Home About us Contact
|
Home About us Contact | ||
|
|
||
Initial Disturbance (initial + disturbance)
Selected AbstractsPost-European settlement response gradients of river sensitivity and recovery across the upper Hunter catchment, AustraliaEARTH SURFACE PROCESSES AND LANDFORMS, Issue 7 2009Kirstie Fryirs Abstract Most analyses of river adjustment have focused on parts of catchments where metamorphosis has occurred. This provides a non-representative view of river responses to human-disturbance. Although many rivers have been subjected to systematic land-use change and disturbance, significant variability is evident in the form, extent and consequences of adjustment. This study documents the catchment-wide distribution of river sensitivity and adjustment in the upper Hunter catchment, New South Wales, Australia in the period since European settlement. The spatial distribution and timing of lateral, vertical and wholesale river adjustments are used to assess river sensitivity to change. The type and pattern of rivers, influenced largely by valley setting, have induced a fragmented pattern of river adjustment in the upper Hunter catchment. Adjustments have been largely non-uniform and localized, reflecting the predominance of bedrock-controlled rivers which have limited capacity to adjust and are resilient to change. Less than 20% of river courses have experienced metamorphosis. Phases of reach-scale geomorphic adjustment to human disturbance are characterized as a gradient of primary, secondary and tertiary responses. In general terms, primary responses such as cutoffs or straightening were followed by secondary responses such as channel expansion. These secondary responses occurred between 50,70 years after initial disturbance. A subsequent tertiary phase of river recovery, denoted as a transition from predominantly erosional to predominantly depositional geomorphic processes such as channel contraction, occurred around 70,120 years after initial disturbance. Such responses are ongoing across much of the upper Hunter catchment. Copyright © 2009 John Wiley & Sons, Ltd. [source] Core Inflation and Monetary PolicyINTERNATIONAL FINANCE, Issue 3 2001Marianne Nessén What are the implications of targeting different measures of inflation? We extend a basic theoretical framework of optimal monetary policy under inflation targeting (Svensson 1997) to include several components of CPI inflation, and analyse the implications of using different measures of inflation as the target variable , headline CPI inflation, core inflation, and CPI excluding interest rates. Our main results are the following. First, barring the interest rate component, temporary shocks to inflation do not affect optimal monetary policy under any regime. Second, indirect (second-round) effects of disturbances on target variables need to be accounted for properly. Simply excluding seemingly temporary disturbances from the reaction function risks leading to inappropriate policy responses. Third, it may be optimal to respond to changes in one measure of inflation even if the target is defined in terms of another. Fourth, the presence of the direct interest rate component in the CPI tends to push optimal monetary policy in an expansionary direction. The net effect, considering also the traditional channel, however, depends on the nature of the initial disturbance. [source] Maintaining diversity through intermediate disturbances: evidence from rodents colonizing rehabilitating coastal dunesAFRICAN JOURNAL OF ECOLOGY, Issue 4 2000S. M. Ferreira Abstract Rodents inhabit the coastal dune forests of KwaZulu-Natal, South Africa. Here habitat rehabilitation following mining of dunes has resulted in coastal dune forest succession similar to that recorded in nonmined forests. We investigated the colonization of rehabilitating stands and evaluate the role of disturbance in maintaining rodent diversity. A trapping programme was established between July 1993 and February 1995 during which rodent colonization, local extinction and species richness were recorded for rehabilitating stands of different ages. Trends in these variables were closely associated with one of three possible outcomes for a disturbed patch over time, with no intervening disturbances following the initial disturbance. Colonization was initially high which led to an increase in species richness. Extinction was lower than colonization, but became higher when the habitat was 3 years old, which led to a decline in richness. We extrapolate this result assuming negligibly small disturbances after the initiation of rehabilitation and suggest that intermediate levels of disturbance maintain rodent species richness in coastal dune forests. Furthermore, our results illustrated species turnover, a prediction of the recorded outcome, with young stands dominated by Mastomys natalensis and older stands by Saccostomus campestris or Aethomys chrysophilus. Résumé Il y a des rongeurs dans les forêts des dunes côtières du KwaZulu-Natal, en Afrique du Sud. Là, la réhabilitation de l'habitat après l'exploitation minière des dunes a abouti à une succession de forêts côtières des dunes semblable à celle qui est observée dans les forêts non exploitées. Nous avons étudié la colonisation des endroits en voie de réhabilitation et évalué le rôle des perturbations dans le maintien de la diversité des rongeurs. On a mis au point un programme de piégeage entre juillet 1993 et février 1995, pendant lequel on a noté la colonisation par les rongeurs, l'extinction locale et la richesse en espèces pour les endroits à différents stades de réhabilitation. Les tendances pour ces variables étaient étroitement associées à l'un des trois résultats possibles que peut conna,^tre avec le temps un endroit qui a été perturbé, lorsque aucune autre perturbation ne suit la première. La colonisation a d'abord été forte, ce qui a causé un enrichissement des espèces. Les extinctions étaient moins fréquentes que les colonisations, mais elles ont augmenté lorsque le nouvel habitat a atteint l'âge de trois ans, ce qui a entra,^né une perte de richesse en espèces. Nous extrapolons ce résultant en supposant que ce sont de petites perturbations négligeables après le démarrage de la réhabilitation et nous suggérons que des taux moyens de perturbation maintiennent la richesse spécifique des rongeurs dans les forêts des dunes côtières. De plus, nos résultats illustrent une rotation des espèces, une prédiction des résultats rapportés, avec les endroits les plus jeunes dominés par Mastomys natalensis et les plus anciens par Saccostomus campestris et Aethomys chrysophilus. [source] Stability of travelling wave solutions to a semilinear hyperbolic system with relaxationMATHEMATICAL METHODS IN THE APPLIED SCIENCES, Issue 4 2009Yoshihiro Ueda Abstract We study a semilinear hyperbolic system with relaxation and investigate the asymptotic stability of travelling wave solutions with shock profile. It is shown that the travelling wave solution is asymptotically stable, provided the initial disturbance is suitably small. Moreover, we show that the time convergence rate is polynomially (resp. exponentially) fast as t,, if the initial disturbance decays polynomially (resp. exponentially) for x,,. Our proofs are based on the space,time weighted energy method. Copyright © 2008 John Wiley & Sons, Ltd. [source] Dynamics of grazing lawn formation: an experimental test of the role of scale-dependent processesOIKOS, Issue 10 2008Joris P. G. M. Cromsigt Grazing lawns are characteristic for African savanna grasslands, standing out as intensely grazed patches of stoloniferous grazing-tolerant grass species. Grazing lawn development has been associated with grazing and increased nutrient input by large migratory herds. However, we argue that in systems without mass migrations disturbances, other than direct grazing, drive lawn development. Such disturbances, e.g. termite activity or megaherbivore middens, also increase nutrient input and keep the bunch vegetation down for a prolonged time period. However, field observations show that not all such disturbances lead to grazing lawns. We hypothesize that the initial disturbance has to be of a minimal threshold spatial scale, for grazing intensity to be high enough to induce lawn formation. We experimentally tested this idea in natural tall savanna grassland. We mowed different-sized plots to simulate initial disturbances of different scales (six times during one year) and applied fertilizer to half of the plots during two years to simulate increased nutrient input by herbivores or termite activity. Allowing grazing by naturally occurring herbivores, we followed the vegetation development over more than three years. Grazing kept bunch grass short in coarser, fertilized plots, while grasses grew out toward their initial height in fine-scale and unfertilized plots. Moreover, lawn grasses strongly increased in cover in plots with an increased nutrient input but only after coarser scale disturbance. These results support our hypothesis that an increased nutrient input in combination with grazing indeed induces grazing lawn formation, but only above a threshold scale of the initial disturbance. Our results provide an alternative mechanism for the development of grazing lawns in systems that lack mass migrating herds. Moreover, it gives a new spatial dimension to the processes behind grazing lawn development, and hence help to understand how herbivores might create and maintain spatial heterogeneity in grassland systems. [source] Disturbance Propagation in the Melt Spinning ProcessPROCEEDINGS IN APPLIED MATHEMATICS & MECHANICS, Issue 1 2009Anthony McVeigh This work deals with the propagation of a disturbance acting on a fibre in the melt-spinning process, where molten polymer is ejected vertically downwards from an orifice before being drawn onto a wind-up spool. The disturbance may be produced by a sudden horizontal draught of air impinging on a part of the fibre. The flow is modeled as a wave propagating on a moving string under the influence of damping and variable tension. The amplitude of the disturbance is obtained in closed-form along the characteristics which emanate from the boundaries of the localised initial disturbance; the general solution of the damped disturbance is determined numerically. An important aspect of the model is the ability to predict the magnitude of this disturbance close to the orifice, where the extruded polymer is molten and therefore extremely sensitive to disturbances. (© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Barotropic instability in the tropical cyclone outer regionTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 641 2009Jiayi Peng Abstract The growth of asymmetric perturbations and their interactions with the symmetric flow are investigated for wind profiles in a tropical cyclone with instability in its outer region. Three tangential wind profiles are examined: TC1, a strong barotropic instability profile in the outer region; TC2, a stable wind profile; and TC3, a weaker instability profile comparing to TC1 with a larger distance between the inner negative and the outer positive vorticity gradient centres. An eigenvalue analysis indicates that azimuthal wave-number two is the most unstable mode in both TC1 and TC3, with an e-folding time-scale of about 1 and 9 days, respectively. Numerical simulations using a linear barotropic model, with an initial asymmetry specified in the outer region, confirm the eigenvalue analysis. A mechanism is provided to explain the difference between simulations in TC1 and TC2. In both the stable and unstable case, an inner asymmetry is induced by the initial outer asymmetry acting on the symmetric vorticity gradient. Subsequently, the newly generated inner asymmetry feeds back positively to the outer asymmetry with the unstable profile. Because of this positive feedback, the inner and the outer asymmetries maintain an up-shear phase tilting, leading to a continuous energy transfer from the symmetric flow to the asymmetric perturbation. In the stable TC2, the inner asymmetry could not amplify the outer initial asymmetry as there is no basic-state radial vorticity gradient there. Also due to this feedback process, disturbances grow faster where the (absolute) basic-state vorticity gradients are large. Therefore, the position of an initial disturbance plays a minor role in determining the outcome of the system. Simulations with a nonlinear barotropic model and a primitive equation model further confirm the significant weakening of the maximum tangential wind due to the positive feedback process in TC1. Simulations for TC3 show a smaller change of the symmetric tangential wind, as expected. Copyright © 2009 Royal Meteorological Society [source] Dynamics of grazing lawn formation: an experimental test of the role of scale-dependent processesOIKOS, Issue 10 2008Joris P. G. M. Cromsigt Grazing lawns are characteristic for African savanna grasslands, standing out as intensely grazed patches of stoloniferous grazing-tolerant grass species. Grazing lawn development has been associated with grazing and increased nutrient input by large migratory herds. However, we argue that in systems without mass migrations disturbances, other than direct grazing, drive lawn development. Such disturbances, e.g. termite activity or megaherbivore middens, also increase nutrient input and keep the bunch vegetation down for a prolonged time period. However, field observations show that not all such disturbances lead to grazing lawns. We hypothesize that the initial disturbance has to be of a minimal threshold spatial scale, for grazing intensity to be high enough to induce lawn formation. We experimentally tested this idea in natural tall savanna grassland. We mowed different-sized plots to simulate initial disturbances of different scales (six times during one year) and applied fertilizer to half of the plots during two years to simulate increased nutrient input by herbivores or termite activity. Allowing grazing by naturally occurring herbivores, we followed the vegetation development over more than three years. Grazing kept bunch grass short in coarser, fertilized plots, while grasses grew out toward their initial height in fine-scale and unfertilized plots. Moreover, lawn grasses strongly increased in cover in plots with an increased nutrient input but only after coarser scale disturbance. These results support our hypothesis that an increased nutrient input in combination with grazing indeed induces grazing lawn formation, but only above a threshold scale of the initial disturbance. Our results provide an alternative mechanism for the development of grazing lawns in systems that lack mass migrating herds. Moreover, it gives a new spatial dimension to the processes behind grazing lawn development, and hence help to understand how herbivores might create and maintain spatial heterogeneity in grassland systems. [source] | |||||||||||||