Showing papers on "Disturbance (geology) published in 1999"

Wetland Restoration, Flood Pulsing, and Disturbance Dynamics

Abstract: RESTORATION AND DISTURBANCE: BACKGROUND. Disturbance Dynamics in Wetlands. DISTURBANCE DYNAMICS AND LIFE HISTORY STRATEGIES IN WETLANDS. Restoration Theory. From Seed to Adult: Missing Links in Restoration. RESTORING DISTURBANCE DYNAMICS IN WETLANDS. Restoration and Disturbance Dynamics in Restored Landscapes. Revegetation Alternatives. Case Histories. Glossary. References. Appendices. Index.

Fast reference governors for systems with state and control constraints and disturbance inputs

Abstract: Reference governors are applied to closed-loop tracking systems that are linear and discrete time and have constraints on state and control variables. Earlier results are extended in signi\"cant ways. Disturbance inputs, whose values belong to a speci\"ed set, are allowed and a general class of reference governors is introduced. Each governor in the class guarantees constraint satisfaction for all reference and disturbance inputs. Moreover, if the reference input is ultimately con\"ned to a neighbourhood of a constraint-admissible constant input, the eventual action of the reference governor reduces to a unit delay. By appropriately selecting reference governors from the allowed class it is possible to simplify signi\"cantly their implementation. The increase in on-line speed of operation overcomes prior limits on the practical application of reference governors. Algorithmic procedures are described which facilitate design of the reference governors. Several examples are presented. They illustrate the design process and the excellence of response to large inputs. Copyright ( 1999 John Wiley & Sons, Ltd.

Ecological diversity and resilience of Mediterranean vegetation to disturbance

Abstract: In the Mediterranean region, plant agricultural fields of southern France to investigate the communities are highly diverse with high total and mechanisms underlying the resilience of Mediterranean local species richness, and a high spatial heterogeneity. herbaceous communities to soil disturbance. These They are also highly resilient to disturbance, which has suggest that ecological diversity contributes to resilience been related to their evolutionary history of in at least three ways. disturbance, and conceptually, although without 1. Post-disturbance dynamics involve shifts in formal proof, to their high ecological diversity. These abundances of response groups to disturbance. ideas take on a new importance in the context of global 2. Within response groups, processes of lottery change. In order to predict potential eVects of future competition account for the dynamic maintenance land use change, mechanisms underlying the resilience of numerous species. of ecosystems need to be understood. However, a 3. At the landscape scale, the fine grained distribution review of the recent literature shows that confusion of communities at diVerent successional stages abounds as to the nature and measurement of resilience. allows for patch dynamics to operate with dispersal In this paper we present a brief conceptual and of species between patches. methodological clarification and propose avenues for Further progress in the understanding of the the analysis of mechanisms underlying the relationships relationships between ecological diversity and resilience between diversity and resilience, using Mediterranean

Vegetation patterns in heterogeneous landscapes: The importance of history and environment

Abstract: Throughout the eastern United States, plant species distributions and community patterns have developed in re- sponse to heterogeneous environmental conditions and a wide range of historical factors, including complex histories of natural and anthropogenic disturbance. Despite increased rec- ognition of the importance of disturbance in determining forest composition and structure, few studies have assessed the relative influence of current environment and historical factors on moder vegetation, in part because detailed knowl- edge of prior disturbance is often lacking. In the present study, we investigate moder and historical factors that control veg- etation patterns at Harvard Forest in central Massachusetts, USA. Similar to the forested uplands throughout the north- eastern United States, the site is physiographically heteroge- neous and has a long and complex history of natural and anthropogenic disturbance. However, data on forest composi- tion and disturbance history collected over the past > 90 years allow us to evaluate the importance of historical factors rigor- ously, which is rarely possible on other sites. Soil analyses and historical sources document four catego- ries of historical land use on areas that are all forested today: cultivated fields, improved pastures/mowings, unimproved pastures, and continuously forested woodlots. Ordination and logistic regressions indicate that although species have re- sponded individualistically to a wide range of environmental and disturbance factors, many species are influenced by three factors: soil drainage, land use history, and C:N ratios. Few species vary in accordance with ionic gradients, damage from the 1938 hurricane, or a 1957 fire. Contrary to our expectation that the effects of disturbance will diminish over time, histori- cal land use predicts 1992 vegetation composition better than 1937 composition, perhaps because historical woodlots have become increasingly differentiated from post-agricultural

Distribution of plant functional types along gradients of disturbance intensity and resource supply in an agricultural landscape

Abstract: . In this study, plant functional types are understood as groups of plants with similar biological traits displaying significant optima or maxima on a gradient plane of resource supply and disturbance intensity. The biological traits refer to expansion, vegetative regeneration, generative reproduction, dispersal and seed bank longevity. 129 vegetation samples were taken in an agricultural landscape in southwestern Germany, covering a wide range of terrestrial vegetation types – but with the exception of forests and wetlands. For each site, also soil data were recorded. Mean daily soil moisture was estimated with a simple model. Soil moisture, balanced nitrogen supply and available phosphorus were combined into a factor ‘resource supply’. In addition, disturbance intensity was estimated for each site. This factor was based on (1) frequency of disturbance, (2) disturbance depth below or above the soil surface, and (3) proportion of the area affected by a discrete disturbance event. 30 plant groups with similar biological characteristics resulted from a cluster analysis, based on a compilation of 19 biological traits for a regional species pool. Logistic regression on a gradient plane of disturbance intensity and resource supply yielded response curves for 28 groups. The dependent variable was defined as the probability of encountering all members of a group in a sample. 17 groups display a significant response curve on the gradient plane. Plants with a potential for long- range dispersal are concentrated on sites with low or high disturbance intensities (e.g. fallow land, fields, lawns). On sites with medium disturbance intensity (e.g. meadows) and low to medium resource supply, small-range dispersal predominates. There are no distinct trends concerning seed bank longevity. The potential for vertical and lateral expansion increases with decreasing disturbance intensity. Only at medium disturbance intensities does vertical expansion correlate positively with resource supply. Rapid detachment of daughter individuals occurs more often on productive sites than on less productive sites. Diversity of groups with similar biological traits is highest on sites with medium disturbance intensities.

Plant functional types and disturbance dynamics – Introduction

Abstract: . Plant functional traits and types are useful concepts in relation to disturbance responses of natural and managed ecosystems. To explore their applicability in greater depth, a set of 12 papers presents a broad range of issues from methodologies to the results of particular trait studies in the field, and modelling approaches. So far, empirical studies have only allowed us to identify a few functional traits that are consistently associated with disturbance. To determine the trait variations associated with climate, disturbance history and current disturbance regime as well as the interactions between these factors, global-scale comparisons of numerous individual studies are required. Significant advances toward this ambitious goal are presented in these papers, and include: (1) the articulation of experimental and analytical methodologies for individual studies that could usefully contribute to a global comparison; (2) the identification of core traits that can be used in the further search for disturbance-related traits common to a range of environments; (3) further information on vegetation response to disturbance in terms of trait representation, and the identification of attribute syndromes; (4) the identification of issues for modelling disturbance dynamics using functional types.

The influence of ice on polar nearshore benthos

Abstract: Polar benthic communities are subject to a range of disturbance levels from a variety of sources, principal amongst which is ice. This occurs in four main forms: the ice-foot, ice scour, anchor ice and fast ice, each of which influences benthos in a very different temporal and spatial manner. The four described forms of ice disturbance are all seasonal, but combined, influence communities throughout the year. The magnitude of ice mediated disturbance is often catastrophic and as a result both dominates benthic community structure and makes recolonization and development rates critical. This disturbance extremity results in high temporal and spatial heterogeneity, very low intertidal zone diversity and in places low mid–sublittoral diversity. It may also, however, be important in generating and maintaining the typically high sublittoral zone diversity observed at many polar localities. Intermediate frequencies or magnitudes of disturbance have been controversially discussed as important in maintaining diversity by prevention of space monopolization by overgrowth dominants in such environments as the deep sea. The shelf areas examined to date certainly suggest intermediate disturbance is important in maintaining polar sublittoral zone diversity. The combination of slow colonization and development with high frequencies of disturbance means most polar nearshore environments that have been described are permanently in a state of change or recovery.

Plant species responses along a grazing disturbance gradient in Australian grassland

Abstract: Floristic data from paired roadside-paddock analyses from grassland in central Queensland, Australia, were ordinated. The mean direction of the vectors between these pairs was almost perfectly aligned with the indirect gradient represented by the first axis of Non-metric Multi-dimensional Scaling. It confirms anecdotal evidence of a trend from infrequently grazed roadsides to constantly grazed paddocks. The increasing abundance of annual herbs and grasses along this putative gradient is consistent with documented trends from elsewhere in the world. The response patterns of individual species along the disturbance gradient is consistent with ecological theory predicting unimodal peaks in abundance along physical environmental gradients. The ancestral perennial dominants of the grasslands, Dichanthium sericeum and D. queenslandicum, exhibited a declining response to grazing disturbance. Even the generally unpalatable perennial grass Aristida leptopoda declined considerably in the upper segments of the grazing disturbance gradient. A suite of herbaceous trailing legumes had peaks in their abundance near the middle of the grazing disturbance gradient, trends that can be readily explained by the combination of their palatability and intolerance to competition from tall perennial grasses. Several species including the noxious exotic herb Parthenium hysterophorus showed increasing abundance along the grazing disturbance gradient. The methodology may have application as a rapid method of assessing disturbance impacts elsewhere, and is most suited where a management differential between paired plots can be reliably generalized and where the physical environment is relatively monotonous.

Predictive feedback and feedforward control for systems with unknown disturbances

Abstract: This paper presents the theory and implementation of a hybrid controller for general linear systems by incorporating a feedforward path in the feedback control. The generalized predictive control is extended to include a feedfoward path in the multi‐input multi‐output cases. There are cases in acoustic‐induced vibration where the disturbance signal is not available to be used by the hybrid controller, but a disturbance model is available. In this case the disturbance model may be used in the feedback controller to enhance performance. In practice, however, neither the disturbance signal nor the disturbance model is available. This paper presents the theory of identifying and incorporating the noise model into the feedback controller. Implementations are performed on a test plant and regulation improvements over the case where no noise model is used are demonstrated.

Development and validation of empirical and analytical reaction wheel disturbance models

Abstract: Accurate disturbance models are necessary to predict the effects of vibrations on the performance of precision space-based telescopes, such as the Space Interferometry Mission (SIM) and the Next-Generation Space Telescope (NGST). There are many possible disturbance sources on such a spacecraft, but the reaction wheel assembly (RWA) is anticipated to be the largest. This thesis presents three types of reaction wheel disturbance models. The first is a steady-state empirical model that was originally created based on RWA vibration data from the Hubble Space Telescope (HST) wheels. The model assumes that the disturbances consist of discrete harmonics of the wheel speed with amplitudes proportional to the wheel speed squared. The empirical model is extended for application to any wheel through the development of a MATLAB toolbox that extracts the model parameters from steady-state RWA data. Experimental data obtained from wheels manufactured by Ithaco Space Systems are used to illustrate the empirical modeling process and provide model validation. The model captures the harmonic disturbances of the wheel quite well, but does not include interactions between the harmonics and the structural modes of the wheel which result in large disturbance amplifications at some wheel speeds. Therefore the second model, an analytical model, is created using principles from rotor dynamics to model the structural wheel modes. The model is developed with energy methods and captures the internal flexibilities and fundamental harmonic of an imbalanced wheel. A parameter fitting methodology is developed to extract the analytical model parameters from steady-state RWA vibration data. Data from an Ithaco E type wheel are used to illustrate the parameter matching process and validate the analytical model. It is shown that this model provides a much closer prediction to the true nature of RWA disturbances than the empirical model. Finally, an extended model, which combines features of both the empirical and analytical models, is introduced. This model captures all the wheel harmonics as well as the disturbance amplifications that occur due to excitation of the structural wheel modes by the harmonics. In addition, preliminary analyses that explore the dynamic coupling between RWA and spacecraft are presented and a plan for laboratory testing to gain insight into the effects of coupling and provide disturbance model validation is outlined.

Variability in reactions of Pacific harbor seals, Phoca vitulina richardsi, to disturbance

Abstract: In many locations, disturbance is an important factor affecting the haulout patterns of harbor seals, Phoca vitulina. Disturbance is defined as any activity that alters normal behavior. In the United States, disturbance of marine mammals by humans is regulated by the Marine Mammal Protection Act of 1972. In contrast to pelagic marine mammals, changes in the behavior of pinnipeds on haul-out sites related to disturbance is relatively simple to measure. Long-term effects of disturbance, however, are often difficult to assess. The effects of disturbance may be quite mild or may cause displacement and even mortality. Bighorn sheep (Ovis canadensis) and whitetailed deer exposed to snowmobile traffic have shown increased heart rate but no visible change in behavior (MacArthur et al., 1982; Moen et al., 1982). Humpback whale (Megaptera novaeangliae) femalecalf pairs in Hawaii have avoided nearshore areas of intense human recreational activities (Salden, 1988; Glockner-Ferrari and Ferrari 1 ). Disturbance-related mortality in harbor seals can result from stampeding and pup abandonment (Johnson 2 ). Disturbance from low-flying aircraft may have caused mortality of more than 200 (10%) harbor seal pups on Tugidak Island, Alaska, in

Plant community responses to disturbance by mechanized military maneuvers

Abstract: The effects of 10 yr of military training exercises on vegetation structure were assessed across plant communities that differed in physiognomy and soil texture at Pinon Canyon Maneuver Site (PCMS), Colorado, after release from previous grazing management. Covariate analyses aided in separating temporal trends due to both release from grazing and imposition of training disturbance from the direct effect of training. The shift in land use had both synergistic and antagonistic impacts on successional trajectories of communities, and on horizontal and vertical structural heterogeneity. Vegetation basal cover declined with increasing intensity of disturbance by tracked vehicles, but release from grazing acts additively in this ecosystem. Litter cover increased following release from grazing, even though it declined with increasing levels of disturbance. Vehicular maneuvering generally reduced woody life forms in tall-height classes to a greater extent than short-height classes. Low growing cacti were susceptible to crushing. Species and functional group responses to vehicular disturbance were sometimes dependent on community type. Long-lived perennials declined, but were replaced by short-lived perennials in only the shrub-grassland community. Annuals and exotics did not show relationships with intensity of disturbance, though some weed species increased. Community-wide species dissimilarity did not show large shifts, and patterns in species diversity or richness were not related to intensity of disturbance. The PCMS appears to be in a transient stage where release from grazing has had as much or more impacts as did the imposition of military training. Fine textured soils may be more susceptible to the cumulative effects of vehicular loads.

Effects of logging and fire on runoff and erosion on highly erodible granitic soils in Tasmania

Abstract: A set of rainfall simulator experiments was carried out in a logged forest in Tasmania, Australia, to quantify the impact of logging and fire on runoff and soil loss. A large, portable rainfall simulator was developed for the experiments to help overcome problems associated with spatially varying hydraulic and erosion properties of the soil. Simulated rainfall events with intensities of approximately 35, 75, and 150 mm h−1 were applied to four 300 m2 plots with different levels of surface disturbance: severely burnt, logged and burnt with high mechanical disturbance, logged with low mechanical disturbance, and undisturbed. The experiments indicated that the amount of runoff and soil loss from the plots depended on the type and degree of disturbance to the natural biotic crust developed on this soil. Erodibility per unit runoff was greatest on the severely burnt plot; but runoff on the mechanically disturbed site was so great that it produced the highest total sediment yield on an event basis. Runoff production was similar on the three plots where the natural biotic crust was largely intact and was low relative to the plot with high mechanical disturbance. Total sediment yield was typically higher on logged plots than on unlogged plots.

Effects of sample disturbance on the stress-induced microfracturing characteristics of brittle rock

Abstract: The effects of sampling disturbance on the laboratory-derived mechanical properties of brittle rock were measured on cored samples of Lac du Bonnet granite taken from three different in situ stress...

Infant sleep disturbance: Description of a problem behaviour process

Abstract: Clinicians are commonly consulted by parents of infants aged 6–24 months about infant sleep disturbance (ISD). Considerable research over the last two decades has contributed to our knowledge of ISD but has failed to lead to a comprehensive aetiological explanation. This research is reviewed and identifies: (i) individual factors associated with sleep disturbance; (ii) interactive mechanisms linking these associated factors; and (iii) the relationship between learning and infant sleep disturbance. The findings of this research are incorporated into models describing proximal factors involved in the three processes considered important in the development of sleep disturbance: (i) the development of sleep self-initiation at around 3 months of age; (ii) the development of primary sleep disturbance by 6 months of age; and (iii) the development of secondary sleep disturbance later in the first or second year. Description of the influence that proximal factors have on sleep disturbance, organized in these three models, is the most appropriate way of presenting this complex material for the clinician. Changing these proximal factors is important in both prevention and management.  1999 Harcourt Publishers Ltd

Analysis of dynamic disturbance on rock burst

Abstract: The accumulation of elastic strain energy in the rock is the main internal cause of rock burst. However, dynamic disturbance is an important external factor, frequently triggering this phenomenon. The paper discusses the mechanism of dynamic disturbance and its contribution to rock burst using numerical simulation based on an example from a hydropower station in China.

On the spatial evolution of a wall-imposed periodic disturbance in pipe Poiseuille flow at Re = 3000. Part 1. Subcritical disturbance

Abstract: We have performed a numerical study on the transition of a cylindrical pipe flow under the influence of a localized disturbance in the form of periodic suction and blowing (PSB) applied at the pipe wall. We focus here on the so-called receptivity problem where the spatial evolution of this disturbance is studied as it travels downstream through the pipe. The study is carried out by means of two techniques: an eigenmode expansion solution (EES) and a full nonlinear direct numerical simulation (DNS). The EES is based on an analytical expansion in terms of the eigenfunctions of the linear operator which follows from the equations of motion expressed in a cylindrical coordinate system. The DNS is formulated in terms of a spectral element method.We restrict ourselves to a so-called subcritical disturbance, i.e. the flow does not undergo transition. For very small amplitudes of the PSB disturbance the results of the EES and DNS techniques agree excellently. For larger amplitudes nonlinear interactions come into play which are neglected in the EES method. Nevertheless, the results of both methods are consistent with the following transition scenario. The PSB excites a flow perturbation that has the same angular wavenumber and frequency as the imposed disturbance itself. This perturbation is called the fundamental mode. By nonlinear self-interaction of this fundamental mode higher-order harmonics, both in the angular wavenumber and frequency, are generated. It is found that the harmonic with angular wavenumber 2, i.e. twice the wavenumber of the fundamental mode, and with zero frequency grows strongly by a linear process known as transient growth. As a result the (perturbed) pipe flow downstream of the disturbance region develops extended regions of low velocity, known as low-speed streaks. At large disturbance amplitudes these low-speed streaks show the development of high wavenumber oscillations and it is expected that at even higher disturbance amplitudes these oscillations become unstable and turbulent flow will set in.Our result agrees (at least qualitatively) with the transition scenario in a plane Poiseuille flow as discussed by Reddy et al. (1998) and Elofson & Alfredson (1998).

Time-frequency analysis of power system disturbance signals for power quality

Abstract: In this paper, we utilize fundamental concepts of time-frequency analysis to analyze transient disturbance signals in power systems. Within the restriction of the uncertainty principle, time-frequency analysis provides localized information of the disturbance signal as a function of time and frequency. Using simulation and actual power disturbance data, we show that the so-called reduced interference distribution (RID) appears to be most suitable for investigating the time-frequency characteristics of power disturbances. In this paper, the potential assessment of power quality via application of time-frequency analysis is also briefly discussed.

Should the wind disturbance patterns observed in natural forests be mimicked in planted forests in the British uplands

Abstract: Summary Recent developments in UK forestry policy require the adoption of management practices that maintain and improve the biodiversity of managed forests. One approach is to use natural disturbance in unmanaged forests as a template for setting the scale, frequency and pattern of forest operations in managed forests. This review considers the relevance of this approach for conifer plantations in upland Britain. The dynamics of British planted forests are compared with the disturbance dynamics of analogous natural forests with particular reference to disturbance by strong winds. Western hemlock‐Sitka spruce (Tsuga heterophylla‐Picea sitchensis) forests in the Pacific North-west of North America and particularly South-east Alaska provide the most promising comparison. There are few reports on disturbance in these forests, but the regime includes both gap-phase and stand replacement dynamics due to wind. However, the landscape proportion and pattern of resulting structural types are not well defined. The dynamics of planted forests in Britain are dominated by rotational patch clearfelling which results in regular stand replacement and little possibility of the stands developing beyond the stage of stem exclusion towards old-growth. The pattern and timing of felling is driven by economic and visual amenity considerations rather than by an attempt to mimic natural disturbance patterns. Moreover, the structural complexity and remnant elements (such as deadwood, large trees, vegetation patches) left after large scale disturbance are rarely found after conventional timber harvesting. The authors conclude that natural wind disturbance regimes have potential as a reference point for management in British upland forests but at present are not relevant as a model to mimic explicitly. This is because the biodiversity benefits of adopting a ‘natural’ approach in planted forests are unclear compared with management guided by other criteria such as rarity. Furthermore, the spatial and temporal pattern to be mimicked is not sufficiently well understood. Improved knowledge could inform decisions on the scale and distribution of harvesting across a landscape, and modify silvicultural operations to create and maintain the structures and patterns associated with natural disturbance. However, further research is needed to quantify the spatial and temporal characteristics of wind disturbance in upland forests in Britain and in natural forests elsewhere.

Finite-time control for uncertain linear systems with disturbance inputs

Abstract: We consider the static output feedback, finite-time disturbance rejection problem for linear systems with time-varying norm-bounded uncertainties. The first result provided in the paper is a sufficient condition for finite-time state feedback disturbance rejection in the presence of constant disturbances. This condition requires the solution of an LMI. Then we consider the more general output feedback case, which is shown to be reducible to the solution of an optimization problem involving bilinear matrix inequalities. Finally we deal with the case in which the disturbance is time-varying and generated by a linear system.

Recovery of Productivity and Species Diversity in Tussock Tundra following Disturbance

Abstract: Tundra ecosystems appear to recover slowly from disturbance, but little long-term data concerning plant diversity has been available. We examined recovery of tundra vegetation in Alaska, U.S.A., 23 yr after fire and 24 yr after bulldozing. Primary productivity, depth of thaw, and vascular plant diversity were compared between disturbed and undisturbed tundra to determine whether recovery was complete. Productivity, species richness, and diversity did not differ between burned and unburned plots. Depth of thaw, however, remained greater in burned relative to unburned plots. In contrast, depth of thaw was the only characteristic that did not differ between bulldozed and control plots. Productivity and species richness were greater in bulldozed plots, but diversity was less than control plots. The differences between the two disturbances suggest that, ultimately, recovery depends more on the impact of disturbance on vegetation than changes in the abiotic environment. Vegetative propagules persisted in the soil after fire, but not bulldozing. Therefore, recolonization after fire included plants from the seed bank and vegetative propagules. Vegetation on bladed plots was dominated only by seed bank species. Thus, more than two decades after disturbance, recovery of tundra vegetation appeared to be a function of the nature of the disturbance.

Understory tree characteristics and disturbance history of a central Appalachian forest prior to old-growth harvesting

Abstract: To better understand the dynamics of red oak regeneration, we evaluated the composition of understory woody species and recruitment characteristics of a mixed mesophytic forest in the central Appalachian region at the time of old-growth logging. We also evaluated canopy disturbance history during both the old-growth and second-growth periods. Stemwood radial growth patterns were used to evaluate the frequency of disturbance. Levels of red oak radial growth change were greatest from 1906 to 1911, which is indicative of a major disturbance. The oak regeneration in all sampled areas that developed into the second-growth forest was dominated by seedling-size individuals that were recruited from 1902 to 1910. The stand-wide median canopy disturbance interval was 30.83 years between 1797 and 1983.