Savanna Structure Research Articles

Variation in the composition and structure of tropical savannas as a function of rainfall and soil texture along a large‐scale climatic gradient in the Northern Territory, Australia

Abstract. Variation in structural and compositional attributes of tropical savannas are described in relation to variation in annual rainfall and soil texture along a subcontinental‐scale gradient of rainfall in the wet‐dry tropics of the Northern Territory, Australia. Rainfall varies along the gradient from over 1500 mm p.a. in the Darwin region (c. 12° S) to less than 500 mm in the Tennant Creek region (c. 18° S). Soils are patchy, and sands, loams and clays may occur in all major districts within the region. We utilized a large data set (1657 quadrats ° 291 woody species; with numerous measured and derived sample variables) covering an area of 0.5 million km2. Correlations between floristic composition of woody species and environmental variables were assessed using DCA ordination and vector fitting of environmental variables. Vectors of annual rainfall and soil texture were highly correlated with variation in species composition. Multiple regression analyses incorporating linear and quadratic components of mean annual rainfall and topsoil clay content were performed on three structural attributes (tree height, tree cover, tree basal area) and two compositional attributes (woody species richness, deciduous tree species richness). Tree height declined with decreasing rainfall; cover, basal area, woody species richness and deciduous species richness all declined with decreasing rainfall and increasing soil clay content. Regression models accounted for between 17% and 45% of the variation in the data sets. Variation in other factors such as soil depth, landscape position and recent land‐use practices (for which there were no data on an individual quadrat basis) are likely to have contributed to the large residual variation in the data set.

Ecosystem structure and growth of the African savanna

Ecosystems are highly structured, interlocking sets of organismic and environmental interactions. Particular ecosystems reappear in the geological record, structured similarly regardless of their evolutionary history. Ulanowicz has hypothesized that energy throughput and compartmentalization are an important part of the evolutionary growth and development of ecosystems, and that redundancy within compartments can shield an ecosystem from perturbation. The compartments studied in this paper are mammalian guilds and redundancy has been interpreted as guild depth, defined as the number of genera within a particular guild. The fossil history and paleocology of the African savanna ecosystem, from potential precursor systems in the Early Miocene, through the protosavanna, the flowering of the savanna, and the severe depletion of this system about 1.5 Ma, show that redundancy and compartmentalization in this ecosystem did increase with time, and that redundancy did, at 1.5 Ma, protect the system's survival. A similarly redundant savanna system in North America destructure and restructured to prairie at about 5 Ma, apparently due to even more extreme climatic perturbations.

Environmental degradation of the Obra-Renukoot-Singrauli Area, India, and its impact on natural and derived ecosystems

Vast stretches of the Obra—Renukoot—Singrauli region were once covered with natural forests. The tribal dwellings were small and interspersed. However, there has been rapid industrialisation in recent years. Quarrying for limestone, the establishment of a cement factory, thermal power stations and the construction of the G.B. Pant Sagar reservoir have resulted in a rapid build up of human population, the displacement of the original population, deforestation and conversion of natural forest ecosystems into savanna and marginal croplands. The converted ecosystems are under immense biotic stresses such as lopping, grazing, etc.The rainfall is meagre and erratic, the soils are highly weathered and impoverished, consequently the natural forests, as well as the derived ecosystems, are fragile. Signs of desertification are widespread. A rapid depletion in the wildlife has occurred. The establishment of thermal power stations and chemical and cement factories has also resulted in large scale gaseous air pollution, particularly of SO2 and HF, pollution due to particulate matter through fly ash and cement dust, and that due to liquid effluents. Surface coal mining has caused extensive damage to the natural ecosystems with growing dumps of overburden. The latter needs to be stabilised. This integrated research project on environmental degradation and its impact on air, soil, water and vegetation was divided into five sub-projects, each supervised by a principal investigator. These sub-projects were: (i) Industrial air pollutants and their effects on ecosystem structure and function; (ii) Analysis of structure and functioning of forest ecosystems; (iii) Analysis of structure and functioning of savanna and agroecosystems; (iv) Analysis of structure and functioning of watershed and water-bodies; and (v) Impact of land-use system, socio-economic- demographic conditions, and environmental perception and behavioural management. In these studies dynamics of plant biomass, productivity and nutrients, and biochemical—physiological responses of plants to pollution were emphasised. Environmental perceptions of the native and displaced populations were also studied. The paper outlines a range of recommendations which should help to improve the environmental quality of the region.

Nigerian Man and Biosphere research plots in Guinea savanna: floristics and structure of the vegetation

SummaryA study was made of the vegetation of three plots used in the Nigerian Man and Biosphere Research Programme for Savanna Studies. The aim is to provide detailed information on the vegetation of the plots which may possibly contribute to an understanding of factors influencing savanna structure and relative abundances of forbs, grasses and woody species. Results show that there are differences in the species composition of the plots. There are more forb species than grass species in each plot. The woody basal areas and crown areas of the plots do not depend entirely on the density but also on the size of the woody species. There are differences in herbage yield in the plots that could be attributed to differences in soil properties, species composition and level of human and animal activities between the plots.

Late Quaternary extinct ungulates of East Africa and palaeoenvironmental implications

UNGULATE communities of two East African savannas, the Serengeti and Athi-Kapiti Plains, are dominated by wildebeest (Connochaetes taurinus) supplemented by zebra (Equus burchelli), topi (Damaliscus lunatus), hartebeest (Alcelaphus buselaphus), buffalo (Syncerus caffer) eland (Taurotragus oryx) and gazelles (Gazella grand and G. thomsoni)1–3. Before this research, little was known of East African large mammal communities in the Late Pleistocene and early to middle Holocene. We document an extinct impala-sized alcelaphine antelope that is numerically dominant in Late Pleistocene archaeofaunal assemblages from the Athi-Kapiti Plains. The extinct giant buffalo Pelorovis antiquus is present, and a number of arid-adapted regionally extinct species are common. The small alcelaphine is rare in northern Tanzania, but regionally extinct arid-adapted species are present in Late Pleistocene deposits. These data indicate that as recently as 12,000 years ago, the large mammal community structure of East African savannas was very different and dry grasslands and arid-adapted ungulates expanded at least as far south as northern Tanzania during the Last Glacial Maximum.

Plant Ecology in West Africa: Systems and Processes.

This work brings together for the very first time within one volume authoritative and up-to-date accounts of the principal types of vegetation found in West Africa and describes the main processes that occur within them. It deals with such varied topics as the complex inter-relationships between plants and animals, the breeding systems found in West African plants, the structure and dynamics of forest, savanna, desert, coastal and montane vegetation, and the urgent need to conserve this natural environment. Plant Ecology in West Africa reviews and summarises our present knowledge of this plant environment in a comprehensive manner and from an ecological viewpoint. It will be essential reading for the student of tropical botany and the research worker in plant ecology, and will provide understanding of basic processes so necessary to tackle problems of agriculture and land use in the tropics, as well as offering a valuable reference work to all those concerned with the development and management of the natural resources of the third world.

Presettlement forest structure as a factor in urban forest development

Characteristics of presettlement forests at Menlo Park and South Lake Tahoe, CA were compared with the present characteristics of the urban forests in these cities. Urbanization of forest types in both cities led to decreased tree density (in Menlo Park from 279 to 43/ha; in South Lake Tahoe from 761 to 373/ha), decreased tree cover (in Menlo Park from 92 to 34%; in South Lake Tahoe from 57% to 19%). In contrast, urbanization of the oak savannas at Menlo Park has resulted in an increase in tree density (from 4 to 35/ha) crown cover (from 14 to 25%) and the number of species (from 3 to 130). The number of tree species has increased in both cities as a result of urbanization of presettlement forests (in Menlo Park from 5 to 145; in South Lake Tahoe from 1 to 6). The uneven-aged structure of presettlement oak savannas at Menlo Park has been modified to an all-aged structure as a result of tree planting and the mortality of older age classes. The all-aged structure of the presettlement oak forest at Menlo Park was not changed in the transition to the present urban forest. At South Lake Tahoe the even-aged structure of the presettlement Jeffrey pine forest has been modified into a uneven-aged structure by tree planting. Presettlement forest age structure is recognized as the most significant characteristic for the identification of possible management problems in the urban forest.

The Effects of Thirteen Years of Annual Prescribed Burning on a Quercus Ellipsoidalis Community in Minnesota

A Quercus ellipsoidalis community in central Minnesota has been prescribed burned annually since 1965 in an attempt to restore the area to its presettlement oak savanna structure and composition. By 1979 density and basal area of the overstory were significantly lower in the burned area than in an adjacent unburned area but were still higher than estimated savanna values because of the persistence of stems @>25 cm diameter at breast height (dbh). A tall—shrub/small—tree layer was totally lacking in the burned area but averaged 19% cover in the unburned area. Understory richness was significantly higher in the burned area than in the unburned area. Most of the species that showed a significant difference between the two areas peaked in the burned area; this was especially true for grasses and forbs. These results indicate that annual prescribed burning is gradually restoring the area to savanna but that the restoration is not yet complete. Complete restoration may not be possible with annual burning because such burning seems to have little effect on large—tree (@>25 cm dbh) mortality.