Disturbance and landscape dynamics in a changing world

Disturbance regimes are changing rapidly, and the consequences of such changes for ecosystems and linked social-ecological systems will be profound. This paper synthesizes current understanding of disturbance with an emphasis on fundamental contributions to contemporary landscape and ecosystem ecology, then identifies future research priorities. Studies of disturbance led to insights about heterogeneity, scale, and thresholds in space and time and catalyzed new paradigms in ecology. Because they create vegetation patterns, disturbances also establish spatial patterns of many ecosystem processes on the landscape. Drivers of global change will produce new spatial patterns, altered disturbance regimes, novel trajectories of change, and surprises. Future disturbances will continue to provide valuable opportunities for studying pattern-process interactions. Changing disturbance regimes will produce acute changes in ecosystems and ecosystem services over the short (years to decades) and long-term (centuries and beyond). Future research should address questions related to (1) disturbances as catalysts of rapid ecological change, (2) interactions among disturbances, (3) relationships between disturbance and society, especially the intersection of land use and disturbance, and (4) feedbacks from disturbance to other global drivers. Ecologists should make a renewed and concerted effort to understand and anticipate the causes and consequences of changing disturbance regimes.     

Comparing modern and presettlement forest dynamics of a subboreal wilderness: does spruce budworm enhance fire risk?

Insect disturbance is often thought to increase fire risk through enhanced fuel loadings, particularly in coniferous forest ecosystems. Yet insect disturbances also affect successional pathways and landscape structure that interact with fire disturbances (and vice-versa) over longer time scales. We applied a landscape succession and disturbance model (LANDIS-II) to evaluate the relative strength of interactions between spruce budworm (Choristoneura fumiferana) outbreaks and fire disturbances in the Boundary Waters Canoe Area (BWCA) in northern Minnesota (USA). Disturbance interactions were evaluated for two different scenarios: presettlement forests and fire regimes vs. contemporary forests and fire regimes. Forest composition under the contemporary scenario trended toward mixtures of deciduous species (primarily Betula papyrifera and Populus spp.) and shade-tolerant conifers (Picea mariana, Abies balsamea, Thuja occidentalis), with disturbances dominated by a combination of budworm defoliation and high-severity fires. The presettlement scenario retained comparatively more "big pines" (i.e., Pinus strobus, P. resinosa) and tamarack (L. laricina), and experienced less budworm disturbance and a comparatively less-severe fire regime. Spruce budworm disturbance decreased area burned and fire severity under both scenarios when averaged across the entire 300-year simulations. Contrary to past research, area burned and fire severity during outbreak decades were each similar to that observed in non-outbreak decades. Our analyses suggest budworm disturbances within forests of the BWCA have a comparatively weak effect on long-term forest composition due to a combination of characteristics. These include strict host specificity, fine-scaled patchiness created by defoliation damage, and advance regeneration of its primary host, balsam fir (A. balsamea) that allows its host to persist despite repeated disturbances. Understanding the nature of the three-way interaction between budworm, fire, and composition has important ramifications for both fire mitigation strategies and ecosystem restoration initiatives. We conclude that budworm disturbance can partially mitigate long-term future fire risk by periodically reducing live ladder fuel within the mixed forest types of the BWCA but will do little to reverse the compositional trends caused in part by reduced fire rotations.     

Postfire Management on Forested Public Lands of the Western United States

Abstract:  Forest ecosystems in the western United States evolved over many millennia in response to disturbances such as wildfires. Land use and management practices have altered these ecosystems, however, including fire regimes in some areas. Forest ecosystems are especially vulnerable to postfire management practices because such practices may influence forest dynamics and aquatic systems for decades to centuries. Thus, there is an increasing need to evaluate the effect of postfire treatments from the perspective of ecosystem recovery. We examined, via the published literature and our collective experience, the ecological effects of some common postfire treatments. Based on this examination, promising postfire restoration measures include retention of large trees, rehabilitation of firelines and roads, and, in some cases, planting of native species. The following practices are generally inconsistent with efforts to restore ecosystem functions after fire: seeding exotic species, livestock grazing, placement of physical structures in and near stream channels, ground-based postfire logging, removal of large trees, and road construction. Practices that adversely affect soil integrity, persistence or recovery of native species, riparian functions, or water quality generally impede ecological recovery after fire. Although research provides a basis for evaluating the efficacy of postfire treatments, there is a continuing need to increase our understanding of the effects of such treatments within the context of societal and ecological goals for forested public lands of the western United States.     

A tree and climate assessment tool for modelling ecosystem response to climate change

Understanding how vulnerable forest ecosystems are to climate change is a key requirement if sustainable forest management is to be achieved. Modelling the response of species in their regeneration niche to phenological and biophysical processes that are directly influenced by climate is one method for achieving this understanding. A model was developed to investigate species resilience and vulnerability to climate change within its fundamental-regeneration niche. The utility of the developed model, tree and climate assessment (TACA), was tested within the interior Douglas-fir ecosystem in south-central British Columbia. TACA modelled the current potential tree species composition of the ecosystem with high accuracy and modelled significant responses amongst tree species to climate change. The response of individual species suggests that the studied ecosystem could transition to a new ecosystem over the next 100 years. TACA showed that it can be an effective tool for identifying species resilience and vulnerability to changes in climate within the most sensitive stage of development, the regeneration phase. The TACA model was able to identify the degree of change in phenological and biophysical variables that control tree establishment, growth and persistence. The response to changes in one or more of these variables resulted in changes in the climatic suitability of the ecosystem for species and enabled a measure of vulnerability to be quantified. TACA could be useful to forest managers as a decision support tool for adaptation actions and by researchers interested in modelling stand dynamics under climate change.     

Preemptive and Salvage Harvesting of New England Forests: When Doing Nothing Is a Viable Alternative

Abstract:  One unexpected consequence of natural disturbances in forested areas is that managers often initiate activities that may impose greater ecosystem impacts than the disturbances themselves. By salvage logging areas affected by windstorms or other impacts, by harvesting host trees in advance of insect infestation or disease, or by preemptively harvesting forests in an attempt to improve their resilience to future disturbances and stresses, managers initiate substantial changes in the ecosystem structure and function. Much of this activity is undertaken in the absence of information on the qualitative and quantitative differences between disturbance impacts and harvesting. To provide insight for such decisions we evaluated the ecosystem consequences of two major disturbance processes in New England (U.S.A.)—intense windstorms and invasive pests and pathogens—and contrasted them with impacts from preemptive and salvage harvesting. Despite dramatic physical changes in forest structure resulting from hurricane impacts and insect infestation, little disruption of biogeochemical processes or other ecosystem functions typically follows these disturbances. Indeed, the physical and organic structures produced by these disturbances are important natural features providing habitat and landscape heterogeneity that are often missing due to centuries of land use. From an ecosystem perspective there are strong arguments against preemptive and salvage logging or the attempt through silvicultural means to improve the resistance or resilience of forests to disturbance and stress. There are often valid motivations for salvage or preemptive logging including financial considerations, human safety, and a desire to shape the long-term composition and resource-production characteristics of forests. Nonetheless, there are many ecological benefits derived from leaving forests alone when they are affected or threatened by disturbances and pest and pathogen outbreaks.     

Spatial patterns of forest characteristics in the western United States derived from inventories.

In the western United States, forest ecosystems are subject to a variety of forcing mechanisms that drive dynamics, including climate change, land-use/land-cover change, atmospheric pollution, and disturbance. To understand the impacts of these stressors, it is crucial to develop assessments of forest properties to establish baselines, determine the extent of changes, and provide information to ecosystem modeling activities. Here we report on spatial patterns of characteristics of forest ecosystems in the western United States, including area, stand age, forest type, and carbon stocks, and comparisons of these patterns with those from satellite imagery and simulation models. The USDA Forest Service collected ground-based measurements of tree and plot information in recent decades as part of nationwide forest inventories. Using these measurements together with a methodology for estimating carbon stocks for each tree measured, we mapped county-level patterns across the western United States. Because forest ecosystem properties are often significantly different between hardwood and softwood species, we describe patterns of each. The stand age distribution peaked at 60-100 years across the region, with hardwoods typically younger than softwoods. Forest carbon density was highest along the coast region of northern California, Oregon, and Washington and lowest in the arid regions of the Southwest and along the edge of the Great Plains. These results quantify the spatial variability of forest characteristics important for understanding large-scale ecosystem processes and their controlling mechanisms. To illustrate other uses of the inventory-derived forest characteristics, we compared them against examples of independently derived estimates. Forest cover compared well with satellite-derived values when only productive stands were included in the inventory estimates. Forest types derived from satellite observations were similar to our inventory results, though the inventory database suggested more heterogeneity. Carbon stocks from the Century model were in good agreement with inventory results except in the Pacific Northwest and part of the Sierra Nevada, where it appears that harvesting and fire in the 20th century (processes not included in the model runs) reduced measured stand ages and carbon stocks compared to simulations.     

From single fine roots to a black alder forest ecosystem: How system behaviour emerges from single component activities

Based on empirical findings in a natural black alder ecosystem in Northern Germany we developed an individual based model that integrates components of a black alder ecosystem interacting on different levels of organisation. The factors determining seasonal fine root biomass development of forest ecosystems are not yet fully understood.We used an object oriented model approach to investigate this complex matter for black alder trees. Processes like growth, storage, respiration, transport, nutrient mineralisation and uptake as well as interactions among these factors are described on the level of functionally differentiated plant organs (fine roots, coarse roots, stem, branches, leaves) and soil units. The object structure of the model is determined by spatial relations between plant modules as well as between plant modules and their local environment modules.As results of model application we found that (i) on the organ level, spatio-temporal plasticity of (root) growth allocation is related to spatio-temporal variation of resource availability, (ii) on the plant level, balanced root:shoot growth appears in response to variation of available resources light and nutrients, (iii) on the population level, tree stand development (population structure, self-thinning) resulted from coexistence and competition between plant individuals.For the understanding of the root compartment it seems relevant that the model implementation of local scale fine root dynamics is consistent with a self-organised large scale spatial heterogeneity of fine root activity pattern. On the other hand, fine-root dynamics cannot be explained as a result of autonomous dynamics. A reference to above-ground processes is a necessary condition and the overall plant seems to act as an integrator providing boundary conditions for local activity pattern. At the same time fine-root characteristics are of some importance for properties on hierarchically higher levels, e.g. co-existence in a tree population or element cycling in the ecosystem.As a conclusion, modelling of the spatio-temporal dynamics of tree root systems appears as a paradigmatic example of scale and organisation level integrating processes.     

Ecosystem services as a counterpart of emergy flows to ecosystems

A generic input-state-output scheme has been used to represent ecosystem dynamics. Systemic approaches to ecosystems use functions that are based either on inputs, state or outputs of the system. Some examples of approaches that use a combination of functions have been recently proposed. For example the use of eco-exergy to emergy flow can be seen as a mixed input-state approach; more recently, to connect the state to the output of the ecosystem, the relation of eco-exergy and ecosystems services has been proposed. This paper studies the link between the useful output of an ecosystems and its input through the relation between ecosystem services and emergy flow, in a kind of grey/black box scheme (i.e., without considering the state and the structure of the ecosystem). No direct connection between the two concepts can be determined, but identifying and quantifying the emergy flows feeding an ecosystem and the services to humans coming from them facilitate the sustainable conservation of Nature and its functions. Furthermore, this input-output relation can be established in general by calculating the ratio of the value of the ecosystem services to the emergy flow that supports the system. In particular, the ratio of the world ecosystem services to the emergy flow supporting the entire biosphere has been calculated showing that, at least at the global level, Nature is more efficacious in producing “money” (in form of ecosystem services) than economic systems (e.g., national economies and their GDP).     

Analysis of neighborhood dynamics of forest ecosystems using likelihood methods and modeling.

Advances in computing power in the past 20 years have led to a proliferation of spatially explicit, individual-based models of population and ecosystem dynamics. In forest ecosystems, the individual-based models encapsulate an emerging theory of "neighborhood" dynamics, in which fine-scale spatial interactions regulate the demography of component tree species. The spatial distribution of component species, in turn, regulates spatial variation in a whole host of community and ecosystem properties, with subsequent feedbacks on component species. The development of these models has been facilitated by development of new methods of analysis of field data, in which critical demographic rates and ecosystem processes are analyzed in terms of the spatial distributions of neighboring trees and physical environmental factors. The analyses are based on likelihood methods and information theory, and they allow a tight linkage between the models and explicit parameterization of the models from field data. Maximum likelihood methods have a long history of use for point and interval estimation in statistics. In contrast, likelihood principles have only more gradually emerged in ecology as the foundation for an alternative to traditional hypothesis testing. The alternative framework stresses the process of identifying and selecting among competing models, or in the simplest case, among competing point estimates of a parameter of a model. There are four general steps involved in a likelihood analysis: (1) model specification, (2) parameter estimation using maximum likelihood methods, (3) model comparison, and (4) model evaluation. Our goal in this paper is to review recent developments in the use of likelihood methods and modeling for the analysis of neighborhood processes in forest ecosystems. We will focus on a single class of processes, seed dispersal and seedling dispersion, because recent papers provide compelling evidence of the potential power of the approach, and illustrate some of the statistical challenges in applying the methods.     

Modelling coral reef ecosystems with limited observational data

Ecosystem models represent potentially powerful tools for coral reef ecosystem managers. They can provide insight into ecosystem dynamics not achievable through alternative means allowing coral reef managers to assess the potential outcome of any given management decision. One of the main limitations in the applicability of ecosystem models is that they often require detailed empirical data and this can restrict their applicability to ecosystems that are either currently well studied or have the resources available to collect the required data. This study describes the development of a coral reef ecosystem model that can be calibrated to an ecosystem with limited empirical data. Based on the assumption that coral reef ecological structure is generic across all tropical coral reefs and that the magnitude of the interactions between ecological components is reef specific, the dynamics of the ecosystem can be replicated based on limited empirical data. The model successfully replicated the dynamics of three individual reef systems including an inshore and oceanic reef within the Great Barrier Reef and a Caribbean reef system. It highlighted the importance of understanding the specific dynamics of a given reef and that a positive management intervention in one system may result in a negative outcome for another. The model was also used to assess the importance of various interactions within coral reef ecosystems. It identified the interactions between hard corals and other non-algal benthic components as being an important (but currently understudied) facet of coral reef ecology. The development of this modelling approach provides access to ecosystem modelling tools for coral reef managers previously excluded due to a lack of resources or technical expertise.     

Salvage Logging, Ecosystem Processes, and Biodiversity Conservation

Abstract:  We summarize the documented and potential impacts of salvage logging—a form of logging that removes trees and other biological material from sites after natural disturbance. Such operations may reduce or eliminate biological legacies, modify rare postdisturbance habitats, influence populations, alter community composition, impair natural vegetation recovery, facilitate the colonization of invasive species, alter soil properties and nutrient levels, increase erosion, modify hydrological regimes and aquatic ecosystems, and alter patterns of landscape heterogeneity. These impacts can be assigned to three broad and interrelated effects: (1) altered stand structural complexity; (2) altered ecosystem processes and functions; and (3) altered populations of species and community composition. Some impacts may be different from or additional to the effects of traditional logging that is not preceded by a large natural disturbance because the conditions before, during, and after salvage logging may differ from those that characterize traditional timber harvesting. The potential impacts of salvage logging often have been overlooked, partly because the processes of ecosystem recovery after natural disturbance are still poorly understood and partly because potential cumulative effects of natural and human disturbance have not been well documented. Ecologically informed policies regarding salvage logging are needed prior to major natural disturbances so that when they occur ad hoc and crisis-mode decision making can be avoided. These policies should lead to salvage-exemption zones and limits on the amounts of disturbance-derived biological legacies (e.g., burned trees, logs) that are removed where salvage logging takes place. Finally, we believe new terminology is needed. The word salvage implies that something is being saved or recovered, whereas from an ecological perspective this is rarely the case.     

气候变暖背景下森林土壤碳循环研究进展

由人类活动引起的温室效应以及由此造成的气候变暖对森林牛态系统的影响已引起人们的普遍关注.森林土壤碳循环作为全球碳循环的重要组成部分,是决定未来陆地牛物嘲表现为碳源/碳汇的关键环节,揭示这一作用对于准确理解全球变化背景下陆地生态系统碳循环过程具有重要的指导意义.本文主要通过论述影响土壤碳循环过程的5个方面(土壤呼吸、土壤微生物、土壤酶活性、凋落物输入与分解、土壤碳库),综述了近10 a来全球气候变暖对土壤碳循环过程的影响.近年来,尽管已开展了大量有关土壤碳循环对气候变暖的响应及反馈机制的研究,并取得了一定的成果,但研究结果仍然存在很大的不确定性.整合各种密切关联的全球变化现象,完善研究方法和实验手段,加强根际微生态系统碳循环过程与机理研究将是下一步研究的方向和重点.参70     

On the role of entropy in water quality modelling

The use of the entropy principle in phenomenological water quality models is not only necessary, but also of great advantage. A deterministic ecosystem model must obey the 2nd law of thermodynamics. Gibb's equation is a constraint additional to the balances of mass, energy and momentum. The entropy principle supports the unified treatment of physical, chemical and biological processes in water bodies, offers stability criteria and controls the further development of the aquatic ecosystems. Thermodynamic criteria also allow the determination of the bifurcation points of the model equations. Especially near these points the state and structure of the ecosystem can be strongly changed by fluctuations of the variables and parameters of the ecosystem.Results of the thermodynamic theory of selforganizing systems (Glansdorff and Prigogine, 1971; Nicolis and Prigogine, 1977) are of very great importance for water quality modelling. Furthermore, the entropy principle bridges the phenomenological, stochastic and cybernetic approaches to water quality modelling.While the paper deals with general aspects of the role of entropy in water quality modelling, the basic system of equations, taking the entropy principle into account, can be found in a previous paper (Mauersberger, 1978).     

The effects of forest harvest intensity in combination with wind disturbance on carbon dynamics in Lake States Mesic Forests

Total forest carbon (C) storage is determined by succession, disturbances, climate, and the edaphic properties of a site or region. Forest harvesting substantially affects C dynamics; these effects may be amplified if forest harvesting is intensified to provide biofuel feedstock. We tested the effects of harvest intensity on landscape C using a simulation modeling approach that included C dynamics, multiple disturbances, and successional changes in composition. We developed a new extension for the LANDIS-II forest landscape disturbance and succession model that incorporates belowground soil C dynamics derived from the CENTURY soil model. The extension was parameterized and calibrated using data from an experimental forest in northeastern Wisconsin, USA. We simulated a 9800 ha forested landscape over 400 years with wind disturbance combined with no harvesting, harvesting with residual slash left on site (‘standard harvest’), and whole-tree harvesting. We also simulated landscapes without wind disturbance and without eastern hemlock (Tsuga canadensis) to examine the effects of detrital quantity and quality on C dynamics. We estimated changes in live C, detrital C, soil organic C, total C, and forest composition. Overall, the simulations without harvesting had substantially greater total C and continued to sequester C. Standard harvest simulations had more C than the whole tree harvest simulations. Under both harvest regimes, C accrual was not evident after 150 years. Without hemlock, SOC was reduced due to a decline in detritus and a shift in detrital chemistry. In conclusion, if the intensity of harvesting increases we can expect a corresponding reduction in potential C storage. Compositional changes due to historic circumstances (loss of hemlock) may also affect forest C although to a lesser degree than harvesting. The modeling approach presented enabled us to consider multiple, interacting drivers of landscape change and the subsequent changes in forest C.     

Condition-dependent individual decision-making determines cyprinid partial migration

Partial migration is a common phenomenon among many animals and occurs in many types of ecosystems. Understanding the mechanisms behind partial migration is of major importance for the understanding of population dynamics and, eventually, ecosystem processes. We studied the effects of food availability on the seasonal partial migration of cyprinid fish from a lake to connected streams during winter by the use of passive telemetry. Fish with increased access to food were found to migrate in higher proportion, earlier in the season, and to reside in the streams for a longer period compared to fish with decreased access to food. Furthermore, fewer unfed migrants returned to the lake, indicating higher overwinter mortality. Our results suggest that individual fish trade off safety from predation and access to food differently depending on their body condition, which results in a condition-dependent partial migration. Hence, our main conclusion is that individual decision-making is based on assessment of own condition which offers a mechanistic explanation to partial migration. Moreover, this may be of high importance for understanding population responses to environmental variation as well as ecosystem dynamics and stability.     

Predicting a ‘tree change’ in Australia's tropical savannas: Combining different types of models to understand complex ecosystem behaviour

In this study, key ecological modelling limitations of a process-based simulation model and a Bayesian network were reduced by combining the two approaches. We demonstrate the combined modelling approach with a case study investigating increases in woody vegetation density in northern Australia's tropical savannas. We found that by utilising the strengths of a simulation model and a Bayesian network we could both forecast future change in woody vegetation density and diagnose the reasons for current vegetation states. The local conditions of climate, soil characteristics and the starting population of trees were found to be more important in explaining the likelihood of change in woody vegetation density compared to management practices such as grazing pressure and fire regimes. We conclude that combining the strengths of a process and BN model allowed us to produce a simple model that utilised the ability of the process model to simulate ecosystem processes in detail and over long time periods, and the ability of the BN to capture uncertainty in ecosystem response and to conduct scenario, sensitivity and diagnostic analysis. The overall result was a model that has the potential to provide land managers with a better understanding of the behaviour of a complex ecosystem than simply utilising either modelling approach in isolation.     

A simulation model to evaluate the impacts of invasive earthworms on soil carbon dynamics

Recent studies have reported that earthworm invasions alter native communities and impact nutrient cycling in terrestrial ecosystems. We developed a simulation model to evaluate the potential impacts of earthworm invasions on carbon dynamics, taking into consideration earthworm feeding strategies and priming effects on the microorganisms through their casting activities. Responses of carbon stocks (forest litter, soil organic matter, microbial biomass and earthworm populations) and carbon fluxes (litter decomposition, earthworm consumption, and microbial respiration) were used to evaluate an earthworm invasion of a forest ecosystem. Data from a northern temperate forest (Arnot Forest, New York) were adapted for model calibration and evaluation. Simulation results suggest that the impact and outcome of earthworm invasions are affected by pre-invasion resource availability (litter and soil organic matter), invasive earthworm assemblages (particularly feeding strategy), and invasion history (associated with earthworm population dynamics). The abovementioned factors may also determine invasion progress of earthworm species. The accuracy of the model could be improved by the addition of environmental modules (e.g., soil water regimes), precise parameters accounting for individual species attributes under different environmental conditions (e.g. utilization ability of different types of food resources), as well as earthworm population dynamics (size and structure) and interactions with predators and other invasive/indigenous species during the invasion progress. Such an earthworm invasion model could provide valuable evaluation of the complicated responses of carbon dynamics to earthworm invasions in a range of forest ecosystems, particularly under global change scenarios.     

Efficient and sustainable management of complex forest ecosystems

A large range of models has been developed for the analysis of optimal forest management strategies, with the well-known Faustmann models dating back to the mid-19th century. To date, however, there has been relatively little attention for the implications of complex ecosystem dynamics for optimal forest management. This paper examines the implications of irreversible ecosystem responses for efficient and sustainable forest management. The paper is built around two forest models that comprise two ecosystem components, forest cover and topsoil, the interactions between these components, and the supply of the ecosystem services ‘wood’ and ‘erosion control’. The first model represents a forest that responds in a reversible way to overharvesting. In the second model, an additional ecological process has been included and the ecosystem irreversibly collapses below certain thresholds in forest cover and topsoil depth. The paper presents a general model, and demonstrates the implications of pursuing efficient as well as sustainable forest management for the two forest ecosystems. Both fixed and variable harvesting cycles are examined. Efficient and sustainable harvesting cycles are compared, and it is shown that irreversible ecosystem behaviour reduces the possibilities to reconcile efficient and sustainable forest management through a variable harvesting cycle.     

What Is Ecosystem Management?

The evolving concept of ecosystem management is the focus of much current debate. To clarify discussion and provide a framework for implementation, I trace the historical development of ecosystem management, provide a working definition, and summarize dominant themes taken from an extensive literature review. The general goal of maintaining ecological integrity is discussed along with five specific goals: maintaining viable populations, ecosystem representation, maintaining ecological process (ie., natural disturbance regimes), protecting evolutionary potential of species and ecosystems, and accommodating human use in light of the above. Short-term policy implications of ecosystem management for several groups of key actors (scientists, policymakers, managers, citizens) are discussed. Long-term (> 100 years) policy implications are also reviewed including reframing environmental values, fostering cooperation, and evaluating success. Ecosystem management is not just about science nor is it simply an extension of traditional resource management; it offers a fundamental reframing of bow humans may work with nature.