The effect of landscape pattern on Mediterranean vegetation dynamics: A modelling approach using functional types

Abstract. In the framework of land use changes in the Mediterranean area, I asked to what extent different landscape structures might determine long‐term dynamics in Mediterranean ecosystems. To answer this question, a spatially explicit model was developed (the Melca model), incorporating two functional types of woody species dominant in Mediterranean ecosystems: a resprouter (R) and a non‐resprouter fire‐recruiter (seeders, S). The model was used as a tool for generating hypotheses on the possible consequences of different landscape scenarios. Thus, five different hierarchically structured random landscapes were generated, all having the same cover for the two functional types but different landscape structure (ranging from highly heterogeneous to homogeneous landscapes). After a 100‐yr simulation, plant cover and spatial pattern had changed and the changes were different for the different initial spatial configurations, suggesting that long‐term vegetation dynamics is spatially dependent (the resultant dynamics are sensitive to the initial spatial structure). In the landscapes where R‐type species had a low number of large patches and S‐species had a large number of small patches, the number of R‐patches increased and their size decreased, while the number of S‐patches decreased. In these cases, the final cover of the two types changed little from the initial cover. Landscapes with a large number of small R‐patches interspersed with S‐patches had a decrease in the number of R‐patches, an increase in the number of S‐patches and a decrease in the size of S‐patches. In these landscapes, final cover was significantly changed, increasing in R‐type and decreasing in S‐type species. These results suggest that low spatial autocorrelation (low aggregation) favours R‐type species. Implications for land management are also discussed.     

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.     

Advances in spatial,individual‐based modelling of forest dynamics

Many individual‐based models of forest dynamics lack spatial complexity. Although, in certain cases, spatially simple models may not be substantially inferior to spatially complex models, advances in vegetation science indicate potential weaknesses, particularly the lack of consideration of propagule availability in horizontal space, and varying patch (or canopy gap) dimensions. Models with vertical and horizontal spatial complexity can address these issues, but, thus far, evidence that they outperform patch (or gap) models is limited. Comparison of projections from models that differ only in their spatial complexity is needed to address the effects of propagule availability in space, spatial pattern of canopy tree mortality, and spatial resolution.     

Dissipative particle dynamics simulation of a gold nanoparticle system

Dissipative particle dynamics (DPD) was carried out to study systems containing gold atoms, organic ether (oligohydroquinonyl ether terminated with a thiol group) and organic solvents. The components in the simulated system are very different in size and chemical nature. Our simulation showed that the reproduction of the macroscopic experimental phase separation, properly dividing the polymeric molecule into beads, selecting the size of gold bead, and choosing the appropriate interaction parameters between beads are crucial. In addition, the solvent effect was the dominant factor for the formation of spherical aggregates of Au atoms and organic ether molecules. We report the interaction strengths between the solvent and gold clusters. Our work has demonstrated that DPD methods can be applied to the study of complex meso-scale systems.     

Probing the effects of the well-mixed assumption on viral infection dynamics

Viral kinetics have been extensively studied in the past through the use of spatially well-mixed ordinary differential equations describing the time evolution of the diseased state. However, emerging spatial structures such as localized populations of dead cells might adversely affect the spread of infection, similar to the manner in which a counter-fire can stop a forest fire from spreading. In a previous publication [Beauchemin, C., Samuel, J., Tuszynski, J., 2005. A simple cellular automaton model for influenza A viral infections. J. Theor. Biol. 232(2), 223-234], a simple two-dimensional cellular automaton model was introduced and shown to be accurate enough to model an uncomplicated infection with influenza A. Here, this model is used to investigate the effects of relaxing the well-mixed assumption. Particularly, the effects of the initial distribution of infected cells, the regeneration rule for dead epithelial cells, and the proliferation rule for immune cells are explored and shown to have an important impact on the development and outcome of the viral infection in our model.     

植物群落动态的模型分析

植物群落的动态是植物群落学的中心问题之一,包括更新、波动、演替、进化等主要内容。空间格局对种群和群落的动态起着至关重要的作用,种群空间格局和群落空间结构是群落中各种过程相互作用的产物。模型是描述群落动态、认识植物群落组建和维持机理的有效工具。本文阐述和比较了描述群落动态的四种具有代表性的经验模型,即镶嵌循环模型、随意游走模型、同资源种团比例模型、空间抢先占有模型及其机理。四种经验模型的空间性及缺陷分别是:(1)“镶嵌循环模型”考虑到了相邻斑块之间的植被空间结合在群落动态中的作用,而另外三种模型没有考虑到这一点;(2)在一定程度上,四种植物群落动态模型对各自针对的植物群落可能是适合的,但要作为描述群落动态发展的一般性模型还需要不断完善和发展;因为四种模型均没有考虑到自然干扰和人类干扰对植物群落动态的影响。作者对将来植物群落动态的研究及实践意义做出以下展望:(1)在不同空间尺度上,更加有效地评价控制群落动态变化的各种过程的相对重要性,并进一步将它们之间的复杂相互作用整合到群落动态模型中;(2)充分认识植物群落中存在的各种自然环境条件和生物群体的结构配置对植物群落动态发展的重要性;(3)重视植物群落动态发展中自然干扰过程和人类干扰过程的整合以     

Computational modeling of combined cell population dynamics and oxygen transport in engineered tissue subject to interstitial perfusion

This work presents a computational model of tissue growth under interstitial perfusion inside a tissue engineering bioreactor. The model accounts both for the cell population dynamics, using a model based on cellular automata, and for the hydrodynamic microenvironment imposed by the bioreactor, using a model based on the Lattice–Boltzmann equation and the convection-diffusion equation. The conditions of static culture versus perfused culture were compared, by including the population dynamics along with oxygen diffusion, convective transport and consumption. The model is able to deal with arbitrary complex geometries of the spatial domain; in the present work, the domain modeled was the void space of a porous scaffold for tissue-engineered cartilage. The cell population dynamics algorithm provided results which qualitatively resembled population dynamics patterns observed in experimental studies, and these results were in good quantitative agreement with previous computational studies. Simulation of oxygen transport and consumption showed the fundamental contribution of convective transport in maintaining a high level of oxygen concentration in the whole spatial domain of the scaffold. The model was designed with the aim to be computationally efficient and easily expandable, i.e. to allow straightforward implementability of further models of complex biological phenomena of increasing scientific interest in tissue engineering, such as chemotaxis, extracellular matrix deposition and effect of mechanical stimulation.     

Spatial pattern analysis in forest dynamics: deviation from power law and direction of regeneration waves

Grid-based models have been used to understand spatial heterogeneity of the vegetation height in forests and to analyze spatio-temporal dynamics of the forest regeneration process. In this report, we present two methods of identifying lattice models when spatio-temporal data are given. The first method detects directionality of regeneration waves based on the timing of local disturbance events. The second evaluates the forest pattern by recording the fraction of high and low vegetation areas at multiple spatial scales. We illustrate these methods by applying them to patterns generated using three simple stochastic lattice models: (1) two-state model, distinguishing sites with high and low vegetation, (2) three-state model, in which sites can be in an additional disturbed state, and (3) Shimagare model, which considers a continuous range of states. The combination of the two methods provides efficient means of distinguishing the models. The first method has a more direct ecological meaning, while the second is useful when forest data are limited in time.     

A spatial model of forest dynamics

Effects of spatial processes on temperate deciduous forest structure and dynamics were investigated with a spatial simulator derived from a forest gap model. The multi-species neighborhood model accounted for competitive interactions and endogenous disturbance in the form of small canopy gaps. Simulated and actual spatial pattern of old-growth stands were compared. The 400 yr simulations produced a pattern scale (0.07–0.2 ha patches) similar to that of an actual stand; simulated pattern intensity was greater than actual intensity, however. Distances to nearest neighbor were somewhat similar for trees in the simulated and actual stands; yet the frequency distributions of distance to nearest neighbor values differed substantially. The simulated stand patterns were generally less random than the actual patterns. Spatial pattern changed markedly during the course of simulated succession. Pattern approached a random dispersion in early succession. Intensity peaked at mid-succession (ca. 150 yr) with a hyperdispersed overstory and a strongly clumped understory. Pattern intensity diminished in late succession as a mixed size structure developed. Old-growth patch size was greater than the neighborhood (or gap) size, suggesting the gap-sized areas do not behave independently.     

Simulation of forest carbon dynamics based on a dry-matter production model

Effects of dry season upon a tropical rainforest ecosystem are analysed by employing the microcomputer model developed in a previous paper (Oikawa, 1985). Surplus production (P _s) illustrated in a three-dimensional figure linearly decreases with the length of dry season (λ). Simulation experiments demonstrate that all the three strata constituting the tropical rainforest ecosystem model are able to exist stably and perpetually unless λ exceeds four months. However, it is suggested that the dry season lasting five months brings about a transition from a tropical rainforest to a subtropical deciduous forest, because the upper story fails in a stable and perpetual occurrence at λ =5 months. Though the total living phytomass gradually decreases with prolonged dry season within four months, supplementary increase of living phytomass is observed in the middle story. A considerable amount of soil organic dead accumulated is also observed with increasing dry season, so that fairly constant ecosystem biomass including soil organic dead is predicted independent of the length of dry season so long as it is less than five months.     

Spatial modelling of succession-disturbance dynamics in forest ecosystems: Concepts and examples

Over the last few decades it has become increasingly obvious that disturbance, whether natural or anthropogenic in origin, is ubiquitous in ecosystems. Disturbance-related processes are now considered to be important determinants of the composition, structure and function of ecological systems. However, because disturbance and succession processes occur across a wide range of spatio-temporal scales their empirical investigation is difficult. To counter these difficulties much use has been made of spatial modelling to explore the response of ecological systems to disturbance(s) occurring at spatial scales from the individual to the landscape and above, and temporal scales from minutes to centuries. Here we consider such models by contrasting two alternative motivations for their development and use: prediction and exploration, with a focus on forested ecosystems. We consider the two approaches to be complementary rather than competing. Predictive modelling aims to combine knowledge (understanding and data) with the goal of predicting system dynamics; conversely, exploratory models focus on developing understanding in systems where uncertainty is high. Examples of exploratory modelling include model-based explorations of generic issues of criticality in ecological systems, whereas predictive models tend to be more heavily data-driven (e.g. species distribution models). By considering predictive and exploratory modelling alongside each other, we aim to illustrate the range of methods used to model succession and disturbance dynamics and the challenges involved in the model-building and evaluation processes in this arena.     

珍稀濒危植物桫椤种群结构与动态研究

采用空间序列代替时间变化的方法,对7个地段中的桫椤种群大小结构进行分析,运用C、K、m^*、m^*/m和Iδ5种聚集度指标测定不同地段内桫椤种群的空间分布格局及其动态,并根据植株个体点位图,考察桫椤种群在不同取样尺度上的空间分布格局.结果表明,不同地段中的桫椤种群结构存在增长型、稳定型、成熟型和衰退型4种类型;不同地段中的桫椤种群空间分布格局有所差异,表现为集群分布或随机分布;在桫椤种群生长过程中,分布格局从集群型向随机型转变;种群扩散型指数Iδ随取样尺度的增大而减小.     

Consumer pressure, seed versus safe-site limitation, and plant population dynamics

Plants often suffer reductions in fecundity due to insect herbivory. Whether this loss of seeds has population-level consequences is much debated and often unknown. For many plants, particularly those with long-lived seedbanks, it is frequently asserted that herbivores have minimal impacts on plant abundance because safe-site availability rather than absolute seed number determines the magnitude of future plant recruitment and hence population abundance. However, empirical tests of this assertion are generally lacking and the interplay between herbivory, spatio-temporal variability in seed- or safe-site-limited recruitment, and seedbank dynamics is likely to be complex. Here we use a stochastic simulation model to explore how changes in the spatial and temporal frequency of seed-limited recruitment, the strength of density-dependent seedling survival, and longevity of seeds in the soil influence the population response to herbivory. Model output reveals several surprising results. First, given a seedbank, herbivores can have substantial effects on mean population abundance even if recruitment is primarily safe-site-limited in either time or space. Second, increasing seedbank longevity increases the population effects of herbivory, because annual reductions in seed input due to herbivory are accumulated in the seedbank. Third, population impacts of herbivory are robust even in the face of moderately strong density-dependent seedling mortality. These results imply that the conditions under which herbivores influence plant population dynamics may be more widespread than heretofore expected. Experiments are now needed to test these predictions. Received: 3 November 1999 / Accepted: 15 February 2000     

松嫩平原淡黄虻种群日吸血活动的时空动态研究

淡黄虻是松嫩平原虻虫中的优势种.对其吸血活动的观测发现,其种群数量与温度、湿度呈显著相关;吸血高峰时间出现在14:00左右,种群的聚集度和扩散度也在此时达到最大;种群不仅呈聚集分布,而且呈聚集型扩散;种群聚集分布以疏松的小集团形式存在,其个体呈随机分布.     

Simulation of the effects of deer browsing on forest dynamics

I introduced forest dynamics to a deterministic herbivore-vegetation model to examine the effects of vertically stratified forest structure on the dynamics of the herbivore–vegetation system, the resilience and stable states of vegetation, and the interactions between deer populations and vegetation. I constructed a model based on data from field studies performed in Hokkaido, northern Japan. Three phases of state were identified for a given deer density: (1) understory vegetation is maintained with a equilibrium proportion of canopy gaps in the absence of deer; (2) if the equilibrium proportion of canopy gaps is greater than that in the equilibrium state in the absence of deer, the understory vegetation can be maintained; and (3) the understory vegetation cannot be maintained. At the boundary between phases 2 and 3, the herbivore population level had discontinuous effects on vegetation. When the deer density was held below the threshold, forest vegetation had resilience to recover to the equilibrium stable state at the given deer density, although the equilibrium canopy gap ratio and vegetation biomass differed with deer density. However, the forest vegetation–herbivore system could not be maintained in a stable state without artificial deer population management if food limitation was the only mechanism to keep the deer population at low levels. The deer population must be kept below the boundary between phases 1 and 2 to maintain the forest regeneration processes. The level cannot be determined by observing the deer population; careful observation of forest regeneration processes is required.     

松嫩平原淡黄虻群日吸血活动的空动研究

淡黄虻是松嫩平原虻虫中的优势种,对其吸血活动的观测发现,其种群数量与温度,湿度呈显著相关;吸血高峰时间出现在14：00左右,种群的聚集度和扩散度也在此时达到最大,种群不仅呈聚集分布,而且呈聚集型扩散,种群聚集分布以疏松的小集团形式存在,其个体呈随机分布。     

A theory of the spatial and temporal dynamics of plant communities

An individual-based model of plant competition for light that uses a definition of plant functional types based on adaptations for the simultaneous use of water and light can reproduce the fundamental spatial and temporal patterns of plant communities. This model shows that succession and zonation result from the same basic processes. Succession is interpreted as a temporal shift in species dominance, primarily in response to autogenic changes in light availability. Zonation is interpreted as a spatial shift in species dominance, primarily in response to the effect of allogenic changes in water availability on the dynamics of competition for light. Patterns of succession at different points along a moisture gradient can be used to examine changes in the ecological roles of various functional types, as well as to address questions of shifts in patterns of resource use through time.Our model is based on the cost-benefit concept that plant adaptations for the simultaneous use of two or more resources are limited by physiological and life history constraints. Three general sets of adaptive constraints produce inverse correlations in the ability of plants to efficiently use (1) light at both high and low availability, (2) water at both high and low availability, and (3) both water and light at low availabilities.The results of this type of individual-based model can be aggregated to examine phenomena at several levels of system organization (i.e., subdisciplines of ecology), including (1) plant growth responses over a range of environmental conditions, (2) population dynamics and size structure, (3) experimental and field observations on the distribution of species across environmental gradients, (4) studies of successional pattern, (5) plant physiognomy and community structure across environmental gradients, and (6) nutrient cycling.     

Fractal geometry and root system structures of heterogeneous plant communities

Above-ground plant growth is widely known in terms of structural diversity. Likewise, the below-ground growth presents a mosaic of heterogeneous structures of differing complexity. In this study, root system structures of heterogeneous plant communities were recorded as integral systems by using the trench profile method. Fractal dimensions of the root images were calculated from image files by the box-counting method. This method allows the structural complexity of such associations to be compared between plant communities, with regard to their potentials for soil resource acquisition and utilization. Distinct and partly significant differences are found (fractal dimension between 1.46±0.09 and 1.71±0.05) in the below-ground structural complexity of plant communities, belonging to different biotope types. The size of the heterogeneous plant community to be examined has an crucial influence on the fractal dimension of the root system structures. The structural heterogeneity becomes particularly evident (fractal dimensions between 1.32 and 1.77) when analysing many small units of a complex root system association. In larger plant communities, a broad variety of below-ground structures is recorded in its entirety, integrating the specific features of single sub-structures. In that way, extreme fractal dimensions are lost and the diversity decreases. Therefore, the analysis of larger units of root system associations provides a general knowledge of the complexity of root system structures for heterogeneous plant communities.     

亚热带山地森林珍稀植物群落的结构与动态

为探讨亚热带山地常绿落叶阔叶混交林中珍稀植物群落的性质和特征,调查了长江三峡大老岭地区６类珍稀树种占优势的森林群落,分析它们植物区系与物种多样性、群落结构、种间关系及其空间格局、珍稀树种的种群结构。结果表明：①这些珍稀植物群落的区系构成中,东亚、东亚－北美和中国特有成份的比例更高;②群落内物种非常丰富,乔木层和草本层多样性尤高;③各珍稀植物群落的生活型谱相似,却与亚热带常绿阔叶林和退化次生的混交林