The intent of this thesis is to set out new data relating to the climate of the savanna in the Rupununi district of Guyana, and to comment thereon with reference to the causes and continuing existence of that savanna. To indicate the significance of the data and the pertinence of the comments it will be necessary to review recent discussion of the nature and causes of savanna in general. [...]

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The majority of the vegetation types occurring on Malilangwe Estate, in the south-eastern lowveld of Zimbabwe, are dominated by Colophospermum mopane (mopane). Over the past 30-50 years the stand density of these mopane vegetation types has increased, and an investigation was undertaken to determine the effect of season of burning and type of fire on mopane woodlands. From this study the following was ascertained: 1) A single predictive equation cannot be used over all seasons to estimate standing crop (fuel load) using the standard disc pasture meter procedure. The calibration equations developed using this procedure accounted for between 39 and 72% of the variation in standing crop, illustrating the high variation in basal cover of the grass sward, as well as the variation between months. Although the revised procedure, developed for areas with low basal cover, accounts for a lot more of the variation in standing crop, this procedure was not used to estimate standing crop over the study period because the calibration equation covered a number of vegetation types, and was not specific to the mopane woodlands. 2) Standing crop tracks effective rainfall (monthly rainfall divided by monthly pan evaporation) closely, with a lag period of less than one month. Standing crop can be estimated using a predictive equation that utilizes effective rainfall from the previous month. There is a positive relationship between peak standing crop and rainfall. A predictive equation was developed to estimate peak standing crop, using annual rainfall. Standing crop declines through the dry season as effective rainfall decreases, and this 'decrease function' allows for the estimation of the standing crop for a particular month, after peak standing crop is reached. 3) Two leaf quantification equations were developed for mopane trees in the south-eastern lowveld of Zimbabwe, one for coppicing and for non-coppicing individuals. These allow for the estimation of leaf dry mass from measured canopy volume. 4) There was no significant difference between the fire intensities attained for the three seasons of burning. Over all seasons, head fires were significantly more intense than back fires. 5) Percentage topkill after late dry season burns was significantly higher than topkill after early dry season burns. There was no significant difference between mid and late dry season burns, and head fires led to significantly more topkill than back fires. Plants < 150 cm experienced significantly more topkill (80 %) than did individuals > 150 cm (44%). 6) Fire per se led to an increase in stand density over all seasons and types of fire, but this change was not significant. Fire did not influence the nett recruitment of new individuals. Height class one (0-50 cm) and three (151-350 cm) were impacted most by fire. This reflects a change in tree structure, with an increase in the amount of leaf material in height class three, and a subsequent decrease in the amount of material in height class one. 7) The effect of season of burning on the change in tree height was significant, whereas the effect of type of fire was not significant. All treatments, except early dry season back fires, led to a reduction in tree height, whereas trees in the no burn areas increased in height. 8) Burning in any season, and implementing either type of fire, led to an increase in the number of stems. Mid dry season burns led to the highest increase in number of stems. However, the more intense the fire the smaller the increase in number of stems. 9) All three seasons of burning (head and back fires) led to a significant decrease in maximum canopy diameter per tree, while the maximum canopy diameter of trees in the no burn areas increased. Mid dry season burns resulted in the greatest decrease in canopy diameter. 10) The effect of burning on the change in leaf dry mass per tree was highly significant. All three seasons of burning led to a decrease in leaf dry mass, while there was no difference between head and back fires. Leaf dry mass in the control areas increased however. High fire intensities led to the greatest decrease in leaf dry mass, late dry season head fires having the greatest decrease. This study suggests that mopane plants face a constraint due to fire and/or browsing, and a tradeoff occurs between canopy volume, canopy diameter, canopy area; and number of stems. Fire leads to an increase in the number of stems through coppicing, while canopy volume and leaf dry mass decreases. This decrease is either (i) a tradeoff in response to increasing stem number, or (ii) a reduction in canopy because additional leaves on the new stems contribute to photosynthesis. The most important response to season of burning is the altered phenophase (phenological stage) of the plant. Early dry season burns cause the trees to be leafless during the early dry season (when unburnt trees are carrying full leaf), and then to be in leaf at the end of the dry season (when unburnt trees are leafless). It would appear that fire disturbance initiates leaf senescence after burning, and then leaf expansion earlier than normal i.e the whole leaf senescence/growth process is brought forward. Trees in late dry season burn areas remain leafless at the start of the rains, while trees in unburnt areas are carrying leaf. Being leafless these trees do not photosynthesize during this time, and it is proposed that the grass sward is advantaged by the reduced competition from the tree component. The consequences of these two changes in phenophase could not be addressed in this study, but are pertinent questions that must be answered if mopane woodland dynamics are to be more fully understood. Management recommendations for (1) the removal of unacceptable moribund grass material, or (2) the reduction of encroachment by woody species on Malilangwe Estate are also given. In an attempt to combat the increase in stand density of mopane it is recommended that high intensity head fires be implemented, when standing crop (fuel load) is sufficient and climatic conditions are conducive to maintaining high intensity fires. These should be carried out at the end of the dry season, before the onset of the rains. / Thesis (M.Sc.)-University of Natal, Pietermaritzburg, 2000.

Diss. (sammanfattning). Umeå : Sveriges lantbruksuniv., 2003. / Härtill 5 uppsatser.

Diss. (sammanfattning) Umeå : Sveriges lantbruksuniv., 2007. / Härtill 5 uppsatser.

Diss. (sammanfattning) Umeå : Sveriges lantbruksuniv., 2008. / Härtill 4 uppsatser.

Les savanes sont des écosystèmes complexes pilotées par plusieurs mécanismes ascendant (ex: les nutriments du sol ou pluviométrie) ou descendant (ex: feu ou herbivorie), mais l'importance relative de ces mécanismes reste largement débattue. En particulier, le rôle des herbivores brouteurs (browsers) reste mal compris en tant que source de perturbation, et donc de force de pression descendante influant sur la dynamique des savanes. Dans cette étude, deux approches ont été développées pour aborder le rôle des perturbations dans la dynamique des savanes. Dans un première partie, j'ai utilisé une approche comparative inter-site pour explorer les réponses des communautés de plantes, et des principaux traits de ces plantes associés aux feuilles, branches, architecture et défense, aux variations de quatre facteurs : les nutriments dans le sol, la pluviométrie, la pression d'herbivorie et l'intensité du feu. Seize sites de savane, en Afrique du Sud et au Zimbabwe, ont été sélectionnés sur des gradients de chacun de ces facteurs. Les espèces ligneuses dominantes (>80 % de la biomasse) sur chaque site ont été identifiées et échantillonnées, afin de mesurer les traits des feuilles et des branches associés à l'appétence, architecture, ainsi qu'aux défenses physiques et chimiques de ces plantes. Des mesures ont également été faites pour estimer les effets des meso-brouteurs et mega-brouteurs. Des transects ont permis d'estimer la fréquence et l'intensité du feu sur chaque site, et l'effet sur les plantes. En préambule à l'analyse, et devant le manque de protocole standard pour estimer la fertilité des sols dans la littérature écologique, je propose une méthode et un échantillonnage afin de définir de manière robuste la fertilité des sols sur chaque site. Dans cette partie inter-site, huit traits principaux ont été comparés sur le gradient de qualité de sol et de pluviométrie, et bien que quelques relations statistiques existent entre les traits des feuilles, le sol et la pluviométrie, ces relations sont très faibles comparées à celle trouvées dans les méta analyses inter-biomes publiées dans la littérature. Cependant, ces approches interbiome sont dominées par des sites tempérés qui ont des niveaux de perturbations bien inférieurs à ceux des savanes africaines. L'évaluation des effets des meso-brouteurs et mega-brouteurs le long des gradients de sol et de pluviométrie sur vingt traits associés aux défenses structurelles et chimiques des plantes montre que les défenses structurelles sont plus corrélées aux caractéristiques du sol que les défenses chimiques, mais que seules les défenses structurelles sont fortement corrélées à l'impact par les brouteurs. Le niveau d'utilisation des plantes par les mesobrouteurs apparaît plus prévisible en fonction des traits des plantes que celui par les mégabrouteurs. Dans une deuxième partie présentent des résultats de deux études basées sur des expériences d'exclos. Dans le parc national de Kruger, la composition de la communauté, l'abondance et la démographie des ligneux dominants ont été estimées à l'intérieur et à l'extérieur de trois exclos de 40 ans, et les brouteurs apparaissent comme ayant un impact significatif sur la distribution, la densité et la structure des populations des espèces arbustives et arborées ayant des traits préférés : forte concentration en azote foliaire et faible teneur en défenses chimiques. L'interaction entre les effets des brouteurs et du feu semble aussi affecter le recrutement des juvéniles ligneux dans les grandes classes de taille. Dans le parc de Hluhluwe-iMfolozi, cinq exclos ont été utilisé pour tester l'effet des brouteurs sur l'architecture, la croissance, les défenses chimiques et structurelles des jeunes individus de sept espèces d'acacia. Des différences nettes apparaissent entre les espèces d'acacia de savane semi-aride et plus humide dans les traits associés à l'appétence, l'architecture et les défenses … [etc] / Savannas are complex ecosystems affected by several bottom-up (e.g. soil nutrient availability and rainfall) and top-down (e.g. fire and herbivory) drivers. However, the relative importance of bottom-up vs. top-down drivers in influencing savanna dynamics is still widely debated. Within the top-down (disturbance) category of drivers, the role of mammal browsers in particular in driving savanna functioning is still not well understood. Two approaches were adopted to determine the role of disturbance in savannas. Firstly, by using a comparative approach, I attempted to address the so-called ‘savanna problem’ by investigating how savanna woody plant community compositions and key plant traits relating to the leaves, stems, architecture, and defence are influenced by soil nutrient status, rainfall, fire and browsing. Sixteen sites were selected along gradients of these four drivers from savanna parks throughout South Africa and Zimbabwe. The dominant woody species (species that accounted for >80% of standing biomass) at each site were identified and sampled for the key leaf and stem traits relating to plant functioning, palatability, architecture, physical and chemical defences. Measurements were undertaken for each species in order to determine both meso-browser and mega browser impact. Transects were undertaken in order to determine the relative abundance and the effects of fire on each species at each site. Due to the current lack of standardized soil sampling protocols in the ecological literature, and uncertainty around the definition of what denotes a fertile or infertile soil, I propose a number of standardized protocols and sampled according to these established protocols in order to accurately determine the soil nutrient status at each site. Following this, the relationships between climatic variables and soil nutrients with both species means and community weighted means for eight key leaf traits were explored. Although some significant relationships were found between savanna leaf traits of woody plants, climate, soil nutrients and their interactions, these tended to be weaker than those found in meta-analyses. These broad-scale studies usually include sites from many biome types, many of which are from temperate regions where inherent levels of disturbance are typically much lower than in African savannas. The high levels of disturbance typically found in African savannas are thought to partially account for the high within site variability found in leaf traits and the weak relationships found between leaf traits, soil nutrients and rainfall. To assess the importance of resources vs. disturbance in savannas functioning, the effects of soil nutrients, rainfall, fire and both meso-browser and mega-browser impact on twenty savanna woody plant traits relating to plant palatability, chemical and structural defences were explored. Structural defences were found to be more strongly correlated with soil characteristics than chemical defences, while browser impact was found to be strongly correlated with structural defences but not with chemical defences. Actual browser utilisation tended to be more predictable for meso-browsers than mega-browsers. Secondly using an experimental approach, two sets of herbivore exclosures were utilized to directly test how mammal browsers influenced woody species distributions, abundance, population structure and plant traits relating to palatability and defence. The effects of three longterm herbivore exclosures in the Kruger National Park on savanna woody plant community compositions, population demographics and densities were determined. Browsers were found to have significant impacts on species distributions, densities and population structures by actively selecting for species with favourable traits, particularly higher leaf N. An interaction between browsers and fire which limited the recruitment of seedlings and saplings into larger size classes was also demonstrated… [etc]

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Este trabalho foi realizado com o objetivo de determinar o padrão de distribuição do carbono/biomassa das espécies arbóreo-arbustivas presentes em duas áreas de savana no município de Boa Vista em Roraima, Água Boa (AB) e Monte Cristo (MC), em função dos atributos químicos e físicos do solo. A metodologia consistiu em fazer um levantamento fitossociológico em parcelas permanentes estabelecidas nas duas áreas e, com isso, estimar a biomassa e o estoque de carbono dos indivíduos arbóreo-arbustivos pelo método indireto / The objective of this study was to determine the distribution pattern of carbon/biomass of species of trees and schrubs in two savanna areas in the municipality Boa Vista, Roraima, Água Boa (AB) and Monte Cristo (MC), according to the soils chemical and physical characteristics. Methodology consisted in a phytosociological survey in permanent plots established in the two areas, thus estimating the biomass and carbon stock of the tree and schrub specimens by indirect method

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / O objetivo deste estudo foi o de entender os padrões de restauração e autorregeneração sob a influência da área da copa de quatro espécies arbóreas (árvores-berçário ou nucleadoras) dominantes na região da savana (lavrado) de Roraima, extremo norte da Amazônia brasileira: Curatella americana L., Byrsonima crasssifolia (L.) H. B. K, Byrsonima coccolobifolia Kunth e Roupala montana Aubl. O estudo levou em conta a composição, a riqueza (S) e a diversidade (Shannon-Weaver) de plantas arbóreo-arbustivas encontradas sob a copa destas espécies nucleadoras, além de fazer uma comparação dos resultados através do índice de similaridade de Sørensen. Também foram avaliados os estágios de vida (plântula, jovem e estabelecido) e o padrão de distribuição espacial dos indivíduos arbóreo-arbustivos sob as copas das espécies avaliadas. Os resultados demonstraram que as árvores-berçário estudadas possuem preferências de estabelecimento fortemente vinculadas aos fatores edáficos, notadamente textura e saturação por alumínio, e que isto se reflete na diversidade de plantas sob seu domínio de copa. Quanto ao efeito nucleador das espécies estudadas, constatou-se que a auto-regeneração é marcante, principalmente nas espécies simpátricas do gênero Byrsonima (48% para B. crassifolia e 46% para B. coccolobifolia) e em menor grau em R. montana (12 %) e C. americana (6%). A riqueza e a diversidade de plantas arbóreo-arbustivas foram maiores sob a copa de C. americana (S = 16; H = 1,389-1,824), enquanto que as demais tiveram índices menores: R. montana (S = 13; H = 1,453-1,992), B. coccolobifolia (S = 12; H = 0,669) e B. crassifolia (S = 10; H = 0,864-1,908). A maioria dos indivíduos arbóreo-arbustivos encontrados sob a área de influência da copa de B. coccolobifolia foi de B. coccolobifolia (46%), enquanto que de B. crassifólia, 48% foram de auto-regeneração. Sob C. americana, a espécie Randia formosa esteve presente com 33%; e sob R. montana as espécies Connarus favosus e B. crassifolia possuíam 23%. Das espécies inventariadas sob a copa dos indivíduos nucleadores, foi constatado que B. crassifolia, Eugenia punicifolia, Erythroxilum suberosum e Xylopia aromatica são espécies sem exigência nutricional para seu estabelecimento, enquanto C. americana e Rourea grosourdyana exigem basicidade. Os resultados sugerem que as condições de estresse ambiental constante (sazonalidade marcante do regime pluviométrico e solos de baixa fertilidade natural), associado à forte pressão antropogênica (fogo), transmitem às espécies nucleadoras deste ecossistema um importante papel na manutenção e formação da diversidade vegetal das savanas de Roraima, pois geram efeitos favoráveis ao recrutamento e à manutenção de espécies regionais. A substituição destas espécies-berçário (resistentes e adaptadas às condições locais) por outras exóticas (não adaptadas) pode causar efeitos danosos ao ambiente, com consequências diretas relacionadas à perda da biodiversidade regional. As árvores-berçário das savanas de Roraima são constituintes de todo o sistema de restauração ambiental. / The objective of this study was understand the restoration and self-regeneration patterns under the influence of the crown of four dominant tree species (nursery-tree or nucleation-tree) in the Roraimas savanna (lavrado), northern of Brazilian Amazonia: Curatella americana L., Byrsonima crasssifolia (L.) H.B.K, Byrsonima coccolobifolia Kunth and Roupala montana Aubl. The study took into account the composition, richness (S) and diversity index (Shannon-Weaver) of tree and scrub plants surveyed under the crown of these nucleation species, besides to take a comparison of results through the Sørensen index of similarity. Were also assessment the onthogenic stages (sapling, young and established) and the spatial distribution pattern of these individuals under the crown of observed species. The results showed that the nursery-trees have strong relationship with edaphic factors, specially texture and aluminum saturation, and that this is related to the plant diversity under their crowns. It was verified that the nucleation effect of the studied species was markedly higher to self-regeneration, mainly in the simpatric species of the genus Byrsonima (48% to B. crassifolia and 46% to B. coccolobifolia) and in smaller degree to R. montana (12 %) and C. americana (6%). Richness and diversity of tree and scrub plants was larger under the crow of C. americana (S = 16; H = 1,389-1,824); R. montana (S = 13; H = 1,453-1,992), B. coccolobifolia (S = 12; H = 0,669) and B. crassifolia (S = 10; H = 0,864-1,908). Most of the tree and scrub individuals found under the crown influence area of B. coccolobifolia was of B. coccolobifolia (46%) while for B. crassifolia were of B. crassifolia (48%). Under C. americana the specie Randia formosa (33%) was more yisibie while for R. montana the species more important were Connarus favosus and B. crassifolia with 23%. Some surveyed species under the crown of the nucleation-tree (B. crassifolia, Eugenia punicifolia, Erythroxilum suberosum and Xylopia aromatica) are species without nutritional demand for their establishment, while C. americana and Rourea grosourdyana demanded basicity. The results suggest that conditions of constant environmental stress (high rain sazonality and low fertility soils), associate to strong anthropogenic pressure (fire), transmit to the nucleation species of this ecosystem an important role in the maintenance and formation of the plant diversity in the Roraimas savanna, generating favorable effects to the recruitment and the maintenance of regional plants. The substitution of these nursery-trees (resistant and adapted to the local conditions) for other exotic ones (no adapted) can generate harmful effects to the environment, with direct consequences related to the loss of the regional biodiversity. Nursery-trees of the Roraimas savanna are constituents of the environmental restoration system.

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