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Disturbance Intensity (disturbance + intensity)
Selected AbstractsPlant community properties predict vegetation resilience to herbivore disturbance in the ArcticJOURNAL OF ECOLOGY, Issue 5 2010James D. M. Speed Summary 1.,Understanding the impact of disturbance on vegetation and the resilience of plant communities to disturbance is imperative to ecological theory and environmental management. In this study predictors of community resilience to a simulated natural disturbance are investigated. Responses to disturbance are examined at the community, plant functional type and species level. 2.,Field experiments were set up in seven tundra plant communities, simulating disturbance based on the impact of grubbing by an increasing herbivore population of pink-footed geese (Anser brachyrhynchus). The short-term resilience of communities was assessed by comparing community dissimilarity between control plots and plots subject to three disturbance intensities based on the foraging impact of these geese. Potential for long-term recovery was evaluated across different disturbance patch sizes. 3.,Resilience to disturbance varied between communities; those with higher moss cover and higher soil moisture, such as wetlands and mires, were most resilient to disturbance. 4.,The wetter communities demonstrated greater long-term recovery potential following disturbance. In wetland communities, vegetative recovery of vascular plants and moss was greater in smaller disturbed patches and at the edges of patches. 5.,The response of vegetation to disturbance varied with intensity of disturbance, plant community and plant species. The use of functional type classifications only partially explained the variation in species responses to disturbance across communities, thus their use in predicting community changes was limited. 6.,Synthesis. The impact of disturbance is shown to be plant-community specific and related to the initial abiotic and biotic properties of the community. By showing that resilience is partly predictable, the identification of disturbance-susceptible communities is possible, which is of relevance for ecosystem management. [source] Validation of plant functional types across two contrasting landscapesJOURNAL OF VEGETATION SCIENCE, Issue 2 2002Michael Kleyer Disturbance; Fertility; Logistic regression; Trait; Urban landscape Abstract. The validation of plant functional type models across contrasting landscapes is seen as a step towards the claim that plant functional types should recur regionally or even globally. I sampled the vegetation of an urban landscape on a range of sites representing gradients of resource supply and disturbance intensity. A group of plants with similar attributes was considered a ,functional type', if the species significantly co-occurred in a certain segment of the gradient plane of resource supply and disturbance intensity. Vegetative and regeneration traits were considered. A similar study was performed in a nearby agricultural landscape (Kleyer 1999). The logistic regression models from the urban landscape were applied to the data set of the agricultural landscape and vice versa. Although the overall environment of the two landscapes was very different, recurrent patterns of several functional types were found. At high fertility and high disturbance levels, annual species predominated with a persistent seed bank, high seed output, and short vertical expansion. When disturbances changed from below-ground to above-ground, the sexual regeneration mode was replaced by the vegetative mode, while vertical expansion remained low. At medium disturbance intensities, the vertical expansion and vegetative regeneration increased with fertility, while the seed bank remained mostly transient to short-term persistent and lateral expansion and sexual regeneration was intermediate. At low disturbances and low resource supplies, seed bank longevity, and vertical and lateral expansion tended to be long. Diversity of groups of plants with similar attributes was highest at intermediate disturbance levels and low fertility. These results correspond with Grime's humped-back model and Connell's intermediate disturbance hypothesis. [source] Parameter and state estimation in nonlinear stochastic continuous-time dynamic models with unknown disturbance intensityTHE CANADIAN JOURNAL OF CHEMICAL ENGINEERING, Issue 5 2008M. S. Varziri Abstract Approximate Maximum Likelihood Estimation (AMLE) is an algorithm for estimating the states and parameters of models described by stochastic differential equations (SDEs). In previous work (Varziri et al., Ind. Eng. Chem. Res., 47(2), 380-393, (2008); Varziri et al., Comp. Chem. Eng., in press), AMLE was developed for SDE systems in which process-disturbance intensities and measurement-noise variances were assumed to be known. In the current article, a new formulation of the AMLE objective function is proposed for the case in which measurement-noise variance is available but the process-disturbance intensity is not known a priori. The revised formulation provides estimates of the model parameters and disturbance intensities, as demonstrated using a nonlinear CSTR simulation study. Parameter confidence intervals are computed using theoretical linearization-based expressions. The proposed method compares favourably with a Kalman-filter-based maximum likelihood method. The resulting parameter estimates and information about model mismatch will be useful to chemical engineers who use fundamental models for process monitoring and control. L'estimation des vraisemblances maximums approximatives (AMLE) est un algorithme destiné à l'estimation des états et des paramètres de modèles décrits par les équations différentielles stochastiques (SDE). Dans un précédent travail (Varziri et al., 2008a, 2008b), l'AMLE a été mis au point pour des systèmes SDE dans lesquels les intensités de perturbations et les variances de bruits de mesure sont supposées connues. On propose dans cet article une nouvelle formulation de la fonction objectif de l'AMLE pour le cas où la variance de bruit de mesure est disponible mais où l'intensité des perturbations de procédé n'est pas connue a priori. La formulation révisée fournit des estimations des paramètres de modèle et des intensités de perturbations, comme le démontre une étude de simulation en CSTR non linéaire. Les intervalles de confiance des paramètres sont calculés par ordinateur au moyen d'expressions basées sur la linéarisation théorique. La méthode proposée se compare favorablement à une méthode de vraisemblance maximun reposant sur le filtre de Kalman. Les estimations de paramètres qui en résultent et l'information sur la discordance de modèle seront utiles aux ingénieurs en génie chimique qui utilisent des modèles fondamentaux pour la surveillance et le contrôle des procédés. [source] The relationship between soil seed bank, above-ground vegetation and disturbance intensity on old-field successional permanent plotsAPPLIED VEGETATION SCIENCE, Issue 4 2009Michaela Dölle Abstract Questions: How does disturbance and successional age influence richness, size and composition of the soil seed bank? What is the potential contribution of the soil seed bank to the plant community composition on sites differing in their successional age or disturbance intensity? Location: Experimental Botanical Garden of Göttingen University, central Germany. Methods: Above-ground vegetation and soil seed bank were studied on formerly arable fields in a 36-year-old permanent plot study with five disturbance intensities, ranging from yearly ploughing via mowing to long-term uninterrupted succession. We compared species compositions, seed densities and functional features of the seed bank and above-ground vegetation by using several methods in parallel. Results: The seed bank was mainly composed of early successional species typical of strongly disturbed habitats. The difference between seed bank composition and above-ground vegetation decreased with increasing disturbance intensity. The species of greatest quantitative importance in the seed bank was the non-native forb Solidago canadensis. Conclusions: The ability of a plant community to regenerate from the soil seed bank dramatically decreases with increasing time since abandonment (successional age) and with decreasing disturbance intensity. The present study underlines that plant species typical of grasslands and woodlands are limited by dispersal capacity, owing to low capacity for accumulation of seeds in the soil and the fact that most species do not build up persistent seed banks. Rare and target species were almost absent from the seed bank and will, after local elimination, depend on reintroduction for continuation of their presence. [source] Numerical simulation of turbulent flow through series stenosesINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 7 2003T. S. Lee Abstract The flow fields in the neighbourhoods of series vascular stenoses are studied numerically for the Reynolds numbers from 100 to 4000, diameter constriction ratios of 0.2,0.6 and spacing ratios of 1, 2, 3, 4 and ,. In this study, it has been further verified that in the laminar flow region, the numerical predictions by k,, turbulence model matched those by the laminar-flow modelling very well. This suggests that the k,, turbulence model is capable of the prediction of the laminar flow as well as the prediction of the turbulent stenotic flow with good accuracy. The extent of the spreading of the recirculation region from the first stenosis and its effects on the flow field downstream of the second stenosis depend on the stenosis spacing ratio, constriction ratio and the Reynolds number. For c1 = 0.5 with c2 , c1, the peak value of wall vorticity generated by the second stenosis is always less than that generated by the first stenosis. However, the maximum centreline velocity and turbulence intensity at the second stenosis are higher than those at the first stenosis. In contrast, for c1 = 0.5 with c2 = 0.6, the maximum values at the second stenosis are much higher than those at the first stenosis whether for centreline velocity and turbulence intensity or for wall vorticity. The peak values of the wall vorticity and the centreline disturbance intensity both grow up with the Reynolds number increasing. The present study shows that the more stenoses can result in a lower critical Reynolds number that means an earlier occurrence of turbulence for the stenotic flows. Copyright © 2003 John Wiley & Sons, Ltd. [source] Wind-throw mortality in the southern boreal forest: effects of species, diameter and stand ageJOURNAL OF ECOLOGY, Issue 6 2007ROY L. RICH Summary 1Patterns of tree mortality as influenced by species, diameter and stand age were assessed across a gradient in wind disturbance intensity in a southern boreal forest in Minnesota, USA. Few previous studies have addressed how wind impacts boreal forests where fire was historically the dominant type of disturbance. 2We surveyed 29 334 trees of nine species within a 236 000 ha blowdown in the Boundary Waters Canoe Area Wilderness (BWCAW), in forests that have never been logged and were not salvaged after the windstorm. Within the disturbed area, a range of disturbance severity from zero to complete canopy mortality was present, overlaying an existing mosaic of fire origin stands. For this study, we derived an index of wind disturbance intensity by standardizing the observed disturbance severity using common species with similar diameter at breast height (d.b.h.) distributions. We then used multiple logistic regression to assess patterns of tree mortality across gradients in tree size and wind intensity index, and for three stand ages. 3Probability of mortality was higher with increasing ln d.b.h. for all nine species, with two species (Abies balsamea and Picea mariana) showing much more dramatic shifts in mortality with d.b.h. than the others. As hypothesized, the species most susceptible to windthrow at all d.b.h. classes were early successional and shade intolerant (Pinus banksiana, Pinus resinosa, Populus tremuloides) and those least susceptible were generally shade tolerant (e.g. Thuja occidentalis, Acer rubrum), although the intolerant species Betula papyrifera also had low mortality. 4Mortality rates were higher in mature (c. 90 years old) stands than for old and very old (c. 126,200 years old) stands, probably because old stands had already gone through transition to a multi-aged stage of development. 5Synthesis. Quantification of canopy mortality patterns generally supports disturbance-mediated accelerated succession following wind disturbance in the southern boreal forest. This wind-induced weeding of the forest favoured Thuja occidentalis, Betula papyrifera and Acer rubrum trees of all sizes, along with small Abies balsamea and Picea mariana trees. Overall, the net impact of wind disturbance must concurrently consider species mortality probability, abundance and diameter distributions. [source] Leaf dry matter content and lateral spread predict response to land use change for six subalpine grassland speciesJOURNAL OF VEGETATION SCIENCE, Issue 2 2007Nicolas Gross Abstract Question: Land-use change has a major impact on terrestrial plant communities by affecting fertility and disturbance. We test how particular combinations of plant functional traits can predict species responses to these factors and their abundance in the field by examining whether trade-offs at the trait level (fundamental trade-offs) are linked to trade-offs at the response level (secondary trade-offs). Location: Central French Alps. Methods: We conducted a pot experiment in which we characterized plant trait syndromes by measuring whole plant and leaf traits for six dominant species, originating from contrasting subalpine grassland types. We characterized their response to nutrient availability, shading and clipping. We quantified factors linked with different land usage in the field to test the relevance of our experimental treatments. Results: We showed that land management affected nutrient concentration in soil, light availability and disturbance intensity. We identified particular suites of traits linked to plant stature and leaf structure which were associated with species responses to these environmental factors. Leaf dry matter content separates fast and slow growing species. Height and lateral spread separated tolerant and intolerant species to shade and clipping. Discussion and Conclusion: Two fundamental trade-offs based on stature traits and leaf traits were linked to two secondary trade-offs based on response to fertilization shade and mowing. Based on these trade-offs, we discuss four different species strategies which could explain and predict species distributions and traits syndrome at community scale under different land-uses in subalpine grasslands. [source] The role of the seed bank, seed rain and the timing of disturbance in gap regenerationJOURNAL OF VEGETATION SCIENCE, Issue 1 2005R.J. Pakeman Abstract. Question: Does the degree and timing of disturbance contribute significantly to the pattern and process of regeneration in plant communities as a consequence of the availability and number of species of propagules present? Location: Acid grassland at 230 m a.s.l., eastern Scotland, UK. Methods : Plots were surface disturbed or had their soil profile inverted at monthly intervals at 12 dates during a year. Seed bank and seed rain were assessed at each treatment time. The effect of disturbance intensity and timing on the regenerating vegetation was assessed. Results: Removing the seed bank significantly slowed regeneration, as it contributed 43 % of developing cover after one year where it was present. At an individual seed level, seed in the seed rain had a much higher likelihood of contributing to the regenerating vegetation than a seed in the seed bank. Some species showed a reliance on the seed bank for regeneration, and hence there was a significant difference in the vegetation that developed between plots with the seed bank intact and those where it was removed. Winter disturbed plots (little seed rain) had slower rates of re-vegetation than summer disturbed plots. Timing had little effect on species composition, though a significantly higher cover of perennial forb species developed on the winter disturbed plots. Conclusion: Removing the contribution of the seed bank had a greater effect on the composition of regenerating vegetation than the effect of seasonal variation on the seed rain. [source] Validation of plant functional types across two contrasting landscapesJOURNAL OF VEGETATION SCIENCE, Issue 2 2002Michael Kleyer Disturbance; Fertility; Logistic regression; Trait; Urban landscape Abstract. The validation of plant functional type models across contrasting landscapes is seen as a step towards the claim that plant functional types should recur regionally or even globally. I sampled the vegetation of an urban landscape on a range of sites representing gradients of resource supply and disturbance intensity. A group of plants with similar attributes was considered a ,functional type', if the species significantly co-occurred in a certain segment of the gradient plane of resource supply and disturbance intensity. Vegetative and regeneration traits were considered. A similar study was performed in a nearby agricultural landscape (Kleyer 1999). The logistic regression models from the urban landscape were applied to the data set of the agricultural landscape and vice versa. Although the overall environment of the two landscapes was very different, recurrent patterns of several functional types were found. At high fertility and high disturbance levels, annual species predominated with a persistent seed bank, high seed output, and short vertical expansion. When disturbances changed from below-ground to above-ground, the sexual regeneration mode was replaced by the vegetative mode, while vertical expansion remained low. At medium disturbance intensities, the vertical expansion and vegetative regeneration increased with fertility, while the seed bank remained mostly transient to short-term persistent and lateral expansion and sexual regeneration was intermediate. At low disturbances and low resource supplies, seed bank longevity, and vertical and lateral expansion tended to be long. Diversity of groups of plants with similar attributes was highest at intermediate disturbance levels and low fertility. These results correspond with Grime's humped-back model and Connell's intermediate disturbance hypothesis. [source] Effects of disturbance intensity and frequency on early old-field successionJOURNAL OF VEGETATION SCIENCE, Issue 5 2001B. Collins Radford et al. (1968) Abstract. Early old-field succession provides a model system for examining vegetation response to disturbance frequency and intensity within a manageable time scale. Disturbance frequency and intensity can interact with colonization and competition to influence relative abundance of earlier and later successional species and determine, respectively, how often and how far succession can be reset. We tested the joint effects of disturbance frequency and intensity on vegetation response (species richness, abundance, canopy structure) during the first six years of succession by clipping the dominant species (D) or all species (T) in spring and fall of each year (S), once per year in summer (Y1), each two years in summer (Y2), or each four years in summer (Y4). Vegetation response reflected disturbance effects on expansion of a later monospecific dominant perennial herb, Solidago altissima, and persistence of the early, richer flora of annuals. A more abundant and taller top Solidago canopy developed on plots clipped each 2 yr or less frequently. Plots clipped yearly or seasonally were richer, but had less abundant, shorter, and differently stratified canopy. Disturbance mediated the relative abundance of early and later successional species; however, frequency and intensity effects were not completely congruent. Persistence of a richer early successional flora increased through the most frequent disturbance (S), and was magnified by disturbance intensity. Disturbance as extreme as clipping all vegetation twice yearly did not cause a drop in species richness, but maintained the early successional community over the first six years of succession. We conclude that clipping disturbance influenced the rate of succession, but the early community could rebound through the range of disturbance frequency and intensity tested. [source] Parameter and state estimation in nonlinear stochastic continuous-time dynamic models with unknown disturbance intensityTHE CANADIAN JOURNAL OF CHEMICAL ENGINEERING, Issue 5 2008M. S. Varziri Abstract Approximate Maximum Likelihood Estimation (AMLE) is an algorithm for estimating the states and parameters of models described by stochastic differential equations (SDEs). In previous work (Varziri et al., Ind. Eng. Chem. Res., 47(2), 380-393, (2008); Varziri et al., Comp. Chem. Eng., in press), AMLE was developed for SDE systems in which process-disturbance intensities and measurement-noise variances were assumed to be known. In the current article, a new formulation of the AMLE objective function is proposed for the case in which measurement-noise variance is available but the process-disturbance intensity is not known a priori. The revised formulation provides estimates of the model parameters and disturbance intensities, as demonstrated using a nonlinear CSTR simulation study. Parameter confidence intervals are computed using theoretical linearization-based expressions. The proposed method compares favourably with a Kalman-filter-based maximum likelihood method. The resulting parameter estimates and information about model mismatch will be useful to chemical engineers who use fundamental models for process monitoring and control. L'estimation des vraisemblances maximums approximatives (AMLE) est un algorithme destiné à l'estimation des états et des paramètres de modèles décrits par les équations différentielles stochastiques (SDE). Dans un précédent travail (Varziri et al., 2008a, 2008b), l'AMLE a été mis au point pour des systèmes SDE dans lesquels les intensités de perturbations et les variances de bruits de mesure sont supposées connues. On propose dans cet article une nouvelle formulation de la fonction objectif de l'AMLE pour le cas où la variance de bruit de mesure est disponible mais où l'intensité des perturbations de procédé n'est pas connue a priori. La formulation révisée fournit des estimations des paramètres de modèle et des intensités de perturbations, comme le démontre une étude de simulation en CSTR non linéaire. Les intervalles de confiance des paramètres sont calculés par ordinateur au moyen d'expressions basées sur la linéarisation théorique. La méthode proposée se compare favorablement à une méthode de vraisemblance maximun reposant sur le filtre de Kalman. Les estimations de paramètres qui en résultent et l'information sur la discordance de modèle seront utiles aux ingénieurs en génie chimique qui utilisent des modèles fondamentaux pour la surveillance et le contrôle des procédés. [source] The relationship between soil seed bank, above-ground vegetation and disturbance intensity on old-field successional permanent plotsAPPLIED VEGETATION SCIENCE, Issue 4 2009Michaela Dölle Abstract Questions: How does disturbance and successional age influence richness, size and composition of the soil seed bank? What is the potential contribution of the soil seed bank to the plant community composition on sites differing in their successional age or disturbance intensity? Location: Experimental Botanical Garden of Göttingen University, central Germany. Methods: Above-ground vegetation and soil seed bank were studied on formerly arable fields in a 36-year-old permanent plot study with five disturbance intensities, ranging from yearly ploughing via mowing to long-term uninterrupted succession. We compared species compositions, seed densities and functional features of the seed bank and above-ground vegetation by using several methods in parallel. Results: The seed bank was mainly composed of early successional species typical of strongly disturbed habitats. The difference between seed bank composition and above-ground vegetation decreased with increasing disturbance intensity. The species of greatest quantitative importance in the seed bank was the non-native forb Solidago canadensis. Conclusions: The ability of a plant community to regenerate from the soil seed bank dramatically decreases with increasing time since abandonment (successional age) and with decreasing disturbance intensity. The present study underlines that plant species typical of grasslands and woodlands are limited by dispersal capacity, owing to low capacity for accumulation of seeds in the soil and the fact that most species do not build up persistent seed banks. Rare and target species were almost absent from the seed bank and will, after local elimination, depend on reintroduction for continuation of their presence. [source] Seed production in fens and fen meadows along a disturbance gradientAPPLIED VEGETATION SCIENCE, Issue 3 2009A. Klimkowska Abstract Question: The seed production in several wetland communities across Europe was investigated and differences in seed output in relation to disturbance intensity were tested. The relationship between the vegetation composition and the seed production profile was examined and the results are discussed in relation to restoration. Location: Poland, Germany and the Netherlands. Methods: The seed production in various plant communities was estimated, based on field counts. In addition, records from available databases were used for missing data. Multivariate methods were used to characterize the vegetation and seed production. Communities were grouped according to level of disturbance and tested for differences in seed production. Similarity between vegetation composition and seed profile was examined using the Sørensen index and Spearman correlation coefficient. Results: It was found that the seed production of the studied communities is large, variable and in general increasing with disturbance intensity. The estimated median seed production was ca. 24 × 103 seeds m,2 in fens, 167 × 103 in fen meadows and 556 × 103 seeds m,2 in degraded meadows. The majority of seeds was produced by just a few species. The similarity between the vegetation composition and the seed production profile was low (similarity 52%, correlation coefficient 0.42, P<0.05) and slightly increased with disturbance intensity. Conclusions: Increased disturbance enhances seed production at the community level. The composition of the vegetation is a poor predictor of the seed output. It is estimated that the number of seeds transferred with hay is much lower than the seed production in fens and fen meadows. [source] | ||||||||||