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Explaining woody invasions in riparian systems with agent-based simulations: Implications for conservation management
Abstract
Riparian forests are among the most vulnerable ecosystems to biological invasions. Effective management
strategies require a nuanced understanding of both species invasiveness and environmental conditions, but
analysing these interactions is complex. We designed an Agent-Based Model (ABM) to simulate the spatio-
temporal spread of Gleditsia triacanthos in the riparian forest of the Esteros de Farrapos e Islas del Río
Uruguay National Park; Uruguay. This is a parsimonious and generalisable model that incorporates distinctive
features of woody invasions in riparian systems—such as environmental heterogeneity and flooding regimes—as
well as common characteristics of woody invaders. Management simulations suggest that if control measures are
maintained over time, the abundance of the invader will remain constant at low levels; thus, the invasion can be
controlled, even if not completely eradicated. The results also indicate that without control, the invasion process
would spread even into unsuitable areas. The SWIRS model facilitates the evaluation of targeted management
strategies, ultimately providing a framework to enhance biodiversity conservation efforts in riparian ecosystems.
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Grazing is one of the principal drivers triggering woody invasion in grasslands and savannas worldwide, but evidence is opposing. Cattle can promote or limit woody establishment by direct and indirect mechanisms, like consumption, trampling or relaxing grass competition. Despite its increasing relevance, it is unknown if intensively managed cattle grazing can curb woody establishment. We evaluated the effect of cattle grazing with short cycles of high stocking rate and resting periods on Gleditsia triacanthos establishment, an invasive tree in grasslands. By two field experiments in pastures, we manipulated cattle grazing (with or without) and: Exp. (1) initial age-related size of G. triacanthos (small plants: 3 mo.; or large plants: 9 mo.), and Exp. (2) neighbouring vegetation (clipped or intact). Overall, grazing had a negative effect due to tree consumption and a positive effect by relaxing grass competition on tree establishment, but the first effect outweighed the second. Grazing reduced by 25% G. triacanthos survival and by 40% its biomass. Consumption was independent of plant size and neighbouring vegetation, but large plants had more resprouting capacity than small plants. Regardless of grazing, small plants had higher mortality than large plants. Intact neighbouring vegetation did not modify survival but decreased tree biomass which may affect middle or long-term tolerance and survival. Intensively managed rotational grazing with resting periods for pasture recovery may limit woody establishment, but this management strategy should be applied immediately after tree germination. The increasing intensification of production systems requires to evaluate the impact of management on invasion processes.
The high ecological impacts of many invasive alien trees have been well documented. However, to date, we lacked synthesis of their economic impacts, hampering management actions. Here, we summarize the cost records of invasive trees to (I) identify invasive trees with cost information and their geographic locations, (II) investigate the types of costs recorded and sectors impacted by invasive trees and (III) analyze the relationships between categories of uses of invasive trees and the invasion costs attributed to these uses. We found reliable cost records only for 72 invasive trees, accumulating a reported total cost of 3.5 billion). Close attention to the ornamental sector is important for reducing the economic impact of invasive trees, since most invasive trees with cost records were introduced for that use. Despite massive reported costs of invasive trees, there remain large knowledge gaps on most invasive trees, sectors, and geographic scales, indicating that the real cost is severely underestimated. This highlights the need for further concerted and widely-distributed research efforts regarding the economic impact of invasive trees.
SOSA, B.; FERNÁNDEZ, G.; ACHKAR, M. Relación entre la propagación del proceso invasivo de gleditsia triacanthos y la deposición de arena en bosques del Río Uruguay. GeoUECE (online), v. 09, n. 16, p. 110-122, 2020.
Fluvial riparian vegetation (RV) links fluvial and terrestrial ecosystems. It is under significant pressure from anthropogenic activities, and, therefore, the management and restoration of RV are increasingly important worldwide. RV has been investigated from different perspectives, so knowledge on its structure and function is widely distributed. An important step forward is to convert existing knowledge into an overview easily accessible-for example, for use in decision-making and management. We aim to provide an overview of ecosystem services provided by RV by adopting a structured approach to identify the ecosystem services, describe their characteristics, and rank the importance of each service. We evaluate each service within four main riparian vegetation types adopting a global perspective to derive a broad concept. Subsequently, we introduce a guided framework for use in RV management based on our structured approach. We also identify knowledge gaps and evaluate the opportunities an ecosystem service approach offers to RV management.
The Overview, Design concepts and Details (ODD) protocol for describing Individual- and Agent-Based Models (ABMs) is now widely accepted and used to document such models in journal articles. As a standard- ized document for providing a consistent, logical and readable account of the structure and dynamics of ABMs, some research groups also find it useful as a workflow for model design. Even so, there are still limitations to ODD that obstruct its more widespread adoption. Such limitations are discussed and addressed in this paper: the limited availability of guidance on how to use ODD; the length of ODD documents; limitations of ODD for highly complex models; lack of sufficient details of many ODDs to enable reimplementation without access to the model code; and the lack of provision for sections in the document structure covering model design ratio- nale, the model’s underlying narrative, and the means by which the model’s fitness for purpose is evaluated. We document the steps we have taken to provide better guidance on: structuring complex ODDs and an ODD summary for inclusion in a journal article (with full details in supplementary material; Table 1); using ODD to point readers to relevant sections of the model code; update the document structure to include sections on model rationale and evaluation. We also further advocate the need for standard descriptions of simulation ex- periments and argue that ODD can in principle be used for any type of simulation model. Thereby ODD would provide a lingua franca for simulation modelling.
Flood and fire can harm plants but they can have survival strategies, such as the seed bank. We aimed to determine the influence of fire and flood on the richness, abundance and diversity of the seed bank. Sampling was carried out in October/2013, year of prolonged drought, and October/2014, year of a heavy flood, in ten areas along the Paraguay River. The areas were selected in satellite images, five with old burn (2010, three years before sampling) and five with recent burn (2013, three months before sampling). In each area, we marked a 20 m long transect with ten 20 × 20 cm quadrats where we collected 5 cm deep topsoil samples, five with and five without litter. Seed bank richness and abundance were determined by seedling emergence. Old burn areas presented greater abundance than recent burn. The drier year presented greater abundance, richness and diversity than flood year. Removal of litter reduced the richness only in the wetter year. There was no difference in richness in the drier year. The removal of the litter did not affect the abundance and diversity. Interaction between fire and flood did not affect richness, abundance and diversity of the seed bank.
Systematic conservation planning is intended to inform spatially explicit decision making. Doing so requires that it be integrated into complex regulatory and governance processes, and there are limited instances where this has been achieved effectively. South Africa is a global leader in the application of conservation plans, the outputs of which are widely used for spatial planning and decision making in many spheres of government. We aimed to determine how conservation planning in the country progressed from theory to implementation, and to identify practical actions that enabled this transition, by assessing temporal trends in the characteristics of conservation plans (1990–2017, n = 94). Since 2010 conservation planning has entered an operational period characterized by government leadership of plans, administrative rather than ecological planning domains, decreasing size of planning units, increasing emphasis on end‐user products, and scheduled revision of plans. Key actions that enabled this progression include transitioning leadership of plans from scientists to practitioners, building capacity within implementing agencies, creating opportunities to integrate plans in legislative processes, establishing a strong community of practice, adopting implementation‐focused methods, and balancing standardization with innovation. Learning from this model will allow other countries, particularly those with a similar megadiverse, developing context, to operationalize conservation planning into spatial planning and decision making.
In recent years, belowground plant ecology has experienced a booming interest. This has resulted in major advances towards a greater understanding of belowground plant and ecosystem functioning focused on fine roots, mycorrhizal associations and nutrient acquisition.
Despite this, other important functions (e.g., on‐spot persistence, space occupancy, resprouting after biomass removal) exerted by different belowground plant organs (e.g., roots, rhizomes, bulbs) remain largely unexplored.
Here, we propose a framework providing a comprehensive perspective on the entire set of belowground plant organs and functions. We suggest a compartment‐based approach. We identify two major belowground compartments, that is, acquisitive and nonacquisitive, associated with biomass allocation into these functions. Also, we recommend the nonacquisitive compartment to be divided into structural (e.g., functional roles carried out by rhizomes, such as sharing of resources, space occupancy) and nonstructural (e.g., functional roles exerted by carbohydrates reserve affecting resprouting ability, protection against climate adversity) subcompartments. We discuss methodological challenges—and their possible solutions—posed by changes in biomass allocation across growth forms and ontogenetic stages, and in relation to biomass partitioning and turnover.
We urge the implementation of methods and approaches considering all the belowground plant compartments. This way, we would make sure that key, yet less‐studied functions would be incorporated into the belowground plant ecology research agenda. The framework has potential to advance the understanding of belowground plant and ecosystem functioning by considering relations and trade‐offs between different plant functions and organs. At last, we identify four major areas where using the proposed compartment‐based approach would be particularly important, namely (a) biomass scaling, (b) clonality‐resource acquisition relations, (c) linkages between resprouting and changing environmental conditions and (d) carbon sequestration.
A plain language summary is available for this article.
Riparian zones are among the natural habitats more prone to be invaded by exotic plants. In this study we review the causes and consequences of these invasions on fluvial and riparian ecosystems, as well as the effects described for the Iberian Peninsula so far. Riparian zones receive a high propagule pressure of exotic plants, their abiotic conditions are benign for plant life, and their biotic resistance from native vegetation is released by natural (floods) and anthropic (hydrological changes) disturbances. The convergence of these factors explains the high invasion rate of riparian zones. An eventual replacement of native by nonnative vegetation might alter the fire regime, the depth of the water table, nutrient cycles and organic matter processing, soil properties, communities of detritivore invertebrates and vertebrates dwelling in rivers and riparian zones. In the Iberian Peninsula we found that the effects of non-native riparian plants were more often negative (e.g. alteration of the structure and activity of microbial communities) than neutral (e.g. similar decomposition rate between native and non-native litter), and rarely positive for primary production (e.g. faster litter decomposition). Our review also highlights a strong bias in the selection of target non-native species (mostly Eucalyptus globulus, Ailanthus altissima and Robinia pseudoacacia) and in the studied effects (mostly litter decomposition and structure and activity of microbial communities). Therefore, a reliable evaluation of the effects of non-native plants in Iberian riparian ecosystems requires extending the number of studied species and of response variables.
Gleditsia triacanthos L. is a woody species native to North America that has invaded Uruguay, Spain, Australia, South Africa and several countries of Central and Eastern Europe. In Argentina, it has become one of the most important invasive woody species and has a high potential to continue spreading. In this study, we review different aspects of G. triacanthos invasion in Argentina that include descriptions of invaded ecoregions and environments, intrinsic characteristics of the species, invasion dynamics and impacts. In addition, we discuss mechanisms that potentially explain its success, control strategies and natural barriers to its invasion. We reviewed a total of 91 articles and book chapters, of which 62 were developed in Argentina. Studies reported that the invasion of G. triacanthos in different ecoregions was favoured by intrinsic characteristics of the species, together with the interaction with cattle and disturbances, which cause negative impacts on flora, fauna and ecosystem processes. Disturbances were proposed as the main mechanism to explain this species’ invasion, but other hypotheses such as the release of natural enemies and/or propagule pressure might also be important. Further studies are required, mainly on the impacts on ecosystem processes and on the control, production of organic compounds and mutualistic interactions (with nitrogen-fixing bacteria and mycorrhizal fungi).
Ecosystem and community ecology have evolved along different pathways, with little overlap. However, to meet societal demands for predicting changes in ecosystem services, the functional and structural view dominating these two branches of ecology, respectively, must be integrated. Biodiversity–ecosystem function research has addressed this integration for two decades, but full integration that makes predictions relevant to practical problems is still lacking. We argue that full integration requires going, in both branches, deeper by taking into account individual organisms and the evolutionary and physico-chemical principles that drive their behavior. Individual-based models are a major tool for this integration. They have matured by using individual-level mechanism to replace the demographic thinking which dominates classical theoretical ecology. Existing individual-based ecosystem models already have proven useful both for theory and application. Still, next-generation individual-based models will increasingly use standardized and re-usable submodels to represent behaviors and mechanisms such as growth, uptake of nutrients, foraging, and home range behavior. The strategy of pattern-oriented modeling then helps make such ecosystem models structurally realistic by developing theory for individual behaviors just detailed enough to reproduce and explain patterns observed at the system level. Next-generation ecosystem scientists should include the individual-based approach in their toolkit and focus on addressing real systems because theory development and solving applied problems go hand-in-hand in individual-based ecology.
Clonal plants are widespread throughout the plant kingdom and dominate in diverse habitats. Spatiotemporal heterogeneity of environment is pervasive at multiple scales, even at scales relevant to individual plants. Clonal integration refers to resource translocation and information communication among the ramets of clonal plants. Due to clonal integration, clonal plant species possess a series of peculiar attributes: plasticity in response to local and non-local conditions, labor division with organ specialization for acquiring locally abundant resources, foraging behavior by selective placement of ramets in resource-rich microhabitats, and avoidance of intraclonal competition. Clonal integration has very profound ecological consequences for clonal plants. It allows them to efficiently cope with environmental heterogeneity, by alleviating local resource shortages, buffering environmental stresses and disturbances, influencing competitive ability, increasing invasiveness, and altering species composition and invasibility at the community level. In this paper, we present a comprehensive review of research on the ecological consequences of plant clonal integration based on a large body of literature. We also attempt to propose perspectives for future research.
Over the last two decades spatial point pattern analysis (SPPA) has become increasingly popular in ecological research. To direct future work in this area we review studies using SPPA techniques in ecology and related disciplines. We first summarize the key elements of SPPA in ecology (i.e., data types, summary statistics and their estimation, null models, comparison of data and models, and consideration of heterogeneity); second, we review how ecologists have used these key elements; and finally, we identify practical difficulties that are still commonly encountered and point to new methods that allow current key questions in ecology to be effectively addressed.
Our review of 308 articles published over the period 1992-2012 reveals that a standard canon of SPPA techniques in ecology has been largely identified and that most of the earlier technical issues that occupied ecologists, such as edge correction, have been solved. However, the majority of studies underused the methodological potential offered by modern SPPA. More advanced techniques of SPPA offer the potential to address a variety of highly relevant ecological questions. For example, inhomogeneous summary statistics can quantify the impact of heterogeneous environments, mark correlation function can include trait and phylogenetic information in the analysis of multivariate spatial patterns, and more refined point process models can be used to realistically characterize the structure of a wide range of patterns. Additionally, recent advances in fitting spatially-explicit simulation models of community dynamics to point pattern summary statistics hold the promise for solving the longstanding problem of linking pattern to process. All these newer developments allow ecologists to keep up with the increasing availability of spatial data sets provided by newer technologies, which allow point patterns and environmental variables to be mapped over large spatial extents at increasingly higher image resolutions.
Las especies invasoras constituyen una amenaza para la conservación de la biodiversidad, por lo que el desarrollo de programas para su control requiere de una evaluación cuidadosa. La invasión de Gleditsia triacanthos es una de las principales amenazas en el Parque Nacional Esteros de Farrapos e Islas del Río Uruguay. Evaluamos la efectividad del control de esta leñosa mediante la aplicación de dos herbicidas (Glifotec y Tordón) en perforaciones practicadas en el tronco, y su impacto sobre la regeneración del componente leñoso de la vegetación. Luego de dos años de la aplicación, la mortalidad media por parcela para los tratamientos con Tordón y Glifotec fue 66% y 60%, respectivamente. Sin embargo, la regeneración de otras leñosas se encuentra comprometida ya que G. triacanthos representó el 92% del total de los individuos. El método de control no afectó la abundancia ni el número de géneros de las plántulas del resto de las leñosas. El desarrollo de un programa de control químico de G. triacanthos resulta viable, pero su restauración requerirá medidas de gestión activa.
This paper presents and illustrates FlowLogo, an interactive modelling environment for developing coupled agent-based groundwater models (GW-ABMs). It allows users to simulate complex socio-environmental couplings in groundwater systems, and to explore how desirable patterns of groundwater and social development can emerge from agent behaviours and interactions. GW-ABMs can be developed using a single piece of software, addressing common issues around data transfer and model analyses that arise when linking ABMs to existing groundwater codes. FlowLogo is based on a 2D finite-difference solution of the governing groundwater flow equations and a set of procedures to represent the most common types of stresses and boundary conditions of regional aquifer flow. The platform is illustrated using a synthetic example of an expanding agricultural region that depends on groundwater for irrigation. The implementation and analysis of scenarios from this example highlight the possibility to: (i) deploy agents at multiple scales of decision-making (farmers, waterworks, institutions), (ii) model feedbacks between agent behaviours and groundwater dynamics, and (iii) perform sensitivity and multi-realisation analyses on social and physical factors. The FlowLogo interface allows interactively changing parameters using ‘tuneable’ dials, which can adjust agent decisions and policy levers during simulations. This flexibility allows for live interaction with audiences (role-plays), in participatory workshops, public meetings, and as part of learning activities in classrooms. FlowLogo’s interactive features and ease of use aim to facilitate the wider dissemination and independent validation of GW-ABMs.
Larrea tridentata is a dominant desert shrub throughout the warm deserts of North America. We studied a Larrea population we believed to be in the process of invading adjacent grasslands. To determine whether our hypothesis was correct we developed a set of predictions about characteristics of invading populations and examined the relationship between Larrea density and demographic variation. For an invading Larrea population we predicted that: 1) individuals in low density areas would exhibit attributes indicating higher intrinsic rates of increase-individuals would have greater fruit production, 2) individuals in high density areas would exhibit the effects of negative density-dependence-plants would have canopies in poor condition (with at least 20% dead branches) and be small in size, and 3) low density areas would be below carrying capacity-plots would have a lower total Larrea biomass than high density areas. We measured height, average width, canopy condition, and fruit set for 2,000 plants sampled from 40 plots varying in density. We also estimated the total Larrea biomass for each plot. We collected data on two measures of microenvironmental variation, cover of the common grassland species, and cover of gravel on the soil surface. All plant characters were significantly correlated with Larrea density and both measures of microenvironmental variation were significantly correlated with density. In contrast, results for total Larrea biomass did not conform to our predition. Although the relationship between Larrea density and the plant characters supported our predictions for an invading Larrea population, results for total Larrea biomass did not. An alternative explanation that the pattern of demographic variation within the study population was determined by microenvironmental variation was more strongly supported.
Se estudió la composición florística y distribución de los bosques ribereños de dos sistemas hidrográficos en una región subtropical andina. Utilizando técnicas uni y multivariadas, se probó la hipótesis que existe un bosque ribereño diferenciable, compuesto por vegetación nativa típica de la provincia fitogeográfica de Yungas, y que la distribución de la vegetación varió significativamente con las características geomorfológicas. Transectos paralelos a lo largo de los cursos de agua se utilizaron para recolectar los datos de presencia-ausencia de vegetación en once sitios. El Análisis de Correspondencia definió un grupo de especies ribereñas común para el área estudiada (Solanum riparium, Phenax laevigatus, Tipuana tipu, Cestrum parqui, Carica quercifolia, Acacia macracantha, Celtis iguanaea, Juglans australis, Pisoniella arborescens, Baccharis salicifolia, Cinnamomum porphyrium y Eugenia uniflora) e identificó dos sitios de referencia. La distribución de la vegetación ribereña varió significativamente con las características geomorfológicas a lo largo de los sitios estudiados. Los hábitats ribereños estuvieron compuestos por especies nativas y exóticas. Una flora ribereña distintiva, diferente en estructura y función a la vegetación terrestre adyacente, no pudo ser identificada. Especies ribereñas fueron similares a los estratos terrestres adyacentes. Estas especies no estarían limitadas por la proximidad al río. Impactos antropogénicos fueron factores importantes que regulan la introducción y el incremento de la vegetación exótica. La falta de regulación de algunas actividades en la zona podría causar problemas graves en la integridad de este ecosistema.
For this study, we chose eight alien ligneous plant species: Elaeagnus angustifolia; Acer negundo; Ailanthus altissima; Amorpha fruticosa; Robinia pseudoacacia; Lycium barbarum; Fraxinus pennsylvanica and Gleditsia triacanthos in order to identify their way of adaptation in Danube Delta. The swot attempts a comprehensive analysis of 375 G.P.S. points correlated with different maps. The results reveal that there are not significant differences among species with concern to pH, soil, salinity, alcalinity. However, there are certain differences in the frequency and the spreading of these species in Danube Delta, namely species such as Amorpha fruticosa, Ailanthus altissima and Robinia pseudoacacia are predominantly found in the fluvial delta and, on the other hand, Lycium barbarum and Elaeagnus angustifolia are frequently found in the fluvial-maritime delta.
Attention is drawn to the proliferation of Gleditsia triacanthos L. (honey locust tree) at several locations in south east Queensland and New South Wales. Although promoted and planted as an attractive garden ornamental and as a high protein fodder tree, this thorny, exotic tree has the potential to smother pastures and replace native riparian vegetation. G. triacanthos appears well suited to sub-tropical and temperate climates and could become a serious pest on alluvial soils throughout the south-eastern states of Australia. In Queensland, G. triacanthos was declared noxious in March 1993 and an eradication program has been implemented. Where feasible, early control of this species in other States is recommended.
The northern region of Uruguay undergoes land transformation processes, and degradation of native forest. The existing information on the structure and floristic composition of the forests in that region, as a basis for studies about dynamics, is scarce. Species distribution, floristic composition (wooden species) and forest management, was assessed in the Rivera Region, Uruguay. The study area (33 000 ha) was delimited by 2 simultaneous and complementary approaches, the first one natural (basin) and the other one socioeconomic (reference city). Data from floristic inventories and surveys of producers were captured into a geographic information system. Forests were grouped according to their water gradient associated to contour lines. Native forest area occupies 13.9% of the total study area in Rivera, which means 3 times the national average. Sixty woody species were found, with the Anacardiaceae (26.5%) and Euphorbiaceae (14.7%) as the most frequent families. Anacardiaceae are more common in subxerophytic woods, and Euphorbiaceae in more hydrophilic forests. All forests are vulnerable due to the weak integration between forestry and farming, resulting in no forest management. Non native species constitute a source of degradation of native forests, specifically in areas close to the city.
Recent data on biological invasion show that range expansion is driven by various modes of dispersal such as neighborhood diffusion and long-distance dispersal that occur side by side within a species. In such a stratified dispersal process, the initial range expansion mainly occurs by neighborhood diffusion. However, as the range of the founder population expands, new colonies created by long-distance migrants increase in number to cause an accelerating range expansion in the later phase. We classify several well-documented examples of geographical expansions into three major types depending on the nonlinearity of the range-versus-time curve. To examine how long-distance dispersal produces accelerating range expansion, we construct a stratified diffusion model, which describes the dynamics of the size distribution of colonies created by long-distance migrants. The model consists of a von Foerster equation combined with a Skellam model. Analyzing the model provides an estimate of range expansion in terms of the rate of expansion due to neighborhood diffusion, the leap distance, and the colonization rate of long-distance migrants. The results explain various types of nonlinear range expansion observed in biological invasions.
Environmental degradation is often defined as a public goods problem, emerging when property rights are not clearly defined and costs are externalized to other parties. Proposing corrective regulation that enforces technological fixes or market-based approaches is often met with political resistance and doubts about its effectiveness. This is partly due to the complexity of interacting physical and socio-economic components that obscure the impacts of human decision-making on environmental functions. Yet, understanding the complexity of integrated human-environmental systems can help planners and stakeholders frame environmental problems, view their role in them and design effective policies to address them. This article examines the potential and limitations of agent-based models as metaphors that can contribute to the understanding of such complex systems, illustrating the argument with a hypothetical application in groundwater management.
Riparian zones possess an unusually diverse array of species and environmental processes. The ecological diversity is related to variable flood regimes, geograph-ically unique channel processes, altitudinal climate shifts, and upland influences on the fluvial corridor. The resulting dynamic environment supports a variety of life-history strategies, biogeochemical cycles and rates, and organisms adapted to disturbance regimes over broad spatial and temporal scales. Innovations in riparian zone management have been effective in ameliorating many ecological issues related to land use and environmental quality. Riparian zones play essential roles in water and landscape planning, in restoration of aquatic systems, and in catalyzing institutional and societal cooperation for these efforts.
Summary • This article reviews the application of some summary statistics from current theory of spatial point processes for extracting information from spatial patterns of plants. Theoretical measures and issues connected with their estimation are described. Results are illustrated in the context of specific ecological questions about spatial patterns of trees in two forests. • The pair correlation function, related to Ripley's K function, provides a formal measure of the density of neighbouring plants and makes precise the general notion of a ‘plant's-eye’ view of a community. The pair correlation function can also be used to describe spatial relationships of neighbouring plants with different qualitative properties, such as species identity and size class. • The mark correlation function can be used to describe the spatial relationships of quantitative measures (e.g. biomass). We discuss two types of correlation function for quantitative marks. Applying these functions to the distribution of biomass in a temperate forest, it is shown that the spatial pattern of biomass is uncoupled from the spatial pattern of plant locations. • The inhomogeneous pair correlation function enables first-order heterogeneity in the environment to be removed from second-order spatial statistics. We illustrate this for a tree species in a forest of high topographic heterogeneity and show that spatial aggregation remains after allowing for spatial variation in density. An alternative method, the master function, takes a weighted average of homogeneous pair correlation functions computed in subareas; when applied to the same data and compared with the former method, the spatial aggregations are smaller in size. • Synthesis. These spatial statistics, especially those derived from pair densities, will help ecologists to extract important ecological information from intricate spatially correlated plants in populations and communities.
Rivers are conduits for materials and energy; this, the frequent and intense disturbances that these systems experience, and their narrow, linear nature, create problems for conservation of biodiversity and ecosystem functioning in the face of increasing human influence. In most parts of the world, riparian zones are highly modified. Changes caused by alien plants — or environmental changes that facilitate shifts in dominance creating novel ecosystems — are often important agents of perturbation in these systems. Many restoration projects are underway. Objective frameworks based on an understanding of biogeographical processes at different spatial scales (reach, segment, catchment), the specific relationships between invasive plants and resilience and ecosystem functioning, and realistic endpoints are needed to guide sustainable restoration initiatives. This paper examines the biogeography and the determinants of composition and structure of riparian vegetation in temperate and subtropical regions and conceptualizes the components of resilience in these systems. We consider changes to structure and functioning caused by, or associated with, alien plant invasions, in particular those that lead to breached abiotic- or biotic thresholds. These pose challenges when formulating restoration programmes. Pervasive and escalating human-mediated changes to multiple factors and at a range of scales in riparian environments demand innovative and pragmatic approaches to restoration. The application of a new framework accommodating such complexity is demonstrated with reference to a hypothetical riparian ecosystem under three scenarios: (1) system unaffected by invasive plants; (2) system initially uninvaded, but with flood-generated incursion of alien plants and escalating invasion-driven alteration; and (3) system affected by both invasions and engineering interventions. The scheme has been used to derive a decision-making framework for restoring riparian zones in South Africa and could guide similar initiatives in other parts of the world.
and the native dominant Lithraea ternifolia in montane forests of central Argentina, considering life history and demographic traits of both the alien and the native species and different site conditions for population growth (good and bad sites). Matrix models are applied to project the consequences of differences in vital rates for population growth. Analyzing these models helps identify which life cycle transitions contributed most to population growth. Obtained population growth rates are considered to assess predicted rates of spread using the reaction-diffusion (R-D) model. G. triacanthos presents many of the life history traits that confer plants high potential for invasiveness: fast growth, clonal and sexual reproduction, short juvenile period, high seed production, and high seed germinability. These traits would ensure G. triacanthos invasive success and the displacement of the slow-growing, relatively less fecund native L. ternifolia. However, since disturbance and environmental heterogeneity complicate the invasibility pattern of G. triacanthos in these montane forests, the outcome of the invasion process is not straightforward as could be if only life history traits were considered.
Great variation in demographic parameters was observed between populations of each species at good and bad sites. Though both good and bad sites signified increasing or at least stable populations for G. triacanthos, for L. ternifolia bad sites represented local extinction. Analyzing the results of matrices models helps design the optimal management for the conservation of L. ternifolia populations while preventing the invasion by G. triacanthos. The predicted asymptotic rate of spread for G. triacanthos at the good site was fourfold greater than the predicted one for L. ternifolia, although the difference was much smaller considering the bad site. The usefulness of the R-D model to study this invasion system is discussed.
Microtopography, or the small‐scale variation in ground surface height (10⁻¹–10⁰ m) over short (10⁰–10² m) spatial scales, is a ubiquitous feature of wetlands globally. This variation in elevation, characterized by local high (“hummocks”) and low (“hollows”) patches, is more structured than what is observed in uplands, and is intertwined with concordantly structured spatiotemporal variability in hydrologic regimes and associated ecological processes. The importance of microtopography in wetlands is manifold, with critical influence on local hydrological, biogeochemical, and biological processes. Further, the creation and maintenance of wetland microtopography is a balance between activation processes (i.e., those that induce random elevation variation) and autogenic reinforcement processes (i.e., those that provide the feedbacks necessary for the persistence of microtopography). While there are many mechanisms that create vertical structure (e.g., tree falls, accumulation of roots and soil organic matter, and sediment deposition), they all yield a similar core feedback to enhance and sustain microtopographic structure. Finally, microtopography contributes to spatial patterning that confers emergent ecosystem‐scale functions such as hydrologic storage and flows, carbon cycling, organism dispersal, and biodiversity. There is an ongoing need to study the origins and implications of this fine‐scale variation in elevation, as well as the utility of including microtopography in model predictions and ecological restoration efforts.
This article is categorized under:
• Water and Life > Conservation, Management, and Awareness
Abstract
In wetlands, a little relief or microtopographic variation in the soil surface influences numerous hydrological and biogeochemical processes, creating strong feedbacks with ecological patterns, and functions that are important for management and conservation.
We present Hydroman, a flexible spatially explicit model coupling human and hydrological processes to explore shallow water tables and land cover interactions in flat agricultural landscapes, modeled after the Argentine Pampas. With fewer parameters, Hydroman aligned well with established hydrological models, and was validated with observed water table patterns and crop yield data. Simulations with different climate, phreatic and land cover conditions confirmed that climate remains the main driver, but crops also influence water levels and yields, depending on the growing cycle. We also examined the impacts of two alternative sowing strategies. Risk aversion proves robust in minimizing crop losses, but often results in less sowing, exacerbating flooding. Strict rotators risk more, but help stabilize the groundwater levels. Reintroducing pasture further stabilizes the system. Future work will engage farmers to derive and assess land cover strategies that maximize yield and minimize losses, and transfer our modeling approach to other applications.
Enlace descarga pdf completo hasta 29 marzo 2018: https://authors.elsevier.com/c/1WWr81L~GwCrnH
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Exotic species have different strategies to invade a new area. Invasive species limited by establishment have a wide dispersion range and high seedling mortality rate. Riparian forests are harsh environments for the establishment of plants. In this sense, we tested the hypothesis that the invasive process of G. triacanthos is limited by establishment in a riparian forest of the Uruguay River and that it begins in the fluvial bank at the transition zone between forest and grassland and moves towards the shore border. We expected that: (1) the distribution pattern of adults of G. triacanthos would be decoupled from the distribution pattern of seedlings and saplings (prereproductive stages); (2) the population would have a cluster distribution pattern related to the occurrence of punctual events that favoured the development to adult stages; and (3) the abundance of seedlings, saplings and adults would be higher in the transition zone (between forest and grassland) than in the shore border of the fluvial bank. The spatial pattern of G. triacanthos along the Uruguay River was analyzed using spatial point pattern analysis: autocorrelation indexes, hot spots and wavelet analysis. The effect of the fluvial bank position on the abundance of G. triacanthos was evaluated using non-metric multidimensional scaling (NMDS). This methodology allowed us to detect a primary invasion area, an invasion focus and the dispersal area. This pattern corresponded to an infiltration invasion pattern limited by establishment. It did not seem to be affected by position in the fluvial bank. This information could be relevant to establish correct decision-making in the control of an invasive process and provide an applicable methodology for any other invasion process, especially in riparian ecosystems.
Invasive species are a worldwide issue, both ecologically and economically. A large body of work focuses on various aspects of invasive species control, including how to allocate control efforts to eradicate an invasive population as cost effectively as possible: There are a diverse range of invasive species management problems, and past mathematical analyses generally focus on isolated examples, making it hard to identify and understand parallels between the different contexts. In this study, we use a single spatiotemporal model to tackle the problem of allocating control effort for invasive species when suppressing an island invasive species, and for long-term spatial suppression projects. Using feral cat suppression as an illustrative example, we identify the optimal resource allocation for island and mainland suppression projects. Our results demonstrate how using a single model to solve different problems reveals similar characteristics of the solutions in different scenarios. As well as illustrating the insights offered by linking problems through a spatiotemporal model, we also derive novel and practically applicable results for our case studies. For temporal suppression projects on islands, we find that lengthy projects are more cost effective and that rapid control projects are only economically cost effective when population growth rates are high or diminishing returns on control effort are low. When suppressing invasive species around conservation assets (e.g., national parks or exclusion fences), we find that the size of buffer zones should depend on the ratio of the species growth and spread rate.
Planners making groundwater plans often use scientific hydrological forecasts to estimate long term the risk of water depletion. We study a group of Chicago planners and stakeholders who learned to use and helped develop agent-based models (ABM) of coupled land-use change and groundwater flow, to explore the effects of resource use and policy on future groundwater availability. Using discourse analysis, we found planners learned to play with the ABM to judge complex interaction effects. The simulation results challenged prior policy commitments, and instead of reconsidering those commitments to achieve sustainability, participants set aside the ABM and the lessons learned with them. Visualizing patterns of objections and agreements in the dialogue enabled us to chart how clusters of participants gradually learned to grasp and interpret the simulated effects of individual and policy decisions even as they struggled to incorporate them into their deliberations.
County and regional government agencies use stakeholder committees to ensure representation of diverse interests when planning, but these representatives often are not trained to understand the complexity inherent to human-environmental issues (e.g., groundwater management). Planning professionals use computer models to simulate the interaction effects of different planrelated policies, but most are "black boxes" to stakeholders, who, therefore, are forced to trust the simulations without understanding human-environmental complexity, reducing the opportunities for effective solution building. Agent-based models can represent decisions and environmental dynamics in a rule-based form that invites nonexpert users' involvement in both developing the model and meaningfully interpreting its outputs. We conducted a series of collaborative and developmental agent-based modeling meetings with stakeholders and planners in a rapidly suburbanizing area facing groundwater shortages. Stakeholders learned how to use the models, understand the relationships among the components, interpret the outputs based on these relationships, and suggested modifications with new insights. The enhanced understanding of complex interactions reduced early commitments to policy solutions as stakeholders jointly explored the range of possible outcomes. These results show that agent-based modeling holds promise for use in collaborative planning exercises.
Eradication of an invasive species is a holy grail sought by land managers, scientists and policy makers alike. This prize is particularly attractive to funding bodies that foresee a one-off investment to solve a problem. We evaluate a 20-year eradication project on the annual weed Martynia annua L. from remote Gregory (Jutburra) National Park in northern Australia. M. annua was regionally introduced in the 1860s and has since become naturalised and locally abundant on some pastoral properties. When land use changed from grazing to national park in the mid 1980s, M. annua was thought to be a serious problem. An eradication project was started in the late 1980s. Eradication of all individuals from within the National Park has not been successful but there have been other benefits of the project. We analysed operational, biological, social and economic criteria to find that the principal barriers to eradication were: occasional inaccessibility during the crucial seed production window; many widely dispersed small infestations; a perennial seed bank; and long-distance dispersal mechanisms. The two successes of the project were control of the weed to a level where ecological impact was negligible; and extensive community engagement. A novel approach adopted by the National Park, a biannual event called the Devil’s Claw Festival, has trained and educated hundreds of local, national and international people about biological invasions and conservation issues in remote northern Australia. Long-term institutional leadership and investment have been crucial for this project. We offer recommendations to policy makers embarking on eradication projects of widespread rangeland weeds.
We used an individual-based model to identify how localized patterns of woodland invasions by exotic shrubs are likely influenced by (1) observed variation in age at first reproduction and fecundity, (2) hypothesized effects of canopy gaps on these life-history traits and dispersal, and (3) initial establishment location. Rates of spread accelerated nearly twofold as age at first reproduction decreased from eight to three years or fecundity increased from 3 to 20 offspring per year, illustrating the need to better understand the factors that influence these life-history traits. Canopy gaps facilitated spread by influencing these life-history traits, but not through their effects on dispersal. Invasions starting at the woodland center spread more rapidly than do those starting along the woodland edge. These findings suggest that managers should not only prioritize the removal of shrubs that reproduce the earliest or produce the most offspring, but they should also focus on the invasions in woodlands with high canopy openness and/or that are located in woodland interiors. Investigated factors also affected other invasion characteristics, often in surprising ways. For example, those changes in age at first reproduction and fecundity that increased the rate of spread produced nonparallel patterns of change in the proportions of invasion reproducing, whether or not invasions exhibited clumped or scattered spatial arrangements, and invasional lag. Additionally, canopy gaps influenced these characteristics by increasing fecundity, but not by decreasing age at first reproduction or altering dispersal, suggesting that canopy gaps affect local patterns of exotic-shrub invasions primarily through their positive effects on fruit production.
Understand How to Analyze and Interpret Information in Ecological Point Patterns
Although numerous statistical methods for analyzing spatial point patterns have been available for several decades, they haven’t been extensively applied in an ecological context. Addressing this gap, Handbook of Spatial Point-Pattern Analysis in Ecology shows how the techniques of point-pattern analysis are useful for tackling ecological problems. Within an ecological framework, the book guides readers through a variety of methods for different data types and aids in the interpretation of the results obtained by point-pattern analysis.
Ideal for empirical ecologists who want to avoid advanced theoretical literature, the book covers statistical techniques for analyzing and interpreting the information contained in ecological patterns. It presents methods used to extract information hidden in spatial point-pattern data that may point to the underlying processes. The authors focus on point processes and null models that have proven their immediate utility for broad ecological applications, such as cluster processes.
Along with the techniques, the handbook provides a comprehensive selection of real-world examples. Most of the examples are analyzed using Programita, a continuously updated software package based on the authors’ many years of teaching and collaborative research in ecological point-pattern analysis. Programita is tailored to meet the needs of real-world applications in ecology. The software and a manual are available online.
This paper considers key issues in plant invasion ecology, where findings published since 1990 have significantly improved our understanding of many aspects of invasions. The review focuses on vascular plants invading natural and semi-natural ecosystems, and on fundamental ecological issues relating to species invasiveness and community invasibility. Three big questions addressed by the SCOPE programme in the 1980s (which species invade; which habitats are invaded; and how can we manage invasions?) still underpin most work in invasion ecology. Some organizing and unifying themes in the field are organism-focused and relate to species invasiveness (the tens rule; the concept of residence time; taxonomic patterns and Darwin's naturalization hypothesis; issues of phenotypic plasticity and rapid evolutionary change, including evolution of increased competitive ability hypothesis; the role of long-distance dispersal). Others are ecosystem-centred and deal with determinants of the invasibility of communities, habitats and regions (levels of invasion, invasibility and propagule pressure; the biotic resistance hypothesis and the links between diversity and invasibility; synergisms, mutualisms, and invasional meltdown). Some theories have taken an overarching approach to plant invasions by integrating the concepts of species invasiveness and community invasibility (a theory of seed plant invasiveness; fluctuating resources theory of invasibility). Concepts, hypotheses and theories reviewed here can be linked to the naturalization-invasion continuum concept, which relates invasion processes with a sequence of environmental and biotic barriers that an introduced species must negotiate to become casual, naturalized and invasive. New research tools and improved research links between invasion ecology and succession ecology, community ecology, conservation biology and weed science, respectively, have strengthened the conceptual pillars of invasion ecology.
It is widely accepted that exotic invasive species are one of the most important ecological and economic problems. Reproductive and establishment traits are considered key features of a population expansion process, but few works have studied many of these simultaneously. This work examines how large the differences are in reproductive and establishment traits between two Fabaceae, the exotic invasive, Gleditsia triacanthos and the native, Acacia aroma. Gleditsia is a serious leguminous woody invader in various parts of the world and Acacia is a common native tree of Argentina. Both species have similar dispersal mechanisms and their reproductive phenology overlaps. We chose 17 plants of each species in a continuous forest of the Chaco Serrano Forest of Córdoba, Argentina. In each plant we measured fruit production, fruit removal (exclusion experiments), seed predation (pre- and post-dispersal), seed germination, seed bank (on each focal tree, three sampling periods during the year), and density of seedlings (around focal individuals and randomly in the study site). Gleditsia presented some traits that could favour the invasion process, such as a higher number of seeds per plant, percentage of scarified seed germination and density of seedlings around the focal individuals, than Acacia. On the other hand, Gleditsia presented a higher percentage of seed predation. The seed bank was persistent in both species and no differences were observed in fruit removal. This work highlights the importance of simultaneously studying reproductive and establishment variables involved in the spreading of an exotic invasive species. It also gives important insight into the variables to be considered when planning management strategies. The results are discussed from the perspective of some remarkable hypotheses on invasive species and may contribute to rethinking some aspects of the theory on invasive species.
This article argues that the agent-based computational model permits a distinctive approach to social science for which the term “generative” is suitable. In defending this terminology, features distinguishing the approach from both “inductive” and “deductive” science are given. Then, the following specific contributions to social science are discussed: The agent-based computational model is a new tool for empirical research. It offers a natural environment for the study of connectionist phenomena in social science. Agent-based modeling provides a powerful way to address certain enduring—and especially interdisciplinary—questions. It allows one to subject certain core theories—such as neoclassical microeconomics—to important types of stress (e.g., the effect of evolving preferences). It permits one to study how rules of individual behavior give rise—or “map up”—to macroscopic regularities and organizations. In turn, one can employ laboratory behavioral research findings to select among competing agent-based (“bottom up”) models. The agent-based approach may well have the important effect of decoupling individual rationality from macroscopic equilibrium and of separating decision science from social science more generally. Agent-based modeling offers powerful new forms of hybrid theoretical-computational work; these are particularly relevant to the study of non-equilibrium systems. The agentbased approach invites the interpretation of society as a distributed computational device, and in turn the interpretation of social dynamics as a type of computation. This interpretation raises important foundational issues in social science—some related to intractability, and some to undecidability proper. Finally, since “emergence” figures prominently in this literature, I take up the connection between agent-based modeling and classical emergentism, criticizing the latter and arguing that the two are incompatible. ! 1999 John Wiley & Sons, Inc.
Many tree species in the South are adapted to periodic and/or prolonged soil waterlogging. However, artificial disturbances of natural water regimes sometimes cause flooding to occur at abnormal times or the flood water to be deeper and waterlogging longer in duration than is normal. As a consequence, it is difficult for forest managers to predict how a species will respond to such disturbances or to decide how to manage an area where the water regime has been significantly altered. This paper discusses some factors which influence the waterlogging tolerance of tree species, compiles several classification systems, indicates the pertinent literature, and offers a new relative waterlogging-tolerance rating for southern lowland tree species.
A major challenge in ecology is to link patterns and processes across different spatial and temporal scales. Flood plains are ideal model ecosystems to study (i) the processes that create and maintain environmental heterogeneity and (ii) to quantify the effects of environmental heterogeneity on ecosystem functioning and biodiversity. Fluvial processes of cut-and-fill alluviation create new channels, bars and benches within a flood plain that in turn provides new surface for subsequent vegetative recruitment and growth resulting in a shifting mosaic of interconnected aquatic and terrestrial habitat patches. Composition and spatial arrangement of these habitat patches control the movement of organisms and matter among adjacent patches; and the capacity of a habitat to process matter depends on the productivity of adjacent patches and on the exchange among these patches. The exchange of matter and organisms among habitats of different age and productivity is often pulsed in nature. Small pulses of a physical driver (e.g. short-term increase in flow) can leach large amounts of nutrients thereby stimulating primary production in adjacent aquatic patches, or trigger mass emergence of aquatic insects that may in turn impact recipient terrestrial communities. Hence, biodiversity in a river corridor context is hierarchically structured and strongly linked to the dynamic biophysical processes and feedback mechanisms that drive these chronosequences over broad time and space scales. Today, the active conversion of degraded ecosystems back to a more heterogeneous and dynamic state has become an important aspect of restoration and management where maintaining or allowing a return to the shifting habitat mosaic dynamism is the goal with the expected outcome greater biodiversity and clean water among other valuable ecosystem goods and services. Copyright © 2009 John Wiley & Sons, Ltd.
A large number of methods for the analysis of point pattern data have been developed in a wide range of scientific fields. First-order statistics describe large-scale variation in the intensity of points in a study region, whereas second-order characteristics are summary statistics of all point-to-point distances in a mapped area and offer the potential for detecting both different types and scales of patterns. Second-order analysis based on Ripley's K-function is increasingly used in ecology to characterize spatial patterns and to develop hypothesis on underlying processes; however, the full range of available methods has seldomly been applied by ecologists. The aim of this paper is to provide guidance to ecologists with limited experience in second-order analysis to help in the choice of appropriate methods and to point to practical difficulties and pitfalls. We review (1) methods for analytical and numerical implementation of two complementary second-order statistics, Ripley's K and the O-ring statistic, (2) methods for edge correction, (3) methods to account for first-order effects (i.e. heterogeneity) of univariate patterns, and (4) a variety of useful standard and non-standard null models for univariate and bivariate patterns. For illustrative purpose, we analyze examples that deal with non-homogeneous univariate point patterns. We demonstrate that large-scale heterogeneity of a point-pattern biases Ripley's K-function at smaller scales. This bias is difficult to detect without explicitly testing for homogeneity, but we show that it can be removed when applying methods that account for first-order effects. We synthesize our review in a number of step-by-step recommendations that guide the reader through the selection of appropriate methods and we provide a software program that implements most of the methods reviewed and developed here.
The primary objective of this publication is to provide an overview of the species identity, invasion status, geographical extent, and abundance of alien plants in South Africa, Swaziland and Lesotho, based on fi eld records from 1979 to the end of 2000. The dataset is all the species records for the study area in the Southern African Plant Invaders Atlas (SAPIA) database during this time period. A total of 548 naturalized and casual alien plant species were catalogued and invasion was recorded almost throughout the study area. Most invasion, in terms of both species numbers and total species abundance, was recorded along the southern, southwestern and eastern coastal belts and in the adjacent interior. This area includes the whole of the Fynbos and Forest Biomes, and the moister eastern parts of the Grassland and Savanna Biomes. This study reinforces previous studies that the Fynbos Biome is the most extensively invaded vegetation type in South Africa but it also shows that parts of Savanna and Grassland are as heavily invaded as parts of the Fynbos. The Fabaceae is prominent in all biomes and Acacia with 17 listed species, accounts for a very large proportion of all invasion. Acacia mearnsii was by far the most prominent invasive species in the study area, followed by A. saligna, Lantana camara, A. cyclops, Opuntia fi cus-indica, Solanum mauritianum, Populus alba/×canescens, Melia azedarach, A. dealbata and species of Prosopis.
1 We mapped the locations of live and dead trees in a large forest plot dominated by pioneer Douglas fir (Pseudotsuga menziesii) with an understorey of the invading late-successional species western hemlock (Tsuga heterophylla) and western red cedar (Thuja plicata) on Vancouver Island, British Columbia, Canada, to test for intra- and interspecific density-dependent effects on tree survival.
2 We analysed both the spatial patterning of trees in the plot and the relationships between neighbourhood density and tree survival. We also examined the effects of additional variables (principally elevation) as covariates in our neighbourhood analyses.
3 Both the spatial and initial neighbourhood analyses suggested strong intra- and interspecific density-dependent effects on tree survival. Douglas fir survival was significantly higher in less dense patches of conspecifics and non-random tree death led to regularly spaced survivors, as expected from intraspecific competition. The significantly lower survival of western hemlock in denser patches of Douglas fir and the resulting negative spatial association between surviving trees of these two species were consistent with interspecific competition.
4 However, having controlled for the influence of elevation on tree survival (probably mediated by variation in soil moisture) in neighbourhood analyses, although the survival of the pioneer Douglas fir trees was still subject to strong density-dependent effects, variation in its density in the overstorey no longer appeared to influence the survival of the invading late-successional species. There was, however, evidence for asymmetric interspecific density dependence between the two late-successional species since western hemlock mortality tended to be higher in denser patches of western red cedar.
5 Our results emphasize the importance of considering confounding factors in studies that seek evidence for density dependence.
Summary • Rhizomatous clonal plants frequently colonize and stabilize dunes on sea and lake shores, and in inland deserts and desertified areas, where sand burial is common. To date, little attention has been paid to how clonal integration affects their ability to withstand sand burial. • In an inland dune Psammochloa villosa ramets were buried under 0, 20, 40 and 60 cm of sand, and the rhizomes at the edges of the 50 × 50 cm2 treatment quadrats were either severed or left connected. • With increasing burial depth the surviving ramets of P. villosa decreased markedly both in number and in size (number of leaves and above-ground biomass). In the connected quadrats, however, sand burial resulted in significantly smaller decreases than in the severed quadrats of the number, but not in size, of the surviving P. villosa ramets. • We conclude that clonal integration increased the ability of P. villosa to withstand sand burial, and that P. villosa could emerge from deep burial probably by elongating vertical structures with the help of the energy imported from the connected, unburied ramets. ©New Phytologist (2004) doi: 10.1111/j.1469-8137.2004.01073.x
Populus nigra L. var betulifolia and Salix alba L. var alba are early successional riparian tree species threatened throughout Continental Europe by significant changes to the natural physical processes governing their natural habitat – geomorphologically active floodplains. River management activities have dramatically altered natural patterns of river flow and rates of sediment delivery along rivers, with possible consequences for the balance between sexual and asexual regeneration strategies in these species. Conservation strategies will benefit from a greater understanding of the ways in which dynamic physical processes on the floodplain influence sexual and asexual recruitment. This paper describes a field survey investigating the relative abundance and spatial distributions of P. nigra and S. alba sexual and asexual recruits during the first years of establishment along a braided gravel bed river. Sexual and asexual recruits were identified by excavation along transects in a wet and a dry field season and distributional differences were described in terms of elevation on the floodplain, local sediment type and exposure to floodwaters. Regeneration was overwhelmingly from seed in the first 2–3 years following recruitment, but poor survival rates among sexual recruits saw a shift in the relative abundance of regeneration strategies over time. In relating hydrological data to recruitment, unseasonal flood disturbances had a negative effect on recruitment from seed and a positive effect on vegetative regeneration. Seedlings were associated with fine sediment deposits and were restricted primarily to low elevations on the flood plain, while asexual recruits had a wider spatial distribution. Certain microsite types were unique to either regeneration strategy.