Dynamics and spatial organization of plant communities in water-limited systems

A mathematical model for plant communities in water-limited systems is introduced and applied to a mixed woody-herbaceous community. Two feedbacks between biomass and water are found to be of crucial importance for understanding woody-herbaceous interactions: water uptake by plants' roots and increased water infiltration at vegetation patches. The former acts to increase interspecific competition while the latter favors facilitation. The net interspecific interaction is determined by the relative strength of the two feedbacks. The model is used to highlight new mechanisms of plant-interaction change by studying factors that tilt the balance between the two feedbacks. Factors addressed in this study include environmental stresses and patch dynamics of the woody species. The model is further used to study mechanisms of species-diversity change by taking into consideration tradeoffs in species traits and conditions giving rise to irregular patch patterns.     

Competition, traits and resource depletion in plant communities

Although of primary importance to explain plant community structure, general relationships between plant traits, resource depletion and competitive outcomes remain to be quantified across species. Here, we used a comparative approach to test whether instantaneous measurements of plant traits can capture both the amount of resources depleted under plant cover over time (competitive effect) and the way competitors perceived this resource depletion (competitive response). We performed a large competition experiment in which phytometers from a single grass species were transplanted within 18 different monocultures grown in a common-garden experiment, with a time-integrative quantification of light and water depletion over the phytometers’ growing season. Resource-capturing traits were measured on both phytometers (competitive response traits) and monocultures (competitive effect traits). The total amounts of depleted light and water availabilities over the season strongly differed among monocultures; they were best estimated by instantaneous measurements of height and rooting depth, respectively, performed when either light or water became limiting. Specific leaf area and leaf water potential, two competitive response traits measured at the leaf level, were good predictors of changes in phytometer performance under competition, and reflected the amount of light and water, respectively, perceived by plants throughout their lifespan. Our results demonstrated the relevance of instantaneous measures of plant traits as indicators of resource depletion over time, validating the trait-based approach for competition ecology. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Abiotic and biotic resistance to grass invasion in serpentine annual plant communities

Biological invasions severely impact native plant communities, causing dramatic shifts in species composition and the restriction of native species to spatially isolated refuges. Competition from resident species and the interaction between resource limitation and competition have been overlooked as mechanisms of community resistance in refugia habitats. We examined the importance of these factors in determining the resistance of California serpentine plant communities to invasion by three common European grasses, Avena barbata, Bromus diandrus, and Hordeum murinum. We added seeds of each of these grasses to plots subjected to six levels of resource addition (N, P, Ca, H₂O, all resources together, and a no-addition control) and two levels of competition (with resident community present or removed). Resource limitation and competition had strong effects on the biomass and reproduction of the three invaders. The addition of all resources together combined with the removal of the resident community yielded individual plants that were fourfold to 20-fold larger and sixfold to 20-fold more fecund than plants from control plots. Competitor removal alone yielded invaders that were twofold to sevenfold larger and twofold to ninefold more fecund. N addition alone or in combination with other resources led to a twofold to ninefold increase in the biomass and fecundity of the invaders. No other resource alone significantly affected native or invader performance, suggesting that N was the key limiting resource during our experiment. We found a significant interaction between abiotic and biotic resistance for Bromus, which experienced increased competitive suppression in fertilized plots. The threefold increase in resident biomass with N addition was likely responsible for this result. Our results confirm that serpentine plant communities are severely N limited, which, in combination with competition from resident species, promotes the resistance of these systems to invasions. Our work suggests that better understanding the relative sensitivities of invaders and residents to the physical environment is critical to predicting how abiotic and biotic factors interact to determine community resistance.     

A mathematical approach to spatial distribution and temporal succession in plant communities

The Volterra equations which represent competitions between two species are utilized to examine the phenomenon of boundary formation between two species of plants. The set of stable stationary points for these equations is determined and is illustrated in a product space of parameters and dynamical variables. The stages of boundary appearance and succession are visualized by considering slow changes of the parameters as functions of time and space.     

Shedding light on plant competition: modelling the influence of plant morphology on light capture (and vice versa)

A plant's morphology is both strongly influenced by local light availability and, simultaneously, strongly influences this local light availability. This reciprocal relationship is complex, but lies at the heart of understanding plant growth and competition. Here, we develop a sub-individual-based simulation model, cast at the level of interacting plant components. The model explicitly simulates growth, development and competition for light at the level of leaves, branches, etc., located in 3D space. In this way, we are able to explore the manner in which the low-level processes governing plant growth and development give rise to individual-, cohort-, and community-level phenomena. In particular, we show that individual-level trade-offs between growing up and growing out arise naturally in the model, and robustly give rise to cohort-level phenomena such as self-thinning, and community processes such as the effect of ecological disturbance on the maintenance of biodiversity. We conclude with a note on our methodology and how to interpret the results of simulation models such as this one.     

Effects of spatial pattern and relatedness in an experimental plant community

Many plant species show limited dispersal resulting in spatial and genetic substructures within populations. Consequently, neighbours are often related between each other, resulting in sibling competition. Using seed families of the annuals Capsella bursa-pastoris and Stachys annua we investigated effects of spatial pattern (i.e. random versus aggregated) on total and individual performance at the level of species and seed families under field conditions. At the level of species, we expected that inferior competitors increase, while superior competitors decrease their performance within neighbourhoods of conspecifics. Thus, we expected a species by spatial pattern interaction. Sibling competition, however, might reduce the performance of competitors, when genetically related, rather than non-related individuals are competing. Therefore, aggregations at the level of seed families could decrease the performance of competitors. Alternatively, if the opposite outcome would be observed, kin selection might be hypothesized to have occurred in the past. Because heavy seeds are expected to disperse less than light seeds, we further hypothesized that kin selection might be more likely to occur in superior competitors with heavy, locally dispersed seeds (e.g. Stachys) compared to inferior competitors with light, more distantly dispersed seeds (e.g. Capsella). We found a significant species by spatial pattern interaction. Indeed, the inferior competitor, Capsella, showed increased reproductive biomass production in aggregated compared to random patterns. Whereas, the performance of the superior competitor, Stachys, was to some extent decreased by intraspecific aggregation. Although statistically not significant, effects of intrafamily aggregations tended to be rather negative in Capsella but positive in Stachys. Our results confirmed that spatial patterns affect growth and reproduction of plant species promoting coexistence in plant communities. Although, we could not provide strong evidence for sibling competition or kin selection, our results suggested that competition among relatives was more severe for Capsella (lighter seeds) compared to Stachys (heavier seeds).     

The role of reproductive plant traits and biotic interactions in the dynamics of semi-arid plant communities

The dynamics of semi-arid plant communities are determined by the interplay between competition and facilitation among plants. The sign and strength of these biotic interactions depend on plant traits. However, the relationships between plant traits and biotic interactions, and the consequences for plant communities are still poorly understood. Our objective here was to investigate, with a modelling approach, the role of plant reproductive traits on biotic interactions, and the consequences for processes such as plant succession and invasion. The dynamics of two plant types were modelled with a spatially-explicit integrodifferential model: (1) a plant with seed dispersal (colonizer of bare soil) and (2) a plant with local vegetative propagation (local competitor). Both plant types were involved in facilitation due to a local positive feedback between vegetation biomass and soil water availability, which promoted establishment and growth. Plants in the system also competed for limited water. The efficiency in water acquisition (dependent on reproductive and growth plant traits) determined which plant type dominated the community at the steady state. Facilitative interactions between plant types also played an important role in the community dynamics, promoting establishment in the driest conditions and recovery from low biomass. Plants with vegetative propagation took advantage of the ability of seed dispersers to establish on bare soil from a low initial biomass. Seed dispersers were good invaders, maintained high biomass at intermediate and high rainfall and showed a high ability in taking profit from the positive feedback originated by plants with vegetative propagation under the driest conditions. However, seed dispersers lost competitiveness with an increasing investment in fecundity. All together, our results showed that reproductive plant traits can affect the balance between facilitative and competitive interactions. Understanding this effect of plant traits on biotic interactions provides insights in processes such as plant succession and shrub encroachment.     

Modelling potential impacts of climate change on the spatial distribution of zonal forest communities in Switzerland

Abstract. A spatially explicit, climate-sensitive vegetation model is presented to simulate both present and future distribution of potential natural vegetation types in Switzerland at the level of zonal forest communities. The model has two versions: (1) a ‘basic’ version using geographical region, aspect, bedrock (represented by soil pH), and elevation, and (2) a ‘climate-sensitive’ version obtained by replacing elevation (complex environmental gradient) with temperature (climatic factor). Version 2 is used to predict vegetation response under different (today's and projected) climatic conditions. Two regional climate scenarios are applied: (1) assuming an annual mean temperature increase of 1.1 — 1.4 °C, and (2) assuming an increase of 2.2 — 2.75 °C. Both scenarios result in significant changes of the spatial vegetation patterns as compared with today's climatic conditions. In scenario 1, ca. 33 % of the sample points remain unchanged in terms of the simulated zonal forest community; in scenario 2, virtually all sample points change. The most noticeable changes occur on the Swiss Plateau with Carpinion forests (zonal vegetation of present colline belt) expanding to areas that are occupied today by submontane and low-montane Fagus forests. To estimate the reliability of the simulation, quantitative (comparison with field mapping) and qualitative (comparison with climate types in the Alpine region) tests are performed and the main limitations of the approach are evaluated.     

A spatial simulation model for vegetation dynamics

A fine-resolution, spatially explicit, stochastic model was developed to simulate the dynamics of species cover abundance and pattern in a single vegetation layer wherein neighbouring individuals are assumed to compete for growing space. Each species in the model is characterized by a small number of morphological and life-history parameters, which enter into equations that stand for a minimal set of vegetation processes. The model performed well in reproducing post-fire successional trends among the three codominant dwarf shrubs in a Dutch heathland community as recorded in an annually mapped permanent quadrat. Program inputs, outputs and an example of sensitivity analysis are illustrated. With suitable changes, the model could potentially describe any plant community in which the vertical structure is simple and community dynamics are determined by spatial interactions among neighbouring plants.Jacques de Smidt provided data, information and stimulus for this project. The model was developed during ICP's visit to Utrecht, arranged by Marinus Werger and financed by The Netherlands Science Research Council (ZWO). We also thank David Glenn-Lewin and Ernst Lippe for discussion and cooperation.     

Plant zonation at salt marshes of the endangered cordgrass <Emphasis Type="Italic">Spartina maritima</Emphasis> invaded by <Emphasis Type="Italic">Spartina densiflora</Emphasis>

The South American cordgrass Spartina densiflora is invading European salt marshes getting into contact with the indigenous and endangered low-marsh dominant, Spartina maritima. This work describes the evolution of the plant zonation during 7 years in a marsh of S. maritima invaded by S. densiflora. S. maritima appeared throughout the whole intertidal gradient from 1.72 to 3.33 m over Spanish Hydrographic Zero (SHZ), showing its higher biomasses and shoot densities at low elevations. In contrast, S. densiflora only invaded upper areas (>+2.59 m SHZ) at the centre of circular tussocks of S. maritima. Above-ground biomass of S. maritima dropped drastically at maximum occupation of space by the alien, and its shoot density and above-ground biomass decreased at S. densiflora zone during the study. The competitive potential of S. densiflora was reflected in high above- and below-ground biomass and shoot densities, accompanied by elevated wrack accumulation and the absence of other marsh plants presented together with S. maritima from areas dominated by S. densiflora. S. densiflora altered the native vegetational zonation pattern through the invasion of the centre of S. maritima tussocks; however, the alien invasion may be limited by the presence of the autochthonous cordgrass at lower elevations. Handling editor: Luis Mauricio Bini     

Effects of temporal and spatial heterogeneities created by consumer-driven nutrient recycling on algal diversity

A spatially explicit plant-herbivore model composed of planktonic herbivores, algal preys and nutrients was constructed to examine the effects of consumer-driven nutrient recycling (CNR) on the algal species richness with and without spatial structure. The model assumed that either of two essential nutrients (N and P) limited growth of algal populations and that consumer individuals moved randomly in the lattice and grazed all the algal species with the same efficiency. The results showed that when there was no CNR, the number of persistent algal species was affected by neither supply rates of external nutrients nor spatial structure and was consistently low. When consumers recycled nutrients according to their stoichiometry, the algal species richness changed with supply rates of external nutrients depending on spatial structure: the algal species richness decreased with increasing nutrient loadings when there were no spatial structure because CNR increased the probability of stochastic extinction of algal species by amplifying the oscillation of algae-consumer dynamics. However, when spatial structures were created by the migration of consumers, CNR increased the algal species richness in a range of nutrient loadings because spatial variation of grazing pressure functioned to stabilize the algal-consumer dynamics. The present study suggests that through grazing and nutrient recycling, consumer individuals can create ephemeral heterogeneity in growth environments for algal species and that this ephemerality is one of the keys to understanding algal species in nature.     

On the dynamics of vegetation: Succession in model communities

Summary Successional change is thought to be at least partially driven by forees originating from within the community, namely by reaction and competition. Both processes operate through changes in the environment, but from the literature on the subject it is not clear how they differ.To clarify these issues successiens of model communities are studied. This leads us to conclude that competition represent an instantaneous interaction, whereas reaction has historical aspects since it relies on cumulative changes in the environment. The three models considered-one relying on reaction to cause vegetational change, one relying on competition and differential growth rates, and a hybrid third one-yield very similar predictions: roughly bell-shaped curves displaced along the time axis. This shows that the mere fit of a certain model to successional data may easily be spurious (recently some workers have empirically fitted models identical to one derived here from first principles). The three models do behave radically different under perturbation, however: any model relying completely or partially on historical interactions cannot account for the well known possibility of artificially arresting succession. Even if the importance of historical interactions in succession (i.e. the Markovian character of succession) cannot easily be ascertained, one can nevertheless ask whether historical interactions are at all necessary for the explanation of successional change. It is argued here that succession can be entirely understood in terms of instantaneous interactions, notably competition. The argument rests upon the well known relationship between colonizing and competitive ability, and on the fact, proven here, that stress, defined as expressing itself in severe random fluctuations in the growth parameters, is negatively correlated with competition intensity.The author gratefully acknowledges the critical support be received from Professor László Orlóci. This paper forms part of a wider research program supported by a NCR grant to Proc. Orlóci.     

A capture-rate model of net-spinning caddisfly communities

Summary Empirical research suggests that net-spinning caddisflies require two basic resources, suspended particulate foods, and the currents which deliver them. I present a theoretical model of caddisfly communities based on quantitative differences in the capture rate produced by different catchnet designs. It assumes that catchnet architecture reflects a tradeoff between water filtration rate (flux through the net) and capture efficiency (the proportion of suspended items retained), and that the marginal resource concentration required by species with different catchnet morphologies should reflect the product of these parameters. The model hypothesizes a) that downstream changes in the physical morphology of the stream channel cause a shift in the relative importance of population limitations imposed by food and current-substrate availability, b) that the interaction of these physical changes with the filtering biota results in a seston resource gradient, and c) that the distribution of each taxon along this resource gradient reflects a marginal resource requirement determined by the functional morphology of its catchnet.     

The role of mycorrhiza in plant populations and communities

Summary There is a large body of literature concerning the value of mycorrhiza to plant growth. Recent emphasis on the potential benefits of the fungi to natural ecosystems conservation and productive agriculture has focused attention on the roles and underlying mechanisms of the association. In parallel, recognition that isolates/ species of vesicular arbuscular mycorrhizal fungi have variable life-history traits has resulted in investigations focused on the symbiotic fungi rather than the host, a perspective which may reflect the fungal evolutionary strategies. This paper discusses progress in understanding interactions amongst hyphae, which in one form or another are a major component of mycorrhiza and each phase of the life-history.     

Modelling Cooperation between fishermen with a Cellular automaton. A framework for fishing effort spatial dynamics

In many fisheries sharing information between vessels is an important characteristic of fishermen's behaviour rarely modelled or analyzed. A Cellular Automaton is designed in an attempt to understand circumstances that favour group formation. The simulated world is toroidal with a static fishing resource distributed in patches. Movement decisions are random in the case of fishermen in a local scale. After a certain time interval, sharing information is possible between fishermen in a dynamic Moore's neighbourhood of cells at a broader scale and movement to adjacent areas may occur according to a set of rules. The dynamic neighbourhood is a novel concept defined in this work within the framework of Cellular Automata. Decision making by each fisherman is a function of the influence other fishermen (neighbours) exert on them as well as on personal knowledge, to form an opinion of the areas (cells) quality, and take action consequently.     

Economically and ecologically important plant communities in high altitude coniferous forest of Malam Jabba,Swat, Pakistan

A study on the economically important plant communities was carried out during summer 2008 in various parts of Malam Jabba valley, Swat. The principal aim of the study was phytosociological evaluation with special reference to the occurrence of commercially important medicinal plant species in coniferous forest of the study area. Secondly to prepare ethnobotanical inventory of the plant resources of the area, as well as to evaluate the conservation status of important medicinal and aromatic plants (MAPs) through rapid vulnerable assessment (RVA) procedure. The study documented 90 species of ethnobotanical importance, out of these 71 spp used as medicinal plant, 20 spp fodder plant, 10 spp vegetables, 14 spp wild fruit, 18 spp fuel wood, 9 spp furniture and agricultural tools, 9 spp thatching, fencing and hedges, 4 spp honey bee, 2 spp evil eyes, 2 spp religious and 3 spp as poison. Phytosociologically six plant communities were found, comprising five herbs-shrubs-trees communities and one meadow community. Further study is, therefore, required to quantify the availability of species and to suggest suitable method for their production and conservation. Recommendations are also given in the spheres of training in identification, sustainable collection, value addition, trade monitoring and cooperative system of marketing.     

Effects of temporal variability of disturbance on the succession in marine fouling communities in northern-central Chile

We investigated the effects of temporal variability in a disturbance regime on fouling communities at two study sites in a northern-central Chilean bay. Fouling assemblages grown on artificial settlement substrata were disturbed by mechanical removal of biomass at different time intervals. Using one single disturbance frequency (10 disturbance events over 5 months) we applied 7 different temporal disturbance treatments: a constant disturbance regime (identical intervals between disturbance events), and 6 variable treatments where both variableness and sequences of intervals between disturbance events were manipulated. Two levels of temporal variableness (low and high, i.e. disturbance events were either dispersed or highly clumped in time) in the disturbance regime were applied by modifying the time intervals between subsequent disturbance events. To investigate the temporal coupling between disturbance events and other ecological processes (e.g. larval supply and recruitment intensity), three different sequences of disturbance intervals were nested in each of the two levels of temporal variableness. Species richness, evenness, total abundance, and structure of communities that experienced the various disturbance regimes were compared at the end of the experiment (15 days after the last disturbance event). Disturbance strongly influenced the community structure and led to a decrease in evenness and total abundance but not species richness. In undisturbed reference communities, the dominant competitor Pyura chilensis (Tunicata) occupied most available space while this species was suppressed in all disturbed treatments. Surprisingly, neither temporal variableness in the disturbance regime nor the sequence of intervals between disturbance events had an effect on community structure. Temporal variability in high disturbance regimes may be of minor importance for fouling communities, because they are dominated by opportunistic species that are adapted to rapidly exploit available space.     

A master equation for a spatial population model with pair interactions

We derive a closed master equation for an individual-based population model in continuous space and time. The model and master equation include Brownian motion, reproduction via binary fission, and an interaction-dependent death rate moderated by a competition kernel. Using simulations we compare this individual-based model with the simplest approximation, the spatial logistic equation. In the limit of strong diffusion the spatial logistic equation is a good approximation to the model. However, in the limit of weak diffusion the spatial logistic equation is inaccurate because of spontaneous clustering driven by reproduction. The weak-diffusion limit can be partially analyzed using an exact solution of the master equation applicable to a competition kernel with infinite range. This analysis shows that in the case of a top-hat kernel, reducing the diffusion can increase the total population. For a Gaussian kernel, reduced diffusion invariably reduces the total population. These theoretical results are confirmed by simulation.     

Empirical and virtual investigation of the population dynamics of an alien plant under the constraints of local carrying capacity: Heracleum mantegazzianum in the Czech Republic

During the last decades, invasive alien species have become a global concern because of their ecological and economic impact. Heracleum mantegazzianum (Apiaceae), is a tall monocarpic perennial native to Caucasus and invasive in Europe since the 1950s. Within an interdisciplinary EU project aimed at assessing suitable management strategy, we analysed the demography and ecology of this species in its invasive range. The monitoring of population dynamics in the Czech Republic led to the result that in the observed sites the species showed decreasing populations. To find an explanation for this unexpected result, two types of models were parameterized, based on the empirical data: (1) a stage-based transition matrix model, which projected a continuous negative development, and (2) a spatially explicit individual-based model (IBM), including individual variation. This second model was able to create a population with steady individual numbers. Analyses of the simulation showed that in more than 54% of the simulated years (n=5000) the growth rate was smaller than one. Still, population increase in the remaining years was sufficient to sustain a population. Nevertheless long-term observations document an invasive behaviour of the observed populations. Hence, we could assume temporal changes in the course of an invasion and thus wanted to evaluate the probability of sampling negative growth in dependence of time since first invasion. By using a method from ‘Virtual Ecology’, we approached the question: first we create an invasive population, based on the empirical data of H. mantegazzianum and second empirical sampling techniques were mimicked using the Virtual Ecologist approach. The results demonstrate how the probability of sampling negative growth increases with time since first invasion. Hence, we assume that the studied populations have already reached a maximum of their local invasive potential and thus stagnate in their size.