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Woody Vegetation (woody + vegetation)
Terms modified by Woody Vegetation Selected AbstractsThe role of environmental gradients in non-native plant invasion into burnt areas of Yosemite National Park, CaliforniaDIVERSITY AND DISTRIBUTIONS, Issue 2 2006Rob Klinger ABSTRACT Fire is known to facilitate the invasion of many non-native plant species, but how invasion into burnt areas varies along environmental gradients is not well-understood. We used two pre-existing data sets to analyse patterns of invasion by non-native plant species into burnt areas along gradients of topography, soil and vegetation structure in Yosemite National Park, California, USA. A total of 46 non-native species (all herbaceous) were recorded in the two data sets. They occurred in all seven of the major plant formations in the park, but were least common in subalpine and upper montane conifer forests. There was no significant difference in species richness or cover of non-natives between burnt and unburnt areas for either data set, and environmental gradients had a stronger effect on patterns of non-native species distribution, abundance and species composition than burning. Cover and species richness of non-natives had significant positive correlations with slope (steepness) and herbaceous cover, while species richness had significant negative correlations with elevation, the number of years post-burn, and cover of woody vegetation. Non-native species comprised a relatively minor component of the vegetation in both burnt and unburnt areas in Yosemite (percentage species = 4%, mean cover < 6.0%), and those species that did occur in burnt areas tended not to persist over time. The results indicate that in many western montane ecosystems, fire alone will not necessarily result in increased rates of invasion into burnt areas. However, it would be premature to conclude that non-native species could not affect post-fire succession patterns in these systems. Short fire-return intervals and high fire severity coupled with increased propagule pressure from areas used heavily by humans could still lead to high rates of invasion, establishment and spread even in highly protected areas such as Yosemite. [source] Processes and forms of an unstable alluvial system with resistant, cohesive streambeds ,EARTH SURFACE PROCESSES AND LANDFORMS, Issue 7 2002Andrew Simon Abstract As a response to channelization projects undertaken near the turn of the 20th century and in the late 1960s, upstream reaches and tributaries of the Yalobusha River, Mississippi, USA, have been rejuvenated by upstream-migrating knickpoints. Sediment and woody vegetation delivered to the channels by mass failure of streambanks has been transported downstream to form a large sediment/debris plug where the downstream end of the channelized reach joins an unmodified sinuous reach. Classification within a model of channel evolution and analysis of thalweg elevations and channel slopes indicates that downstream reaches have equilibrated but that upstream reaches are actively degrading. The beds of degrading reaches are characterized by firm, cohesive clays of two formations of Palaeocene age. The erodibility of these clay beds was determined with a jet-test device and related to critical shear stresses and erosion rates. Repeated surveys indicated that knickpoint migration rates in these clays varied from 0·7 to 12 m a,1, and that these rates and migration processes are highly dependent upon the bed substrate. Resistant clay beds of the Porters Creek Clay formation have restricted advancement of knickpoints in certain reaches and have caused a shift in channel adjustment processes towards bank failures and channel widening. Channel bank material accounts for at least 85 per cent of the material derived from the channel boundaries of the Yalobusha River system. Strategies to reduce downstream flooding problems while preventing upstream erosion and land loss are being contemplated by action agencies. One such proposal involves removal of the sediment/debris plug. Bank stability analyses that account for pore-water and confining pressures have been conducted for a range of hydrologic conditions to aid in predicting future channel response. If the sediment/debris plug is removed to improve downstream drainage, care should be taken to provide sufficient time for drainage of groundwater from the channel banks so as not to induce accelerated bank failures. Published in 2002 John Wiley & Sons, Ltd. [source] INTEGRATED LANDSCAPE ANALYSES OF CHANGE OF MIOMBO WOODLAND IN TANZANIA AND ITS IMPLICATION FOR ENVIRONMENT AND HUMAN LIVELIHOODGEOGRAFISKA ANNALER SERIES A: PHYSICAL GEOGRAPHY, Issue 1 2009LENNART STRÖMQUIST ABSTRACT. Landscapes bear witness to past and present natural and societal processes influencing the environment and human livelihoods. By analysing landscape change at different spatial scales over time the effects on the environment and human livelihoods of various external and internal driving forces of change can be studied. This paper presents such an analysis of miombo woodland surrounding the Mkata plains in central Tanzania. The rich natural landscape diversity of the study area in combination with its historical and political development makes it an ideal observation ground for this kind of study. The paper focuses on long-term physical and biological changes, mainly based on satellite information but also on field studies and a review of documents and literature. The miombo woodlands are highly dynamic semi-arid ecosystems found on a number of nutrient-poor soil groups. Most of the woodlands are related to an old, low-relief geomorphology of erosion surfaces with relatively deep and leached soils, or to a lesser extent also on escarpments and steep Inselberg slopes with poor soils. Each period in the past has cast its footprints on the landscape development and its potential for a sustainable future use. On a regional level there has been a continual decrease in forest area over time. Expansion of agriculture around planned villages, implemented during the 1970s, in some cases equals the loss of forest area (Mikumi-Ulaya), whilst in other areas (Kitulangalo), the pre-independence loss of woodland was small; the agricultural area was almost the same during the period 1975,1999, despite the fact that forests have been lost at an almost constant rate over the same period. Illegal logging and charcoal production are likely causes because of the proximity to the main highway running through the area. Contrasting to the general regional pattern are the conditions in a traditional village (Ihombwe), with low immigration of people and a maintained knowledge of the resource potential of the forest with regards to edible plants and animals. In this area the local community has control of the forest resources in a Forest Reserve, within which the woody vegetation has increased in spite of an expansion of agriculture on other types of village land. The mapping procedure has shown that factors such as access to transport and lack of local control have caused greater deforestation of certain areas than during the colonial period. Planned villages have furthermore continued to expand over forest areas well after their implementation, rapidly increasing the landscape fragmentation. One possible way to maintain landscape and biodiversity values is by the sustainable use of traditional resources, based on local knowledge of their management as illustrated by the little change observed in the traditionally used area. [source] Shrub expansion stimulates soil C and N storage along a coastal soil chronosequenceGLOBAL CHANGE BIOLOGY, Issue 7 2010STEVEN T. BRANTLEY Abstract Expansion of woody vegetation in grasslands is a worldwide phenomenon with implications for C and N cycling at local, regional and global scales. Although woody encroachment is often accompanied by increased annual net primary production (ANPP) and increased inputs of litter, mesic ecosystems may become sources for C after woody encroachment because stimulation of soil CO2 efflux releases stored soil carbon. Our objective was to determine if young, sandy soils on a barrier island became a sink for C after encroachment of the nitrogen-fixing shrub Morella cerifera, or if associated stimulation of soil CO2 efflux mitigated increased litterfall. We monitored variations in litterfall in shrub thickets across a chronosequence of shrub expansion and compared those data to previous measurements of ANPP in adjacent grasslands. In the final year, we quantified standing litter C and N pools in shrub thickets and soil organic matter (SOM), soil organic carbon (SOC), soil total nitrogen (TN) and soil CO2 efflux in shrub thickets and adjacent grasslands. Heavy litterfall resulted in a dense litter layer storing an average of 809 g C m,2 and 36 g N m,2. Although soil CO2 efflux was stimulated by shrub encroachment in younger soils, soil CO2 efflux did not vary between shrub thickets and grasslands in the oldest soils and increases in CO2 efflux in shrub thickets did not offset contributions of increased litterfall to SOC. SOC was 3.6,9.8 times higher beneath shrub thickets than in grassland soils and soil TN was 2.5,7.7 times higher under shrub thickets. Accumulation rates of soil and litter C were highest in the youngest thicket at 101 g m,2 yr,1 and declined with increasing thicket age. Expansion of shrubs on barrier islands, which have low levels of soil carbon and high potential for ANPP, has the potential to significantly increase ecosystem C sequestration. [source] Implications of future climate and atmospheric CO2 content for regional biogeochemistry, biogeography and ecosystem services across East AfricaGLOBAL CHANGE BIOLOGY, Issue 2 2010RUTH M. DOHERTY Abstract We model future changes in land biogeochemistry and biogeography across East Africa. East Africa is one of few tropical regions where general circulation model (GCM) future climate projections exhibit a robust response of strong future warming and general annual-mean rainfall increases. Eighteen future climate projections from nine GCMs participating in the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment were used as input to the LPJ dynamic global vegetation model (DGVM), which predicted vegetation patterns and carbon storage in agreement with satellite observations and forest inventory data under the present-day climate. All simulations showed future increases in tropical woody vegetation over the region at the expense of grasslands. Regional increases in net primary productivity (NPP) (18,36%) and total carbon storage (3,13%) by 2080,2099 compared with the present-day were common to all simulations. Despite decreases in soil carbon after 2050, seven out of nine simulations continued to show an annual net land carbon sink in the final decades of the 21st century because vegetation biomass continued to increase. The seasonal cycles of rainfall and soil moisture show future increases in wet season rainfall across the GCMs with generally little change in dry season rainfall. Based on the simulated present-day climate and its future trends, the GCMs can be grouped into four broad categories. Overall, our model results suggest that East Africa, a populous and economically poor region, is likely to experience some ecosystem service benefits through increased precipitation, river runoff and fresh water availability. Resulting enhancements in NPP may lead to improved crop yields in some areas. Our results stand in partial contradiction to other studies that suggest possible negative consequences for agriculture, biodiversity and other ecosystem services caused by temperature increases. [source] Dynamical effects of the statistical structure of annual rainfall on dryland vegetationGLOBAL CHANGE BIOLOGY, Issue 5 2006CHRISTOPHER A. WILLIAMS Abstract In this study, we extend a model of daily dryland dynamics by parameterizing a modified version of a minimalistic annual model to examine how the statistical structure of annual rainfall and grazing intensity interact to influence dryland vegetation. With a Monte Carlo approach, an ensemble outcome provides a statistical description of likely dryland vegetation dynamics responding to variations in rainfall structure and grazing intensity. Results suggest that increased rainfall variability decreases the average and increases the variability of grass cover leading to more frequent degradation of the grass resource. Vegetation of drier regions is found to be more sensitive to interannual variability in rainfall. Concentrating this variability into an organized periodic mode further decreases the mean and increases the variability of grass cover. Hence, a shift toward lower, more variable, or more inter-annually correlated annual rainfall will likely lead to a general decrease in the grass resource and increased dryland vulnerability to degradation. Higher grazing intensity or lower annual rainfall both lead to more frequent and longer duration degradation of the grass condition. We note an interesting interaction in the response of grass biomass to grazing intensity and rainfall variability, where increased rainfall variability leads to longer duration degradation for low grazing, but shorter periods of degradation for high grazing. Once grass reaches a degraded condition, we find that woody vegetation strongly suppresses recovery even if successive rainfall is high. Overall, these findings suggest that the projected increase in interannual rainfall variability will likely decrease grass cover and potentially lead to more frequent, longer lasting degradation of dryland vegetation, particularly if enhanced rainfall variability is concentrated in long period (e.g. decadal) modes. [source] Climatic signals in growth and its relation to ENSO events of two Prosopis species following a latitudinal gradient in South AmericaGLOBAL CHANGE BIOLOGY, Issue 5 2006BERNAT C. LÓPEZ Abstract Semiarid environments throughout the world have lost a major part of their woody vegetation and biodiversity due to the effects of wood cutting, cattle grazing and subsistence agriculture. The resulting state is typically used for cattle production, but the productivity of these systems is often very low, and erosion of the unprotected soil is a common problem. Such dry-land degradation is of great international concern, not only because the resulting state is hardly productive but also because it paves the way to desertification. The natural distribution of the genus Prosopis includes arid and semiarid zones of the Americas, Africa and Asia, but the majority of the Prosopis species are, however, native to the Americas. In order to assess a likely gradient in the response of tree species to precipitation, temperature and their connection to El Niño southern oscillation (ENSO) events, two Prosopis species were chosen along a latitudinal gradient in Latin America, from northern Peru to central Chile: Prosopis pallida from a semi-arid land in northern and southern Peru and P. chilensis from a semiarid land in central Chile. Growth rings of each species were crossdated at each sampling site using classical dendrochronological techniques. Chronologies were related with instrumental climatic records in each site, as well as with SOI and N34 series. Cross-correlation, spectral and wavelet analysis techniques were used to assess the relation of growth with precipitation and temperature. Despite the long distance among sites, the two Prosopis species presented similar responses. Thus, the two species' growth is positively correlated to precipitation, while with temperature it is not. In northern Peru, precipitation and growth of P. pallida present a similar cyclic pattern, with a period of around 3 years. On the other hand, P. pallida in southern Peru, and P. chilensis also present this cyclic pattern, but also another one with lower frequency, coinciding with the pattern of precipitation. Both cycles are within the range of the ENSO band. [source] Habitat influences on urban avian assemblagesIBIS, Issue 1 2009KARL L. EVANS Urbanization is increasing across the globe and there is growing interest in urban ecology and a recognition that developed areas may be important for conservation. We review the factors influencing urban avian assemblages, focusing on habitat type and anthropogenic resource provision, and analyse data from a common bird monitoring scheme to assess some of these issues. The review suggests that (1) local factors are more important than regional ones in determining the species richness of urban avian assemblages, raising the potential for the management of urban sites to deliver conservation; (2) habitat fragmentation frequently influences urban avian assemblages, with the effects of patch size being greater than those of isolation, and (3) urban bird assemblages appear to respond positively to increasing the structural complexity, species richness of woody vegetation and supplementary feeding, and negatively to human disturbance. Data from Britain's Breeding Bird Survey, combined with habitat data obtained from aerial photographs, were used to assess a number of these issues at the resolution of 1-km squares. Green-space constituted 45% of these squares, and domestic gardens contributed 50% of this green-space, though their contribution to large continuous patches of green-space was negligible. There was no significant positive correlation between the densities of individual species in urban areas and surrounding rural areas. Rural species richness declined with increasing latitude, but urban species richness was not correlated with latitude. This contrast contributes to slightly higher avian species richness in rural squares in Southern England than urban ones. Occupancy and abundance were strongly positively correlated in urban avian assemblages, and some indicator species of conservation concern occurred in few urban areas and at low densities. Such species will require conservation action to be precisely targeted within urban areas. Of the urban indicators of conservation concern, only the House Sparrow Passer domesticus and Common Starling Sturnus vulgaris were more abundant in urban than rural areas. Moreover, the densities of these two species were strongly and positively correlated, indicating that they may be limited by shared resources, such as nest-sites or supplementary food. There was little evidence that high densities of nest-predating corvids were associated with reduced densities of their prey species. Species richness and the densities of individual species frequently declined with an increasing number of buildings. Current trends for the densification of many British urban areas are thus likely to be detrimental for many bird species. [source] Conundrums in mixed woody,herbaceous plant systemsJOURNAL OF BIOGEOGRAPHY, Issue 11 2003Joanna I. House Abstract Aims To identify approaches to improve our understanding of, and predictive capability for, mixed tree,grass systems. Elucidation of the interactions, dynamics and determinants, and identification of robust generalizations that can be broadly applied to tree,grass systems would benefit ecological theory, modelling and land management. Methods A series of workshops brought together scientific expertise to review theory, data availability, modelling approaches and key questions. Location Ecosystems characterized by mixtures of herbaceous and woody plant life-forms, often termed ,savannas', range from open grasslands with few woody plants, to woodlands or forests with a grass layer. These ecosystems represent a substantial portion of the terrestrial biosphere, an important wildlife habitat, and a major resource for provision of livestock, fuel wood and other products. Results Although many concepts and principles developed for grassland and forest systems are relevant to these dual life-form communities, the novel, complex, nonlinear behaviour of mixed tree,grass systems cannot be accounted for by simply studying or modelling woody and herbaceous components independently. A more robust understanding requires addressing three fundamental conundrums: (1) The ,treeness' conundrum. What controls the relative abundance of woody and herbaceous plants for a given set of conditions at given site? (2) The coexistence conundrum. How do the life-forms interact with each other? Is a given woody,herbaceous ratio dynamically stable and persistent under a particular set of conditions? (3) The net primary productivity (NPP) conundrum. How does NPP of the woody vegetation, the herbaceous vegetation, and the total ecosystem (woody + herbaceous) change with changes in the tree,grass ratio? Tests of the theory and conceptual models of determinants of mixed woody,herbaceous systems have been largely site- or region-specific and have seldom been broadly or quantitatively evaluated. Cross-site syntheses based on data and modelling are required to address the conundrums and identify emerging patterns, yet, there are very few data sets for which either biomass or NPP have been quantified for both the woody and the herbaceous components of tree,grass systems. Furthermore, there are few cross-site comparisons spanning the diverse array of woody,herbaceous mixtures. Hence, initial synthesis studies should focus on compiling and standardizing a global data base which could be (1) explored to ascertain if robust generalizations and consistent patterns exist; and (2) used to evaluate the performance of savanna simulation models over a range of woody,herbaceous mixtures. Savanna structure and productivity are the result of complex and dynamic interactions between climate, soils and disturbances, notably fire and herbivory. Such factors are difficult to isolate or experimentally manipulate in order to evaluate their impacts at spatial and temporal scales appropriate for assessing ecosystem dynamics. These factors can, however, be evaluated with simulation models. Existing savanna models vary markedly with respect to their conceptual approach, their data requirements and the extent to which they incorporate mechanistic processes. Model intercomparisons can elucidate those approaches most suitable for various research questions and management applications. Conclusion Theoretical and conceptual advances could be achieved by considering a broad continuum of grass,shrub,tree combinations using data meta-analysis techniques and modelling. [source] The irreversible cattle-driven transformation of a seasonally flooded Australian savannaJOURNAL OF BIOGEOGRAPHY, Issue 5 2003Ben R. Sharp Abstract Aim ,Anecdotal historical and photographic evidence suggests that woody vegetation is increasing dramatically in some northern Australian savanna habitats. Vegetation change in savannas has important implications for pastoral land-use, conservation management, and landscape-scale carbon storage, and informs theoretical debates about ecosystem function. This study seeks to determine the nature, extent and cause(s) of woody vegetation change in a seasonally flooded alluvial savanna habitat. Location ,The study area is located within the seasonally inundated alluvial zone of the tidal portion of the Victoria River, Northern Territory, Australia. The study area has been grazed by domestic stock since c. 1900, prior to which the area was inhabited and more likely regularly burnt by Aboriginal people for thousands of years. Methods ,Digital georeferenced aerial photographic coverages were used to examine and quantify woody vegetation change between 1948 and 1993. Transect surveys of woody and herbaceous vegetation were carried out to ground-truth air-photo results and determine the nature and causes of observed vegetation changes. Results ,There has been a dramatic increase in woody vegetation cover throughout the study area. Vegetation change patterns are roughly uniform across the full range of edaphic habitat variation and are unrelated to the depositional age of fluvial sediments. Two woody species, Eucalyptus microtheca and Excoecaria parvifolia, are predominantly responsible for observed increases. Demographic analyses reveal that woody invasions have been episodic and indicate that in most locations peak woody species establishment occurred in the mid-1970s. Grasses are almost absent in a majority of habitats within the study area. Instead, large areas are covered by scalded soil, dense invasive weed populations, and unpalatable forbs and sedges. What grasses do occur are predominantly of very low value for grazing. The condition of the herbaceous layer renders most of the study area almost completely non-flammable; what fires do burn are small and of low intensity. Main conclusions ,Multiple working hypotheses explaining observed patterns of woody vegetation increase were considered and rejected in turn. The only hypothesis consistent with the evidence is as follows: (1) observed changes are a direct consequence of extreme overgrazing by cattle, most likely when stocking rates peaked in the mid-1970s; (2) prolonged heavy grazing effected the complete transformation of much of the herbaceous vegetation to a new state that is not flammable; and (3) in the absence of regular fire mortality, woody vegetation increased rapidly. The relatively treeless system that existed in 1948 was apparently stable and resilient to moderate grazing levels, and perhaps also to episodic heavy grazing events. However, grazing intensity in excess of a sustainable threshold has forced a transition that is irreversible in the foreseeable future. Stable-state transitions such as this one inform debates at the heart of ecological theory, such as the nature of stability, resilience, equilibrium and carrying capacity in dynamic savanna ecosystems. [source] Impact of habitat disturbance in the wetland forests of East Usambara, TanzaniaAFRICAN JOURNAL OF ECOLOGY, Issue 2 2010Ramadhani Senzota Abstract We evaluated habitat characteristics of East Usambara wetland forests. The abundance and species composition in the tree, shrub and herbaceous layers were enumerated in two sets of nested plots, one set in a natural wetland forest and the second in a wetland forest that had been disturbed by small-scale gold mining activities. Each set had thirty-six 1 m × 1 m plots for herbs, inside nine 5 m × 5 m plots for shrubs, in three 20 m × 20 m plots for trees. The habitat profile of herbaceous , shrub , tree layers was found to be sharply different from one obtained in previous studies at the surrounding nonwetland forests as were species composition and abundance. Unlike in the nonwetland forests, the herbaceous layer was thick, the shrub layer very thin and the woody species density and richness much lower. Disturbance significantly reduced woody cover and changed species composition in the herbaceous layer. Recovery of the woody vegetation was low. Wetland forests in the East Usambaras form a small fraction of the total area, but they are a unique biodiversity repository, they appear to be an important carbon dioxide sink and to reserve and purify water. They are sensitive to disturbance and need protection. Résumé Nous avons évalué les caractéristiques de l'habitat des forêts humides d'East Usambara. L'abondance et la composition des espèces dans les étages d'arbres, d'arbustes et d'herbes furent dénombrées dans deux ensembles de parcelles emboitées, un situé dans une forêt humide naturelle, l'autre dans une forêt humide qui avait été perturbée par les activités d'orpaillage à petite échelle. Chaque ensemble se composait de 36 carrés de 1 m² pour les herbes, situés dans neuf carrés de 5 m × 5 m pour les buissons, eux-mêmes situés à l'intérieur de trois carrés de 20 m × 20 m pour les arbres. Le profil des habitats pour les trois couches d'herbes , d'arbustes , d'arbres s'est révélé très différent de celui qui avait été obtenu lors d'études précédentes réalisées dans des forêts environnantes non humides; il en était de même pour la composition et l'abondance des espèces. Contrairement aux forêts non humides, la couche herbeuse était épaisse, la couche des arbustes très claire et la densité et la richesse des espèces ligneuses étaient beaucoup plus faibles. La perturbation avait significativement réduit le couvert ligneux et changé la composition des espèces de l'étage herbacé. La restauration de la végétation ligneuse était faible. Les forêts humides des East Usambara ne constituent qu'une petite fraction de la superficie totale, mais elles sont un sanctuaire unique pour sa biodiversité, elles semblent être un puits de carbone important et aussi retenir et purifier l'eau. Elles sont sensibles à toute perturbation et doivent être protégées. [source] The impact of elephants, Loxodonta africana, on woody vegetation through selective debarking in Samburu and Buffalo Springs National Reserves, KenyaAFRICAN JOURNAL OF ECOLOGY, Issue 1 2010Festus W. Ihwagi Abstract The impact of elephants on the woody plant community through debarking was investigated in Samburu and Buffalo Springs National Reserves, Kenya. Acacia elatior Brenan, the most abundant tree species in the riverine zone, accounted for 68% (n = 1375) of woody plants. A. tortilis (Forsskal) Hayne dominated plots away from the river. Debarking incidences were significantly higher for A. elatior than for other species indicating selective utilization. The riverine zone by virtue of having more trees of the preferred species, A. elatior, had the highest debarking incidences. Presence of very few saplings along the river is attributed to both elephant trampling and herbivory by other species. An estimated 38.5% and 22.5% of the riverine A. elatior and A. tortilis trees respectively, were bound to die within the next 4,5 years because of severe debarking, ,75% of bark circumference. Debarking was positively correlated with stem circumference; the medium-sized trees being the worst affected by the elephants' selective debarking behaviour. Intense debarking incidences were recorded during the dry season. Through the elephants' selective debarking, the riverine habitat is bound to open up gradually, leading to considerable habitat change in the near future. Elephant impact on vegetation is less away from the river and increases with their densities. Résumé L'impact des éléphants sur la communauté des plantes ligneuses, à cause de l'enlèvement des écorces, a étéétudié dans les Réserves Nationales de Samburu et de Buffalo Springs, au Kenya. L'Acacia elatior Brenan, le plus abondant de la zone riveraine, comptait pour 68% (n = 1375) des plantes ligneuses. A. tortilis (Forsskal) Haine dominait les parcelles plus éloignées de la rivière. Les cas d'écorçage étaient significativement plus nombreux pour A. elatior que pour les autres espèces, ce qui indique une utilisation sélective. La zone riveraine, du fait qu'elle compte plus d'arbres de l'espèce préférée, A. elatior, comptait aussi le plus grand nombre de cas d'écorçage. La présence de très peu de jeunes plants le long de la rivière est attribuée au piétinement par les éléphants et à la consommation par d'autres espèces herbivores. On estime que respectivement 38,5% et 22,5% des A. tortilis et A. elatior de la zone riveraine devraient mourir dans les 4,5 années à venir, en raison d'écorçages sévères qui peuvent être de ,75% de la circonférence de l'arbre. L'écorçage était positivement liéà la circonférence du tronc, les arbres de diamètre moyen étant les plus gravement affectés par le comportement sélectif des éléphants. Des cas d'écorçage intense ont été enregistrés au cours de la saison sèche. En raison de l'écorçage sélectif des éléphants, l'habitat riverain est condamnéà s'ouvrir progressivement, ce qui entraînera un changement considérable dans un proche avenir. L'impact des éléphants sur la végétation est moindre lorsqu'on s'éloigne de la rivière, et il augmente en fonction de leur densité. [source] Effects of water dependence on the utilization pattern of woody vegetation by elephants in the Tembe Elephant Park, Maputaland, South AfricaAFRICAN JOURNAL OF ECOLOGY, Issue 1 2010Jerome Y. Gaugris Abstract The utilization of vegetation and particularly trees in enclosed small reserves where elephant populations are confined is a contentious conservation issue. In Tembe Elephant Park in Maputaland, the diverse Sand Forest is considered the most valuable feature to conserve; yet it is considered at risk from increasing elephant utilization of the park's vegetation in general. The mean canopy removal by elephants across the park was studied over two periods: a recent period including the twelve months before the study and an older period >12 months earlier. Age of utilization was determined from the degree of decay observed on woody fibres. The relationships between intensity of utilization, vegetation unit selection and distance to water were evaluated. Results show that utilization patterns have shifted in the recent drier period, during which elephants used vegetation communities closer to permanent water. Concurringly, a significant decrease in utilization intensity was correlated to an increasing distance from water points in the park during that period, while this effect was not observed during the wetter old period. We debate that active water availability management may be a way to limit elephant utilization in small fenced reserves. Résumé L'utilisation de la végétation, et particulièrement des arbres, dans de petites réserves clôturées où sont confinées des populations d'éléphants est un problème de conservation controversé. Dans le Tembe Elephant Park, au Maputaland, la Sand Forest, très variée, est considérée comme la caractéristique la plus intéressante à préserver; pourtant, on estime qu'elle est en danger en raison de l'utilisation croissante que font les éléphants de la végétation du parc en général. Pendant deux périodes de temps, on a étudié l'enlèvement moyen de la canopée par les éléphants dans tout le parc : une période récente qui incluait les 12 mois qui ont précédé cette étude et une période plus ancienne, plus de 12 mois auparavant. La date de l'utilisation fut déterminée à partir du degré de décomposition observé sur les fibres ligneuses. Les relations entre l'intensité de l'utilisation, la sélection du type de végétation et la distance par rapport à l'eau ont étéévaluées. Les résultats montrent que les schémas d'utilisation se sont modifiés lors de la période sèche récente, au cours de laquelle les éléphants ont fréquenté des communautés végétales plus proches des points d'eau permanents. Dans le même temps, une réduction significative de l'intensité d'utilisation était corrélée à l'éloignement croissant par rapport aux points d'eau dans le parc, alors que cet effet ne fut pas constaté au cours de la première période, plus humide. Nous discutons du fait que la gestion de la disponibilité en eau pourrait être un moyen de limiter la fréquentation des petites réserves clôturées par les éléphants. [source] Differences in woody vegetation are unrelated to use by African elephants (Loxodonta africana) in Mkhuze Game Reserve, South AfricaAFRICAN JOURNAL OF ECOLOGY, Issue 1 2010Angela M. White Abstract The reintroduction of African elephants into fenced game reserves throughout South Africa has presented managers with several challenges. Although elephants are a natural part of southern African ecosystems, their confinement to fenced protected areas in South Africa has exacerbated their potential to impact their habitats negatively. However, many studies investigating the impact of elephants have failed to control for the effects of other browsers on the vegetative community. In this study, we used location data on an elephant herd to delineate high-use and low-use areas. This paired design allowed us to minimize confounding factors that could explain differences in the structure, diversity and utilization of woody species. We found little evidence to suggest elephant-mediated change in, or selection for, the structure or diversity of woody species; however, our results suggest that elephants may be altering the composition of species by preferentially using areas with higher canopy diversity and by enhancing sapling recruitment. Although stripping of bark was higher in high-use areas, there was no evidence of differential mortality of tree species. Therefore, in our study area, and over the current time scale, elephants are having a negligible impact on the vegetative community. Résumé La réintroduction d'éléphants africains dans des réserves de faune clôturées dans toute l'Afrique du Sud représente plusieurs défis pour les gestionnaires. Bien que les éléphants fassent naturellement partie des écosystèmes d'Afrique australe, leur confinement dans des aires protégées clôturées en Afrique du Sud a exacerbé leur capacité d'avoir des impacts négatifs sur leurs habitats. Pourtant, les nombreuses études qui ont recherché l'impact des éléphants ont omis de contrôler l'effet des autres herbivores sur la communauté végétale. Dans cette étude, nous avons utilisé des données sur la localisation des hardes d'éléphants pour délimiter les zones plus ou moins fréquentées. Cette étude couplée nous a permis de minimiser les facteurs prêtant à confusion qui pouvaient expliquer des différences de structure, de diversité et d'utilisation des espèces ligneuses. Nous avons trouvé peu de preuves qui auraient indiqué que les éléphants changeaient ou sélectionnaient la structure ou la diversité des espèces ligneuses; cependant, nos résultats suggèrent que les éléphants pourraient modifier la composition des espèces en choisissant de préférence les zones où la diversité de la canopée est plus élevée et en stimulant le recrutement de jeunes plants. Même si les arrachages d'écorces étaient plus nombreux dans les zones très fréquentées, il n'y avait aucune preuve d'une mortalité différentielle selon les espèces d'arbres. C'est pourquoi, dans l'aire où nous avons travaillé, et pour le moment, on peut dire que les éléphants n'ont qu'un impact négligeable sur la communauté végétale. [source] The use of structural species size classes in the description of the woody vegetation of a nature reserveAFRICAN JOURNAL OF ECOLOGY, Issue 4 2004L. R. Brown Abstract The need for a scientifically based wildlife management plan and more knowledge on the vegetation ecology of the Borakalalo Nature Reserve prompted an ecological investigation of the Reserve. One of the aims was to develop a structural classification of the woody component using species size (SPIZE) classes. A further aim was to compare the various structural classes identified with the recognized floristically derived plant communities of the Reserve. The frequency, density, percentage crown cover and importance value for each woody species were calculated. A classification of the woody component was done using a TWINSPAN classification algorithm on this structural density data. Fifteen structural SPIZE classes were identified, described and compared with the described plant communities. The results of this study indicate that structural SPIZE classes could also be used to explain the spatial distribution of woody species within and between various plant communities. Résumé Le besoin de concevoir un plan d'aménagement scientifique, et un désir d'en savoir plus sur l'écologie de la végétation dans la Réserve Naturelle de Borakalalo, ont menéà une étude de cette Réserve. Un des buts principaux était d'établir une classification structurale de la partie boisée en fonction des catégories de taille des espèces (des catégories dites ,SPIZE', ou ,species size classes'). L'étude avait aussi comme but de comparer les diverse catégories structurelles identifiées avec les communautés végétales reconnues de la Réserve de dérivation floristique. La fréquence, la densité, le pourcentage de couverture de la cime et l'importance de chaque espèce boisée ont été calculés. Une classification de la partie boisée a été réalisée en se servant d'un algorithme dit ,TWINSPAN' pour évaluer les données sur la densité structurale. Quinze catégories SPIZE ont été identifiées, décrites et comparées avec les communautés végétales décrites. Les résultats de cette étude indiquent que les catégories SPIZE structurelles pourraient être utilisées davantage pour expliquer la distribution spatiale des espèces boisées entre et parmi les diverses communautés végétales. [source] Fresh insights into long-term changes in flora, vegetation, land use and soil erosion in the karstic environment of the Burren, western IrelandJOURNAL OF ECOLOGY, Issue 5 2009Ingo Feeser Summary 1. ,The study focuses on species-rich, upland, heathy vegetation with arctic-alpine floristic affinities and Sesleria grasslands in the karstic Burren region, western Ireland. The investigations aimed at reconstructing the long-term development of these high conservation-value communities and the role of farming in their formation and long-term survival. 2. ,The methods used included pollen analysis and 14C-dating of short monoliths and investigation of grykes (fissures in karstic limestone) for evidence of soil erosion. Special attention was paid to fossil, coprophilous fungal spores as indicators of local grazing. The strong local character of the pollen records facilitated identification of inter-site differences as well as regional patterns. It is shown that open pine woodland characterized the Cappanawalla uplands between c. 1500 BC and 500 BC. It is proposed that such woodlands, with floristic affinities to Scandinavian open pine woodlands on calcareous soils, provided a suitable environment for the present-day, open heath vegetation with species such as Dryas octopetala, Arctostaphylos uva-ursi, Geranium sanguineum and Empetrum nigrum. 3. ,Burning of vegetation as a management tool was important in the uplands over most of the last two millennia. Firing seems to have ceased with the onset of more intensive grazing regimes in the 18th century. 4. ,Synthesis. Upland palaeoarchives, derived from shallow peaty deposits, show that the upland Burren supported mainly plagioclimax Corylus -dominated woody vegetation and grasslands from c. 1500 BC (beginning of present record), until possibly as late as the 17th century AD. In the uplands of the north-western Burren, open, species-rich pinewoods with hazel dominated. The northern-arctic elements in the present-day upland flora survived clearances, involving initially Pinus sylvestris (c. 500 BC) and subsequently Corylus avellana (c. AD 1600). Colluvial material retrieved from grykes supports the idea of considerable soil loss occurring as late as the first and early 2nd millennium AD. The investigations highlight the potential of upland palaeoarchives, consisting of short sequences, for elucidating vegetation and land-use dynamics in karstic environments such as the Burren. [source] Phenology of fine roots and leaves in forest and grasslandJOURNAL OF ECOLOGY, Issue 6 2008Diego F. Steinaker Summary 1The phenology of temperate vegetation is advancing in association with climate warming. Most phenology data, however, comes from flowers and tree leaves. We tested the generality of results from shoot phenology by expanding data collection in two new directions. We related forest leaf phenology to root phenology, and to phenology in a second habitat, grassland. 2We measured leaf and root phenology simultaneously in aspen forest and adjacent native grassland. Root growth accounts for 80,90% of productivity in these habitats. Seasonal variation in soil moisture and temperature were also measured. 3Forest leaf production was greatest about 45 days before peak root production, resulting in a significant negative correlation between leaf and root production in forest. Grassland leaf production was greatest about 15 days before peak root production, and grassland leaf and root production were significantly positively correlated. The duration of root production was 40% greater than that of shoot production. 4Forest leaf production increased significantly with increasing soil moisture, but not temperature. In contrast, the production of forest roots, grassland roots and grassland leaves increased significantly with soil temperature. 5Synthesis. The most commonly measured aspect of phenology, forest leaves, is out of step with the majority of production in forest, as well as phenology in grassland. The invasion of grassland by woody vegetation is characterized by a decoupling of root and shoot phenology, a result that has not been reported previously. Given the global nature of woody plant encroachment, this decoupling may influence our general understanding of productivity and carbon sequestration in response to warming. [source] Predicting Root Density in Streambanks,JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION, Issue 2 2008Candice Piercy Abstract:, Roots of riparian vegetation increase streambank erosion resistance and structural stability; therefore, knowledge of root density and distribution in streambanks is useful for stream management and restoration. The objective of this study was to compare streambank root distributions for herbaceous and woody vegetation and to develop empirical models to predict root density. Root length density, root volume ratio, soil physical and chemical properties, and above-ground vegetation densities were measured at 25 sites on six streams in southwestern Virginia. The Mann-Whitney test was used to determine differences in root density along stream segments dominated by either woody or herbaceous vegetation. Multiple linear regression was used to develop relationships between root density and site characteristics. Study results showed that roots were evenly distributed across the bank face with the majority of roots having diameters less than 2 mm. Soil bulk density and above-ground vegetation were key factors influencing root density. While significant relationships were developed to predict root density, the predictive capabilities of the equations was low. Because of the highly variable nature of soil and vegetation properties, it is recommended at this time that soil erodibility and root density be measured in the field for design and modeling purposes, rather than estimated based on empirical relationships. [source] Fire and ice: volcanic and glacial impacts on the phylogeography of the New Zealand forest fern Asplenium hookerianumMOLECULAR ECOLOGY, Issue 21 2007LARA D. SHEPHERD Abstract In the Southern Hemisphere there has been little phylogeographical investigation of forest refugia sites during the last glacial. Hooker's spleenwort, Asplenium hookerianum, is a fern that is found throughout New Zealand. It is strongly associated with forest and is a proxy for the survival of woody vegetation during the last glacial maximum. DNA sequence data from the chloroplast trnL-trnF locus were obtained from 242 samples, including c. 10 individuals from each of 21 focal populations. Most populations contained multiple, and in many cases unique, haplotypes, including those neighbouring formerly glaciated areas, while the predominant inference from nested clade analysis was restricted gene flow with isolation by distance. These results suggest that A. hookerianum survived the last glacial maximum in widespread populations of sufficient size to retain the observed phylogeography, and therefore that the sheltering woody vegetation must have been similarly abundant. This is consistent with palynological interpretations for the survival in New Zealand of thermophilous forest species at considerably smaller distances from the ice sheets than recorded for the Northern Hemisphere. Eastern and central North Island populations of A. hookerianum were characterized by a different subset of haplotypes to populations from the remainder of the country. A similar east,west phylogeographical pattern has been detected in a diverse array of taxa, and has previously been attributed to recurrent vulcanism in the central North Island. [source] Ecological distribution of terrestrial orchids in a south Brazilian Atlantic regionNORDIC JOURNAL OF BOTANY, Issue 1 2010Fernando Souza Rocha Terrestrial orchids comprise a taxonomically and ecologically varied group, ranging from tropical to subpolar regions and from wet marshy to dry sand-dune environments. An ecological survey of these plants was performed in a natural region of Rio Grande do Sul, south Brazil, namely the northern coastal plain, between the abrupt slopes of the Serra Geral and the coastal line of the Atlantic Ocean (29°01,S to 30°00,S, 49°43,W to 50°16,W). The study area presents a humid subtropical climate of the Cfa type. Mean annual temperature and precipitation range from 17.5 to 20.0°C and from 1200 to 1700,mm, respectively. The occurrence of indigenous terrestrial orchid species was recorded for six major habitats or vegetation types: bogs and marshes, peat forests, rain forests, dune forests, Butia -palm stands and coastal sand-dunes. The ecological range was defined for 39 species belonging to 23 genera, based on literature, herbarium revisions and extensive collecting along the studied area. Species richness ranged from nine species, in both coastal dunes and palm-groves, to 17 species in peat forests. A negative correlation was observed between species richness and ecological range, showing a general tendency for terrestrial orchids to be confined to one or a few habitats. Multivariate analyses indicated light (herbaceous vs woody vegetation) as a primary ecological factor, and soil drainage (sandy vs peaty substrates) as a secondary factor controlling terrestrial orchid distribution. [source] Management actions are required to improve the viability of the rare grassland herb Carlina biebersteiniiNORDIC JOURNAL OF BOTANY, Issue 1-2 2008Satu Ramula Small population size of many rare or endangered plant species makes a quantitative assessment of population status challenging because of the lack of detailed demographic data on different life-history stages. However, an urgent assessment is often required to start possible management actions. We performed a count-based population viability analysis (PVA) using discontinuous time series to quantitatively assess the viability of a rare, monocarpic, grassland herb Carlina biebersteinii Bernh. ex Hornem. (synonyms: C. vulgaris L. ssp. longifolia, C. vulgaris L. ssp. stricta) and examined demographic and environmental factors contributing to its viability. Based on 12 abundance counts of eight C. biebersteinii populations in Finland, we found that seven out of the eight population sizes declined during the observation period, and that annual population growth rates were slightly synchronised among the populations. Synchrony in annual population growth rates declined with increasing geographic distances among the populations, while fluctuations in the number of flowering plants were unrelated to geographic distances among the populations. According to stochastic simulations, the risk of losing all flowering individuals during the next 20,years will be high for unmanaged populations. To prevent the populations from gradually declining, our results suggest that summer grazing or removal of woody vegetation is required to increase habitat openness and consequently, to improve fecundity and seedling recruitment. [source] Effects of permafrost degradation on woody vegetation at arctic treeline on the Seward Peninsula, AlaskaPERMAFROST AND PERIGLACIAL PROCESSES, Issue 2 2003Andrea H. Lloyd Abstract Permafrost degradation leads to substantial changes in soil thermal and hydrologic characteristics. We investigated the effects of changes in active layer thickness and soil drainage on vegetation distribution near the arctic treeline on the Seward Peninsula, Alaska. We measured active layer thickness, soil moisture, density of tall shrub species, cover of low shrub species, and reconstructed white spruce establishment history along transects across the banks of a network of thaw ponds. We found that active layer thickness did not vary along our transects, but soils on thaw pond banks were significantly drier than those on level tundra or in thaw-pond channels. Thaw-pond banks were the only sites in which trees successfully established, and shrub communities on thaw-pond banks were taller and more dominated by tall shrub species like willow and shrub birch. The data suggest that the establishment of tree and tall shrub species at the arctic treeline can be limited by the availability of well-drained microsites, and the response of these species to regional climatic changes will be constrained by the availability of such microsites and thus contingent upon further degradation of the permafrost. Copyright © 2003 John Wiley & Sons, Ltd. [source] Forest Regeneration in a Chronosequence of Tropical Abandoned Pastures: Implications for Restoration EcologyRESTORATION ECOLOGY, Issue 4 2000T. Mitchell Aide Abstract During the mid-1900s, most of the island of Puerto Rico was deforested, but a shift in the economy from agriculture to small industry beginning in the 1950s resulted in the abandonment of agricultural lands and recovery of secondary forest. This unique history provides an excellent opportunity to study secondary forest succession and suggest strategies for tropical forest restoration. To determine the pattern of secondary succession, we describe the woody vegetation in 71 abandoned pastures and forest sites in four regions of Puerto Rico. The density, basal area, aboveground biomass, and species richness of the secondary forest sites were similar to those of the old growth forest sites (>80 yr) after approximately 40 years. The dominant species that colonized recently abandoned pastures occurred over a broad elevational range and are widespread in the neotropics. The species richness of Puerto Rican secondary forests recovered rapidly, but the species composition was quite different in comparison with old growth forest sites, suggesting that enrichment planting will be necessary to restore the original composition. Exotic species were some of the most abundant species in the secondary forest, but their long-term impact depended on life history characteristics of each species. These data demonstrate that one restoration strategy for tropical forest in abandoned pastures is simply to protect the areas from fire, and allow natural regeneration to produce secondary forest. This strategy will be most effective if remnant forest (i.e., seed sources) still exist in the landscape and soils have not been highly degraded. Patterns of forest recovery also suggest strategies for accelerating natural recovery by planting a suite of generalist species that are common in recently abandoned pastures in Puerto Rico and throughout much of the neotropics. [source] The influence of increasing population size and vegetation productivity on elephant distribution in the Kruger National ParkAUSTRAL ECOLOGY, Issue 3 2009KIM D. YOUNG Abstract Decisions to reduce the impacts of large herbivores on biodiversity in protected areas are often based on controlling their numbers. However, numbers per se may not be the foremost consideration when managing impacts. This is because density-related changes in distribution can also affect habitat utilization and hence, impact. In this study we tested whether changes in the distribution of African elephants are associated with increasing population size. We used spatially explicit count data collected during the dry seasons from 1998 to 2004 in South Africa's Kruger National Park. We did this at five spatial scales and in landscapes defined by vegetation, geology, climate and soils. We then investigated whether observed distributions and grid-cell-specific densities were associated with the remotely sensed Normalized Difference Vegetation Index (NDVI) as a measure of productivity and therefore food resource availability at the landscape scale. Consistent with density mediated changes, we found that elephant grid-cell occupancy increased with population size, while grid-cell-specific density became less variable. In addition, the combined distribution of bull groups and breeding herds became less clumped with increasing population size. We further found that within landscapes elephants were present on grid-cells with higher NDVI values, but that the influence of NDVI during the dry season on densities among landscapes may be weak. These results suggest that NDVI was more indicative of structural habitat choices such as woody vegetation than food availability per se. Our study highlights the need to consider factors other than population size alone when formulating management decisions to reduce large herbivore impacts on biodiversity in protected areas. [source] Responses to Fire in Selected Tropical Dry Forest Trees,BIOTROPICA, Issue 5 2006Sarah M. Otterstrom ABSTRACT Fire is a frequent disturbance in the tropical dry forests of Central America, yet very little is known about how native species respond to such events. We conducted an experimental burn in a tropical dry forest of western Nicaragua to evaluate plant responses to fire with respect to survivorship and recruitment. Measurements of woody vegetation of all size classes were carried out prior to the prescribed burn and three successive years post fire. We selected the 15 most abundant species <10 cm DBH to assess percent survivorship and sprouting responses post fire. Changes in seedling densities for these 15 most abundant species and the 15 least abundant species were analyzed using a repeated measure ANOVA. We also assessed changes in seedling densities for three species of international conservation concern. We found three major fire-coping strategies among common dry forests plants: resisters (low fire-induced mortality), resprouters (vigorous sprouting), and recruiters (increased seeding post-fire). While survivorship was generally high relative to tropical moist forest species, those species with lower survivorship used either seeding or sprouting as an alternative strategy for persisting in the forest community. Seed dispersal mechanisms, particularly wind dispersal, appear to be an important factor in recruitment success post-fire. Burn treatment led to a significant increase in the density of seedlings for two species of conservation concern: Guaiacum sanctum and Swietenia humilis. Results of this study suggest that common dry forest species in western Nicaragua are fire tolerant. Further study of individual species and their fire responses is merited. RESUMEN El incendio forestal causa perturbación frecuente en los bosques secos tropicales de Centroamérica, sin embargo se conoce poco del comportamiento de las especies nativas a ésta perturbación. Nosotros llevamos a acabo una quema experimental en un bosque seco tropical de Nicaragua occidental para evaluar el comportamiento de la flora con respecto a la sobrevivencia y reclutamiento de las especies después de la quema. Se midió todas las clases diamétricas de la vegetación leñosa antes de la quema controlada y durante 3 años consecutivos después de la quema. Seleccionamos las 15 especies más abundantes en la clase diametrica <10 dap para evaluar el porcentaje de sobrevivencia y la actividad de rebrote después de la quema. Cambios en la densidad de plántulas para las 15 especies más abundantes y para las 15 especies menos abundantes fueron analizadas utilizando el análisis de varianza de medición repetido (ANOVA). También, evaluamos cambios en la densidad de plántulas para tres especies de interés a nivel internacional con respecto a su conservación. En la flora común del bosque seco se encontró tres estrategias principales utilizadas por las especies para contender con los incendios forestales, estas fueron: resistores (baja mortalidad por el incendio), rebrotadores (rebrotamiento vigoroso), y reclutores (reclutamiento elevado pos-incendio). Mientras la sobrevivencia de especies en el bosque seco en general fue relativamente alta a la de especies de bosque húmedo tropical, las especies con menor sobrevivencia utilizaron el reclutamiento ó rebrotamiento como estrategia para persistir en la comunidad boscosa. El mecanismo de dispersión de semilla, particularmente la dispersión eólica, parece ser un factor importante en el éxito de reclutamiento después de la quema. Las quemas controladas produjeron un aumento significativo en la densidad de plántulas para dos de las especies de interés para la conservación: Guaiacum sanctum L. (Zygophyllaceae) y Swietenia humilis Zucc. (Meliaceae). Resultados de esta investigación sugieren que las especies comunes del bosque seco de Nicaragua son tolerantes al fuego. Por lo tanto, es merecido hacer más estudios de estas especies y su comportamiento ante la perturbación del fuego. [source] | ||||||||||||||||||||||||||||