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Major Disturbance (major + disturbance)
Selected AbstractsPostfire response of North American boreal forest net primary productivity analyzed with satellite observationsGLOBAL CHANGE BIOLOGY, Issue 8 2003Jeffrey A. Hicke Abstract Fire is a major disturbance in the boreal forest, and has been shown to release significant amounts of carbon (C) to the atmosphere through combustion. However, less is known about the effects on ecosystems following fire, which include reduced productivity and changes in decomposition in the decade immediately following the disturbance. In this study, we assessed the impact of fire on net primary productivity (NPP) in the North American boreal forest using a 17-year record of satellite NDVI observations at 8- km spatial resolution together with a light-use efficiency model. We identified 61 fire scars in the satellite observations using digitized fire burn perimeters from a database of large fires. We studied the postfire response of NPP by analyzing the most impacted pixel within each burned area. NPP decreased in the year following the fire by 60,260 g C m,2 yr,1 (30,80%). By comparing pre- and postfire observations, we estimated a mean NPP recovery period for boreal forests of about 9 years, with substantial variability among fires. We incorporated this behavior into a carbon cycle model simulation to demonstrate these effects on net ecosystem production. The disturbance resulted in a release of C to the atmosphere during the first 8 years, followed by a small, but long-lived, sink lasting 150 years. Postfire net emissions were three times as large as from a model run without changing NPP. However, only small differences in the C cycle occurred between runs after 8 years due to the rapid recovery of NPP. We conclude by discussing the effects of fire on the long-term continental trends in satellite NDVI observed across boreal North America during the 1980s and 1990s. [source] Season of grazing and stocking rate interactively affect fuel loads in Baikiaea plurijuga Harms woodland in northwestern ZimbabweAFRICAN JOURNAL OF ECOLOGY, Issue 4 2008J. Gambiza Abstract Wildfire is a major disturbance in Baikiaea plurijuga Harms woodland savannas. We tested the hypothesis that the timing and intensity of herbivory influence fuel loads. We used three stocking rates namely light (three cows and four goats ha,1), medium (six cows and eight goats ha,1) and heavy (eleven cows and sixteen goats ha,1) and three times of grazing namely early-, middle- and late-growing seasons. Season of grazing and stocking rate influenced herbaceous phytomass. Phytomass was generally the highest (53.5 g DM m,2) in paddocks grazed during the early growing season and the lowest (27.8 g DM m,2) in those grazed during the late growing season. Phytomass was also generally the highest (40.4 g DM m,2) in lightly stocked paddocks and the lowest (32.7 g DM m,2) in heavily stocked ones. Litter mass was the lowest (160.8 g DM m,2) in paddocks grazed during the early season whereas there were no differences in ungrazed paddocks and those grazed during either mid- or late growing seasons (205.4 g DM m,2). There was a negative relationship between litter mass and stocking rate. Baikiaea Benth. woodlands should be grazed during either the mid- or late-growing season at stocking rates greater than 0.1 LU ha,1 to reduce grass fuel loads. Résumé Les feux de brousse sont une perturbation majeure dans les savanes arborées àBaikiaea plurijuga Harms. Nous avons testé l'hypothèse selon laquelle le timing et l'intensité de la consommation par les herbivores influenceraient la quantité de combustible. Nous avons utilisé trois taux de pâturage: léger (trois vaches et quatre chèvres par hectare), moyen (six vaches et huit chèvres par hectare) et élevé (11 vaches et 16 chèvres par hectare), et trois périodes de pâturage, à savoir au début, au milieu et à la fin de la période de croissance. La période de pâturage et le taux de pâturage influençaient la phytomasse herbacée. La phytomasse était généralement la plus élevée (53.5 g MS m,2) dans les enclos pâturés au début de la saison de croissance, et la plus faible (27.8 g MS m,2) dans ceux qui sont pâturés en fin de période de croissance. La phytomasse était aussi généralement la plus grande (40.4 g MS m,2) dans les enclos légèrement pâturés et la plus basse (32.7 g MS m,2) dans les enclos très pâturés. La biomasse végétale (litière) était la plus faible (160.8 g MS m,2) dans les enclos pâturés au début de la saison, alors qu'il n'y avait pas de différence entre les enclos non pâturés et ceux qui l'étaient au milieu ou à la fin de la période de croissance (205.4 g MS m,2). Il y avait une relation négative entre la masse de litière et le taux de pâturage. Les forêts àBaikiaea Benth. devraient être pâturés en milieu ou en fin de période de croissance, et à des taux plus élevés que 0.1 UFL ha,1, pour réduire la quantité de combustible herbacé. [source] Fruit dispersal and seed banks in Atriplex sagittata: the role of heterocarpyJOURNAL OF ECOLOGY, Issue 2 2001Bohumil Mandák Summary 1Atriplex sagittata, an arly succesional, annual species of disturbed habitats in Central Europe, produces three types of fruits. We studied the differences in dispersal between the fruit types in order to investigate their ecological roles. 2The typical positive relationship between dispersal ability and germinability is not found in this species. Type A (ebracteate) fruits produced deeply dormant seeds and had the lowest dispersal potential, but of the two fruits with bracts, type B, with dormant seeds, was better dispersed by both water and wind than type C, which produces non-dormant seeds. 3Wind, temperature and precipitation have significant effects on fruit release but their effects differ between fruit types. The release of fruit types with bracts was positively correlated with wind whereas all fruit types tended to be negatively correlated with precipitation and temperature range. 4Type C fruit, which contains non-dormant seed, are absent from the soil in summer and have a Type II transient seed bank. Type A and B fruits, containing dormant seeds, form a persistent seed bank. 5Heterocarpy, where fruit types show distinct ecological behaviour, enables colonizing species such as A. sagittata to survive both major disturbance (by ensuring that some seeds persist) and unfavourable conditions (by spreading germination over a long period). 6In A. sagittata, seed dynamics can be explained by the germination behaviour of seeds produced by the three types of fruit. All fruit types mature in autumn, but few of Type A fall from the mother plant until spring, when germination is probably inhibited because of insufficient stratification. Type C fruit, however, show peak dispersal in winter and the majority of these non-dormant seeds are able to germinate as soon as conditions become more suitable. [source] Is the productivity of vegetation plots higher or lower when there are more species?OIKOS, Issue 2 2003Variable predictions from interaction of the, competitive dominance effect' on the habitat templet, sampling effect' Using a habitat templet model, we predict that the productivity (total biomass) of plots within a plant community may be positively, negatively or not at all related to variation in the number of species per plot, depending on successional stage (time since major disturbance) and habitat carrying capacity (reflecting the total resource supplying power of the habitat). For plots of a given size, a positive relationship between productivity and species richness is predicted in recently disturbed habitats because local neighbourhoods here will have been assembled largely stochastically, usually from a pool of available species with a right-skewed size frequency distribution. Hence, in the earliest stages of succession, plots will have relatively high total biomass only if they contain at least some of the relatively uncommon larger species which will, in turn, be more likely in those neighbourhoods that contain more species (the sampling effect). Among these will also be some of the more common smaller species; hence, these high biomass, species-rich plots should have relatively low species evenness, in contrast to what is predicted under effects involving species complementarity. In late succession, the plots with high total biomass will still be those that contain relatively large species but these plots will now contain relatively few species owing to increased competitive exclusion over time (the competitive dominance effect). In intermediate stages of succession, no relationship between plot productivity and species richness is predicted because the opposing sampling and competitive dominance effects cancel each other out. We predict that the intensity of both the sampling and competitive dominance effects on the productivity/species richness relationship will decrease with decreasing habitat carrying capacity (e.g. decreasing substrate fertility) owing to the inherently lower variance in between-plot productivity that is predicted for more resource-impoverished habitats. [source] Alcohol and violence and the possible role of serotoninCRIMINAL BEHAVIOUR AND MENTAL HEALTH, Issue 1 2003Abdulla A.-B. Background There is undisputed evidence linking alcohol consumption and violence and other forms of aggressive behaviour, and also linking aggression with dysfunction of the brain indolylamine serotonin (5-hydroxytryptamine or 5-HT). Alcohol consumption also causes major disturbances in the metabolism of brain serotonin. In particular, acute alcohol intake depletes brain serotonin levels in normal (non-alcohol-dependent) subjects. On the basis of the above statements, it is suggested that, at the biological level, alcohol may induce aggressive behaviour in susceptible individuals, at least in part, by inducing a strong depletion of brain serotonin levels. Aims In this article, evidence supporting these interrelationships and interactions will be summarized and discussed, the alcohol,serotonin,aggression hypothesis will be reiterated, and potential intervention strategies will be proposed. Copyright © 2003 Whurr Publishers Ltd. [source] Projecting future fire activity in AmazoniaGLOBAL CHANGE BIOLOGY, Issue 5 2003MANOEL F. CARDOSO Abstract Fires are major disturbances for ecosystems in Amazonia. They affect vegetation succession, alter nutrients and carbon cycling, and modify the composition of the atmosphere. Fires in this region are strongly related to land-use, land-cover and climate conditions. Because these factors are all expected to change in the future, it is reasonable to expect that fire activity will also change. Models are needed to quantitatively estimate the magnitude of these potential changes. Here we present a new fire model developed by relating satellite information on fires to corresponding statistics on climate, land-use and land-cover. The model is first shown to reproduce the main contemporary large-scale features of fire patterns in Amazonia. To estimate potential changes in fire activity in the future, we then applied the model to two alternative scenarios of development of the region. We find that in both scenarios, substantial changes in the frequency and spatial patterns of fires are expected unless steps are taken to mitigate fire activity. [source] Patterns of genetic variation do not correlate with geographical distance in the reef-building coral Pocillopora meandrina in the South PacificMOLECULAR ECOLOGY, Issue 7 2005H. MAGALON Abstract Dispersal may be a critical factor in the ability of reef-building corals to recover after major disturbances. We studied patterns of geographical structure using four microsatellite markers in seven South Pacific populations of Pocillopora meandrina, a major coral species from Polynesia. Variation within populations showed evidence of heterozygote deficiency. Genetic differentiation between populations was detected at a large scale (2000 km) between the Tonga and the Society Islands. Within the Society Islands, four of the five studied populations from Bora Bora, Moorea and Tahiti were not significantly different from each other. Unexpectedly, one of the three populations surveyed in Moorea was genetically different from the other two populations of this island (that were 5 and 10 km apart), and from the populations of the other two surveyed islands in this archipelago. We cannot rule out the possibility that this pattern is an equilibrium state, whereby short-range dispersal is locally more differentiating than long-range dispersal, as has been suggested by similar patterns reported in other studies. An alternative explanation that is globally consistent with all observations is that this is the signature of a large-scale destruction event, as for instance a bleaching event, followed by the recent restoration of populations by new colonists. [source] | ||||||||||