Hierarchical plant responses and diversity loss after nitrogen addition: testing three functionally-based hypotheses in the Inner Mongolia grassland

Background

Numerous studies have shown that nitrogen (N) deposition decreases biodiversity in terrestrial ecosystems. To explain the N-induced species loss, three functionally based hypotheses have been proposed: the aboveground competition hypothesis, the belowground competition hypothesis, and the total competition hypothesis. However, none of them is supported sufficiently by field experiments. A main challenge to testing these hypotheses is to ascertain the role of shoot and root competition in controlling plant responses to N enrichment. Simultaneously examining both aboveground and belowground responses in natural ecosystems is logistically complex, and has rarely been done.

Methodology/Principal Findings

In a two-year N addition experiment conducted in a natural grassland ecosystem, we investigated both above- and belowground responses of plants at the individual, species, and community levels. Plants differed significantly in their responses to N addition across the different organizational levels. The community-level species loss was mainly due to the loss of perennial grasses and forbs, while the relative abundance of plant species was dependent mainly on individual-level responses. Plasticity in biomass allocation was much smaller within a species than between species, providing a biological basis for explaining the functionally based species loss. All species increased biomass allocation to aboveground parts, but species with high belowground allocations were replaced by those with high aboveground allocations, indicating that the increased aboveground competition was the key process responsible for the observed diversity loss after N addition in this grassland ecosystem.

Conclusions/Significance

Our findings shed new light on the validity of the three competing hypotheses concerning species loss in response to N enrichment. They also have important implications for predicting the future impacts of N deposition on the structure and functioning of terrestrial ecosystems. In addition, we have developed a new technique for ascertaining the roles of aboveground and belowground competition in determining plant responses to N fertilization.     

Local Adaptation of Aboveground Herbivores towards Plant Phenotypes Induced by Soil Biota

Background

Soil biota may trigger strong physiological responses in plants and consequently induce distinct phenotypes. Plant phenotype, in turn, has a strong impact on herbivore performance. Here, we tested the hypothesis that aboveground herbivores are able to adapt to plant phenotypes induced by soil biota.

Methodology and Principal Findings

We bred spider mites for 15 generations on snap beans with three different belowground biotic interactions: (i) no biota (to serve as control), (ii) arbuscular mycorrhizal fungi and (ii) root-feeding nematodes. Subsequently, we conducted a reciprocal selection experiment using these spider mites, which had been kept on the differently treated plants. Belowground treatments induced changes in plant biomass, nutrient composition and water content. No direct chemical defence through cyanogenesis was detected in any of the plant groups. Growth rates of spider mites were higher on the ecotypes on which they were bred for 15 generations, although the statistical significance disappeared for mites from the nematode treatment when corrected for all multiple comparisons.

Conclusion/Significance

These results demonstrate that belowground biota may indeed impose selection on the aboveground insect herbivores mediated by the host plant. The observed adaptation was driven by variable quantitative changes of the different separately studied life history traits (i.e. fecundity, longevity, sex-ratio, time to maturity).     

Prevailing negative soil biota effect and no evidence for local adaptation in a widespread Eurasian grass

Background

Soil biota effects are increasingly accepted as an important driver of the abundance and distribution of plants. While biogeographical studies on alien invasive plant species have indicated coevolution with soil biota in their native distribution range, it is unknown whether adaptation to soil biota varies among populations within the native distribution range. The question of local adaptation between plants and their soil biota has important implications for conservation of biodiversity and may justify the use of seed material from local provenances in restoration campaigns.

Methodology/Principal Findings

We studied soil biota effects in ten populations of the steppe grass Stipa capillata from two distinct regions, Europe and Asia. We tested for local adaptation at two different scales, both within (ca. 10–80 km) and between (ca. 3300 km) regions, using a reciprocal inoculation experiment in the greenhouse for nine months. Generally, negative soil biota effects were consistent. However, we did not find evidence for local adaptation: both within and between regions, growth of plants in their ‘home soil’ was not significantly larger relative to that in soil from other, more distant, populations.

Conclusions/Significance

Our study suggests that negative soil biota effects can prevail in different parts of a plant species'' range. Absence of local adaptation points to the possibility of similar rhizosphere biota composition across populations and regions, sufficient gene flow to prevent coevolution, selection in favor of plasticity, or functional redundancy among different soil biota. From the point of view of plant - soil biota interactions, our findings indicate that the current practice of using seeds exclusively from local provenances in ecosystem restoration campaigns may not be justified.     

Contribution of above- and below-ground plant traits to the structure and function of grassland soil microbial communities

Background and Aims

Abiotic properties of soil are known to be major drivers of the microbial community within it. Our understanding of how soil microbial properties are related to the functional structure and diversity of plant communities, however, is limited and largely restricted to above-ground plant traits, with the role of below-ground traits being poorly understood. This study investigated the relative contributions of soil abiotic properties and plant traits, both above-ground and below-ground, to variations in microbial processes involved in grassland nitrogen turnover.

Methods

In mountain grasslands distributed across three European sites, a correlative approach was used to examine the role of a large range of plant functional traits and soil abiotic factors on microbial variables, including gene abundance of nitrifiers and denitrifiers and their potential activities.

Key Results

Direct effects of soil abiotic parameters were found to have the most significant influence on the microbial groups investigated. Indirect pathways via plant functional traits contributed substantially to explaining the relative abundance of fungi and bacteria and gene abundances of the investigated microbial communities, while they explained little of the variance in microbial activities. Gene abundances of nitrifiers and denitrifiers were most strongly related to below-ground plant traits, suggesting that they were the most relevant traits for explaining variation in community structure and abundances of soil microbes involved in nitrification and denitrification.

Conclusions

The results suggest that consideration of plant traits, and especially below-ground traits, increases our ability to describe variation in the abundances and the functional characteristics of microbial communities in grassland soils.     

Changes in plant species richness induce functional shifts in soil nematode communities in experimental grassland

Background

Changes in plant diversity may induce distinct changes in soil food web structure and accompanying soil feedbacks to plants. However, knowledge of the long-term consequences of plant community simplification for soil animal food webs and functioning is scarce. Nematodes, the most abundant and diverse soil Metazoa, represent the complexity of soil food webs as they comprise all major trophic groups and allow calculation of a number of functional indices.

Methodology/Principal Findings

We studied the functional composition of nematode communities three and five years after establishment of a grassland plant diversity experiment (Jena Experiment). In response to plant community simplification common nematode species disappeared and pronounced functional shifts in community structure occurred. The relevance of the fungal energy channel was higher in spring 2007 than in autumn 2005, particularly in species-rich plant assemblages. This resulted in a significant positive relationship between plant species richness and the ratio of fungal-to-bacterial feeders. Moreover, the density of predators increased significantly with plant diversity after five years, pointing to increased soil food web complexity in species-rich plant assemblages. Remarkably, in complex plant communities the nematode community shifted in favour of microbivores and predators, thereby reducing the relative abundance of plant feeders after five years.

Conclusions/Significance

The results suggest that species-poor plant assemblages may suffer from nematode communities detrimental to plants, whereas species-rich plant assemblages support a higher proportion of microbivorous nematodes stimulating nutrient cycling and hence plant performance; i.e. effects of nematodes on plants may switch from negative to positive. Overall, food web complexity is likely to decrease in response to plant community simplification and results of this study suggest that this results mainly from the loss of common species which likely alter plant – nematode interactions.     

A functional trait perspective on plant invasion

Background and Aims

Global environmental change will affect non-native plant invasions, with profound potential impacts on native plant populations, communities and ecosystems. In this context, we review plant functional traits, particularly those that drive invader abundance (invasiveness) and impacts, as well as the integration of these traits across multiple ecological scales, and as a basis for restoration and management.

Scope

We review the concepts and terminology surrounding functional traits and how functional traits influence processes at the individual level. We explore how phenotypic plasticity may lead to rapid evolution of novel traits facilitating invasiveness in changing environments and then ‘scale up’ to evaluate the relative importance of demographic traits and their links to invasion rates. We then suggest a functional trait framework for assessing per capita effects and, ultimately, impacts of invasive plants on plant communities and ecosystems. Lastly, we focus on the role of functional trait-based approaches in invasive species management and restoration in the context of rapid, global environmental change.

Conclusions

To understand how the abundance and impacts of invasive plants will respond to rapid environmental changes it is essential to link trait-based responses of invaders to changes in community and ecosystem properties. To do so requires a comprehensive effort that considers dynamic environmental controls and a targeted approach to understand key functional traits driving both invader abundance and impacts. If we are to predict future invasions, manage those at hand and use restoration technology to mitigate invasive species impacts, future research must focus on functional traits that promote invasiveness and invader impacts under changing conditions, and integrate major factors driving invasions from individual to ecosystem levels.     

Developing X-ray Computed Tomography to non-invasively image 3-D root systems architecture in soil

Background

The positive relationship between biodiversity and ecosystem functioning (BEF) is due mainly to complementarity between species. Most BEF studies primarily focused on plant interactions; however, plants are embedded in a dense network of multitrophic interactions above and below the ground, which are likely to play a crucial role in BEF relationships.

Scope

In the present review I point out the relevance of aboveground–belowground interactions as a source of complementarity effects in grassland biodiversity experiments. A review of the current knowledge on the role of decomposers, arbuscular mycorrhizal fungi, rhizobia, plant growth promoting rhizobacteria, invertebrate ecosystem engineers, herbivores, pathogens and predators in biodiversity experiments, indicates that soil biota can drive both positive and negative complementarity between plant species via a multitude of mechanisms.

Conclusions

I pose four main processes by which aboveground–belowground interactions determine positive complementarity effects: enlarging biotope space, mediating legume effects, increasing plant community resistance, and maintaining plant diversity. By contrast, soil biota may also reinforce negative complementarity effects by competing with plants for nutrients or by exerting herbivore or pathogen pressure, thereby reducing community productivity. Thus, considering aboveground–belowground interactions as well as interactions between antagonistic and mutualistic consumers may improve the mechanistic understanding of complementarity effects in plant diversity–ecosystem functioning experiments and should inspire future research.     

Genetic identity affects performance of species in grasslands of different plant diversity: an experiment with Lolium perenne cultivars

Background and Aims

Recent biodiversity research has focused on ecosystem processes, but less is known about responses of populations of individual plant species to changing community diversity and implications of genetic variation within species. To address these issues, effects of plant community diversity on the performance of different cultivars of Lolium perenne were analysed.

Methods

Populations of 15 genetic cultivars of Lolium perenne were established in experimental grasslands varying in richness of species (from 1 to 60) and functional groups (from 1 to 4). Population sizes, mean size of individual plants, biomass of individual shoots and seed production were measured in the first and second growing season after establishment.

Key Results

Population sizes of all cultivars decreased with increasing community species richness. Plant individuals formed fewer shoots with a lower shoot mass in more species-rich plant communities. A large proportion of variation in plant size and relative population growth was attributable to effects of community species and functional group richness, but the inclusion of cultivar identity explained additional 4–7 % of variation. Cultivar identity explained most variation (28–51 %) at the shoot level (biomass of individual tillers and reproductive shoots, seed production, heading stage). Coefficients of variation of the measured variables across plant communities were larger in cultivars with a lower average performance, indicating that this variation was predominantly due to passive growth reductions and not a consequence of larger adaptive plastic responses. No single cultivar performed best in all communities.

Conclusions

The decreasing performance of Lolium perenne in plant communities of increasing species richness suggests a regulation of competitive interactions by species diversity. Genetic variation within species provides a base for larger phenotypic variation and may affect competitive ability. However, heterogeneous biotic environments (= plant communities of different species composition) are important for the maintenance of intra-specific genetic variation.Key words: Biodiversity, competition, genetic variation, growth reduction, Lolium perenne, phenotypic plasticity, species richness     

Soil Communities Promote Temporal Stability and Species Asynchrony in Experimental Grassland Communities

Background

Over the past two decades many studies have demonstrated that plant species diversity promotes primary productivity and stability in grassland ecosystems. Additionally, soil community characteristics have also been shown to influence the productivity and composition of plant communities, yet little is known about whether soil communities also play a role in stabilizing the productivity of an ecosystem.

Methodology/Principal Findings

Here we use microcosms to assess the effects of the presence of soil communities on plant community dynamics and stability over a one-year time span. Microcosms were filled with sterilized soil and inoculated with either unaltered field soil or field soil sterilized to eliminate the naturally occurring soil biota. Eliminating the naturally occurring soil biota not only resulted in lower plant productivity, and reduced plant species diversity, and evenness, but also destabilized the net aboveground productivity of the plant communities over time, which was largely driven by changes in abundance of the dominant grass Lolium perenne. In contrast, the grass and legumes contributed more to net aboveground productivity of the plant communities in microcosms where soil biota had been inoculated. Additionally, the forbs exhibited compensatory dynamics with grasses and legumes, thus lowering temporal variation in productivity in microcosms that received the unaltered soil inocula. Overall, asynchrony among plant species was higher in microcosms where an unaltered soil community had been inoculated, which lead to higher temporal stability in community productivity.

Conclusions/Significance

Our results suggest that soil communities increase plant species asynchrony and stabilize plant community productivity by equalizing the performance among competing plant species through potential antagonistic and facilitative effects on individual plant species.     

Biomass and Its Allocation in Relation to Temperature,Precipitation, and Soil Nutrients in Inner Mongolia Grasslands,China

Aim

Understanding and predicting ecosystem functioning such as biomass accumulation requires an accurate assessment of large-scale patterns of biomass distribution and partitioning in relation to climatic and soil environments.

Methods

We sampled above- and belowground biomass from 26 sites spanning 1500 km in Inner Mongolian grasslands, compared the difference in aboveground, belowground biomass and below-aboveground biomass ratio (AGB, BGB, and B/A, respectively) among meadow steppe, typical steppe, and desert steppe types. The relationships between AGB, BGB, B/A and climatic and soil environments were then examined.

Results

We found that AGB and BGB differed significantly among three types of grasslands while B/A did not differ. Structural equation model analyses indicated that mean annual precipitation was the strongest positive driver for AGB and BGB. AGB was also positively associated with soil organic carbon, whereas B/A was positively associated with total soil nitrogen.

Conclusions

These results indicated that precipitation positively influence plant production in Inner Mongolian grasslands. Contrary to the prediction from the optimal partitioning hypothesis, biomass allocation to belowground increased with soil total nitrogen, suggesting that more productive sites may increase belowground allocation as an adaptive strategy to potentially high fire frequencies.     

Relative importance of biotic and abiotic soil components to plant growth and insect herbivore population dynamics

Background

Plants are affected by several aspects of the soil, which have the potential to exert cascading effects on the performance of herbivorous insects. The effects of biotic and abiotic soil characteristics have however mostly been investigated in isolation, leaving their relative importance largely unexplored. Such is the case for the dune grass Ammophila, whose decline under decreasing sand accretion is argued to be caused by either biotic or abiotic soil properties.

Methodology/Principal Findings

By manipulating dune soils from three different regions, we decoupled the contributions of region, the abiotic and biotic soil component to the variation in characteristics of Ammophila arenaria seedlings and Schizaphis rufula aphid populations. Root mass fraction and total dry biomass of plants were affected by soil biota, although the latter effect was not consistent across regions. None of the measured plant properties were significantly affected by the abiotic soil component. Aphid population characteristics all differed between regions, irrespective of whether soil biota were present or absent. Hence these effects were due to differences in abiotic soil properties between regions. Although several chemical properties of the soil mixtures were measured, none of these were consistent with results for plant or aphid traits.

Conclusions/Significance

Plants were affected more strongly by soil biota than by abiotic soil properties, whereas the opposite was true for aphids. Our results thus demonstrate that the relative importance of the abiotic and biotic component of soils can differ for plants and their herbivores. The fact that not all effects of soil properties could be detected across regions moreover emphasizes the need for spatial replication in order to make sound conclusions about the generality of aboveground-belowground interactions.     

Plant Trait Diversity Buffers Variability in Denitrification Potential over Changes in Season and Soil Conditions

Background

Denitrification is an important ecosystem service that removes nitrogen (N) from N-polluted watersheds, buffering soil, stream, and river water quality from excess N by returning N to the atmosphere before it reaches lakes or oceans and leads to eutrophication. The denitrification enzyme activity (DEA) assay is widely used for measuring denitrification potential. Because DEA is a function of enzyme levels in soils, most ecologists studying denitrification have assumed that DEA is less sensitive to ambient levels of nitrate (NO₃ ⁻) and soil carbon and thus, less variable over time than field measurements. In addition, plant diversity has been shown to have strong effects on microbial communities and belowground processes and could potentially alter the functional capacity of denitrifiers. Here, we examined three questions: (1) Does DEA vary through the growing season? (2) If so, can we predict DEA variability with environmental variables? (3) Does plant functional diversity affect DEA variability?

Methodology/Principal Findings

The study site is a restored wetland in North Carolina, US with native wetland herbs planted in monocultures or mixes of four or eight species. We found that denitrification potentials for soils collected in July 2006 were significantly greater than for soils collected in May and late August 2006 (p<0.0001). Similarly, microbial biomass standardized DEA rates were significantly greater in July than May and August (p<0.0001). Of the soil variables measured—soil moisture, organic matter, total inorganic nitrogen, and microbial biomass—none consistently explained the pattern observed in DEA through time. There was no significant relationship between DEA and plant species richness or functional diversity. However, the seasonal variance in microbial biomass standardized DEA rates was significantly inversely related to plant species functional diversity (p<0.01).

Conclusions/Significance

These findings suggest that higher plant functional diversity may support a more constant level of DEA through time, buffering the ecosystem from changes in season and soil conditions.     

Non-Additive Effects of Genotypic Diversity Increase Floral Abundance and Abundance of Floral Visitors

Background

In the emerging field of community and ecosystem genetics, genetic variation and diversity in dominant plant species have been shown to play fundamental roles in maintaining biodiversity and ecosystem function. However, the importance of intraspecific genetic variation and diversity to floral abundance and pollinator visitation has received little attention.

Methodology/Principal Findings

Using an experimental common garden that manipulated genotypic diversity (the number of distinct genotypes per plot) of Solidago altissima, we document that genotypic diversity of a dominant plant can indirectly influence flower visitor abundance. Across two years, we found that 1) plant genotype explained 45% and 92% of the variation in flower visitor abundance in 2007 and 2008, respectively; and 2) plant genotypic diversity had a positive and non-additive effect on floral abundance and the abundance of flower visitors, as plots established with multiple genotypes produced 25% more flowers and received 45% more flower visits than would be expected under an additive model.

Conclusions/Significance

These results provide evidence that declines in genotypic diversity may be an important but little considered factor for understanding plant-pollinator dynamics, with implications for the global decline in pollinators due to reduced plant diversity in both agricultural and natural ecosystems.     

Soil substrate utilization pattern and relation of functional evenness of plant groups and soil microbial community in five low mountain NATURA 2000

Background and aims

Species rich, semi-natural grassland systems provide several ecosystem functions. The goal was to assess how aboveground composition and evenness affects soil substrate utilization pattern and soil microbial functional evenness.

Methods

At five German NATURA 2000 grassland sites, the interactions of plant functional groups (graminoids, forbs and legumes) and belowground microbial functional evenness were investigated in relation to soil properties and sampling date. Functional evenness of soil microorganisms was measured with high spatial resolution by community level physiological profiling (CLPP) using multi-SIR (substrate-induced respiration) at three sampling dates during the vegetation period. Evenness indices were used to compare plant functional group diversity and soil microbial functional diversity.

Results

All sites differed in the consistently high soil microbial functional evenness, which was strongly predicted by soil pH, but not by plant functional groups or aboveground plant dry matter production. However, soil microbial functional evenness was particularly decreased by an increasing legume proportion and showed seasonal changes, probably driven by shifts in resource availability and soil water content.

Conclusions

Our results suggest that changes in soil chemical properties or in a single key plant functional group may have stronger effects on soil microbial functional evenness than changes in plant functional group evenness.     

Long-term functional plasticity in plant hydraulic architecture in response to supplemental moisture

Background and Aims

Plasticity in structural and functional traits related to water balance may determine plant performance and survival in ecosystems characterized by water limitation or high levels of rainfall variability, particularly in perennial herbaceous species with long generation cycles. This paper addresses whether and the extent to which several such seasonal to long-term traits respond to changes in moisture availability.

Methods

Using a novel approach that integrates ecology, physiology and anatomy, a comparison was made of lifetime functional traits in the root xylem of a long-lived perennial herb (Potentilla diversifolia, Rosaceae) growing in dry habitats with those of nearby individuals growing where soil moisture had been supplemented for 14 years. Traditional parameters such as specific leaf area (SLA) and above-ground growth were also assessed.

Key Results

Individuals from the site receiving supplemental moisture consistently showed significant responses in all considered traits related to water balance: SLA was greater by 24 %; roots developed 19 % less starch storing tissue, an indicator for drought-stress tolerance; and vessel size distributions shifted towards wider elements that collectively conducted water 54 % more efficiently – but only during the years for which moisture was supplemented. In contrast, above-ground growth parameters showed insignificant or inconsistent responses.

Conclusions

The phenotypic changes documented represent consistent, dynamic responses to increased moisture availability that should increase plant competitive ability. The functional plasticity of xylem anatomy quantified in this study constitutes a mechanistic basis for anticipating the differential success of plant species in response to climate variability and change, particularly where water limitation occurs.     

Desert farming benefits from microbial potential in arid soils and promotes diversity and plant health

Background

To convert deserts into arable, green landscapes is a global vision, and desert farming is a strong growing area of agriculture world-wide. However, its effect on diversity of soil microbial communities, which are responsible for important ecosystem services like plant health, is still not known.

Methodology/Principal Findings

We studied the impact of long-term agriculture on desert soil in one of the most prominent examples for organic desert farming in Sekem (Egypt). Using a polyphasic methodological approach to analyse microbial communities in soil as well as associated with cultivated plants, drastic effects caused by 30 years of agriculture were detected. Analysing bacterial fingerprints, we found statistically significant differences between agricultural and native desert soil of about 60%. A pyrosequencing-based analysis of the 16S rRNA gene regions showed higher diversity in agricultural than in desert soil (Shannon diversity indices: 11.21/7.90), and displayed structural differences. The proportion of Firmicutes in field soil was significantly higher (37%) than in the desert (11%). Bacillus and Paenibacillus play the key role: they represented 96% of the antagonists towards phytopathogens, and identical 16S rRNA sequences in the amplicon library and for isolates were detected. The proportion of antagonistic strains was doubled in field in comparison to desert soil (21.6%/12.4%); disease-suppressive bacteria were especially enriched in plant roots. On the opposite, several extremophilic bacterial groups, e.g., Acidimicrobium, Rubellimicrobium and Deinococcus-Thermus, disappeared from soil after agricultural use. The N-fixing Herbaspirillum group only occurred in desert soil. Soil bacterial communities were strongly driven by the a-biotic factors water supply and pH.

Conclusions/Significance

After long-term farming, a drastic shift in the bacterial communities in desert soil was observed. Bacterial communities in agricultural soil showed a higher diversity and a better ecosystem function for plant health but a loss of extremophilic bacteria. Interestingly, we detected that indigenous desert microorganisms promoted plant health in desert agro-ecosystems.     

Animal ecosystem engineers modulate the diversity-invasibility relationship

Background

Invasions of natural communities by non-indigenous species are currently rated as one of the most important global-scale threats to biodiversity. Biodiversity itself is known to reduce invasions and increase stability. Disturbances by ecosystem engineers affect the distribution, establishment, and abundance of species but this has been ignored in studies on diversity-invasibility relationships.

Methodology/Principal Findings

We determined natural plant invasion into 46 plots varying in the number of plant species (1, 4, and 16) and plant functional groups (1, 2, 3, and 4) for three years beginning two years after the establishment of the Jena Experiment. We sampled subplots where earthworms were artificially added and others where earthworm abundance was reduced. We also performed a seed-dummy experiment to investigate the role of earthworms as secondary seed dispersers along a plant diversity gradient. Horizontal dispersal and burial of seed dummies were significantly reduced in subplots where earthworms were reduced in abundance. Seed dispersal by earthworms decreased with increasing plant species richness and presence of grasses but increased in presence of small herbs. These results suggest that dense vegetation inhibits the surface activity of earthworms. Further, there was a positive relationship between the number of earthworms and the number and diversity of invasive plants. Hence, earthworms decreased the stability of grassland communities against plant invasion.

Conclusions/Significance

Invasibility decreased and stability increased with increasing plant diversity and, most remarkably, earthworms modulated the diversity-invasibility relationship. While the impacts of earthworms were unimportant in low diverse (low earthworm densities) and high diverse (high floral structural complexity) plant communities, earthworms decreased the stability of intermediate diverse plant communities against plant invasion. Overall, the results document that fundamental processes in plant communities like plant seed burial and invader establishment are modulated by soil fauna calling for closer cooperation between soil animal and plant ecologists.     

Performance of dryland and wetland plant species on extensive green roofs

Background and Aims

Green roofs are constructed ecosystems where plants perform valuable services, ameliorating the urban environment through roof temperature reductions and stormwater interception. Plant species differ in functional characteristics that alter ecosystem properties. Plant performance research on extensive green roofs has so far indicated that species adapted to dry conditions perform optimally. However, in moist, humid climates, species typical of wetter soils might have advantages over dryland species. In this study, survival, growth and the performance of thermal and stormwater capture functions of three pairs of dryland and wetland plant species were quantified using an extensive modular green roof system.

Methods

Seedlings of all six species were germinated in a greenhouse and planted into green roof modules with 6 cm of growing medium. There were 34 treatments consisting of each species in monoculture and all combinations of wet- and dryland species in a randomized block design. Performance measures were survival, vegetation cover and roof surface temperature recorded for each module over two growing seasons, water loss (an estimate of evapotranspiration) in 2007, and albedo and water capture in 2008.

Key Results

Over two seasons, dryland plants performed better than wetland plants, and increasing the number of dryland species in mixtures tended to improve functioning, although there was no clear effect of species or habitat group diversity. All species had survival rates >75 % after the first winter; however, dryland species had much greater cover, an important indicator of green roof performance. Sibbaldiopsis tridentata was the top performing species in monoculture, and was included in the best treatments.

Conclusions

Although dryland species outperformed wetland species, planting extensive green roofs with both groups decreased performance only slightly, while increasing diversity and possibly habitat value. This study provides further evidence that plant composition and diversity can influence green roof functions.     

Effects of plant functional group removal on structure and function of soil communities across contrasting ecosystems

Loss of plant diversity has an impact on ecosystems worldwide, but we lack a mechanistic understanding of how this loss may influence below‐ground biota and ecosystem functions across contrasting ecosystems in the long term. We used the longest running biodiversity manipulation experiment across contrasting ecosystems in existence to explore the below‐ground consequences of 19 years of plant functional group removals for each of 30 contrasting forested lake islands in northern Sweden. We found that, against expectations, the effects of plant removals on the communities of key groups of soil organisms (bacteria, fungi and nematodes), and organic matter quality and soil ecosystem functioning (decomposition and microbial activity) were relatively similar among islands that varied greatly in productivity and soil fertility. This highlights that, in contrast to what has been shown for plant productivity, plant biodiversity loss effects on below‐ground functions can be relatively insensitive to environmental context or variation among widely contrasting ecosystems.     

Unravelling soil fungal communities from different Mediterranean land-use backgrounds

Background

Fungi strongly influence ecosystem structure and functioning, playing a key role in many ecological services as decomposers, plant mutualists and pathogens. The Mediterranean area is a biodiversity hotspot that is increasingly threatened by intense land use. Therefore, to achieve a balance between conservation and human development, a better understanding of the impact of land use on the underlying fungal communities is needed.

Methodology/Principal Findings

We used parallel pyrosequencing of the nuclear ribosomal ITS regions to characterize the fungal communities in five soils subjected to different anthropogenic impact in a typical Mediterranean landscape: a natural cork-oak forest, a pasture, a managed meadow, and two vineyards. Marked differences in the distribution of taxon assemblages among the different sites and communities were found. Data analyses consistently indicated a sharp distinction of the fungal community of the cork oak forest soil from those described in the other soils. Each soil showed features of the fungal assemblages retrieved which can be easily related to the above-ground settings: ectomycorrhizal phylotypes were numerous in natural sites covered by trees, but were nearly completely missing from the anthropogenic and grass-covered sites; similarly, coprophilous fungi were common in grazed sites.

Conclusions/Significance

Data suggest that investigation on the below-ground fungal community may provide useful elements on the above-ground features such as vegetation coverage and agronomic procedures, allowing to assess the cost of anthropogenic land use to hidden diversity in soil. Datasets provided in this study may contribute to future searches for fungal bio-indicators as biodiversity markers of a specific site or a land-use degree.