Beáta Oborny

Growth rules in clonal plants and environmental predictability - A simulation study

The adaptive nature of clonal growth rules was studied using a Monte Carlo simulation based on a stochastic, spatially explicit growth model. In this model, the development of a clone was controlled by environment-dependent growth rules, acting upon elementary developmental decisions, such as suppression or activation of meristems at certain locations, or modification of internode length. The neutral model, used as a control, simulated a plant of rigid, non-environment-dependent form. Growth proceeded in a spatially and temporally heterogeneous environment. A wide range of environmental types - modelled as mosaics of resource-rich and resource-poor sites - were used for the tests, and analysed in the framework of an information theory model (...)

THE EFFECT OF CLONAL INTEGRATION ON PLANT COMPETITION FOR MOSAIC HABITAT SPACE

Physiological integration between ramets has been observed in several clonal plant species. But consequences of integration on the competitive ability and spatial de- velopment of genets have received little attention so far. This study is an attempt to examine the population- and community-level implications of integration in a spatially explicit model. We have simulated different resource patterns in a cellular automata, varying the pro- portion (p) and size (s) of resource-rich patches and the average resource level in the area (h). We compared the efficiency of integrator to splitter genets in exploring and utilizing the resource patches. In the integrator, ramets that belonged to the same genet were phys- iologically connected and shared the resource taken up by any part of the genet. In the splitter, all ramets were autonomous, i.e., survival and reproduction of each ramet only depended on the local resource conditions. Thus, the integrator sensed and responded to environmental heterogeneity on a larger spatial scale than the splitter. Integration can be interpreted as sharing the risk of genet mortality among interconnected ramets, while split- ting represents risk spreading among autonomous ramets. In each simulated habitat type, we followed the course of competition between splitter and integrator genets for 500 gen- erations. First, we examined the effect of environmental heterogeneity on the result of compe- tition. We fixed the average resource level in the area and changed the spatial distribution of the resource by varying p and s. We found that the smaller the proportion of favorable sites, the better it is to integrate. But already a small deviation from random to clumped resource distribution (i.e., relatively small s) made the integrator competitively inferior to the splitter. In the next step, we varied the overall resource richness in the area. High productivity of the habitat promoted the dominance of integrators. The simulations dem- onstrated that there were habitat types in which one of the strategies (the integrator or the splitter) rapidly excluded its competitor. But at the medium range of environmental param- eters, long-term coexistence of the strategies became possible. As integration proceeded simultaneously with horizontal growth and exploration of new patches, integrators re- markably differed from splitters in their pattern of space occupation. A spatial statistical analysis revealed that splitters maintained strong spatial association with the resource: splitting promoted habitat selection. Integration, on the other hand, led to gap-filling, fu- gitive spatial behavior. For a range of realistic habitat patterns, we need not expect the exclusion of any of the pure strategies—splitters and integrators can coexist.

Spacer length in clonal plants and the efficiency of resource capture in heterogeneous environments: A Monte Carlo simulation

A stochastic, spatially explicit simulation model for clonal growth is presented which relates growth patterns to the pattern of resource availability in the environment in both space and time. The effects of two simple growth rules were examined which affect the length of spacers depending on the local environmental conditions. According to one of the rules, shorter spacers were developed in resource-rich microsites than in resource-poor microsites (growth rule G-). If the other rule acted, the spacers were lengthened in resource-rich sites (growth rule G+). The neutral reference, G0, represented a plant of rigid growth form.A wide range of habitat types was used in the tests and characterized by an information theory model. It was found that the effectiveness of resource capture in most habitat types can be explained by spatio-temporal predictability of the environment, measured on the scale of spacer length. Shortening the spacers in resource-rich microsites, as hypothesized by “foraging theory”, reduced the proportion of misplaced ramets. Lengthening the spacers never reduced this proportion. However, the degree of intraclonal competition was significantly reduced by both shortening and lengthening the spacers in response to site quality. There were certain types of environment where plastic modification of spacers had no effect on the efficiency of resource capture when compared to the reference random (non-environment-dependent) search pattern. Such habitats can be identified exactly on the basis of the information content of habitat pattern, measured here by spatio-temporal predictability.This study emphasizes that a wide range of environmental types should be taken into consideration when examining the adaptive nature of a certain growth pattern. Generalizing from experimental results gained in temporally constant environments may strongly bias our view on morphological adaptation.

Fragmentation of clones: how does it influence dispersal and competitive ability?

We applied individual-based simulations to study the effect of physiological integration among ramets in clonal species that live in patchy habitats. Three strategies were compared: (1) Splitter, in which the genet was fragmented into independent ramets; (2) Transient Integrator, where only groups of ramets were connected; and (3) Permanent Integrator, in which fragmentation did not occur, and the whole genet was integrated. We studied the dynamics of spatial spreading and population growth in these strategies separately and in competition. Various habitat types were modeled by changing the density of favorable habitat patches. We found that the spatial pattern of good patches significantly influenced the growth of the populations. When the resource patches were scarce, a large proportion of the carrying capacity of the habitat was not utilized by any of the strategies. It was the Splitter that proved to be the most severely dispersal-limited. But at the same time, it could compete for the good patches most efficiently. The balance between these two contradictory effects was largely determined by the proportion of favorable to unfavorable areas. When this proportion was low or intermediate (up to ca. 50% good), integration was more advantageous. At higher proportions, fragmentation became beneficial. Fragmentation into groups of ramets (Transient Integration) was not sufficient, only radical splitting could ensure a significant selective advantage. Transient Integrators got fragmented according to the spatial pattern of ramet mortality. It was interesting that the enrichment of the area in good sites did not lead to larger fragment sizes. It merely raised the number of fragments. Nevertheless, these small fragments were more similar to integrated genets (in the Permanent Integrator) than to solitary ramets (in the Splitter) in terms of dispersal and competitive ability. This suggests that even a slightly integrated clonal species can be ecologically considered as an integrator.

Exploration and exploitation of resource patches by clonal growth: A spatial model on the effect of transport between modules

Abstract Modular development in plants facilitates to cope with environmental heterogeneity. Differential natality and mortality of the modules in resource-rich versus poor sites can lead to selective occupancy of favorable habitat patches (foraging for good sites). In some species, especially in clonal plants, the modules are quite autonomous, and perceive and respond to local habitat conditions individually (splitter strategy). In others, the modules are physiologically integrated, and exchange water, nutrients and assimilates. In the simplest case, the resource moves from rich to poor sites, thus, the contrast between habitat patches is evened out within the plant (integrator strategy). Integration can significantly rearrange the pattern of resources that are available to the modules. We studied the effect of integration on the efficiency of foraging. We applied a spatially explicit (cellular automata) model, in which we varied the size and proportion of resource-rich patches in the habitat. In each simulation, a splitter and an integrator species were competing for this patchy resource. We compared the dynamics of module populations. We studied the spatial association between the splitter, the integrator, and the resource by an information statistical method of pattern analysis. Thus, we gained a quantitative description of habitat structure, and a related measure for the efficiency of foraging. We found that splitting always promoted foraging, but at the expense of slower population growth. This became critical when the distribution of resource patches was sparse. In these cases, the characteristically fugitive spatial behavior of the integrator facilitated its quick spreading into the gaps that has been left vacant by the splitter. Better tolerance of the integrator to resource scarcity, and better chance for colonizing distant patches enabled its long-term persistence, occasionally even dominance over the area. Therefore, the adaptive advantage of splitting versus integration depended sensitively on the spatial pattern of resource patches.

Transition from connected to fragmented vegetation across an environmental gradient: scaling laws in ecotone geometry

A change in the environmental conditions across space—for example, altitude or latitude—can cause significant changes in the density of a vegetation type and, consequently, in spatial connectivity. We use spatially explicit simulations to study the transition from connected to fragmented vegetation. A static (gradient percolation) model is compared to dynamic (gradient contact process) models. Connectivity is characterized from the perspective of various species that use this vegetation type for habitat and differ in dispersal or migration range, that is, “step length” across the landscape. The boundary of connected vegetation delineated by a particular step length is termed the “ hull edge.” We found that for every step length and for every gradient, the hull edge is a fractal with dimension 7/4. The result is the same for different spatial models, suggesting that there are universal laws in ecotone geometry. To demonstrate that the model is applicable to real data, a hull edge of fractal dimension 7/4 is shown on a satellite image of a piñon‐juniper woodland on a hillside. We propose to use the hull edge to define the boundary of a vegetation type unambiguously. This offers a new tool for detecting a shift of the boundary due to a climate change.

Spatial constraints masking community assembly rules: A simulation study

The effect of competition on species coexistence is usually strongly modified by other factors especially in non-equilibrium systems of sessile organisms with limited availability of propagules. As a consequence, competition-based assembly rules (even if their existence seems to be unambiguously detected) would result in incomplete understanding of the coexistence of species in plant communities. J. Bastow Wilson suggested measuring variance deficit in the number of co-occurring species as a means to detect niche limitation in a community. The method provides a relatively simple and quick “snap-shot” analysis of a community. However, it has been questioned whether niche limitation is the only factor which might account for variance deficit.The paper presents a spatially explicit simulation experiment in which artificial communities are produced by pre-defined rules for competitive interactions. Then we examine whether these rules can be detected by a proposed method for pattern analysis. Two limiting cases are simulated: (A) all the species share the same niche, and (B) all the species have different niches. The difference between these cases in the variance of species numbers is examined. Using the simulation results, some basic spatial constraints upon species assembly are emphasized.It is argued that the assumptions of Wilson’s approach confine its applicability to species-saturated equilibrium communities. The study of assembly rules in dynamically changing, spatially structured communities requires the consideration of a set of coenological characteristics and the use of careful spatio-temporal scaling to detect their patterns. The use of spatially explicit individual-based models to study the mechanisms and constraints limiting species coexistence at different scales is suggested.

Intermediate landscape disturbance maximizes metapopulation density

The viability of metapopulations in fragmented landscapes has become a central theme in conservation biology. Landscape fragmentation is increasingly recognized as a dynamical process: in many situations, the quality of local habitats must be expected to undergo continual changes. Here we assess the implications of such recurrent local disturbances for the equilibrium density of metapopulations. Using a spatially explicit lattice model in which the considered metapopulation as well as the underlying landscape pattern change dynamically, we show that equilibrium metapopulation density is maximized at intermediate frequencies of local landscape disturbance. On both sides around this maximum, the metapopulation may go extinct. We show how the position and shape of the intermediate viability maximum is responding to changes in the landscape’s overall habitat quality and the population’s propensity for local extinction. We interpret our findings in terms of a dual effect of intensified landscape disturbances, which on the one hand exterminate local populations and on the other hand enhance a metapopulation’s capacity for spreading between habitat clusters.

Resource transport between ramets alters soil resource pattern: a simulation study on clonal growth

Abstract.Clonal plants spread horizontally, and can transport nutrients between ramets. Decaying biomass feeds back nutrients into the soil, but importantly, the place of deposition may differ from the place of uptake. To our knowledge, the present model is the first attempt to couple population dynamics with resource dynamics with the consideration of lateral transport. The simulations start from various initial resource patterns. Six types of clonal plants are compared, which differ in the birth and survival rates of ramets. Size of the ramet population and the amount of translocated resource are recorded over time. In addition, we consider the pattern of gaps in the canopy of the clonal plant from the aspect of two colonizer species: a strong and a weak competitor. The results suggest that the most important factor determining the impact of a clonal plant on its environment is ramet survival; the rate of ramet production is only secondary. Phenotypic plasticity in the production of ramets does not appear to be important: it has only minor effect on resource translocation and on the availability of colonizable gaps.

External and internal control in plant development

Bodies of plants are modularly organized. Development proceeds by adding new modules to open endings with a potential for branching. Each module is autonomous to some extent. Development relies on the self-organized patterns that emerge from the interactions of individual modules. Interactions include both competition and cooperation, and several types of positive and negative feedback loops are involved. Development can be open to external influences, thus enabling the plant to adjust its form to the environment, for example, to the spatial distribution of ecological resources. This paper provides a review on adaptive plasticity in plants.