Cumulative nitrogen enrichment alters the drivers of grassland overyielding

Effects of plant diversity on grassland productivity, or overyielding, are found to be robust to nutrient enrichment. However, the impact of cumulative nitrogen (N) addition (total N added over time) on overyielding and its drivers are underexplored. Synthesizing data from 15 multi-year grassland biodiversity experiments with N addition, we found that N addition decreases complementarity effects and increases selection effects proportionately, resulting in no overall change in overyielding regardless of N addition rate. However, we observed a convex relationship between overyielding and cumulative N addition, driven by a shift from complementarity to selection effects. This shift suggests diminishing positive interactions and an increasing contribution of a few dominant species with increasing N accumulation. Recognizing the importance of cumulative N addition is vital for understanding its impacts on grassland overyielding, contributing essential insights for biodiversity conservation and ecosystem resilience in the face of increasing N deposition.

Environmental morphing enables informed dispersal of the dandelion diaspore

Animal migration is highly sensitised to environmental cues, but plant dispersal is considered largely passive. The common dandelion, Taraxacum officinale, bears an intricate haired pappus facilitating flight. The pappus enables the formation of a separated vortex ring during flight; however, the pappus structure is not static but reversibly changes shape by closing in response to moisture. We hypothesised that this leads to changed dispersal properties in response to environmental conditions. Using wind tunnel experiments for flow visualisation, particle image velocimetry, and flight tests, we characterised the fluid mechanics effects of the pappus morphing. We also modelled dispersal to understand the impact of pappus morphing on diaspore distribution. Pappus morphing dramatically alters the fluid mechanics of diaspore flight. We found that when the pappus closes in moist conditions, the drag coefficient decreases and thus the falling velocity is greatly increased. Detachment of diaspores from the parent plant also substantially decreases. The change in detachment when the pappus closes increases dispersal distances by reducing diaspore release when wind speeds are low. We propose that moisture-dependent pappus-morphing is a form of informed dispersal allowing rapid responses to changing conditions.

Seed dispersal as a search strategy: dynamic and fragmented landscapes select for multi-scale movement strategies in plants

BACKGROUND

Plant dispersal is a critical factor driving ecological responses to global changes. Knowledge on the mechanisms of dispersal is rapidly advancing, but selective pressures responsible for the evolution of dispersal strategies remain elusive. Recent advances in animal movement ecology identified general strategies that may optimize efficiency in animal searches for food or habitat. Here we explore the potential for evolution of similar general movement strategies for plants.

METHODS

We propose that seed dispersal in plants can be viewed as a strategic search for suitable habitat, where the probability of finding such locations has been optimized through evolution of appropriate dispersal kernels. Using model simulations, we demonstrate how dispersal strategies can optimize key dispersal trade-offs between finding habitat, avoiding kin competition, and colonizing new patches. These trade-offs depend strongly on the landscape, resulting in a tight link between optimal dispersal strategy and spatiotemporal habitat distribution.

RESULTS

Our findings reveal that multi-scale seed dispersal strategies that combine a broad range of dispersal scales, including Lévy-like dispersal, are optimal across a wide range of dynamic and patchy landscapes. At the extremes, static and patchy landscapes select for dispersal strategies dominated by short distances, while uniform and highly unpredictable landscapes both select for dispersal strategies dominated by long distances.

CONCLUSIONS

By viewing plant seed dispersal as a strategic search for suitable habitat, we provide a reference framework for the analysis of plant dispersal data. Consideration of the entire dispersal kernel, including distances across the full range of scales, is key. This reference framework helps identify plant species' dispersal strategies, the evolutionary forces determining these strategies and their ecological consequences, such as a potential mismatch between plant dispersal strategy and altered spatiotemporal habitat dynamics due to land use change. Our perspective opens up directions for future studies, including exploration of composite search behaviour and 'informed searches' in plant species with directed dispersal.

Fast and furious: Early differences in growth rate drive short-term plant dominance and exclusion under eutrophication

The reduction of plant diversity following eutrophication threatens many ecosystems worldwide. Yet, the mechanisms by which species are lost following nutrient enrichment are still not completely understood, nor are the details of when such mechanisms act during the growing season, which hampers understanding and the development of mitigation strategies.Using a common garden competition experiment, we found that early-season differences in growth rates among five perennial grass species measured in monoculture predicted short-term competitive dominance in pairwise combinations and that the proportion of variance explained was particularly greater under a fertilization treatment.We also examined the role of early-season growth rate in determining the outcome of competition along an experimental nutrient gradient in an alpine meadow. Early differences in growth rate between species predicted short-term competitive dominance under both ambient and fertilized conditions and competitive exclusion under fertilized conditions.The results of these two studies suggest that plant species growing faster during the early stage of the growing season gain a competitive advantage over species that initially grow more slowly, and that this advantage is magnified under fertilization. This finding is consistent with the theory of asymmetric competition for light in which fast-growing species can intercept incident light and hence outcompete and exclude slower-growing (and hence shorter) species. We predict that the current chronic nutrient inputs into many terrestrial ecosystems worldwide will reduce plant diversity and maintain a low biodiversity state by continuously favoring fast-growing species. Biodiversity management strategies should focus on controlling nutrient inputs and reducing the growth of fast-growing species early in the season.

Spatial early warning signals for impending regime shifts: A practical framework for application in real-world landscapes

Prediction of ecosystem response to global environmental change is a pressing scientific challenge of major societal relevance. Many ecosystems display nonlinear responses to environmental change, and may even undergo practically irreversible 'regime shifts' that initiate ecosystem collapse. Recently, early warning signals based on spatiotemporal metrics have been proposed for the identification of impending regime shifts. The rapidly increasing availability of remotely sensed data provides excellent opportunities to apply such model-based spatial early warning signals in the real world, to assess ecosystem resilience and identify impending regime shifts induced by global change. Such information would allow land-managers and policy makers to interfere and avoid catastrophic shifts, but also to induce regime shifts that move ecosystems to a desired state. Here, we show that the application of spatial early warning signals in real-world landscapes presents unique and unexpected challenges, and may result in misleading conclusions when employed without careful consideration of the spatial data and processes at hand. We identify key practical and theoretical issues and provide guidelines for applying spatial early warning signals in heterogeneous, real-world landscapes based on literature review and examples from real-world data. Major identified issues include (1) spatial heterogeneity in real-world landscapes may enhance reversibility of regime shifts and boost landscape-level resilience to environmental change (2) ecosystem states are often difficult to define, while these definitions have great impact on spatial early warning signals and (3) spatial environmental variability and socio-economic factors may affect spatial patterns, spatial early warning signals and associated regime shift predictions. We propose a novel framework, shifting from an ecosystem perspective towards a landscape approach. The framework can be used to identify conditions under which resilience assessment with spatial remotely sensed data may be successful, to support well-informed application of spatial early warning signals, and to improve predictions of ecosystem responses to global environmental change.

A mechanistic assessment of the relationship between gut morphology and endozoochorous seed dispersal by waterfowl

Many plants and invertebrates rely on internal transport by animals for long-distance dispersal. Their dispersal capacity is greatly influenced by interactions with the animal's digestive tract. Omnivorous birds adjust their digestive tract morphology to seasonally variable diets. We performed feeding trials in waterfowl to unravel how changing organ size, in combination with seed size, affects dispersal potential. We subjected captive mallards to mimics of summer (animal-based), winter (plant-based), and intermediate diets, and analyzed gut passage of seeds before and after the treatment (trials 1 and 2). To test the effect of gut morphology on seed digestion, we measured digestive organ size after euthanasia. Three hours before euthanasia, differently sized seeds were fed to test how seed size affects gut passage by determining their relative position in the digestive tract (trial 3). Trials 1 and 2 showed that intact seed passage was lower in the plant-based than in the animal-based diet group. Retention time changed only within groups, decreasing in the animal-based, and increasing in the plant-based diet group. No post-diet differences in organ size were detected, probably due to large between-individual variation within groups. Digestive tract measures did not explain variation in seed survival or retention time. Trial 3 revealed that small seeds pass the digestive tract rapidly, while large seeds are retained longer, particularly in the gizzard. Differential retention in the gizzard, the section where seeds can be destroyed, is likely why larger seeds have a lower probability to pass the digestive tract intact. Our results confirm that rapid, flexible adaptation to diet shifts affects seed digestion in waterfowl, although we could not conclusively relate this to organ size. Large interindividual variation in digestive efficiency between mallards feeding on the same diet may provide opportunities for seed dispersal in the field throughout the annual cycle.

Interactions between seed traits and digestive processes determine the germinability of bird-dispersed seeds

Waterbirds disperse a wide range of plant seeds via their guts, promoting biotic connectivity between isolated habitat patches. However, the intensity of digestive forces encountered by seeds, and therefore their potential to survive digestive tract passage, varies within and between waterbird species. Here, we investigate under controlled conditions how the interaction between seed traits and digestive strategies affect the germinability of seeds following waterbird-mediated dispersal. We exposed seeds of 30 wetland plant species to the main digestive processes in the dabbling duck digestive system: mechanical, chemical and intestinal digestion. These were simulated by 1) a pressure test and scarification treatment, 2) incubation in simulated gastric juice, and 3) incubation in intestinal contents of culled mallards (Anas platyrhynchos). We evaluated their separate and combined effects on seed germination, and identified the role of seed size and seed coat traits in resisting the digestive forces. Seeds were generally resistant to separate digestive processes, but highly sensitive to a combination. Resistance to mechanical break-down was reduced by up to 80% by chemical pre-treatment, especially for seeds with permeable coats. Scarified seeds were 12–17% more vulnerable to chemical and intestinal digestive processes than undamaged seeds. Large seeds and seeds with thin, permeable coats were particularly sensitive to chemical and intestinal digestion. These results indicate that efficient digestion of seeds requires multiple digestive processes. The gizzard, responsible for mechanical digestion, plays a key role in seed survival. Omnivorous birds, which have relatively light gizzards compared to pure herbivores or granivores, are thus most likely to disperse seeds successfully. Regardless of digestive strategy, small seeds with tough seed coats are most resistant to digestion and may be adapted to endozoochorous dispersal by waterbirds.

Effects of increased flooding on riparian vegetation: Field experiments simulating climate change along five European lowland streams

In many parts of the world, the magnitude and frequency of cold-season precipitation are expected to increase in the near future. This will result in an increased magnitude and duration of winter and spring flooding by rain-fed streams and rivers. Such climate-driven increases in flooding are likely to affect riparian plant communities, but future vegetation changes are hard to predict due to current lack of data. To fill this knowledge gap, we experimentally modified the hydrology of five streams across three countries in north-western Europe during late winter/early spring over a period of 3 years. We assessed the responses in riparian plant species richness, biomass, plant-available nitrogen and phosphorus and seed deposition to increased flooding depth (+18 cm on average at the lowest positions along the riparian gradient) and prolonged flooding duration (6 weeks on average). After 3 years of increased flooding, there was an overall decline in riparian species richness, while riparian plant biomass increased. Extractable soil nitrogen and phosphorus also increased and are likely to have contributed to the increased biomass. Increased flooding resulted in the arrival of more seeds of additional species to the riparian zone, thereby potentially facilitating the shifts in riparian plant species composition we observed. The results of our concerted experimental effort demonstrate that changes in stream riparian plant communities can occur rapidly following increased winter flooding, leading to strong reductions in plant species diversity.

Movement patterns of a keystone waterbird species are highly predictable from landscape configuration

BACKGROUND

Movement behaviour is fundamental to the ecology of animals and their interactions with other organisms, and as such contributes to ecosystem dynamics. Waterfowl are key players in ecological processes in wetlands and surrounding habitats through predator-prey interactions and their transportation of nutrients and other organisms. Understanding the drivers of their movement behaviour is crucial to predict how environmental changes affect their role in ecosystem functioning. Mallards (Anas platyrhynchos) are the most abundant duck species worldwide and important dispersers of aquatic invertebrates, plants and pathogens like avian influenza viruses. By GPS tracking of 97 mallards in four landscape types along a gradient of wetland availability, we identified patterns in their daily movement behaviour and quantified potential effects of weather conditions and water availability on the spatial scale of their movements.

RESULTS

We demonstrate that mallard movement patterns were highly predictable, with regular commuting flights at dusk and dawn between a fixed day roost and one or several fixed nocturnal foraging sites, linked strongly to surface water. Wind and precipitation hardly affected movement, but flight distances and home range sizes increased when temperatures dropped towards zero. Flight distances and home range sizes increased exponentially with decreasing availability of freshwater habitat. Total shoreline length and the number of water bodies in the landscape surrounding the roost were the best predictors of the spatial scale of daily mallard movements.

CONCLUSIONS

Our results show how mallards may flexibly adjust the spatial scale of their movements to wetland availability in the landscape. This implies that mallards moving between discrete habitat patches continue to preserve biotic connectivity in increasingly fragmented landscapes. The high predictability of mallard movement behaviour in relation to landscape features makes them reliable dispersal vectors for organisms to adapt to, and allows prediction of their ecological role in other landscapes.

Riparian plant community responses to increased flooding: a meta-analysis

A future higher risk of severe flooding of streams and rivers has been projected to change riparian plant community composition and species richness, but the extent and direction of the expected change remain uncertain. We conducted a meta-analysis to synthesize globally available experimental evidence and assess the effects of increased flooding on (1) riparian adult plant and seedling survival, (2) riparian plant biomass and (3) riparian plant species composition and richness. We evaluated which plant traits are of key importance for the response of riparian plant species to flooding. We identified and analysed 53 papers from ISI Web of Knowledge which presented quantitative experimental results on flooding treatments and corresponding control situations. Our meta-analysis demonstrated how longer duration of flooding, greater depth of flooding and, particularly, their combination reduce seedling survival of most riparian species. Plant height above water level, ability to elongate shoots and plasticity in root porosity were decisive for adult plant survival and growth during longer periods of flooding. Both 'quiescence' and 'escape' proved to be successful strategies promoting riparian plant survival, which was reflected in the wide variation in survival (full range between 0 and 100%) under fully submerged conditions, while plants that protrude above the water level (>20 cm) almost all survive. Our survey confirmed that the projected increase in the duration and depth of flooding periods is sufficient to result in species shifts. These shifts may lead to increased or decreased riparian species richness depending on the nutrient, climatic and hydrological status of the catchment. Species richness was generally reduced at flooded sites in nutrient-rich catchments and sites that previously experienced relatively stable hydrographs (e.g. rain-fed lowland streams). Species richness usually increased at sites in desert and semi-arid climate regions (e.g. intermittent streams).

Effects of climate-induced increases in summer drought on riparian plant species: a meta-analysis

1. Frequency and duration of summer droughts are predicted to increase in the near future in many parts of the world, with considerable anticipated effects on riparian plant community composition and species richness. Riparian plant communities along lowland streams are characterised by high species richness due to their system-specific environmental gradients. As these streams and their hydrological gradients are mainly rain-fed, they are sensitive to precipitation changes. 2. We conducted a literature survey and meta-analysis to examine the effects of an increase in summer drought on: (i) riparian plant biomass; (ii) riparian seedling survival and (iii) riparian plant species composition and richness. We also aimed to determine whether hydrological thresholds related to drought tolerance can be distinguished for riparian plant species. 3. ISI Web of Knowledge was searched for relevant peer-reviewed studies, and 23 papers were found that met our criteria and contained quantitative study results. To detect overall responses of biomass and seedling survival, a random-effects model was applied using Comprehensive Meta-analysis™ software. Regression curves were then fitted to response ratio data relating the effects on drought-impacted groups to those on control groups. 4. Our results showed that a drought duration of approximately >30 days strongly reduces riparian plant biomass and that a duration of approximately >30-35 days and high drought intensities (starting from 3 to 4 cm water table decline per day) can be detrimental for riparian seedling survival. Especially Populus and Salix seedlings showed a reduced survival in response to drought, in contrast to Tamarix seedlings, which have the ability to rapidly and expansively elongate their roots. The data also revealed that an increase in drought conditions rapidly leads to a decline of riparian species richness and an increased presence of species adjusted to drier conditions. 5. Riparian groundwater level, surface water permanence and certain plant traits, especially plasticity in rooting depth, were mentioned most frequently as factors determining species responses. Very few studies mentioned hydrological thresholds, such as critical values for ground- and/or surface water levels, and so far these results have proved difficult to generalise. 6. Our meta-analysis has shown that the projected increase in the duration and intensity of drought periods, especially intense droughts lasting more than 30 days, can be expected to narrow the riparian wetland zone with typical hydric species and accelerate riparian wetland species losses in the near future. This may require extra efforts in terms of management and restoration of species-rich riparian areas.

Addressing the Impact of Atmospheric Nitrogen Deposition on Western European Grasslands

There is a growing evidence base demonstrating that atmospheric nitrogen deposition presents a threat to biodiversity and ecosystem function in acid grasslands in Western Europe. Here, we report the findings of a workshop held for European policy makers to assess the perceived importance of reactive nitrogen deposition for grassland conservation, identify areas for policy development in Europe and assess the potential for managing and mitigating the impacts of nitrogen deposition. The importance of nitrogen as a pollutant is already recognized in European legislation, but there is little emphasis in policy on the evaluation of changes in biodiversity due to nitrogen. We assess the potential value of using typical species, as defined in the European Union Habitats Directive, for determining the impact of nitrogen deposition on acid grasslands. Although some species could potentially be used as indicators of nitrogen deposition, many of the typical species do not respond strongly to nitrogen deposition and are unlikely to be useful for identifying impact on an individual site. We also discuss potential mitigation measures and novel ways in which emissions from agriculture could be reduced.

Human-mediated dispersal of seeds over long distances

Human activities have fundamental impacts on the distribution of species through altered land use, but also directly by dispersal of propagules. Rare long-distance dispersal events have a disproportionate importance for the spread of species including invasions. While it is widely accepted that humans may act as vectors of long-distance dispersal, there are few studies that quantify this process. We studied in detail a mechanism of human-mediated dispersal (HMD). For two plant species we measured, over a wide range of distances, how many seeds are carried by humans on shoes. While over half of the seeds fell off within 5m, seeds were regularly still attached to shoes after 5 km. Semi-mechanistic models were fitted, and these suggested that long-distance dispersal on shoes is facilitated by decreasing seed detachment probability with distance. Mechanistic modelling showed that the primary vector, wind, was less important as an agent of long-distance dispersal, dispersing seeds less than 250 m. Full dispersal kernels were derived by combining the models for primary dispersal by wind and secondary dispersal by humans. These suggest that walking humans can disperse seeds to very long distances, up to at least 10 km, and provide some of the first quantified dispersal kernels for HMD.

Mechanistic Analytical Models for Long‐Distance Seed Dispersal by Wind

We introduce an analytical model, the Wald analytical long-distance dispersal (WALD) model, for estimating dispersal kernels of wind-dispersed seeds and their escape probability from the canopy. The model is based on simplifications to well-established three-dimensional Lagrangian stochastic approaches for turbulent scalar transport resulting in a two-parameter Wald (or inverse Gaussian) distribution. Unlike commonly used phenomenological models, WALD's parameters can be estimated from the key factors affecting wind dispersal--wind statistics, seed release height, and seed terminal velocity--determined independently of dispersal data. WALD's asymptotic power-law tail has an exponent of -3/2, a limiting value verified by a meta-analysis for a wide variety of measured dispersal kernels and larger than the exponent of the bivariate Student t-test (2Dt). We tested WALD using three dispersal data sets on forest trees, heathland shrubs, and grassland forbs and compared WALD's performance with that of other analytical mechanistic models (revised versions of the tilted Gaussian Plume model and the advection-diffusion equation), revealing fairest agreement between WALD predictions and measurements. Analytical mechanistic models, such as WALD, combine the advantages of simplicity and mechanistic understanding and are valuable tools for modeling large-scale, long-term plant population dynamics.