Successional change in species composition alters climate sensitivity of grassland productivity

Succession theory predicts altered sensitivity of ecosystem functions to disturbance (i.e., climate change) due to the temporal shift in plant community composition. However, empirical evidence in global change experiments is lacking to support this prediction. Here, we present findings from an 8‐year long‐term global change experiment with warming and altered precipitation manipulation (double and halved amount). First, we observed a temporal shift in species composition over 8 years, resulting in a transition from an annual C₃‐dominant plant community to a perennial C₄‐dominant plant community. This successional transition was independent of any experimental treatments. During the successional transition, the response of aboveground net primary productivity (ANPP) to precipitation addition magnified from neutral to +45.3%, while the response to halved precipitation attenuated substantially from ?17.6% to neutral. However, warming did not affect ANPP in either state. The findings further reveal that the time‐dependent climate sensitivity may be regulated by successional change in species composition, highlighting the importance of vegetation dynamics in regulating the response of ecosystem productivity to precipitation change.     

BUGS in the analysis of biodiversity experiments: species richness and composition are of similar importance for grassland productivity

The idea that species diversity can influence ecosystem functioning has been controversial and its importance relative to compositional effects hotly debated. Unfortunately, assessing the relative importance of different explanatory variables in complex linear models is not simple. In this paper we assess the relative importance of species richness and species composition in a multilevel model analysis of net aboveground biomass production in grassland biodiversity experiments by estimating variance components for all explanatory variables. We compare the variance components using a recently introduced graphical Bayesian ANOVA. We show that while the use of test statistics and the R2 gives contradictory assessments, the variance components analysis reveals that species richness and composition are of roughly similar importance for primary productivity in grassland biodiversity experiments.     

Soil microbes regulate ecosystem productivity and maintain species diversity

One of the major goals in ecology is to determine the mechanisms that drive the asymptotic increase in ecosystem productivity with plant species diversity. Niche complementarity, the current paradigm for the asymptotic diversity-productivity pattern, posits that the addition of species to a community increases productivity because each species specializes on different resources and thus can more thoroughly utilize the available resources. At higher diversity the increase in productivity decreases because resources become limiting, resulting in the classic asymptotic diversity-productivity pattern. An alternative but less tested explanation is that density-dependent disease from species-specific soil microbes drive the diversity-productivity relationship by increasing disease and thus decreasing productivity at low diversity. At higher diversity, productivity asymptotes because disease decreases with increasing diversity until it reaches a uniformly low level. Using a series of field experiments, we found that the classic asymptotic diversity-productivity pattern existed only when soil microbes were present. Soil microbes created the well-known pattern by depressing plant growth at low productivity though negative density dependent disease. In contrast, niche complementarity played only a weak role in explaining the diversity-productivity relationship because productivity remained high at low abundance in the absence of soil microbes. Based on our findings, the ongoing loss of species in natural ecosystems will likely increase per capita plant disease and lower ecosystem productivity. Furthermore, recent evidence suggests that negative density dependent disease maintains plant species diversity, and thus this single mechanism appears to link diversity maintenance to the diversity-productivity curve—two important ecological processes.Key words: density dependence, diversity-productivity, negative feedback, pathogens, species richness, soil microbesThe asymptotically saturating increase in ecosystem productivity with increasing diversity is a well know pattern in nature^1–4 (Fig. 1). The pattern has been used as an argument for the importance of species diversity,⁵ and understanding the mechanisms that drive the pattern is critical to determine the potential loss in productivity with ongoing and accelerating species loss in many ecosystems. The cause of the diversity-productivity pattern can be explained by either bottom-up control, such as plant resource competition, or top-down control from plant herbivores or pathogens. Most contemporary explanations for the pattern are centered on the bottom-up concept of niche-based resource competition, in which different species utilize different resources. The commonly accepted explanation, the niche complementarity hypothesis, states that the increase in species diversity increases productivity because each additional species uses a differ set of resources (e.g., nutrients) and thus more thoroughly utilizes whole-ecosystem resources.^3,4,6 At high diversity, however, the resource requirements of additional species overlap with existing ones and thus productivity no longer increases with diversity, resulting in the asymptotic diversity-productivity pattern (Fig. 1).Open in a separate windowFigure 1Theoretical relationship between species number and biomass. As diversity increases, total biomass increases asymptotically.Top-down control from plant enemies may also produce the asymptotic diversity-productivity pattern if the enemies are species-specific and have a strong negative density-dependent effect at low diversity. One general group of enemies is plant pathogens and parasites (bacterial, fungal, viral) that live in the soil and infect plant roots (hereafter referred to as soil pathogens). The specificity of soil pathogens has been shown in various studies and is now generally accepted.^1,7,8 The negative density dependent effect of plant pathogens at low diversity is likely because when diversity is low the relative abundance of each remaining species is high,^9–11 which leads to most individuals growing in close proximity of conspecifics and thus a greater probability of species-specific disease transmission. Unlike other plant enemies, such as foliar pathogens or insect and mammalian herbivores, which can be broadly dispersed, soil-borne pathogens may be a particularly effective driver of negative density dependent effects because they have low mobility and thus are more likely to infect nearby conspecifics, which causes increased disease at low diversity.^9–11 As diversity increases, the effect of soil-borne pathogens decreases because there is a lower likelihood of growing near a conspecific and there are lower concentrations of host-specific soil enemies.¹⁰ Consequently, soil-borne, species-specific disease may limit ecosystem productivity through top-down density-dependent regulation, even in the absence of niche-based explanations. Few studies, however, have considered the role of plant soil pathogens in driving the classic diversity-productivity relationship¹ (see also ref. ²) and, until now, no study has compared the two potential drivers simultaneously.¹We used a modeling approach to first demonstrate that both niche complementarity and species-specific soil pathogens can both theoretically drive the well-known diversity-productivity pattern.¹ We then used a series of complementary field experiments in grasslands in North America (Ontario, Canada and Minnesota, USA) to determine how plant disease and productivity change over a gradient of plant species richness in the presence and absence of soil microbes, and whether feedback between plants and their species-specific soil biota influenced the diversity-productivity pattern.¹ We first tested whether the asymptotic diversity-ecosystem productivity relationship arose in the presence of soil pathogens (a test of the negative density dependence hypothesis) or in the absence of soil pathogens (a test of the niche complementarity hypothesis). We then confirmed that soil biota were species specific and examined the decrease in plant disease and increase in productivity with increasing plant diversity.     

Asymmetric competition between two tit species: a reciprocal removal experiment

1. In a 5-year field experiment, competition for food was tested between great tit ( Parus major L.) and blue tit ( P. caeruleus L.), two common hole-nesters in Central Europe. Experimental ('allopatric') populations of both species were created during the breeding seasons by preventing the nesting or egg laying of one of the competing species.
2. An asymmetric relationship was found between the two tits; blue tits were more successful in the competitive interaction. Detectable effects were found only in nestling condition. Great tits raised lighter nestlings when breeding sympatrically with blue tits.
3. A possible mechanism is suggested that could be responsible for the different competitive abilities of the two species; blue tits are more effective in utilizing the most abundant size categories of caterpillar food supply than great tits.     

Constraints on colonization and species richness along a grassland productivity gradient: the role of propagule availability

An important goal in ecology is to discern under what habitat conditions community structure is primarily regulated by local ecological interactions and under what conditions community structure is more regulated by the pool of available colonists. I conducted a seed addition experiment in successional grassland to evaluate the relative significance of neighbourhood biotic interactions and propagule availability in regulating plant colonization and species richness along a natural gradient of grassland productivity. In undisturbed field plots, seed additions of 34 species led to an increase in species richness in locations of low productivity, an effect that declined in magnitude as productivity increased. In disturbed plots, seed additions led to a relatively constant increase in species richness at all levels of productivity. The results support the hypothesis that the role of propagule availability in regulating colonization dynamics and species richness declines in significance relative to local-scale competitive interactions as habitat productivity increases.     

Intraspecific trait variation and allocation strategies of calcareous grassland species: Results from a restoration experiment

Intra- and interspecific trait variation express the response of plants dealing with different environmental conditions. We measured root and leaf traits on 14 species of calcareous grasslands in a restoration experiment. We aimed at identifying intraspecific differences in biomass allocation and functional plant traits under contrasting soil conditions by comparing plants growing in ancient grassland and two restored grasslands on ex-arable land, one of them with topsoil removal. Relative importance of trait variation within and among species, and among site was assessed by variance partitioning. Interspecific variation was more important than intraspecific variation, but the contribution of the latter to total variation was considerable, especially for specific leaf area. Changes in soil properties due to topsoil removal resulted in lower values of plant height, specific leaf area and specific root length compared to the control (ancient grassland). Soil fertility found in the treatment without top soil removal did not affect plant plasticity compared to the control. The study species showed two allocation strategies in relation to resource stress, while the responses of individual traits to the soil treatments were consistent across species. We conclude that caution must be taken when using mean trait values for plastic species or when working with environmental gradients.     

Post-dispersal seed predation of three grassland species in a plant diversity experiment

Aims Post-dispersal seed predation is an important ecosystem process because it can influence the seed's fate after the initial dispersal from the mother plant and subsequently transform communities. Even at small scales, post-dispersal seed predation can vary greatly depending on seed identity, granivorous taxa or microhabitat structure. However, little is known about the role of plant species richness and functional group richness in post-dispersal seed predation. The overall aim of this study was to test whether increasing plant species richness or plant functional group richness affects the rate and variability of post-dispersal seed predation. We additionally investigated the influence of vegetation structure and seed species identity on the rate and variability of post-dispersal seed predation and whether the influence of different granivorous taxa changed with increasing plant species richness.Methods We conducted seed removal experiments along a long-term experimental plant diversity gradient, comprising plots with monocultures to 60 species mixtures of common grassland species in Jena, Germany, in August 2011. We studied seeds of Onobrychis viciifolia, Pastinaca sativa and Trifolium pratense in exclusion experiments (seed cafeterias), an experimental setup that allowed access either for arthropods or slugs or for all granivorous taxa. Traditionally, seeds removed from seed cafeterias were classified as consumed but we used traceable fluorescent-coloured seeds to obtain more accurate predation rates by subtracting recovered seeds from overall removed seeds. The effect of multiple vegetation variables on mean and variability of seed predation rates was analysed using generalized mixed-effect models and linear regressions, respectively.Important findings Rates of recovered seeds were low but contributed to significant differences between seed predation rates and removal rates of seeds in some treatments. Seed predation rates were not directly correlated with increasing plant species richness or plant functional group richness but were influenced byseed species identity and granivorous taxa. Vegetation variables such as vegetation height and cover were significantly associated with seed predation rates. Depending on the seed species and/or the granivorous taxa, different vegetation variables correlated with seed predation rates. Our results indicate that effects of plant functional group richness and multiple vegetation variables on the magnitude of post-dispersal seed predation varied with seed identity and seed predator taxa. A direct effect of plant species and plant functional group richness could be shown on the variability of post-dispersal seed predation for some seed species and their respective predators. Thus, the changes in magnitude of post-dispersal seed predation with increasing plant species richness could potentially impact the fitness of some plant species and thereby influence plant community structure.     

Abundance, distribution and life histories of grassland plants: a comparative study of 81 species

1 This study examines the abundance and distribution of grassland plant species in particular relation to features affecting colonization. Seed production (inversely related to seed size) and recruitment success (positively related) affect colonization ability, suggesting that seed size can be used as a key trait.
2 Data on seed size, dispersal mode, life form, geographical range size and abundance were gathered for 81 grassland plant species in a field study area in Sweden. Seed production and plant size were estimated for 69 of these species. Analyses were performed both across species, with species treated as independent data points, and for 43 'phylogenetically independent contrasts'.
3 The cross-species analyses suggested that local abundance was related to life forms but not dispersal or plant size. Perennials were generally most abundant, as were clonal species. If abundance reflects colonization we predicted that species with intermediately sized seeds (or intermediate seed production) would be most abundant, and this was supported by the phylogenetic contrast but not by cross-species analyses. In the former analysis, a high abundance of species was significantly associated with a small seed size deviation (and seed number deviation) from the median values of these traits in the community.
4 Local abundance, seed production and seed size deviation from the community median value were positively related to geographical range size in the cross-species analysis, but no relationships were seen in the phylogenetic contrast analysis.
5 We conclude that colonization processes do have a significant influence on abundance patterns in grasslands. Seed size is a key trait for colonizing ability, and the effects of the trade-off of seed size vs. seed number must be considered. No single mechanism can be identified that influences both abundance and geographical distribution range.     

Carotenoid composition and photochemical activity of four sandy grassland species

The photosynthetic pigments and photochemical efficiency of photosystem 2 (PS2) were studied in four constitutive species (Achillea millefolium L., Festuca pseudovina Hack. ex Wiesb., Potentilla arenaria Borkh., and Thymus degenianus Lyka) of a semiarid grassland in South-eastern Hungary. Every species displayed typical sun-adapted traits and substantial plasticity in the composition and functioning of the photosynthetic apparatus. The contents of chlorophylls (Chls) and carotenoids (Cars) on a dry matter basis declined from May to July, however, the amount of total Cars on a Chl basis increased. This increase was the largest in Potentilla (48 %) and the smallest in Achillea (14 %). The pool of xanthophylls (VAZ) was between 25 % and 45 % of the total Car content and was larger in July than in May. The content of β-carotene increased by July, but lutein content did not change significantly. The Chl fluorescence ratio F_v/F_m was reduced by 3–10 % at noon, reflecting the down-regulation of PS2 in the period of high irradiance and high temperature. The occurrence of minimal values of ΔF/F_m’ showed close correlation to the de-epoxidation rate of violaxanthin. Hence in natural habitats these species developed a considerable capacity to dissipate excess excitation energy in the summer period in their photosynthetic apparatus through the xanthophyll cycle pool and a related photoprotective mechanism, when the photochemical utilization of photon energy was down-regulated.     

Species richness effects on grassland recovery from drought depend on community productivity in a multisite experiment

Biodiversity can buffer ecosystem functioning against extreme climatic events, but few experiments have explicitly tested this. Here, we present the first multisite biodiversity × drought manipulation experiment to examine drought resistance and recovery at five temperate and Mediterranean grassland sites. Aboveground biomass production declined by 30% due to experimental drought (standardised local extremity by rainfall exclusion for 72–98 consecutive days). Species richness did not affect resistance but promoted recovery. Recovery was only positively affected by species richness in low‐productive communities, with most diverse communities even showing overcompensation. This positive diversity effect could be linked to asynchrony of species responses. Our results suggest that a more context‐dependent view considering the nature of the climatic disturbance as well as the productivity of the studied system will help identify under which circumstances biodiversity promotes drought resistance or recovery. Stability of biomass production can generally be expected to decrease with biodiversity loss and climate change.     

Dynamics of species richness and composition in a limestone grassland restored after tree cutting

Abstract. After abandonment many limestone grasslands have been overgrown by trees and shrubs; as a result, species-rich communities with many regionally rare and endangered species are vanishing. Some studies suggest that, in cases where grassland is being restored, the species composition and rate of change is strongly dependent on the initial conditions, i.e. the earlier presence of grassland species and the opportunity for colonization of new sites by grassland species. These hypotheses were tested in a five-year restoration experiment after the clearing of a 35-yr-old secondary pine wood developed on abandoned grassland. Tree cutting induced rapid changes in the floristic composition and species cover. The number of grassland species from the class Festuco-Brometea increased significantly in the restored grassland, but their cover was much lower than in the old grassland. Canonical Correspondence Analysis showed significant differences in species composition between the grassland restored in former wood gaps and that developed in former closed wood. In wood gap sites the cover of species from the class Molinio-Arrhenatheretea and tufted perennials was much higher, whereas the cover of Festuco-Brometea species was lower. Significantly more shrubs, woodland species, ruderal and nitrophilous species as well as annual and biennial species occurred in the former closed wood site. It was found that richness and composition of the restored grassland depended strongly on the community composition before tree cutting, as well as on the presence of grassland species in the neighbourhood. Periodical tree cutting enables the maintenance of a temporal-spatial mosaic of scrub-grassland communities in isolated habitats and the preservation of local species diversity.     

Landscape-scale connectivity and fragment size determine species composition of grassland fragments

As a consequence of agricultural intensification and habitat fragmentation since the mid-20th century, biological diversity has declined considerably throughout the world, particularly in Europe. We assessed how habitat and landscape-scale heterogeneity, such as variation in fragment size (small vs. large) and landscape configuration (measured as connectivity index), affect plant and arthropod diversity. We focused on arthropods with different feeding behaviour and mobility, spiders (predators, moderate dispersal), true bugs (mainly herbivores and omnivores with moderate dispersal), wild bees (pollinators with good dispersal abilities), and wasps (pollinators, omnivores with good dispersal abilities). We studied 60 dry grassland fragments in the same region (Hungarian Great Plain); 30 fragments were represented by the grassland component of forest-steppe stands, and 30 were situated on burial mounds (kurgans). Forest-steppes are mosaics of dry grasslands with small forests in a matrix of plantation forests. Kurgans are ancient burial mounds with moderately disturbed grasslands surrounded by agricultural fields. The size of fragments ranged between 0.16–6.88 ha (small: 0.16–0.48 ha, large: 0.93–6.88 ha) for forest-steppes and 0.01–0.44 ha (small: 0.01–0.10 ha and large: 0.20–0.44 ha) for kurgans. Fragments also represented an isolation gradient from almost cleared and homogenous landscapes, to landscapes with relatively high compositional heterogeneity. Fragment size, connectivity, and their interaction affected specialist and generalist species abundances of forest-steppes and kurgans. Large fragments had higher species richness of ground-dwelling spiders, and the effect of connectivity was more strongly positive for specialist arthropods and more strongly negative for generalists in large than in small fragments. However, we also found a strong positive impact of connectivity for generalist plants in small kurgans in contrast to larger ones. We conclude that besides the well-known effect of enhancing habitat quality, increasing connectivity between fragments by restoring natural and semi-natural habitat patches would help to maintain grassland biodiversity.     

Feedback from plant species change amplifies CO2 enhancement of grassland productivity

Dynamic global vegetation models simulate feedbacks of vegetation change on ecosystem processes, but direct, experimental evidence for feedbacks that result from atmospheric CO₂ enrichment is rare. We hypothesized that feedbacks from species change would amplify the initial CO₂ stimulation of aboveground net primary productivity (ANPP) of tallgrass prairie communities. Communities of perennial forb and C₄ grass species were grown for 5 years along a field CO₂ gradient (250–500 μL L^?1) in central Texas USA on each of three soil types, including upland and lowland clay soils and a sandy soil. CO₂ enrichment increased community ANPP by 0–117% among years and soils and increased the contribution of the tallgrass species Sorghastrum nutans (Indian grass) to community ANPP on each of the three soil types. CO₂‐induced changes in ANPP and Sorghastrum abundance were linked. The slope of ANPP‐CO₂ regressions increased between initial and final years on the two clay soils because of a positive feedback from the increase in Sorghastrum fraction. This feedback accounted for 30–60% of the CO₂‐mediated increase in ANPP on the upland and lowland clay soils during the final 3 years and 1 year of the experiment, respectively. By contrast, species change had little influence on the ANPP‐CO₂ response on the sandy soil, possibly because Sorghastrum increased largely at the expense of a functionally similar C₄ grass species. By favoring a mesic C₄ tall grass, CO₂ enrichment approximately doubled the initial enhancement of community ANPP on two clay soils. The CO₂‐stimulation of grassland productivity may be significantly underestimated if feedbacks from plant community change are not considered.     

Arbuscular mycorrhizal community composition associated with two plant species in a grassland ecosystem

Arbuscular mycorrhizal (AM) fungi are biotrophic symbionts colonizing about two-thirds of land plant species and found in all ecosystems. They are of major importance in plant nutrient supply and their diversity is suggested to be an important determinant of plant community composition. The diversity of the AM fungal community composition in the roots of two plant species (Agrostis capillaris and Trifolium repens) that co-occurred in the same grassland ecosystem was characterized using molecular techniques. We analysed the small subunit (SSU) ribosomal RNA gene amplified from a total root DNA extract using AM fungal-specific primers. A total of 2001 cloned fragments from 47 root samples obtained on four dates were analysed by restriction fragment length polymorphism, and 121 of them were sequenced. The diversity found was high: a total of 24 different phylotypes (groups of phylogenetically related sequences) colonized the roots of the two host species. Phylogenetic analyses demonstrate that 19 of these phylotypes belonged to the Glomaceae, three to the Acaulosporaceae and two to the Gigasporaceae. Our study reveals clearly that the AM fungal community colonizing T. repens differed from that colonizing A. capillaris, providing evidence for AM fungal host preference. In addition, our results reveal dynamic changes in the AM fungal community through time.     

Pine species determine fungal microbiome composition in a common garden experiment

The factors shaping the composition of microbial communities in trees remain poorly understood. We evaluated whether the core and satellite fungal communities in five pine species (Pinus radiata, Pinus pinaster, Pinus sylvestris, Pinus nigra, and Pinus uncinata) were shaped by the host species identity. Because the trees had earlier been inoculated with a fungal pathogen (Fusarium circinatum), we also explored the possibilities to detect its presence and potential co-occurrence networks. We found interspecific variation in the fungal community composition and abundance among the different tree species and the existence of a core microbiome that was independent of the host species. The presence of F. circinatum was confirmed in some samples through qPCR but the pathogen did not co-occur with a specific fungal community. The results highlight the importance of host species as a determinant of microbiome assembly in common environments.     

Richness and species composition of arboreal arthropods affected by nutrients and predators: a press experiment

A longstanding goal for ecologists is to understand the processes that maintain biological diversity in communities, yet few studies have investigated the combined effects of predators and resources on biodiversity in natural ecosystems. We fertilized nutrient limited plots and excluded insectivorous birds in a randomized block design, and examined the impacts on arthropods associated with the dominant tree in the Hawaiian Islands, Metrosideros polymorpha (Myrtaceae). After 33 months, the species load (per foliage mass) of herbivores and carnivores increased with fertilization, but rarified richness (standardized to abundance) did not change. Fertilization depressed species richness of arboreal detritivores, and carnivore richness dropped in caged, unfertilized plots, both because of the increased dominance of common, introduced species with treatments. Herbivore species abundance distributions were more equitable than other trophic levels following treatments, and fertilization added specialized native species without changing relativized species richness. Overall, bird removal and nutrient addition treatments on arthropod richness acted largely independently, but with countervailing influences that obscured distinct top-down and bottom-up effects on different trophic levels. This study demonstrates that species composition, biological invasions, and the individuality of species traits may complicate efforts to predict the interactive effects of resources and predation on species diversity in food webs.     

Effects of ozone on species composition in an upland grassland

Northern hemispheric background concentrations of ozone are increasing, but few studies have assessed the ecological significance of these changes for grasslands of high conservation value under field conditions. We carried out a 3-year field experiment in which ozone was released at a controlled rate over three experimental transects to produce concentration gradients over the field site, an upland mesotrophic grassland located in the UK. We measured individual species biomass in an annual hay cut in plots receiving ambient ozone, and ambient ozone elevated by mean concentrations of approximately 4 ppb and 10 ppb in the growing seasons of 2008 and 2009. There was a significant negative effect of ozone exposure on herb biomass, but not total grass or legume biomass, in 2008 and 2009. Within the herb fraction, ozone exposure significantly decreased the biomass of Ranunculus species and that of the hemi-parasitic species Rhinanthus minor. Multivariate analysis of species composition, taking into account spatial variation in soil conditions and ozone exposure, showed no significant ozone effect on the grass component. In contrast, by 2009, ozone had become the dominant factor influencing species composition within the combined herb and legume component. Our results suggest that elevated ozone concentrations may be a significant barrier to achieving increased species diversity in managed grasslands.