Understanding, monitoring and managing savanna ecosystems require characterizing both functional and structural properties of vegetation. Due to functional diversity and structural heterogeneity in savannas, characterizing these properties using remote sensing is methodologically challenging. Focusing on the semi-arid savanna in the central Kalahari, the objective of this dissertation was to combine in situ data with multi-scale satellite imagery and two image analysis approaches (i.e. Multiple Endmember Spectral Mixture Analysis (MESMA) and Object Based Image Analysis (OBIA)) to : (i) determine the superior method for estimating fractional photosynthetic vegetation (fPV), non-photosynthetic vegetation (fNPV) and bare soil (fBS) when high spatial resolution multispectral imagery is used, (ii) examine the suitability of OBIA for mapping vegetation morphology types using a Landsat TM imagery, (iii) examine the impact of changing spatial resolution on magnitude and accuracy of fractional cover and (iv) examine how the fractional cover magnitude and accuracy are spatially associated with vegetation morphology. Using the GeoEye-1 imagery, MESMA provided more accurate fractional cover estimates than OBIA. The increasing segmentation scale in OBIA resulted in a consistent increase in error. While areas under woody cover produced lower errors even at coarse segmentation scales, those with herbaceous cover provided low errors only at the fine segmentation scale. Vegetation morphology type mapping results suggest that classes with dominant woody life forms attained higher accuracy at fine segmentation scales, while those with dominant herbaceous vegetation reached higher classification accuracy at coarse segmentation scales. Contrarily, for bare areas accuracy was relatively unaffected by changing segmentation scale. Multi-scale fractional cover mapping results indicate that increasing pixel size caused consistent increases in variance of and error in fractional cover estimates. Even at a coarse spatial resolution, fPV was estimated with higher accuracy compared to fNPV and fBS. At a larger pixel size, in areas with dominant woody vegetation, fPV was overestimated at the cost of mainly underestimating fBS; in contrast, in areas with dominant herbaceous vegetation, fNPV was overestimated with a corresponding underestimation of both fPV and fBS. These results underscore that structural and functional heterogeneity in savannas impact retrieval of fractional cover, suggesting that comprehensive remote sensing of savannas needs to take both structure and cover into account. / text

<p>Southern Africa harbours one-third of the world&rsquo / s Testudinid species, many of which inhabit arid or semi-arid areas, but ecological information on these species is scant. I studied the habitat, morphology and ecology of Kalahari tent tortoises over 13 months in semi-arid Savanna at Benfontein farm, Northern Cape Province, South Africa. In order to allow continuous monitoring of individuals, I attached radiotransmitters to males and females, split equally between two habitats, sites E (east) and W (west), with apparent differences in vegetation structure. Results of the study were based on data obtained from 27 telemetered tortoises and 161 individuals encountered opportunistically. Female Kalahari tent tortoises were larger than males and the sex ratio did not differ from 1:1. Based on person-hours to capture tortoises, the population appeared to have a low density, with more time required to capture a juvenile (35 hours) than an adult (10-11 hours). The frequency distribution of body size ranges was indicative of recruitment. Relative age, based on annuli counts, suggested that males were younger than females, perhaps because males as the smaller sex are more predation-prone than females. Linear relationships between annuli counts and shell volume indicated that, after reaching sexual maturity, female body size increased faster in volume than did male body size, possibly because a larger volume may enhance female reproductive success. Body condition differed between sites, sexes and among seasons. The hot and dry summer may account for low summer body condition, whereas vegetation differences and size effects, respectively, may account for the low body condition of tortoises in site W and in males. Site E was sandy with grasses, particularly Schmidtia pappophoroides, being the prevalent growth form. This habitat resembled a Savanna vegetation type Schmidtia pappophoroides &ndash / Acacia erioloba described for a neighbouring reserve. Site W was stonier, dominated by shrubs, and was reminiscent of Northern Upper Karoo vegetation (NKu3). Neither site resembled Kimberley Thornveld (SVk4), the designated vegetation type of the area. Differences in substrate and grazing intensity may have contributed to site vegetation differences. Rainfall had an important influence on seasonal vegetation. Short grass abundance correlated with rainfall and annual plants sprouted after spring rain. Refuge use changed according to season and sex. Males selected denser refuges than females did, perhaps because males were smaller and more vulnerable to predation and solar heat. Tortoises selected sparse, short grass as refuges in cool months, probably to maximise basking whilst remaining in protective cover. During hot periods, mammal burrows were preferred to vegetation as refugia. The smaller males spent more time in cover than females, which may be related to predator avoidance or thermoregulation.&nbsp / Females spent more time basking than males, perhaps due to their larger size and to facilitate reproductive processes. Tortoises did not brumate, but through a combination of basking, and orientation relative to the sun in their refuges, managed to attain body temperatures that allowed small bouts of activity. Body temperature for active tortoises was similar among seasons, and was higher for more specialised active behaviours, such as feeding and socialising, than for walking. Increased activity by males in spring could relate to mating behaviour while females were more active in autumn, when they foraged more than males, perhaps due to the high cost of seasonal reproductive requirements. Males displaced further per day than did females, but home range estimates did not differ between sexes. Annual home range estimates varied substantially among individuals: 0.7&ndash / 306 ha for minimum convex polygons and 0.7&ndash / 181 ha for 95% fixed kernel estimates. The ability to&nbsp / cover large areas would assist tortoises in finding resources, e.g., food, in an area where resource distribution may be patchy. Differences among seasonal home ranges and movements probably reflect seasonal climatic change / activity areas shrinking when temperatures were extreme. In order to assess the effects of a semi-arid environment on the morphology of P. oculifer, I compared its morphology to that of its &lsquo / cool-adapted&rsquo / sister taxon Psammobates geometricus, using live and museum specimens. Both P. oculifer and P. geometricus are sexually dimorphic and differences between the two species could indicate environmental or sexual selection effects, or a combination of the two. The shorter bridge length, which allowed more leg space, and wider front feet in P. oculifer cohorts probably represent traits for manoeuvring in a sandy habitat, while wider heads in P. oculifer possibly relate to interspecific differences in diet. The flatter shell in female P. oculifer, relative to P. geometricus, may represent a trade-off between space for reproductive structures, e.g., eggs, and the need to fit into small refuges, e.g., mammal burrows. Male P. oculifer had wider shells, more space around their hind legs, and wider hind feet than P. geometricus males had, all characteristics which may assist males to fight and mate in a sandy environment.</p>

The patch structure of the grass sward at Kroomie (26°25'38"E 33°48'30"S) in a semi-arid savanna in the Eastern Cape, South Africa, was investigated. The study was conducted on long term grazing trials on five treatments varying in stocking rate (SR; recommended (low) and 1.5 x recommended (high), grazing system (continuous and rotational), and animal type (cattle and sheep). The treatments studied were CR (cattle, rotational stocking, low SR), CC (cattle, continuous stocking, low SR), CH (cattle, rotational stocking, high SR), SC (sheep, continuous stocking, low SR), and SR (sheep, rotational stocking, low SR). Rainfall during the two years of the study (1997/98 and 1998/99) was slightly below the mean average rainfall of the area (66 and 84% of the mean of 519 mm, respectively). Analysis of sward height data using Maximum Likelihood Estimation reflected a bimodal height structure in all treatments. Due to a high overlap of the two distributions in some cases, however, the height at which to separate patches (short grass) from non-patches (tall grass) could not be determined. Canonical Correspondence Analysis (CCA) was used to relate species composition to sward height. It emerged that there are two distinct grass communities at Kroomie, and these are associated with sward height (i.e. patches and non-patches). The interface (in cm) between these two communities, as determined using Two Way Indicator Species Analysis (TWINS PAN) was 6 cm, and this value was subsequently used to discriminate between patches and non-patches. Sward structure was affected by treatments. Animals (cattle and sheep) stocked rotationally at low SR's grazed less than a third of the total area, and this grazing was concentrated primarily in small patches (< 6 m; length is used as a linear indicator of patch size). Animals stocked continuously at low SR's grazed approximately half the area, in small and large (up to 40 m) patches. Animals in the CH treatment grazed approximately two-thirds of the area, in both small and large patches. There was an inverse relation between the size of patches and the size of non-patches, as expected. Nine common grass species were related to sward height. Digiteria eriantha, Eragrostis racemosa, Eustachys paspaloides, and Microchloa caffra were associated with short swards, while Cymbopogon pluronodis, Eragrostis chloromelas, and Sporobolus fimbriatus were associated with tall swards. Themeda triandra (themeda), the most abundant grass at Kroomie, was principally associated with tall swards, but was present at all sward heights. Applying CCA demonstrated a considerable difference between the species composition of patches and of non-patches. There was also a difference in composition between treatments, although these were not as pronounced. Patches reflected a higher species diversity than non-patches. There was a significant (P<0.05) effect of treatment, and of an interaction of treatment by sward structure (i.e. patches and non-patches), on the density of themeda plants. The density of themeda plants was positively correlated with patch size, which suggested that themeda plants that have been grazed may suffer fatal competition from ungrazed neighbours. Anecdotal evidence suggested that patches are stable over the medium term, and that non-patches that are grazed during a drought return to a non-patch structure after rainfall. There was no evidence to support the contention that rotational stocking reduced patch-selective grazing, nor that the species composition of rotationally stocked treatments was better than continuously stocked treatments. / Thesis (M.Sc.)-University of Natal, Pietermaritzburg, 2003.

This study is an effort to provide new information on the effects of isolated tree canopies on understory vegetation composition and soil characteristics of northern Indiana black oak savannas. Temperate savannas in the United States have been greatly altered by human activities. Management of these areas is an important consideration for Midwest natural resource agencies. It is hypothesized that isolated trees within a savanna may alter the soil and plants around them, much in the same way that gaps in a forest canopy alter the plant composition and soil characteristics on the forest floor. In this study, isolated trees were selected in three northern Indiana black oak (Quercus velutina) savannas. Plots were located under the tree canopies and in adjacent open areas, in four directions from the tree stem. Populations of herbaceous plants were inventoried and the soil characteristics analyzed for both inside-canopy and outside-canopy plots. The environmental variables measured accounted for only about 20 percent of the variation in plant community between plots according to a Canonical Correspondence Analysis. Most of the variation in plant composition between plots was explained by pH, the amount of rain throughfall, and the thickness of the A horizon. It is clear that these black oak trees do alter the soil and plant composition around them. This information has implications for the long-term management of northern Indiana savannas. / Department of Natural Resources and Environmental Management

Acacia erioloba (synonym: Acacia giraffae) commonly known as Mpatsaka (Sotho), Mokala (Tswana), Kameeldoring (Afrikaans) and camel Thorn (English) is an indigenous leguminous tree that is adapted and commonly found in semi-arid savanna environments of South Africa. Being a leguminous plant, Acacia erioloba can fix atmospheric nitrogen into the soil. The objective of this study was to quantify the concentration of plant nutrients in soils beneath the canopies of Acacia erioloba trees in three land-use practices viz: fallow, grazing and bare land. Three trees were randomly selected in each land-use practice and soil samples were collected from beneath and beyond the tree canopies at depths of 0-10 and 10-20 cm. Consistently, soil analysis revealed that the concentration of nutrients (N, P, Ca, Mg, Zn and Mn) and soil biological properties (organic carbon, particulate organic matter, microbial biomass nitrogen and microbial biomass carbon) among the land-use practices were in the order: fallow > grazing > bare. Differences in the concentration of soil nutrients between the land-use practices seemed to be influenced by management within the land-use practices. Nitrogen, phosphorus, potassium and zinc were significantly higher (p<0.05) in land under grazing while calcium and magnesium, were significantly higher (p<0.05) under fallow land. As expected, bare land had the lowest concentration of all the nutrients studied. The presence of animals around Acacia erioloba trees contributed to higher concentration of nutrients in land under grazing. The differences in the values of biological properties were considered to be likely due to higher plant biomass on the topsoil compared to the subsoil that increased the microbial activity. The improved biological properties are thought to have improved the nutrient concentration through processes such as mineralisation of nutrients from organic matter. With the exception of phosphorus (P), potassium (K) and zinc (Zn), concentration of all other nutrients (N, Ca, Mg and Mn) was not significantly different (p<0.05) in soil from beneath the tree canopy and those from beyond the tree canopies in all land-use practices. Topsoil (0-1 0 cm) had, significantly higher (p<0.05) concentration of all nutrients and biological properties that were measured compared to the sub soil (10-20 cm). This was attributed to the fact that much of the decomposition of soil organic matter takes place on the surface layer of the soil where most of the organic materials are added. Nutrient uptake from deep soil layers by roots of the trees may be another important mechanism that could enrich surface soils beneath Acacia erioloba tree with nutrients. The availability to a wheat crop of the nutrients found beneath the canopies of Acacia erioloba was evaluated using a "Neubauer" seedling technique in a glasshouse. Wheat seedlings were grown into PVC pots filled with soils collected from beneath and beyond the tree canopies. The wheat that was grown in soils collected beneath Acacia erioloba trees had significantly higher (p<O.OS) growth ~d nutrient uptake than that grown in soils from beyond tree canopies. Plant height, dry matter yields and nutrient uptake by the wheat seedlings were in the order fallow>grazing>bare land. The plant height were 20.9, 16.4 and 14.2 cm for fallow, grazing and bare, respectively. This was attributed to the high accumulation of organic matter from different sources such as leaf litter, grass residues and animal wastes under fallow and grazing land. Fallow and grazing promotes large organic matter inputs and therefore create conditions that favour rapid decomposition of organic matter and mineralisation of nutrients. Correlation coefficients between soil nutrients and nutrients in the tissues of wheat show that there was a strong and significant relationship between the two. It was suggested from these results that Acacia erioloba trees has a potential to be used in agroforestry systems within the farming systems of the small-scale farmers in the semi-arid savanna ecosystem of the North-West Province. Such systems could include agrislviculture and sylvopastoral. If Acacia erioloba trees are to be included in agroforestry systems, the densities need to be increased. This would imply increasing their propagation. More techniques of propagating the tree need to be researched. The nitrogen fixing potential of Acacia erioloba needs to be thoroughly investigated. / Thesis (M.Sc (Agric.) North-West University, Mafikeng Campus, 2003

Bush encroachment is a major problem for the savannas supporting livestock in the Eastern Cape. Farmers employ chemical poisoning and mechanical clearing of woody vegetation to improve grass production. This thesis addressed the following questions. 1. Does soil fertility and hence, herbaceous production and/or quality increase beneath or between former bushclumps following woody clearing or poisoning? 2. Are chemical or mechanical control methods effective for all woody species? 3. Does bush density and height decline with increasing competition from the herbaceous layer? 4. Can fire and goats retard or revert woody re-establishment, thereby keeping these savannas open? A trial for assessing different methods of controlling bush was conducted near Kei Road. Initial treatments included chemical poisoning, mechanical clearing and a control, each replicated eight times in 0.36 hectare plots. Follow-up treatments were control, fire, goats, and the combination of fire and goats each replicated twice per primary treatment. Two additional mechanically cleared plots were oversown with Chloris gayana seeds. Herbaceous production, species composition, foliage quality and soil fertility, and mortality, recruitment, height increase and density of woody individuals were monitored for five seasons. Mechanical clearing of the woody vegetation increased soil fertility, except total nitrogen, and that explained the dramatic response in grass production that endured for the first four seasons since clearing. The periphery and ex-bushclump zones were characterised by increased colonization of Panicum maximum while there were no changes in frequencies of other key herbaceous species in all vegetation zones. Acacia karroo and woody "weeds" (Solanum incanum, Berkheya bipinnatifida) established from seed while all other woody species recruited mainly from coppicing. Seedling recruitment and resprouting resulted in high densities of woody stems and individuals by the second season after clearing when compared with the pre-clearing levels. Oversowing cleared areas with pasture grasses did not only increase grass production but also reduced the density of coppicing woody plants and "weeds". Chemical treatments mirrored the controls in terms of grass production, except during a very wet season, and species composition. Although encroaching woody species (e.g. Scutia myrtina, Maytenus heterophylla and Trimeria trinervus) were susceptible to poisoning, woody plant density was not reduced. Multi-stemmed woody individuals were resistant to poisoning. Fire and goats kept most coppicing woody plants short, less than half a metre, after three seasons of browsing and also improved grass production in the dense bushclumps suggesting that these clumps were being opened up. However, high browsing pressure forced goats to graze more hence this effect was masked. This study indicates that chemical and mechanical control of bush are economically unsustainable for beef and mutton production, at least in the medium term. Fire and goats are appropriate as a follow-up strategy for retarding woody regrowth, keeping bushclumps open, improving grass production and economic viability of mechanical clearing. / Thesis (M.Sc.)-University of Natal, Pietermaritzburg, 2000.

Thesis (doctoral)--Universität zu Göttingen, 2003. / Includes bibliographical references (p. 139-155).

Thesis (Ph. D. (Zoology and Entomology))--Rhodes University, 2005.

A terra é um recurso natural necessário à produção agropecuária. A pesquisa sobre as mudanças no uso agropecuário da terra é especialmente relevante para o país e regiões com características como o Cerrado brasileiro, onde existem áreas vulneráveis à conversão agropecuária e nas quais as condições edafoclimáticas são favoráveis à produção animal e vegetal. Nesse contexto, o objetivo geral desta pesquisa é discutir os efeitos do emprego de tecnologia na pecuária de corte no Cerrado, e o uso agropecuário da terra na região. Para atender ao objetivo geral são estabelecidos objetivos específicos, quais sejam: (i) discutir os fatores que afetam o uso agropecuário da terra em geral e no Cerrado brasileiro; (ii) detalhar a tecnologia no contexto econômico e as principais tecnologias aplicadas à bovinocultura de corte, com destaque àquelas mais aplicáveis ao Cerrado; (iii) apresentar e discutir os modelos aplicados para a análise da mudança no uso da terra, com enfoque nos modelos de equilíbrio geral computável; (iv) apresentar e discutir os resultados do cenário atual da produção pecuária e do uso agropecuário da terra; e (v) apresentar e discutir os resultados do emprego de tecnologia na pecuária por meio da aplicação do modelo de equilíbrio geral computável. As principais fontes de dados são as publicações do Censo Agropecuário e as Contas Nacionais publicadas pelo IBGE. Para a obtenção dos resultados foi utilizada a pesquisa bibliográfica, a estatística descritiva e a aplicação do modelo de equilíbrio geral computável. Os principais resultados verificados nessa pesquisa indicam que a ocupação do Cerrado ocorreu devido a diversos fatores, dos quais ressaltam-se: o emprego de tecnologia, os investimentos em capital e o conhecimento específico aplicado à agricultura a fim de promover a expansão agropecuária de forma mais intensiva. Os fatores que afetam as mudanças no uso agropecuário da terra consistem na combinação de grãos geneticamente melhorados, pastagens plantadas, e as políticas governamentais que induziram a ocupação do Cerrado. O cenário atual da pecuária de corte na região revela a importância da bovinocultura de corte, que é responsável por 70% do valor bruto da produção de carnes da região (incluindo a produção em pastagens e em confinamentos). O principal uso agropecuário da terra no Cerrado é destinado à bovinocultura, quatorze vezes superior à área colhida das culturas de soja, milho e cana-de-açúcar somadas (11,55 milhões de hectares). Quanto ao efeito do emprego de tecnologia na atividade pecuária do Cerrado, se constata que causa aumento da renda per capita na região, e nas demais regiões o impacto é negativo. A principal conclusão dessa pesquisa reside na constatação de que o emprego de tecnologia na pecuária, e mais especificamente na bovinocultura de corte, afeta o uso agropecuário da terra no Cerrado brasileiro. De acordo com os resultados apresentados, é muito provável que nos próximos anos continue influenciando a atividade agropecuária como um todo, provicando ainda mudanças na configuração da produção agropecuária atual. Dentre os principais desafios e limitações de coleta de dados referentes à produção animal (mais especificamente para o cálculo do valor bruto da produção – VBP) enfrentados nessa pesquisa se referem às decisões e opções de quais fontes de dados utilizar quando a fonte de dados oficial do censo agropecuário não oferecia o dado (exemplo, valor bruto da produção de bovinos, suínos, caprinos, dentre outros). Optou-se em favor do Censo Agropecuário para todas as variáveis disponíveis por ser uma publicação periódica oficial. Quando necessário, foram utilizadas outras informações complementares oriundas de outras fontes acadêmicas e profissionais do setor. / Agricultural production depends on natural resources such as land. Agricultural land use change research is especially relevant to regions and countries with characteristics (climate, soil and technology conditions) similar to the Brazilian Savannna – Cerrado. Cerrado is a vulnerable region suitable to increasing agricultural conversion. In this context, this research aims to discuss the technology effects in the Cerrado’s beef production in the Cerrado as well as the agricultural land use in the region. In order to complete the general purpose we propose to meet speficic objectives: (i) to discuss the factors affecting agricultural use of land in the Brazilian Savanna (Cerrado); (ii) to detail the technology in the economic context and key technologies applied to beef cattle in the Cerrado; (iii) to present and discuss applied models for land use change analysis; (iv) to present and discuss the results of actual scenario of livestock production and agricultural land use; and (v) to present and discuss the results of technology use in livestock production using a computable general equilibrium (CGE) model. Main data sources are Agricultural Census and the National Accounts reports, published by the Brazilian Institute of Geography and Statistics (IBGE). Descriptive statistics analysis and the CGE model were used to obtain the results. The key findings in this study indicate occupation of the Cerrado was due to several factors, among which stand out: use of technology, investments and expertise applied to agricultural sector to promote the expansion of farming more intensively. The factors affecting the changes in the agricultural land use consist of a combination of genetically improved seeds, planted pastures, and government policies. Beef cattle current scenario in the region reveals the importance of beef cattle, which accounts for 70% of the gross value of meat production in the region (including pasture and feedlots production). Cattle is the main agricultural land use, fourteen times the harvested area of soybean, corn and cane sugar aggregated (11,55 millions of ha). The main conclusion of this research lies in the fact that technology applied in farming (and more specifically in beef cattle) affects agricultural land use in the Brazilian Cerrado. According to results, it is very likely that in coming years technology will continue influencing the agricultural activity as a whole, stimulating further changes in the configuration of current agricultural production. Among main challenges and limitations of animal production data work (more specifically for the calculation of the gross value of production) addressed in this study refer to the decisions of which data sources rely when the official data source (agricultural census) did not contain the needed variable (eg, gross value of production of cattle, pigs, goats, among others). The agricultural census (official periodical publication) was used for all variables available. We used other additional information coming from other sources from academic and industry professionals (detailed in the method chapter).

Made available in DSpace on 2016-06-02T19:29:36Z (GMT). No. of bitstreams: 1 3772.pdf: 1204652 bytes, checksum: 82fe131354521c84cb2f4d3e457d482f (MD5) Previous issue date: 2011-07-11 / Universidade Federal de Sao Carlos / Allelopathy is a process that involves primary or secondary metabolites produced by plants and microorganisms, which influences the growth and development of agricultural and biological systems. The intensity of plant allelopathic effects may depend on the hábitat, age and the biotic and abiotic conditions during the seasons in which they were collected. When allelopathic substances are purified and identified, they can be used to develop alternative products based on natural compounds to control weed species. The Brazilian savanna (cerrado) has very stressful environment conditions, mainly in dry periods, and should favor a greater variety of allelochemicals in Rapanea ferruginea (Ruiz & Pav.) Mez and Rapanea umbellata (Mart.) Mez (Primulaceae), which have leaves with resin canals and minute secretory glands, sparse throughout the abaxial surface, that can accumulate terpenes or tannins. This thesis hypothesis is that there must be allelopathic substances in the species of Rapanea to favor their establishment in the savanna and that they should be influenced by seasonal factors or by plant age. Allelopathy occurs by the release of allelochemicals in the environment and these may have economic importance, serving as molecular models for the production of natural herbicides, less harmful to the environment. In the first chapter, we compared the effect of plant age and season of leave collection on the allelopathic activity of aqueous extracts of R. ferruginea and R. umbelata. For this, germination and initial length bioassays were conducted using commercially bioindicator cultivated species (lettuce, tomato and onion) under the influence of the aqueous leaf extracts of adults and young individuals from wet and dry season at 10 and 5% (weight of leaf powder per volume of distilled water, g/mL). The extract of adult R. umbellata collected in the dry season showed the highest allelopathic activity and was chosen for the other studies of this thesis. In the second chapter, we compared the allelopathic activity of leaf aqueous extract of R. umbellata (10 and 5%) with the synthetic herbicide Oxyfluorfen on germination and initial length of weed species: barnyardgrass (Echinochloa crus-galli L., Poaceae), wild poinsettia (Euphorbia heterophylla L., Euphorbiaceae) and morning-glory (Ipomoea grandifolia Drammer., Convolvulaceae). In the third chapter, bioprospection, purification and isolation of secondary metabolites of agronomic interest was performed with R. umbellata leaves. Bioassays wheat of etiolated coleoptiles and germination and initial length of cress, lettuce, onion and tomato were performed under the effect of extracts of organic solvents for the direction of the compounds purification. The acetone extract was the most active and therefore was used to be fractionated by chromatographic column. It fractions were purified HPLC and Sephadex column. The isolated and purified compounds were identified by NMR1H and 13C spectra. This thesis results permit we to affirm that allelopathy is one way of the R. ferruginea and R. umbellata species to establish themselves in the savanna. The allelopathic activity of its leaves is influenced to a greater degree by the collection season (most active in dry season) than by plant age. The R. umbellata aqueous leaf extract showed distinct action than the Oxyfluorfen synthetic herbicide, being very active on the root part of the weed species while the synthetic herbicide was more active on the arial parts of these species. The bioprospection of R. umbellata leaves conditioned isolation and purification of the substances lutein and (-)- catechin. (-)-Catechin has the potential to be used as a natural origin and biodegradable herbicide. / Alelopatia é o processo que envolve metabólitos secundários ou primários, produzidos pelas plantas e microrganismos, que influencia o crescimento e o desenvolvimento de sistemas agrícolas e biológicos. A intensidade dos efeitos alelopáticos de uma planta pode depender do hábitat, da idade e condições bióticas e abióticas a que estavam sujeitas na época em que forem coletadas. Quando substâncias alelopáticas são purificadas e identificadas, podem ser utilizadas para o desenvolvimento de produtos alernativos baseados em compostos naturais para o controle de espécies infestantes. O ambiente de cerrado possui condições ambientais bastante estressantes, principalmente na época da seca, e deve favorecer uma maior variedade de aleloquímicos nas espécies Rapanea ferruginea (Ruiz & Pav.) Mez e Rapanea umbellata (Mart.) Mez (Primulaceae), que possuem folhas com canais resiníferos e glândulas secretoras diminutas, esparsas por toda face abaxial, que podem acumular terpenos ou taninos. A hipótese desta tese é que devem existir substâncias alelopáticas em espécies de Rapanea que favoreçam o seu estabelecimento no cerrado e que estas devem ser influenciadas por fatores sazonais ou pela idade das plantas. A alelopatia ocorre pela liberação de aleloquímicos no ambiente e estes podem apresentar importância econômica, servindo de modelos para a produção de herbicidas de origem natural, menos agressivos ao meio ambiente. No primeiro capítulo, comparou-se o efeito da idade e da sazonalidade da época de coleta das folhas sobre a atividade alelopática de extratos aquosos de indivíduos de R. ferruginea e R. umbelata. Para tanto, bioensaios de germinação e comprimento inicial de espécies bioindicadoras cultivadas comercialmente (alface, tomate e cebola) foram realizados sob influência dos extratos foliares aquosos de indivíduos adultos e jovens provenientes de estação chuvosa e seca à 10 e 5% (massa de pó de folhas por volume de água destilada, g/mL). O extrato de R. umbellata adulta coletada em estação seca apresentou atividade alelopática elevada e foi escolhido para os demais estudos desta tese. No segundo capítulo, comparou-se a atividade alelopática do extrato aquoso foliar de R. umbellata (10 e 5%) com a de um herbicida sintético (oxifluorfem) sobre a germinação e comprimento inicial de espécies infestantes de culturas agrícolas: capim-arroz (Echinochloa crus-galli L., Poaceae), amendoim-bravo (Euphorbia heterophylla L., Euphorbiaceae) e corda-de-viola (Ipomoea grandifolia Drammer., Convolvulaceae). No terceiro capítulo, a bioprospecção, purificação e isolamento de metabólitos secundários de interesse agronômico foi realizada com folhas de R. umbellata. Para que houvesse um direcionamento na purificação destes compostos, bioensaios de coleóptilos estiolados de trigo, germinação e comprimento inicial de agrião, alface, cebola e tomate foram realizados sob efeito de extratos de solventes orgânicos. O extrato acetônico foi o mais ativo e, portanto, foi utilizado em fracionamento em coluna cromatográfica. Suas frações foram purificadas em CLAE e coluna de Sephadex. Os compostos isolados e purificados foram identificados através de RMN1H e 13C. Os resultados obtidos com esta tese permitem dizer que a alelopatia é uma das formas das espécies de R. ferruginea e R. umbellata se estabelecerem no cerrado. A atividade alelopática de suas folhas é influenciada em maior grau pela época de coleta (mais ativas em estação seca) do que pela idade das plantas. O extrato foliar aquoso de R. umbellata apresentou ação distinta do herbicida sintético oxifluorfem, sendo bastante ativo sobre a parte radicular das espécies infestantes, enquanto o herbicida sintético foi mais ativo sobre a parte aérea destas espécies. A bioprospecção das folhas de R. umbellata levou ao isolamento e a purificação das substâncias luteína e (-)-Catequina. A (-)-Catequina apresenta potencial para ser utilizada como herbicida de origem natural e biodegradável.