Sudan is endowed with a great diversity of tree species; nevertheless the utilization of wood resources has traditionally concentrated on a few species only. Despite of the richness of Sudan in most of basic factors required to establish forest based industries it still almost entirely dependent on imports to satisfy its needs of the products of such industries. There is an urgent need to assess the suitability of the local fibrous raw materials for industrial utilization, this would not only reduce imports, but they would also provide an economic incentive to the forestry and industrial sectors of Sudan. Sudan has a wide variation of climatic zones, thus; great variations are expected in the anatomical and physical properties between and within species grown in each zone. This variation needs to be fully explored in order to suggest best uses for the species. The present study was carried out to assess the suitability of Acacia seyal var. seyal Del and Balanites aegyptiaca (L.) Delile wood for pulp and paper making (PPM) and flooring industry, as well as to investigate the effect of rainfall zones on selected wood properties. For this purpose, a total of thirty trees per species were collected from four states in Sudan, namely: Blue Nile, North Kordofan, South Kordofan and White Nile. The study areas located in two precipitation zones. Zone one with 273 mm mean annual rainfall, and zone two with 701 mm mean annual rainfall. Wood samples in form of disc were obtained from two heights within each tree, which are 10 % and 90 % from the tree merchantable height. Anatomical, physical and mechanical investigations were conducted in order to test the wood properties of the study species. The studied anatomical properties were: fibre and vessel diameter, lumen diameter and wall thickness. In addition to fibre length and three fibre derived values, namely: flexibility coefficient, Runkel ratio and slenderness ratio. The trend of fiber length from pith to bark was determined. The anatomical composition was described. Wood density was investigated as a main physical property. Basic density as well as air dry density were measured in the current study. Additionally, the density was measured using X-ray densitometry method in order to assess its suitability as a valid tool for the study species density determination. The trend of wood basic density from pith to bark was also determined. Brinell hardness strength was measured in the transverse and radial sections. According to the study results, the fibre length of both species considered as medium (900 -1600 μm). However, Acacia seyal has longer fiber. Acacia seyal wood density considered heavy (≥ 720 kg/m³) while that of Balanites aegyptiaca is medium (500 - < 720 kg/m³). Depending upon the mean values of hardness strength in transverse as well as radial sections, the wood of both species can be classified as very hard (up to 146 N/mm2 hardness strength). Fibre length and wood density for both species followed the increase trend from the pith to the bark. The X-ray densitometry technique is considered as a valid tool for wood density determination for both species. For each species, some wood properties (in mature wood) were significantly affected by the water stress in the drier zone. For instance, Acacia seyal fibre length was negatively affected, while vessel wall thickness, basic density as well as hardness strength of the radial section were positively affected. In case of Balanites aegyptiaca the following properties were affected: vessel dimensions (negatively) and basic density (positively). However, the water stress did not affect Acacia seyal fibre and vessel diameter and lumen diameter, fiber wall thickness, flexibility coefficient, Runkel ratio and hardness strength in transverse section. Balanites aegyptiaca fibre characteristics and hardness strength did not show any response to water stress as well. In general, the overall wood properties of the study species considered compatible for PPM and flooring industry. However, trees growing in the more humid zone are preferable for both industries, due to their lower wood density and longer fibres in case of Acacia seyal and lower density in case of Balanites aegyptiaca.

Ce travail de thèse a pour objectif principal de comprendre la dynamique de l’espèce fondatrice de l’écosystème de la région de Bled Talah, ainsi que les mécanismes susceptibles de mieux comprendre ses interactions avec la végétation compagne. Des mesures dendrométriques accomplies par des essais de détermination d’âge, ont fait ressortir que le peuplement de la forêt du Bou Hedma semble naturellement suivre plutôt une certaine dynamique régressive. C’est ainsi que nos résultats démontrent plutôt la dominance des individus de grandes circonférences (des individus âgés). Les obstacles à la faible régénération naturelle de ce taxon sont, essentiellement, l’infestation des graines par les bruches, la possibilité d’un éventuel effet autoallélopathique et la dominance des interactions négatives (compétition) entre la végétation du Parc National du Bou Hedma et les jeunes plants d’Acacia. Concernant, les interactions biotiques, nos résultats ont permis d’enrichir les débats actuels sur l’importance et la direction des interactions le long des gradients de stress et de perturbation. Une dominance des interactions négatives (compétition), le long des deux gradients a été démontrée, avec une chute de l’importance de la compétition en présence d’herbivorie. Les mesures environnementales et biologiques ont permis de conclure, qu’Acacia tortilis subsp. raddiana, a un effet globalement bénéfique pour le sol, ainsi que pour la végétation naturelle environnante. Compte tenu du rôle clef de cette espèce sous les conditions contraignantes du bioclimat aride, la nécessité de minimiser le risque de la faible régénération naturelle de ce taxon est prioritaire dans les grands programmes de reboisement et de restauration des écosystèmes arides. / The main aim of this work were to understand the dynamics of the founding species of the ecosystem of the region of Bled Talah, as well as the mechanisms to understand better its interactions with the vegetation partner. Dendrometric parameters were measured for each tree then determination of age were estimated. This work improves general understanding of the factors explaining the structure of the Acacia tortilis subsp. raddiana. These species naturally seems to follow rather a certain regressive dynamics. The possible reasons for regeneration failure natural regeneration of Acacia tortilis subsp. raddiana were especially the high infestation of seeds by Bruchidius raddianae, the autoallelopathic effetcs and the dominance of the negative interactions (competition) between the vegetation of the National park of Bou Hedma and the young plantations of Acacia tree. The importance and direction of biotic interactions along environmental gradients is still a major debate in plant ecology. Through the study of biotic interactions, our results suggests the dominance of competition between nurses and target species used in this work.The results of this study confirmed the positive effect of Acacia tortilis subsp. raddiana on the understorey vegetation composition and diversity in arid ecosystems. Once, established in the dry area, this species ameliorates microclimate including soil conditions that may be suitable to the regeneration and the growth of other species. In this way, this species will provide beneficial effects to the generating seedlings in the vicinity and therfore will help rehabilitating degraded areas.

Les plantes invasives constituent un phénomène très répandu sur la planète qui entraîne des problèmes environnementaux majeurs aboutissant à des perturbations significatives dans les processus régissant la conservation de la diversité végétale et microbienne des sols. Acacia mearnsii (De Wild) est l'une des espèces d'Acacia ayant un potentiel invasif important. Cette légumineuse d'origine australienne a été introduite dans les subéraies du Parc National d'El-Kala (PNEK), au Nord-Est de l'Algérie, où elle induit des dégradations drastiques sur l'écosystème forestier de chêne-liège Quercus suber (L.) et la diversité végétale et microbienne de cette formation forestière. L'objectif de cette étude a été d'évaluer les transformations induites par A. mearnsii sur les caractéristiques et microbienne du sol (particulièrement les communautés de symbiotes microbiens) et d'en déterminer les conséquences sur le développement de chêne-liège. Une série d'expérimentations a été réalisée sur les sols de trois sites dans le PNEK : forêt naturelle de Q. suber (site non envahi), peuplement mixte Q. suber et A. mearnsii (site envahi récemment par A. mearnsii) et peuplement d'A. mearnsii (site anciennement envahi par A. mearnsii). Les analyses de sol, de la diversité fonctionnelle et structurelle des microorganismes telluriques ont révélé l'existence de modifications liées à la présence de l'espèce envahissante. D'autre part, il a été déterminé que A. mearnsii avait un effet inhibiteur sur le développement du chêne-liège et sur son cortège ectomycorhizien associé. De plus, cette essence semble avoir la capacité de s'adapter facilement au milieu d'introduction, du fait de sa forte mycotrophie vis à vis des champignons mycorhiziens arbusculaires, et de sa facilité de nodulation spontanée avec des bactéries symbiotiques principalement du genre Bradyrhizobium. La présence de symbiotes compatibles avec A. mearnsii dans les habitats envahis représente probablement un des facteurs susceptibles de faciliter le processus d'envahissement de l'espèce. Nos résultats suggèrent que la régression du développement du chêne-liège peut être liée aux modifications induites par A. mearnsii sur le fonctionnement du sol et la structure des microorganismes telluriques. Cet effet est probablement lié à d'autres éléments biotiques et abiotiques du milieu envahi influencé par cet arbre. Il sera donc important d'affiner cette étude, et d'analyser plus précisément les paramètres pouvant être à l'origine du succès de l'invasion par A. mearnsii afin de définir un cadre de lutte contre cette espèce invasive et ainsi sauvegarder la subéraie . / Invasive plants are a global phenomenon causing major environmental problems leading to significant disruptions in the processes governing the conservation of plant diversity and soil microbial communities. Acacia mearnsii (De Wild) is one of the Acacia species with a significant invasive potential. This Australian native legume was introduced in the cork forests of the National Park of El Kala (PNEK), North-eastern Algeria, and induced drastic degradation of the cork oak (Quercus suber (L.)) forest ecosystem, on understorey plant species and soil microbial diversity. The objective of this study was to evaluate changes resulting from the A. mearnsii invasion on chemical characteristics and soil microbiota (especially symbiotic microbial communities) and to determine their impact on cork oak development. Experiments were conducted on soils collected from three sites in the PNEK: Q. suber natural stand (non-invaded site), mixed Q. suber and A. mearnsii stand (recently invaded site) and A. mearnsii stand (fully invaded site). Both chemical and microbiological soil characteristics were affected by the presence of the invasive species. On the other hand, it was determined that A. mearnsii had an inhibitory effect on the development of the cork oak tree and its associated ectomycorrhizal community. In addition, this tree species seems to easily interact with arbuscular mycorrhizal fungi in the introduction area, being abundantly infected. It is also spontaneously nodulated by local symbiotic bacteria, mainly of the genus Bradyrhizobium. The presence of symbionts, compatible with A. mearnsii in invaded habitats are probably one of the factors that facilitate the process of invasion. Our results suggest that the development of the cork oak can be linked to changes induced by A. mearnsii with soil functioning and with the composition of soil microorganism communities. This effect is probably related to other biotic and abiotic components of the environment influenced by this invasive tree species. It seems thus important to analyze more precisely the parameters that cause the success of the invasion by A. mearnsii in order to better control this invasive species and save the cork oak forest in Algeria.

Acacia spirorbis est une espèce endémique de la Nouvelle-Calédonie, capable de former des symbioses avec des microorganismes du sol, notamment des ectomycorhizes et de se développer sur une très large variété de sols. Afin de tester les capacités de cette espèce à permettre d’initier un processus de restauration écologique des écosystèmes dégradés par l’exploitation minière, un essai en pépinière et trois essais sur le terrain ont été mis en place. Ces essais ont permis de mettre en évidence la capacité d’A. spirorbis à jouer, grâce à la facilitation, un rôle de plante nurse pour des espèces cibles de la restauration écologique comme des espèces du genre Tristaniopsis. Ainsi, nous avons tout d’abord décrit la diversité des champignons ectomycorhiziens associés avec A. spirorbis puis, nous avons constatés leur capacité à s’associer sur le terrain aux Tristaniopsis grâce à la présence initiale d’A. spirorbis. Les capacités d’A. spirorbis à faciliter l’implantation d’autres espèces en améliorant leur survie, leur croissance et en leur permettant de disposer de partenaires fongiques ectomycorhiziens diversifiés sont décrites. Ces résultats on conduit à proposer un itinéraire technique de restauration écologique des maquis miniers dégradés, itinéraire fondés sur la facilitation entre espèce utilisant A. spirorbis comme plante nurse. / Acacia spirorbis is an endemic species of New Caledonia, capable of forming symbiosis with soil micro-organisms, including ectomycorrhizae and developing on a very wide variety of soils. In order to test the capacities of this species to initiate a process of ecological restoration of ecosystems degraded by mining activities, a nursery trial and three field trials were setted up. These trials highlight the ability of A. spirorbis to play, through facilitation, a nurse plant role for target species of the genus Tristaniopsis in the frame of ecological restoration processes. Thus, we first described the diversity of the ectomycorrhizal fungi associated with A. spirorbis and then we described their ability to associate in the field with Tristaniopsis thanks to the initial presence of A. spirorbis. The capacities of A. spirorbis to facilitate the implantation of other target species by improving their survival, growth and allowing them to access to a diversified range of ectomycorrhizal fungal partners are described. These results led to propose a technical itinerary for ecological restoration of degraded mining maquis, an itinerary based on facilitation between species using A. spirorbis as a nurse plant.

Acacia spirorbis est une légumineuse endémique de Nouvelle-Calédonie se développant sur des sols calcaires, métallifères et volcano-sédimentaires, établissant des symbioses avec des bactéries fixatrices d’azote. Pour comprendre la contribution de la symbiose dans l’adaptation de la plante à des milieux contrastés et parfois extrêmes, nous avons évalué la fixation d’azote en conditions naturelles, caractérisé les rhizobia associés à cette plante et analysé la réponse adaptative de la plante aux éléments traces métalliques dans ses tissus racinaires, notamment au niveau des nodules.Nous avons mis en évidence que la symbiose rhizobienne fournissait plus de 80% de l’azote total chez des populations naturelles d’A. spirorbis se développant sur des sites d’études présentant des sols calcaires, métallifères et volcano-sédimentaires. Cette valeur est remarquable puisque chez A. mangium, A. melanoxylon et A. mucronata, les valeurs moyennes sont respectivement de 50%, 43% et 58%. Les rhizobia symbiotiques associés à A. spirorbis appartiennent aux alpha- et bêta-protéobactéries, genres Bradyrhizobium et Paraburkholderia, révélant ainsi une très large gamme de symbiontes et une faible sélectivité de partenaire. De manière remarquable, la taxonomie et la phénotypie de ces souches sont structurées et adaptées aux conditions édaphiques. Enfin, les signatures chimiques des tissus internes des nodules reflètent les propriétés chimiques des sols dans lesquels ils se sont développés, indiquant une potentielle gestion des éléments traces métalliques dans ces tissus.Tous ces éléments suggèrent que la symbiose fixatrice d’azote contribue de manière significative dans l’adaptation d’Acacia spirorbis à des sols contrastés et pouvant présenter une toxicité polymétallique extrême. / Acacia spirorbis is a leguminous tree from New Caledonia naturally found on a wide range of soils (calcareous, ultramafic and volcano-sedimentary) and able to establish symbioses with soil microorganisms, including nitrogen-fixing bacteria. The contribution of this symbiosis in the plant adaptation to contrasted edaphic environments, sometimes extremes, has been investigated according to i) the nitrogen-fixing potential of A. spirorbis in its natural ecosystems, ii) the characterization of its rhizobia and iii) the plant adaptive response to heavy metals inside its roots tissues, especially in nodules.Therefore, we revealed that the nitrogen-fixing symbiosis provided more than 80% of the plant total nitrogen in natural population naturally occurring on calcareous, ultramafic and volcano-sedimentary soils. This value is top notch among other Acacia species, where A. mangium, A. melanoxylon and A. mucronata presented mean values of 50%, 43% and 58%, respectively. Acacia spirorbis established nitrogen-fixing symbioses with alpha- and bêta-proteobacteria, genus Bradyrhizobium and Paraburkholderia, respectively, thus revealing a wide range of symbiotic partner and a low selectivity. Noteworthy, the taxonomy and phenotypes of these symbionts are structured and adapted to edaphic parameters. Finally, chemical signatures of internal nodules tissues presented similarities with soils chemical properties, thus indicating a potential management of heavy metals inside these tissues.Altogether, these data suggest that the nitrogen-fixing symbiosis might significantly contributes to Acacia spirorbis adaptation towards contrasted soils with strong edaphic conditions as an extreme polymetallic toxicity.

by Au Pui Sze. / Thesis submitted in: December 2000. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2001. / Includes bibliographical references (leaves 135-150). / Abstracts in English and Chinese. / Abstract --- p.i / Abstract (in Chinese) --- p.iii / Acknowledgement --- p.iv / Table of contents --- p.vi / List of tables --- p.x / List of figures --- p.xi / List of appendices --- p.xii / Chapter Chapter one --- Introduction / Chapter 1.1 --- The role of plantations in ecosystem rehabilitation --- p.1 / Chapter 1.2 --- Reforestation history in Hong Kong --- p.2 / Chapter 1.3 --- Conceptual framework of the study --- p.5 / Chapter 1.4 --- Objectives of the study --- p.15 / Chapter 1.5 --- Scope and significance of the study --- p.16 / Chapter 1.6 --- Organisation of the thesis --- p.17 / Chapter Chapter two --- The Study Area / Chapter 2.1 --- Geographical setting of Hong Kong --- p.18 / Chapter 2.1.1 --- Climate --- p.18 / Chapter 2.1.2 --- Geology --- p.20 / Chapter 2.1.3 --- Soils --- p.20 / Chapter 2.1.4 --- Vegetation --- p.21 / Chapter 2.2 --- Site selection --- p.23 / Chapter 2.3 --- Site description --- p.26 / Chapter 2.4 --- Nature and extent of disturbance prior to planting --- p.30 / Chapter 2.5 --- Planting techniques and post-planting maintenance --- p.31 / Chapter Chapter Three --- Stand Characteristics of Acacia Plantations / Chapter 3.1 --- Introduction --- p.33 / Chapter 3.2 --- Methodology --- p.37 / Chapter 3.2.1 --- Sampling plots design --- p.37 / Chapter 3.2.2 --- Tree density --- p.37 / Chapter 3.2.3 --- Tree growth parameters --- p.38 / Chapter 3.2.4 --- Data processing and statistical analysis --- p.38 / Chapter 3.3 --- Results and discussion --- p.39 / Chapter 3.3.1 --- Growth dynamics of acacias --- p.39 / Chapter 3.3.2 --- Growth characteristics of the reforested species --- p.44 / Chapter 3.3.3 --- Sustained growth and natural regeneration of acacia plantations --- p.48 / Chapter 3.3.4 --- Rehabilitation of degraded lands by exotic species plantations --- p.52 / Chapter 3.4 --- Conclusion --- p.54 / Chapter Chapter Four --- Cover Soil Characteristics of Acacia Plantations / Chapter 4.1 --- Introduction --- p.56 / Chapter 4.2 --- Methodology --- p.60 / Chapter 4.2.1 --- Soil sampling --- p.60 / Chapter 4.2.2 --- Laboratory analysis --- p.61 / Chapter 4.2.3 --- Texture --- p.61 / Chapter 4.2.4 --- Soil pH --- p.61 / Chapter 4.2.5 --- Organic carbon --- p.62 / Chapter 4.2.6 --- Total Kjeldahl nitrogen --- p.62 / Chapter 4.2.7 --- Carbon : nitrogen ratio --- p.63 / Chapter 4.2.8 --- Mineral nitrogen (NH4-N and N03-N) --- p.63 / Chapter 4.2.9 --- Available phosphorus --- p.64 / Chapter 4.2.10 --- Exchangeable cations --- p.64 / Chapter 4.3 --- Data processing and statistical analysis --- p.64 / Chapter 4.4 --- Results and discussion --- p.65 / Chapter 4.4.1 --- Effect of acacias on soil texture and pH --- p.65 / Chapter 4.4.2 --- "Effect of acacias on SOM, TKN and mineral N" --- p.69 / Chapter 4.4.3 --- Effect of acacias on the available P and exchangeable cations… --- p.77 / Chapter 4.4.4 --- Soil nutrient status of the plantations --- p.82 / Chapter 4.5 --- Conclusion --- p.90 / Chapter Chapter Five --- Understorey Vegetation of Acacia Plantations / Chapter 5.1 --- Introduction --- p.92 / Chapter 5.2 --- Methodology --- p.96 / Chapter 5.2.1 --- Understorey plant sampling --- p.96 / Chapter 5.2.2 --- Species identification and nomenclature --- p.98 / Chapter 5.2.3 --- Plant growth parameters --- p.98 / Chapter 5.2.4 --- Data processing and statistical analysis --- p.99 / Chapter 5.3 --- Results and discussion --- p.100 / Chapter 5.3.1 --- Floristic composition of the acacia plantations --- p.100 / Chapter 5.3.2 --- "Species richness, diversity and woody abundance of the understories" --- p.101 / Chapter 5.3.3 --- Species composition and structure of the understories --- p.106 / Chapter 5.3.4 --- Mechanisms and pathway of succession in the exotic plantations --- p.115 / Chapter 5.4 --- Conclusion --- p.116 / Chapter Chapter Six --- Conclusion / Chapter 6.1 --- Summary of findings --- p.118 / Chapter 6.2 --- Implications of the study --- p.122 / Chapter 6.2.1 --- Ecological value of exotic plantations in Hong Kong --- p.122 / Chapter 6.2.2 --- Restoration strategies for borrow areas --- p.125 / Chapter 6.2.3 --- Enrichment planting after fire --- p.128 / Chapter 6.3 --- Limitations of the study --- p.131 / Chapter 6.4 --- Suggestion for future study --- p.132 / References --- p.135 / Appendices --- p.151

Alleycropping is an agroforestry technology of planting crops between rows of trees, preferable legumes to promote an interaction among them with positive benefits in terms of improving soil fertility and hence good crop yields. The technology has been tested with a variety of trees/shrubs species in association with crops (alleycropping) or with grasses (alleygrazing), sometimes with encouraging results and sometimes not, in a wide range of environmental conditions around the world. Research in alleycropping started in late 1970s and sinc~ then many publications have been released. However, little or nothing has been reported about this technology using black wattle and the fever tree, two nitrogen-fixing trees common in South Africa and reported as fast-growing species which produce a considerable amount of biomass within a short period of time. Due to that fact, a two-year trial was established in 2003 at the Ukulinga research farm, Pietermaritzburg, South Africa to evaluate the potential of the two tree species under alleycropping with maize and cowpeas as joint intercrops, under alleycropping with pumpkin, and under alleygrazing with Eragrostis curvula and with Panicum maximum. The trial assessed the crop yields and the biomass production from all the components, and their fodder digestibility using Neutral Detergent Fibre (NDF) and Acid Detergent Fibre (ADF) determinations. Additionally the changes in tree grovlth vaa."'i.ables (difu~eter, total height, total \lollhl1e and biomass) were mortitored to produce regression equations to predict those variables, one from another, using regression analysis. The diameter was taken at ground level (dgl) and at the height of 1.3 m, normally called diameter at breast height (dbh). The results showed that tree growth and biomass production were better in black wattle alleycropping than in association with the fever tree. The average dgl of black wattle after 12 months was 48mm and the average dbh was 36mlll. Over the same period the total tree height was about 406cm. A tree pruning was done to one-year old black wattle in the whole trial and the prunings produced about 5.6t/ha of fresh foliage biomass in the association with maize and cowpea and 4.5t/ha in the association with pumpkin. In alleygrazing the growth variables were similar to those obtained in alleycropping but the biomass production was considerably different. The prunings produced about 7.66t/ha of fresh foliage biomass. The dry matter biomass from the prunings was 1.96t/ha, 1.58t/ha and 2.68t/ha in the association with maize and cowpeas, pumpkin and E. curvula respectively. The dry matter was obtained from 4days- oven-dried samples and it was 35% of the fresh foliage biomass and 60% of the fresh woody biomass. The fever tree did not grow significantly during the study period and due to that fact, the species was discarded from the study. Similarly, because after several endeavours using different seed lots, P. maximum had germinated very unevenly, and this grass was excluded from the experimentation. Values ofNDF and ADF less than 35% are considered good, between 35% and 60% fair and poor if greater than 50%. Using is classification the NDF and ADF values from this study were good in pumpkin (30.5% and 29.9%) and cowpeas (36.5% and 46.9%) biomass, fair in E. curvula (41.9% and 39.9%) and maize stover (53.6% and 42.1%) and poor in black wattle (76.58% and 68.1%) foliage. If black wattle is to be used as fodder, it must be mixed with highly digestible fodder like P. maximum, Digitaria sp., and other legume plants, to increase animal intake and to avoid any risk of it becoming an animal hazard due to tannin effects. The regression equations produced linear relationships between dgl and age, and biomass and dbh. The other interactions were not linear. The best equations were obtained in the interaction between dgl and age (dgl= 4.8*Age -7.03; R2 =0.86; SE= 6.6), dgl and height (h= -0.03dg12 + 10.5dgl - 21.25; R2= 0.96, SE= 42.9, h= height), biomass and dbh (lny = 2.409*lndbh; R2=0.99, SE=O.O, Y= tree foliage biomass). During the study, monkeys, cattle, birds and bushbucks posed a threat to the success of the study due to damage they caused to the crops. It was possible to keep the damage below the critical levels, although at high cost. / Thesis (M.Env.Dev.)-University of KwaZulu-Natal, Pietermaritzburg, 2006.

Leaf area index (LAI) is an important variable in models that attempt to simulate carbon, nutrient, water and energy fluxes for forest ecosystems. LAI can be measured either directly (destructive sampling) or by using indirect techniques that involve estimation of LAI from light penetration through canopies. Destructive sampling techniques are laborious, expensive and can only be carried out for small plots. Although indirect techniques are non-destructive and less time consuming, they assume a random foliage distribution that rarely occurs in nature. Thus a technique is required that would allow for rapid estimation of LAI at the stand level. A means of getting this information is via remotely sensed measurements of reflected energy with an airborne or satellite-based sensor. Such information on an important plant species such as Acacia mearnsii (Black Wattle) is vital as it provides an insight into its water use. Landsat ETM+ images covering four study sites In KwaZulu-Natal midlands encompassing pure stands of Acacia mearnsii were processed to obtain four types of vegetation indices (VIs). The indices included: normalized difference vegetation index (NDVI), ratio vegetation index (RVI), transformed vegetation index (TVI) and vegetation index 3 (VB). Ground based measurements of LAI were made using destructive sampling (actual LAI) and LAI-2000 optical instrument, (plant area index, PAl). Specific leafarea (SLA) and leaf area (LA) were measured in the field for the entire sample stands to estimate their LAI values. The relationships between the various VIs and SLA, actual LAI and PAl values measured by LAI-2000 were evaluated using correlation and regression statistical analyses. Results showed that the overall mean SLA value of Acacia mearnsii was 8.28 m2kg-1 SLA showed strong correlations with NDVI (r=0.71, p<O.Ol) and RVI (r=0.76, p<O.Ol) and a moderate correlation with TVI (r=0.66, p<0.05). Regression analysis revealed that SLA had significant relationship with RVI (R2=0.59) and NDVI (R2=0.51). Actual LAI values showed strong correlation with PAl values (r=0.86) and the analysis revealed that 74 % of the variation in the relationship between actual LAI and PAl values could be explained by regression. PAl values were strongly correlated with NDVI (r=0.75,p<O.Ol) and moderately correlated with RVI (r=O.63, p<O.05) and TVI (r=O.58, p<O.05). Actual LAI was strongly correlated with NDVI (r=O.79, p<O.Ol) and moderately correlated with RVI (r=O.61, p<O.05). Out of the various VIs examined in this study, NDVI was found to have a better relationship with actual LAI values (R2=O.62) and with PAI values (R2=O.56); while VB didn't show any significant relationship with SLA, PAl or actual LAl. In conclusion, preliminary estimate of SLA of Acacia mearnsii could be obtained from RVI or NDVl. The relationship obtained between PAl and actual LAI values was satisfactory, thus the regression equation can be used to calibrate the LAI-2000 plant canopy analyzer. Because NDVI was observed to have a good relationship with actual LAI and PAl, LAI of Acacia mearnsii can be estimated from Landsat ETM+ satellite imagery with a reasonable degree of accuracy. These results can satisfactorily be used as inputs into models that attempt to estimate water use by Acacia mearnsii. / Thesis (M.Sc.)-University of Natal, Pietermaritzburg, 2003.

The present study was conducted in Umfakarin natural forest reserve, South Kordofan, Sudan. The main objective was to investigate the possibility of managing Acacia seyal Del. variety seyal for the production of gum talha. Three stand densities (strata), namely dense, medium, and slight, were distinguished based on the number of trees per hectare. During the sampling phase, the study adopted the method of identifying the competitors (neighboring trees) from the subject one (trees selected for gum production experiments). From the three stand densities, a total of 482 subject trees, covering variable diameter ranges (d= 9-11.5, 13.5-16, 18-20.5 and above 21 cm) were selected, based on the diameter at 0.25 m height (d0.25). In each stratum, competitor trees were identified within a radius equal to the height of subject tree multiplied by a factor (1.25). The diameter at breast height, height to crown base, height, crown radii, and tree coordinates were measured for each of the subject trees and its competitors. Subject trees were exposed to tapping on first of October, the fifteenth of October, and the first of November, using local tools (Sonki and Makmak). Additionally, untapped trees were used as controlling-variables. The initial gum collection was completed fifteen days after the tapping, while the subsequent (7-9 pickings) were done at an interval of fifteen days. Six stand height functions were tested and the results illustrated that the Michailow stand height function was suitable for predicting the height of Acacia seyal in Umfakarin natural forest. The predictive ability of this height function ranged from 19.3% to 24%. The volume function used in this study was able to predict the volume of standing trees with more than 92 percent accuracy. Competition among trees of Acacia seyal was assessed in terms of competition indices. Eight competition indices were quantified using the CroCom program. The relationship between these indices and tree dimensions (diameter at breast height, height and crown diameter) was tested using logarithmic models. Among these indices, the Hegyi_2 index is considered a suitable index to be applied for estimating the degree of competition in natural stands of A. seyal of dense stratum when using diameter at breast height as a predictor. About 70% of the total variability is explained by this logarithmic model. Gum yielded by each subject tree per season was obtained by summing up the gum samples collected from all pickings. Gum production per unit area was also determined. Regression tree, general linear model (GLM) and logistic regression techniques were used for analyzing the obtained data. The results of the study indicated that the gum yield is independent of stand density. Tapping has influence on gum yield. Trees tapped by sonki on the first of October at medium stand density have the highest gum with an average value of about 56 g/tree/season. Significant difference (p = 0.021) was detected between two groups of dates; the first of October and first of November in medium stand density. The results also revealed that the most important variable influencing gum production was found to be diameter at breast height with 23.95 cm threshold. Between 41-53 percent of subject trees produce gum less than 50 g/season. The results indicated that A. seyal species produces a very low quantity of gum talha (3.6-4.8 kg/ha) and for economic reasons, its tapping is not recommended. The findings of the regression analysis revealed to a model which could be used to estimate the yield of gum talha from A. seyal natural stands in the Umfakarin forest, South Kordofan, Sudan. Conducting experiments on the production of gum talha in permanent plot trials in different climatic regions of the Sudan is highly recommended.

Diss. (sammanfattning) Uppsala : Sveriges lantbruksuniversitet, 2005. / Härtill 5 uppsatser.