Showing papers on "Disturbance (geology) published in 2020"

Satellite‐Based Estimates Reveal Widespread Forest Degradation in the Amazon

Abstract: Anthropogenic and natural forest disturbance cause ecological damage and carbon emissions. Forest disturbance in the Amazon occurs in the form of deforestation (conversion of forest to non-forest land covers), degradation from the extraction of forest resources, and destruction from natural events. The crucial role of the Amazon rainforest in the hydrologic cycle has even led to the speculation of a disturbance "tipping point" leading to a collapse of the tropical ecosystem. Here we use time series analysis of Landsat data to map deforestation, degradation, and natural disturbance in the Amazon Ecoregion from 1995 to 2017. The map was used to stratify the study area for selection of sample units that were assigned reference labels based on their land cover and disturbance history. An unbiased statistical estimator was applied to the sample of reference observations to obtain estimates of area and uncertainty at biennial time intervals. We show that degradation and natural disturbance, largely during periods of severe drought, have affected as much of the forest area in the Amazon Ecoregion as deforestation from 1995 to 2017. Consequently, an estimated 17% (1,036,800 ± 24,800 km2 , 95% confidence interval) of the original forest area has been disturbed as of 2017. Our results suggest that the area of disturbed forest in the Amazon is 44%-60% more than previously realized, indicating an unaccounted for source of carbon emissions and pervasive damage to forest ecosystems.

Forests and Water Yield: A Synthesis of Disturbance Effects on Streamflow and Snowpack in Western Coniferous Forests

Abstract: In coniferous western forests, recent widespread tree mortality provided opportunities to test the long-held theory that forest cover loss increases water yield. We reviewed 78 studies of hydrologic response to standing-replacing (severe wildfire, harvest) or nonstand-replacing (drought, insects, low-severity wildfire) disturbances, and reassessed the question: Does water yield or snowpack increase after forest disturbance? Collective results indicate that postdisturbance streamflow and snowpack may increase, not change, or even decrease, and illuminate factors that may help improve predictability of hydrologic response to disturbance. Contrary to the expectation that tree mortality reduces evapotranspiration, making more water available as runoff, postdisturbance evapotranspiration sometimes increased—particularly following nonstand-replacing disturbance—because of (a) increased evaporation resulting from higher subcanopy radiation, and (b) increased transpiration resulting from rapid postdisturbance growth. Postdisturbance hydrologic response depends on vegetation structure, climate, and topography, and new hypotheses continue to be formulated and tested in this rapidly evolving discipline.

On the conceptualization of total disturbance and its profound implications

Abstract: 1key Laboratory of Systems and Control, Academy of Mathematics and Systems Science, Chinese Academy of Sciences, Beijing 100190, China; 2School of Mathematical Sciences, University of Chinese Academy of Sciences, Beijing 100049, China; 3Beijing Aerospace Automatic, Control Institute, Beijing 100854, China; 4Center for Advanced Control Technologies Department of Electrical Engineering and Computer Science, Cleveland State University, Cleveland Ohio 44115, USA.

Fixed-time sliding mode formation control of AUVs based on a disturbance observer

Abstract: In this paper, we investigate formation tracking control of autonomous underwater vehicles ( AUVs ) with model parameter uncertainties and external disturbances. The external disturbances due to the wind, waves, and ocean currents are combined with the model parameter uncertainties as a compound disturbance. Then a disturbance observer ( DO ) is introduced to estimate the compound disturbance, which can be achieved within a finite time independent of the initial estimation error. Based on a DO, a novel fixed-time sliding control scheme is developed, by which the follower vehicle can track the leader vehicle with all the states globally stabilized within a given settling time. The effectiveness and performance of the method are demonstrated by numerical simulations.

Disturbance Rejection and Control System Design Using Improved Equivalent Input Disturbance Approach

Abstract: This paper presents an improved equivalent input disturbance (EID) approach to deal with exogenous disturbances and system nonlinearities. The disturbances and nonlinearities are regarded as a lumped disturbance on the control input channel. The EID approach, which has a state observer and an EID estimator, is used to estimate the lumped disturbance. There is a constraint between the observer and the estimator on the design of the control system for the conventional EID approach. A stable zero was added to the estimator to remove the constraint and to improve the stability of the system in this paper. This ensures that the observer and the estimator can be designed independently. An analysis of the improved EID approach explains the mechanism of disturbance rejection. The simulation and experimental results of position control of a ball-and-beam system demonstrate the validity of the method.

Disturbance refugia within mosaics of forest fire, drought, and insect outbreaks

Abstract: Author(s): Krawchuk, Meg A; Meigs, Garrett W; Cartwright, Jennifer M; Coop, Jonathan D; Davis, Raymond; Holz, Andres; Kolden, Crystal; Meddens, Arjan JH

Disturbance-Compensation-Based Continuous Sliding Mode Control for Overhead Cranes With Disturbances

Abstract: For practical mechanical systems, uncertainties/disturbances, such as unmodeled dynamics and frictions, are nonignorable factors. For existing control methods, these factors are usually neglected or addressed by a robust way. As a consequence, the nominal control performance of these methods is sacrificed. Moreover, there exists the chattering problem for some existing robust methods, such as sliding mode control laws. To deal with these drawbacks, a continuous global sliding mode controller along with a nonlinear disturbance observer is designed for the regulation and disturbance estimation control of the overhead crane system. Specifically, the original crane dynamic model is transformed into a quasi-integrator-chain form through some transformations. Then, a nonlinear disturbance observer is designed and a continuous global sliding mode control method is introduced on the basis of the constructed disturbance observer. The stability and convergence characteristics are proven through rigorous theoretical analysis. Finally, to demonstrate the performance of the designed controller, a series of experimental tests are performed, and a comparison study between the devised method here and an existing method is given. Note to Practitioners —This article is motivated by the desire to deal with the regulation and disturbance rejection of the overhead crane system. In practical applications, uncertainties/disturbances are unavoidable problems for overhead cranes. For most existing methods, these issues are usually addressed in a robust way. To handle these existing problems, a nonlinear disturbance observer and a continuous global sliding mode controller are proposed for the regulation and disturbance estimation control of the overhead crane system. The disturbance observer is introduced to estimate and compensate for uncertain disturbances, and the sliding mode controller is designed to guarantee the convergence of the state variables of the closed-loop system. In the future, we will try to apply this method to practical overhead cranes.

Stabilization of Chaotic Systems With Both Uncertainty and Disturbance by the UDE-Based Control Method

Abstract: This paper investigates the stabilization of a class of chaotic systems with both model uncertainty and external disturbance. By combining the dynamic feedback control method, and the uncertainty and disturbance estimator (UDE)-based control method, a new UDE-based control method is developed. By using this method, the system stabilization can be achieved by three steps. Illustrative examples using numerical simulations verify the soundness and effectiveness of the proposed method.

Consensus Disturbance Rejection for Linear Multiagent Systems With Directed Switching Communication Topologies

Abstract: This paper addresses the consensus disturbance rejection problem for multiple-input multiple-output linear multiagent systems (MASs) with directed fixed as well as switching communication topologies in the presence of deterministic disturbances. Based on the relative output information among neighboring agents, a controller, which incorporates the consensus error estimator, the state estimator with static coupling strength and the disturbance observer, is given such that consensus tracking could be achieved in the considered MASs under directed switching communication topologies provided that the control parameters are suitably chosen and the average dwell time is larger than a positive threshold, while the disturbances are fully rejected by the designed disturbance observer. Furthermore, by designing a state estimator with adaptive coupling law, a controller is given such that consensus disturbance rejection of MASs with directed fixed topology could be achieved in a fully distributed manner. The obtained theoretical results are finally validated by performing simulations on the unmanned aerial vehicles.

Robust Tracking for Hypersonic Reentry Vehicles via Disturbance Estimation-Triggered Control

Abstract: This note presents a novel control framework based on disturbance estimation information to exploit the potential performance improvement. Disturbances in the hypersonic reentry vehicles are revisited, and a disturbance effect indicator (DEI) is defined to demonstrate the pros and cons of disturbances’ influence on the system. Based on the disturbance estimation, a disturbance estimation-triggered control scheme for the attitude tracking is established. Furthermore, the ultimately bounded stability of the closed-loop system is guaranteed. Distinguished from eliminating the disturbance directly in the state-of-the-art disturbance observer-based control, the proposed control switches its structure to counteract or retain the disturbance according to the DEI, and thus is capable of improving the transient performance. Simulation results verify the effectiveness and superiority of the proposed approach.

The Ecology of Disturbance Interactions

Abstract: Global change has been accompanied by recent increases in the frequency and intensity of various ecological disturbances (e.g., fires, floods, cyclones), both natural and anthropogenic in origin. Because these disturbances often interact, their cumulative and synergistic effects can result in unforeseen consequences, such as insect outbreaks, crop failure, and progressive ecosystem degradation. We consider the roles of biological legacies, thresholds, and lag effects responsible for the distinctive impacts of interacting disturbances. We propose a hierarchical classification that distinguishes the patterns and implications associated with random co-occurrences, individual links, and multiple links among disturbances that cascade in chains or networks. Disturbance-promoting interactions apparently prevail over disturbance-inhibiting ones. Complex and exogenous disturbance cascades are less predictable than simple and endogenous links because of their dependency on adjacent or synchronous events. These distinctions help define regional disturbance regimes and can have implications for natural selection, risk assessment, and options for management intervention.

Combined Vector Resonant and Active Disturbance Rejection Control for PMSLM Current Harmonic Suppression

Abstract: A control method that combines a vector resonant controller and an active disturbance rejection control controller is proposed in this article for suppressing the current harmonics of permanent magnet synchronous linear motors. First, the resonant controller is improved by changing its transfer function so that it can suppress current harmonics better. Then, an active disturbance rejection control (ADRC) is designed to suppress the parameter disturbance of motors, which will adversely affect the improved resonant controller. The parameter disturbance is estimated by an extended state observer, and linear feedback control is used for disturbance compensation. The ADRC and the improved resonant controller work together to not only suppress harmonics but also improve resistance against system disturbance. Finally, a linear motor control platform is built, and experimental results are presented to verify the significance and correctness of the proposed approach.

Use of SAR and Optical Time Series for Tropical Forest Disturbance Mapping

Abstract: Frequent cloud cover and fast regrowth often hamper topical forest disturbance monitoring with optical data. This study aims at overcoming these limitations by combining dense time series of optical (Sentinel-2 and Landsat 8) and SAR data (Sentinel-1) for forest disturbance mapping at test sites in Peru and Gabon. We compare the accuracies of the individual disturbance maps from optical and SAR time series with the accuracies of the combined map. We further evaluate the detection accuracies by disturbance patch size and by an area-based sampling approach. The results show that the individual optical and SAR based forest disturbance detections are highly complementary, and their combination improves all accuracy measures. The overall accuracies increase by about 3% in both areas, producer accuracies of the disturbed forest class increase by up to 25% in Peru when compared to only using one sensor type. The assessment by disturbance patch size shows that the amount of detections of very small disturbances (< 0.2 ha) can almost be doubled by using both data sets: for Gabon 30% as compared to 15.7–17.5%, for Peru 80% as compared to 48.6–65.7%.

Fire and grazing determined grasslands of central Madagascar represent ancient assemblages

Abstract: The ecology of Madagascar's grasslands is under-investigated and the dearth of ecological understanding of how disturbance by fire and grazing shapes these grasslands stems from a perception that d...

Effectiveness of fuzzy sliding mode control boundary layer based on uncertainty and disturbance compensator on suspension active magnetic bearing system

Abstract: This paper presents the disturbance and uncertainty suppression by using the nonlinear disturbance observer and an extended state observer for a nonlinear active magnetic bearing system. Otherwise,...

Move to nocturnality not a universal trend in carnivore species on disturbed landscapes

Abstract: Anthropogenic landscape change is a leading driver of biodiversity loss. Preceding dramatic changes such as wildlife population declines and range shifts, more subtle responses may signal impending larger‐scale change. For example, disturbance‐induced shifts to species’ activity patterns may disrupt temporal niche partitioning along the 24‐h time axis, compromising community structure via altered competitive interactions. We investigated the impacts of human landscape disturbance on species’ activity patterns and temporal niche partitioning in the Canadian Rocky Mountain carnivore guild using camera trap images collected across two regions encompassing a wide gradient of human footprint. Applying kernel density estimation techniques, we tested for carnivore species’ activity shifts 1) between a low versus high disturbance landscape, and 2) in relation to site‐scale disturbance. To test our hypothesis that human disturbance impacts species’ temporal niche partitioning, we modelled activity overlap between co‐occurring carnivore species in relation to natural and anthropogenic landscape features, as well as carnivore community composition. Multiple carnivore species altered activity patterns between the low versus high disturbance landscapes and camera sites, but these shifts varied considerably among species. While wolves appeared to increase nocturnal activity in relation to disturbance, coyote activity consistently trended towards cathemerality and marten increased diurnal activity. Detecting effects of landscape disturbance on activity overlap between co‐occurring species was highly sensitive to site‐level detection sample sizes, and our results suggest altered temporal niche partitioning between marten and wolverine in relation to forest cover. This study indicates that mesocarnivores may respond differently and perhaps indirectly to anthropogenic disturbance compared to apex predators. Apex predator shifts to nocturnality may facilitate a ‘behavioural release’ in mesocarnivores. This may be a likely component of mesocarnivore population release, with important management implications for ecological communities on disturbed landscapes.

Anti-Disturbance Control for Nonlinear Systems Based on Interval Observer

Abstract: This paper proposes a disturbance interval estimation approach for nonlinear systems. Distinct from the conventional estimation schemes aiming to estimate the magnitude of the disturbance, the disturbance interval estimation approach is capable of approximating the time-varying upper and lower boundaries for the disturbance. To achieve this, the dynamics of the boundary estimation errors are constructed as positive systems by introducing a time-invariant coordinate transformation. The boundary estimation errors are guaranteed to be nonnegative and bounded. Then, the generated interval is capable of covering all possible values of the current disturbance and the interval width can provide a quantized guidance on the estimation accuracy at any time. Furthermore, a general antidisturbance control scheme is summarized in light of the proposed disturbance interval estimation approach. As an illustrative case, an antidisturbance control method is developed by integrating the proposed estimation approach and the sliding mode control. Finally, the proposed disturbance interval estimation approach and the anti-disturbance control method are demonstrated through both the numerical and the experimental validation.

The response of arboreal marsupials to long‐term changes in forest disturbance

Abstract: Australian Government’s National Environmental Science Program through the Threatened Species Recovery Hub; Victorian Department of Environment, Water and Land Planning; Parks Victoria; private donors.

Application of multidimensional structural characterization to detect and describe moderate forest disturbance

Abstract: The study of vegetation community and structural change has been central to ecology for over a century, yet the ways in which disturbances reshape the physical structure of forest canopies remain relatively unknown.Moderate severity disturbances affect different canopy strata and plant species, resulting in variable structural outcomes and ecological consequences. Terrestrial lidar (light detection and ranging) offers an unprecedented view of the interior arrangement and distribution of canopy elements, permitting the derivation of multidimensional measures of canopy structure that describe several canopy structural traits (CSTs) with known links to ecosystem function.We used lidar-derived CSTs within a machine learning framework to detect and describe the structural changes that result from various disturbance agents, including moderate severity fire, ice storm damage, age-related senescence, hemlock woolly adelgid, beech bark disease, and chronic acidification. We found that fire and ice storms primarily affected the amount and position of vegetation within canopies, while acidification, senescence, pathogen, and insect infestation altered canopy arrangement and complexity. Only two of the six disturbance agents significantly reduced leaf area, counter to common assumptions regarding many moderate severity disturbances. While findings are limited in their generalizability due to lack of replication among disturbances, they do suggest that the current limitations of standard disturbance detection methods—such as optical-based remote sensing platforms, which are often above-canopy perspectives—limit our ability to understand the full ecological and structural impacts of disturbance, and to evaluate the consistency of structural patterns within and among disturbance agents. A more broadly inclusive definition of ecological disturbance that incorporates multiple aspects of canopy structural change may potentially improve the modeling, detection, and prediction of functional implications of moderate severity disturbance aswell as broaden our understanding of the ecological impacts of disturbance.

Anthropogenic Disturbance and Population Viability of Woodland Caribou in Ontario

Abstract: One of the most challenging tasks in wildlife conservation and management is to clarify how spatial variation in land cover due to anthropogenic disturbance influences wildlife demography and long‐ term viability. To evaluate this, we compared rates of survival and population growth by woodland caribou (Rangifer tarandus caribou) from 2 study sites in northern Ontario, Canada that differed in the degree of anthropogenic disturbance because of commercial logging and road development, resulting in differences in predation risk due to gray wolves (Canis lupus). We used an individual‐based model for population viability analysis (PVA) that incorporated adaptive patterns of caribou movement in relation to predation risk and food availability to predict stochastic variation in rates of caribou survival. Field estimates of annual survival rates for adult female caribou in the unlogged (x̄ = 0.90) and logged (x̄ = 0.76) study sites recorded during 2010–2014 did not differ significantly (P> 0.05) from values predicted by the individual‐based PVA model (unlogged: x̄ = 0.87; logged: x̄ = 0.79). Outcomes from the individual‐based PVA model and a simpler stage‐structured matrix model suggest that substantial differences in adult survival largely due to wolf predation are likely to lead to long‐term decline of woodland caribou in the commercially logged landscape, whereas the unlogged landscape should be considerably more capable of sustaining caribou. Estimates of population growth rates (λ) for the 2010–2014 period differed little between the matrix model and the individual‐based PVA model for the unlogged (matrix model x̄ = 1.01; individual‐based model x̄ = 0.98) and logged landscape (matrix model x̄ = 0.88; individual‐based model x̄ = 0.89). We applied the spatially explicit PVA model to assess the viability of woodland caribou across 14 woodland caribou ranges in Ontario. Outcomes of these simulations suggest that woodland caribou ranges that have experienced This is an open access article under the terms of the Creative Commons Attribution‐NonCommercial License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited and is not used for commercial purposes. Received: 21 February 2019; Accepted: 22 December 2019 E‐mail: jfryxell@uoguelph.ca Current affiliation: Department of Wildland Resources, Utah State University, Logan, UT 84322, USA Current affiliation: CBS Biology Teaching/Learning, University of Minnesota Twin Cities, Room 3‐133 MCB, 420 Washington Ave SE, Minneapolis, MN 55455, USA Current affiliation: Department of Wildlife, Fisheries and Aquaculture, Mississippi State University, Box 9690, Mississippi State, MS 39762‐9690, USA Fryxell et al. • Population Viability of Woodland Caribou 1 significant levels of commercial forestry activities in the past had annual growth rates <0.89, whereas caribou ranges that had not experienced commercial forestry operations had population growth rates >0.96. These differences were strongly related to regional variation in wolf densities. Our results suggest that increased wolf predation risk due to anthropogenic disturbance is of sufficient magnitude to cause appreciable risk of population decline in woodland caribou in Ontario. © 2020 The Authors. The Journal of Wildlife Management published by Wiley Periodicals, Inc. on behalf of The Wildlife Society.

Adaptive Control of Uncertain Nonlinear Time-Delay Systems With External Disturbance

Abstract: In this article, adaptive state feedback stabilization is addressed for a class of delayed uncertain systems with external disturbance, where the time delays and the bound parameters of the delayed states and disturbance are assumed to be unknown. By introducing a new Lyapunov-Krasovskii functional, we present a memoryless control strategy to stabilize nonlinear time-delay systems through newly defined control gain function. The main contribution of this article lies in the construction of control gain function. Multiple gain functions can be revealed in a function set. The asymptotic stability of the closed delayed nonlinear system is ensured. Simulation examples are presented to show the effectiveness of the proposed method.