After a brief introduction, a model describing axial flow of liquids through conifer sapwood is described. This model allows the variation of the volumetric flow rate through the tracheid lumen/bordered pit system with the magnitude of the applied pressure differential to be calculated, given a variety of values for physical constants, and tracheid lumen and bordered pit dimensions. The model takes into consideration the viscous resistance to flow (and the kinetic energy correction to such viscous resistance to flow) generated by the tracheid lumen, the pit apertures, the pit margo pores, and the annuli between the edge of the membrane torus and the pit border. The model also takes into account the established fact that the bordered pit membrane displaces in the direction of the descending pressure gradient, towards one of the pit apertures. The computer programme written to execute this somewhat involved analysis includes facilities for testing whether the assumption that flow is occurring in the laminar regime is correct, and for assessing the effect of error in any one of the assumed values of physical or dimensional quantities. Using data for first formed earlywood and centre of latewood tracheids of Scots pine as an example, the following general conclusions are reached: 1. That at-low applied pressure differentials the conductivity of earlywood tracheids is more than an order of magnitude greater than that of latewood tracheids. At high applied pressure differentials the earlywood membranes will deflect completely (aspirate), thus preventing flow, while latewood tracheids continue to conduct. At even higher pressure differentials, even these latewood membranes should, in theory, aspirate; 2. That with the exception of flow through latewood at high pressure differentials, the assumption that flow through such tracheids is laminar appears to be justified; 3. That kinetic energy corrections to viscous flow theory should only be significant with flow through latewood; 4. That more than two structural components in wood should contribute significantly to total resistance to flow. Subsequently presented experimental evidence, based on the study of gas flow through ponded and unponded wood, and through permeability specimens dried successively from different liquids, confirms this last prediction. These studies also provide evidence that the elasticity of the bordered pit membrane varies systematically across the growth ring, as predicted by the model. The presence of more than two structural components contributing to total resistance to flow through wood introduces theoretical shortcomings into previously used methods for the calculation of the radius and number of conducting pores in conifer wood, and for the estimation of the true liquid permeability of wood from gas flow data. A new computer based analysis designed to overcome these difficulties is presented. This is tested with only partial success in the prediction of the liquid permeability of Western Hemlock. There is good reason to believe that this lack of success was not due to a failure of the method (although ways of improving the method are identified) but due to an unexplained failure to measure the true liquid permeability of the specimens under test. Further rather more speculative biological and technological implications of the model are discussed.

This study focuses on the influences of past forest policies in Bangladesh on environmental sustainability in forest land use and in human attitudes towards forest. The study concludes that colonial policy was inimical to sustainability in forest resource use. The colonial legacy and its prolonged persistence in the post-colonial period left a lasting imprint on the peoples' attitude to forest resources. People came to see themselves as resource users and considered that as government owns the forest, creating forest is the government's responsibility. Afterwards, with increasing population such as imprint has turned out to be a severe threat to the sustainability of forest. The study investigated forest land use policies in Bangladesh from their origin in the British colonial period to the present, aiming to inquire into the development of peoples' attitude to forest land use. Evidence suggests that prior to the colonial era, there were forestry concerns in the administration of ancient Bengal. However, there were no recognisable forest policies in conflict with peoples' culture and tradition. Traditional hill people used to practise both lowland cultivation and semi-permanent upland cultivation for subsistence. From the British era to the present, people have remained alienated from the forest. The long alienation of people from the forest has caused loss of the peoples' trust in the Forest Department. Forestry in Bangladesh still displays colonial influences. As a result, although an international movement is pushing environmental perspectives of forest land use, Bangladesh is facing difficult challenges in changing the attitude of people and of administration which originated from the discourse of colonial policy. However, efforts through the NGOs have seen partial success in participatory forestry. However, for the long-term perspective the policy needs to be reviewed and improved, encompassing the traditional forest areas, serving the interest of the target groups, improving institutional standards, updating the law and order situation and encouraging a changed discourse among the people.

The aim of this work was to investigate and compare features of the photosynthetic apparatus of Sitka spruce (Picea sitchensis (Bong.) Carr.) in an attempt to elucidate the causes of variation in photosynthetic capacity in different parts of a forest canopy. Shoots from the top of the canopy had much higher photosynthetic capacities than shoots from the bottom of the canopy. The hypotheses were: 1) shoots from different parts of the canopy behave differently because they have adapted to the prevailing light environment, which varied through the canopy, 2) part of the limitation to photosynthetic capacity occurs in the reactions associated with the photosystems. Measurements to test these hypotheses were made on both forest shoots, corresponding to 'sun' shoots (top of the canopy), partially shaded shoots and 'shade' shoots (bottom of the canopy), and plants grown in controlled environment chambers in each of one of four different photon flux densities (where all other environmental variables were constant). Photosystem activity, expressed per unit leaf area, was higher in the high-light-grown plants and 'sun' shoots. Quantum requirements for photosystem activity were higher in high-light-grown plants and 'sun' shoots. Specific leaf area increased and dry weight fraction decreased with decreasing light availability for growth. Chlorophyll content increased and chl a:b ratio decreased with decreasing cultivation light in the controlled environment chamber plants. In the forest the response of chlorophyll content to shading was variable. In 1974 chlorophyll content increased with increasing shading, in 1975 chlorophyll content remained constant. Chlorophyll a:b ratio was constant throughout the canopy in both years. An attempt to measure the chl:P700 ratio was measured so that rates of photosystem activity could be expressed per unit reaction centre. The basis of expression for the results has been shown to be very important and significantly affects the interpretation of the results. The two hypotheses were supported by the data obtained.

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The study was carried out at a test area in central Scotland, situated within the Queen Elizabeth II Forest Part (lat. 56°10’ N, long. 4° 23’ W). Six plots containing three different species (Norway spruce, European larch and Sessile oak), characterised by their different light regimes, were established within the area for the measurement of forest variables by using a forest inventory approach and hemispherical photography> The remote sensing data consisted of Landsat ETM+ imagery, small footprint multi-return lidar dataset over the study area, one Airborne Thematic Mapper (ATM) image, and aerial photography with same acquisition data as the lidar data. Landsat ETM+ imagery was used for the spectral characterisation of the species under study and the evaluation of phonological change as a factor to consider for future Landsat imagery acquisitions. Three approaches were used for the discrimination between species: raw data, NDVI, and Principal Component Analysis (PCA). Early summer imagery differentiated species best, although no single date is ideal, and a combination of two or three datasets covering phonological cycles will be optimal for the differentiation. Although the approaches used helped the characterization of forest species, especially the discrimination between spruces, larch and the deciduous specie oak, further work is needed in order to define an optimum approach to discriminate between spruces species. In general, the useful ranges of the indices were small, so a careful and accurate preprocessing of the imagery is highly recommended. Lidar, ATM, and aerial photography were analysed for the characterisation of vertical and horizontal forest structure. A slope-based algorithm was developed for the extraction of ground elevation and three heights from multiple return lidar data and the production of a Digital Terrain Model (DTM) and Digital Surface Model (DSM) of the area under study, and for the comparison of the predicted lidar tree heights with the true tree heights, following by the building of a Digital Canopy Model (DCM) for the determination of percentage canopy cover and tree crown delineation. Mean height and individual tree height were estimated for all sample plots. Lidar underestimated tree heights by an average of 1.49 m. The standard deviation of the lidar estimates was 3.58 m and the mean standard error was 0.38. For spruce and larch plots, lidar measurements explained 92% of the variance associated with the mean height of dominant trees. For the deciduous plots, regression models explained 75% of the mean height variance for dominant trees.

Intact Amazonian forests are often considered to be in a steady-state, where gains in growth and recruitment are offset by losses in mortality and where net carbon accumulation is close to zero. However, permanent plot data have shown that this ecosystem removes ca. 0.4 Pg of carbon per year from the atmosphere, approximately 5% of annual anthropogenic emissions. This thesis tests several competing hypothesized drivers of detected shifts in the structure and dynamics of intact forest, by assessing changes in functional and floristic composition over the last 30 years in over 100 long-term tree monitoring plots distributed across the Amazon. I first show that the majority of species are restricted to wetter conditions, indicating that stronger and more frequent droughts could threaten many species (Chapter 3). I generated an index of water-deficit affiliation for more than 500 genera and 1800 species (Chapter 3), and demonstrated that this index can predict drought-induced mortality in several drought experiments (Chapter 4). Finally, I document how floristic and functional composition of Amazonian forests has shifted over the last 30 years: forests are increasingly dominated by large-statured taxa, and further, large trees are becoming even larger in absolute size (Chapters 5 and 6). However, relative gains in basal area were similar across size classes and canopy status. In addition, recruits are increasingly comprised of dry-affiliated genera, while the mortality of wet-affiliated genera has increased in plots where the dry season has become more intense. Communities are becoming more dry- affiliated, although these changes still lag behind the drying trend. Overall, this thesis shows the potential vulnerability of Amazonian biodiversity to an increase in aridity and supports the hypotheses that a changing climate and increased atmospheric CO2 are driving changes in Amazonian floristic and functional dynamics.

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Tropical rainforests have been and will remain subjected to various natural and human pressures, which have already led to rainforest degradation and disappearance. This thesis aims to improve our current knowledge of the likely future trajectory of the persistence of the tropical African rainforest under climate change and fire impact through reconstruction of past vegetation and burning dynamics using palaeoecological records from sedimentary sequences. I used several types of palaeoecological proxies (i.e., fossil charcoal, phytolihs, and fossil pollen) from sediment records collected in the Sangha River region, in the northern part of the Republic of Congo, to explore past fire dynamics between four different forest types (i.e. mixed forest, swamp forest, Marantaceae forest and monodominant Gilbertiodendron forest), and to reconstruct the vegetation history of two endemic types of African forest i.e., the Marantaceae forest and the monodominant Gilbertiodendron forest, for the last 2500 cal BP. Lastly, I combined fossil pollen records from the literature with ecological niche models (ENM) to explore the relationship between climate and rainforest distribution over the last 6000 cal BP. My results demonstrate that the past frequency of predominately human-induced fires was significantly higher in areas currently covered by Marantaceae forest compared to areas currently covered by any of the other forest types, and that this high frequency of fires has therefore contributed to the maintenance of Marantaceae forest. Thus, the two Marantaceae forest sites I studied are of recent formation (around 450-600 cal BP) and were previously covered by the mixed forest type. Out of all the considered forest types, the mixed forest is the most vulnerable to both climatic and fire disturbances while the monodominant Gilbertiodendron forest has been most resilient to past climatic events, with no major change in its composition for the last 2700 cal BP. I also found that monodominant Gilbertiodendron forest has persisted during this time period mainly under the absence of burning which supports the hypothesis of long term minimal burning disturbance being a key factor for monodominant forest persistence. At the biome level, my results show that the relationship between climate and African rainforest has partially shifted over the last 6000 years, thus opposing the idea of climatic niche conservatism of this biome in the African continent. This thesis provides new information of the past ecological responses of African rainforest to major environmental disturbances. African rainforests represent a globally important and largely understudied forest region, and my thesis offers fundamental insights into its ecology and biogeography.