (RDA) with identity of sown species as explanatory variables. In the first year, the effect of sowing was small; the peak of explained variability occurred between third and fifth year. The legacy of sowing was detectable in the natural colonisers for 18years and in the sown species for the whole 20-year period. For some species, the difference between the plots where they were and were not sown remained significant for the whole 20-year period (e.g. Lathyrus pratensis) although the plots were adjacent and the area was mown with the same machine. Other ones (e.g. Trisetum flavescens) colonised all the plots evenly. The long-lasting effect of the initial sowing confirms contingency of successional pathway on the propagule pressure in the time of start of succession due to the priority effects.

Responses of plant, insect and spider biodiversity to local and landscape scale management intensity in cereal crops and grasslands

The aim of this study was to determine the relative effects of landscape scale management intensity, agroecosystem type, local management intensity and edges on diversity patterns of functional groups of plants, carabid beetles, spiders and grasshoppers. Nine landscapes were selected differing in percent intensively used agricultural area (IAA), each with a pair of organic and conventional winter wheat fields and a pair of organic and conventional mown meadows. Within fields, plants were surveyed in the edge and in the interior. Carabid beetles and spiders were captured by funnel traps, while grasshoppers were sweep-netted in the meadows. Diversity patterns of study organisms were affected both by local variables (local management, agroecosystem type and within-field position) and by landscape scale management intensity. Species richness of grasses, presumably because of sowing low-diversity mixtures, and hunting spiders decreased with percent cover of IAA. Meadows differed from wheat fields in that they had higher species richness of forbs and grasses, as well as higher densities of hunting spiders. In contrast, more carabid individuals, especially of non-carnivore species, were captured in wheat fields. In field edges with their reduced management intensity and increased immigration, species richness of plants, carabids and spiders was higher than in the interiors regardless of agroecosystem type and management. Organic management enhanced forb richness and cover in both agroecosystem types. Organic management also increased grass cover in wheat fields, but not in meadows, and promoted species richness of non-carnivore carabids and hunting spiders, but not grasshoppers. The results show that agri-environmental management needs to be targeted to the agroecosystem's field size, because higher edge area led to higher species richness. Organic management affected several functional groups positively (forbs, non-carnivore carabids, hunting spiders), while lower landscape scale management intensity only increased species richness of grasses and spiders. The great differences in responses of functional groups to local cereal and grassland as well as landscape management suggest implementing more scale and group specific targets for agri-environmental schemes to improve their efficiency.

Sowing of low and high diversity seed mixtures in ecological restoration of surface mined‐land

AbstractQuestionsWhat are the differences in speed and pathway of vegetation development on nutrient‐poor, unvegetated slopes sown with low‐diversity seed mixtures of cultivars and high‐diversity seed mixtures of regional species? And how is the susceptibility of the different sowing treatments to immigration events via seed rain? We compared differences in using a low‐diversity seed mixture in comparison with a high‐diversity seed mixture, both sown on nutrient‐deficient raw soil. We assessed differences of speed and pathway of vegetation development of different variants as well as their susceptibility to immigration events via seed rain.LocationCentralGerman Lignite Mining District.MethodsIn December 2004, the trial was established in complete block design with four variants: sowing of a low‐diversity seed mixture with three grass cultivars and sowing of a high‐diversity seed mixture with 40 herbs and 11 grasses of local provenance, both with and without an additional mulch layer. Vegetation surveys were made between 2005 and 2010. Using generalized linear models (GLM), we tested for treatment effects on species diversity, above‐ground biomass, total coverage of vegetation as well as number and coverage of target grassland species. In addition, we estimatedBray‐Curtis distances between treatments using non‐metric‐multidimensional scaling.ResultsSowing of a high‐diversity seed mixture clearly accelerated the vegetation development and led to a significantly higher biomass production in the 1st year. On both variants, the additional mulch layer facilitated the establishment of sown species and led to a higher coverage of the herb layer in the 1st year. After 6 yr, the influence of the mulch layer decreased for the benefit of the seed mixture. Despite species exchange between sites, sites sown with different seed mixtures were still dominated by different sets of species in the final year of our study. Whereas high‐diversity mixtures sped up vegetation development in the direction of highly diverse semi‐natural grasslands, low‐diversity mixtures considerably delayed the successional progress.ConclusionsIn grassland restoration in surface‐mined land, high‐diversity mixtures performed similar or even better than low‐diversity mixtures of grass cultivars with respect to ecosystem services such as biomass production and ground cover (erosion control). An additional mulch layer considerably accelerated the vegetation development in the 1st years on both sowing variants. In contrast to sowing of low‐diversity seed mixtures of grass cultivars, the use of high‐diversity seed mixtures of local provenance contributes to the enhancement of local biodiversity. Although target grassland species were able to migrate into the low‐diversity sites, this process is slow and only possible when appropriate seed sources are nearby.

Diverse marsh plant communities are more consistently productive across a range of different environmental conditions through functional complementarity

Summary1. Understanding the influence of biodiversity on ecosystem functionality is crucial in modern ecosystem management, especially with regard to the resistance and resilience of ecosystems to future environmental changes. In this study, we assessed the effects of three different environmental regimes on the relationship between diversity and biomass production among marsh plants in comparison with a control treatment to elucidate the underlying classes of proximate mechanisms.2. We subjected assemblages of up to 23 marsh plant species to four different treatments (control, drought, salt, shade) for 4 months. We examined the treatment effect on the relationship between species diversity and biomass production and explored the underlying mechanism.3. Biomass production in the manipulated treatments showed a stronger positive effect of biodiversity than the control because of greater declines of biomass production in low diversity mixtures. This effect was owing to an increasingly positive complementarity effect, i.e. a benefit of most species, with increasing diversity, particularly in shade treatment. The selection effect, i.e. a benefit for few species at the expense of the others, was increasingly negative with increasing diversity and dominance by species with lower than average monoculture biomass. The variability of biomass production decreased with increasing species richness in all treatments.4. Synthesis and applications. We show that the productivity of diverse marsh plant communities is more consistent across a range of environmental conditions than that of depauperate communities and that this unexpectedly resulted from complementarity rather than selection effects. Our results demonstrate that loss of vegetation diversity reduces the average biomass production across a range of environmental conditions and emphasizes the importance of maintaining species‐rich biotic assemblages, especially in the face of global change.

Native Perennial Grassland Species for Bioenergy: Establishment and Biomass Productivity

Proposed perennial bioenergy cropping systems include both native grass monocultures and polycultures of grasses and forbs. We determined the effect of species richness and composition on establishment and initial biomass production of native plant polycultures. Twelve treatments with varying levels of species richness (1–24 species) were established. Establishment success and yield varied over eight locations. The number of species established in polyculture increased linearly as the number of species seeded increased. Average biomass yield ranged from 1.2 to 6.0 Mg ha−1 with the highest yielding treatments being grass monocultures or an eight species grass–legume mixture. An increase in species richness from one to eight species increased yield an average of 28%, but increasing species richness from 8 to 12 or 24 species had no yield advantage at most locations. Early successional species, Canada milkvetch (Astragalus canadensis L.) and Maximilian sunflower (Helianthus maximilian Schrad.), were dominant in mixtures and contributed a majority of the biomass to the yield. Even in high diversity plots, biomass was from only a few plant species with a single species dominating the mixture. Our results suggest that selected low diversity mixtures (one to five species) likely offer the best combination of species establishment and high yield during stand establishment. However, we expect that early successional species that were dominant during the establishment phase of our experiment will contribute less biomass as stands mature and later successional species will become dominant and provide greater biomass.

Grassland restoration on former croplands in Europe: an assessment of applicability of techniques and costs

Grasslands used to be vital landscape elements throughout Europe. Nowadays, the area of grasslands is dramatically reduced, especially in industrial countries. Grassland restoration is widely applied to increase the naturalness of the landscape and preserve biodiversity. We reviewed the most frequently used restoration techniques (spontaneous succession, sowing seed mixtures, transfer of plant material, topsoil removal and transfer) and techniques used to improve species richness (planting, grazing and mowing) to recover natural-like grasslands from ex-arable lands. We focus on the usefulness of methods in restoring biodiversity, their practical feasibility and costs. We conclude that the success of each technique depends on the site conditions, history, availability of propagules and/or donor sites, and on the budget and time available for restoration. Spontaneous succession can be an option for restoration when no rapid result is expected, and is likely to lead to the target in areas with high availability of propagules. Sowing low-diversity seed mixtures is recommended when we aim at to create basic grassland vegetation in large areas and/or in a short time. The compilation of high-diversity seed mixtures for large sites is rather difficult and expensive; thus, it may be applied rather on smaller areas. We recommend combining the two kinds of seed sowing methods by sowing low-diversity mixtures in a large area and high-diversity mixtures in small blocks to create species-rich source patches for the spontaneous colonization of nearby areas. When proper local hay sources are available, the restoration with plant material transfer can be a fast and effective method for restoration.

Early vegetation development after grassland restoration by sowing low-diversity seed mixtures in former sunflower and cereal fields

We studied the early vegetation dynamics in former croplands (sunflower and cereal fields) sown with a low-diversity seed mixture (composed of 2 native grass species) in Egyek-Pusztakócs, Hortobágy National Park, East-Hungary. The percentage cover of vascular plants was recorded in 4 permanent plots per field on 7 restored fields between 2006 and 2009. Ten aboveground biomass samples per field were also collected in June in each year. We addressed two questions: (i) How do seed sowing and annual mowing affect the species richness, biomass and cover of weeds? (ii) How fast does the cover of sown grasses develop after seed sowing? Weedy species were characteristic in the first year after sowing. In the second and third year their cover and species richness decreased. From the second year onwards the cover of perennial grasses increased. Spontaneously immigrating species characteristic to the reference grasslands were also detected with low cover scores. Short-lived weeds were suppressed as their cover and biomass significantly decreased during the study. The amount of litter and sown grass biomass increased progressively. However, perennial weed cover, especially the cover of Cirsium arvense increased substantially. Our results suggest that grassland vegetation can be recovered by sowing low diversity mixtures followed up by yearly mowing. Suppression of perennial weed cover needs more frequent mowing (multiple times a year) or grazing.

Long‐term effectiveness of sowing high and low diversity seed mixtures to enhance plant community development on ex‐arable fields

AbstractQuestions: How is succession on ex‐arable land affected by sowing high and low diversity mixtures of grassland species as compared to natural succession? How long do effects persist?Location: Experimental plots installed in the Czech Republic, The Netherlands, Spain, Sweden and the United Kingdom.Methods: The experiment was established on ex‐arable land, with five blocks, each containing three 10 m × 10 m experimental plots: natural colonization, a low‐ (four species) and high‐diversity (15 species) seed mixture. Species composition and biomass was followed for eight years.Results: The sown plants considerably affected the whole successional pathway and the effects persisted during the whole eight year period. Whilst the proportion of sown species (characterized by their cover) increased during the study period, the number of sown species started to decrease from the third season onwards. Sowing caused suppression of natural colonizing species, and the sown plots had more biomass. These effects were on average larger in the high diversity mixtures. However, the low diversity replicate sown with the mixture that produced the largest biomass or largest suppression of natural colonizers fell within the range recorded at the five replicates of the high diversity plots. The natural colonization plots usually had the highest total species richness and lowest productivity at the end of the observation period.Conclusions: The effect of sowing demonstrated dispersal limitation as a factor controlling the rate of early secondary succession. Diversity was important primarily for its‘insurance effect’: the high diversity mixtures were always able to compensate for the failure of some species.

THE ROLE OF PLANT DIVERSITY AND COMPOSITION FOR NITRATE LEACHING IN GRASSLANDS

The relationship between plant diversity and nitrate leaching into groundwater was investigated in a mid-European semi-natural grassland ecosystem. An experimental approach was used to directly manipulate plant diversity in the field, while holding other environmental factors constant. Species loss was simulated by establishing grassland communities of 16, 8, 4, 2, 1, and 0 plant species, composed of 3, 2, or 1 functional groups (grasses, legumes, and non-legume herbs). Every diversity treatment was replicated with several different species mixtures. Nitrate leaching was determined by continuous extraction of soil solution below the rooting zone and modeling of seepage rates. The concentration of nitrate in the soil solution was highly variable within each level of diversity. In bare ground plots and several low-diversity mixtures containing legumes, nitrate concentrations were higher than the official European Union threshold value for drinking water of 50 mg/L, with maximum values of up to 350 mg/L measured in Trifolium pratense monocultures. Total annual loss of nitrate was unaffected by the number of plant species or functional groups, but it was highly dependent on the specific species composition of the communities, and plots with legumes lost significantly more nitrate than plots without them. Aboveground biomass had no influence on nitrate loss, whereas leaching was negatively correlated with increasing root biomass. The abundance of legumes within a community, litter decomposition rates, and net nitrification were all positively correlated with total nitrate loss. However, in those communities containing legumes, leaching decreased with increasing diversity, because higher species richness led to a reduction in legume dominance, to a reduced nitrate supply through nitrification, and to a complementary uptake of nitrate by grasses and non-leguminous herbs. Based on these results, we expect that increasing the diversity of non-leguminous species or functional groups would reduce the risk of nitrate leaching in low-diversity grass–clover mixtures of ley-farming systems, while allowing for a more efficient exploitation of the beneficial fertilization effect provided by legumes. Corresponding Editor: M. Loreau.

Separating the chance effect from other diversity effects in the functioning of plant communities

The effect of plant species diversity on productivity and competitive ability was studied in an experiment carried out simultaneously in five European countries: Czech Republic (CZ), the Netherlands (NL), Sweden (SE), Spain (SP), and United Kingdom (UK). The aim was to separate the ‘chance’ or ‘sampling effect’ (increasing the number of sown species increases the probability that a species able ‘to do a job’ will be included) from the complementarity effect (species‐rich communities are better able to exploit resources and to take care of ecosystem functions than species‐poor communities). In the experiment, low diversity (LD) and high diversity (HD) mixtures of grassland species were sown into fields taken out of arable cultivation. The HD mixture consisted of five grass species, five legumes and five other forbs. The LD mixtures consisted of two grasses, one legume and one other forb, with different plant species combinations in each replicate block. The design of the experiment was constructed in such a way that the total number of seeds of each species over all the replications was exactly the same in HD and LD treatments, and the total number of grass seeds, leguminous seeds and other forb seeds were the same in both LD and HD. The responses measured were the total above‐ground biomass (as a measure of productivity) and the average number of naturally establishing species in a plot (as a measure of the competitive ability of the mixture), both measured in the third year of the experiment. The results show that, on average, the HD plots performed better (i.e., attained higher biomass, had better weed suppression), but that the best LD mixture was as good as the best HD mixture. On the contrary, the worst LD mixture was always less successful than the worst HD replicate. The performance of particular species in the HD mixtures was a good predictor of the success of a certain species combination in a LD mixture (explaining 61% of variability between particular LD mixtures). In all sites, the LD mixture composed of species which were the most abundant in HD mixtures was as efficient in suppressing weeds as the HD mixture itself. It is argued that the performance of a species assemblage is influenced mostly by the identity of species and the diversity effect is mainly due to the ‘chance’ or ‘sampling’ effect: with increasing number of species the probability that an important species will be included in the mixture increases. Caution is urged in interpreting experiments with manipulated diversity and the possible limitations of such experiments are discussed.

Plant secondary metabolite diversity as a resistance trait against insects: a test with Sitophilus granarius (Coleoptera: Curculionidae) and seed secondary metabolites

A hypothesis is tested about the functioning of plant secondary metabolite diversity taking into account the role of concentration of mixtures or single compounds: a high diversity secondary metabolite mixture in high concentration provides a more effective protection against herbivores than single compounds or low diversity mixtures in both low and high concentrations. Additionally the role of two unusual secondary metabolites for a weevil when they are incorporated in its diet is explored. Number of eggs laid and diet consumption are measured in the granary weevil Sitophilus granarius exposed to cereal and non-cereal secondary metabolites incorporated in artificial diets to test the hypothesis. Artificial diets had one, two, four or six compounds. The mixtures or single compounds were offered simultaneously to the weevils in concentrations ranging from 0.05 to 5mg g−1 in a multiple choice experiment. Low oviposition in high concentration six compounds mixture supported the hypothesis. However, low oviposition in a low concentration mixture of four compounds suggests that a moderate diversity may be equally functional in low and high concentrations. Diet consumption by the weevil was not affected by concentration but an increasing number of compounds in the diet reduced it slightly but significantly. The unusual secondary metabolites for the weevil were less or equally effective reducing oviposition and consumption as usual compounds did.

Influence of Nitrogen Loading and Species Composition on the Carbon Balance of Grasslands

In a 12-year experimental study of nitrogen (N) deposition on Minnesota grasslands, plots dominated by native warm-season grasses shifted to low-diversity mixtures dominated by cool-season grasses at all but the lowest N addition rates. This shift was associated with decreased biomass carbon (C):N ratios, increased N mineralization, increased soil nitrate, high N losses, and low C storage. In addition, plots originally dominated by nonnative cool-season grasses retained little added N and stored little C, even at low N input rates. Thus, grasslands with high N retention and C storage rates were the most vulnerable to species losses and major shifts in C and N cycling.