Recent research has suggested that apparently undisturbed tropical forests, remote from areas of deforestation or other significant human influences, are undergoing unexpected changes. These observed changes may already be having significant impacts on terrestrial carbon stocks, fluxes, and biodiversity. However, the findings have proved controversial, partly because a rather limited number of permanent sample plots (PSPs) have been monitored for short periods. This work is centred on tree-by-tree data collated from long-term forest PSPs from several lowland Neotropical sites, spanning a wide gradient of environmental conditions across the Amazon basin, covering variable census intervals. The aim of the analysis is to: (1) investigate the role played by the main environmental variables in determining the observed geographical patterns of rainforest floristic composition, diversity and structure, (2) characterize temporal and regional patterns of forest dynamics, and changes in forest dynamics and basal area (BA), (3) explore potential shifts in species composition over time, due to mechanisms other than natural successional processes. With regard to floristic aspects, the present work reveals that: (1) large-scale patterns in the abundance of the most important tree families are identifiable, with fast-growing families dominating in highly dynamic forest stands studied in western Amazonia (WA); (2) the number of dry months seems to be the best predictor for alpha-diversity across the sites investigated, (3) soil seems to be playing a major role in determining floristic diversity, at local scale; (4) significant structural differences exists between forests stands located in drier and wetter sites. With regard to forest dynamics and BA, the study shows that: (1) turnover is much higher in western than in central Amazonian (CA) plots, (2) stand BA and turnover appear to have been significantly increasing in the last two decades in almost all PSPs examined, regardless of environmental conditions, (3) morality and recruitment have both increased in every region, but the increase is not significant for mortality rates.

The tropical forests of Cameroon contain a wealth of multi-purpose plants from which a range of goods and services is derived, mostly by rural populations. The forests are currently under the pressure of exploitation for timber and other forms of economic development. To counter the effects of deforestation, this study was aimed at the domestication of three species for use in farming systems by rural people. These species, <I>Ricinodendron heudelotii, Irvingia gabonensis</I> and <I>Gnetum africanum</I>, were selected on the basis of their economic importance in south-west Cameroon. Six non-mist propagators were built in the Forestry Research Station at Kumba, Cameroon. Stockplants were derived from cuttings and seedlings obtained from selected trees. Thirteen experiments were completed investigating the effects of applied auxin, leaf area and rooting media on the rooting of leafy stem cuttings of each of the three species. Additional experiments investigated the effects of fertiliser addition, shading and pollard height on stockplants of <I>Ricinodendron heudelotii</I>. Results indicated that optimal rooting percentages of <I>Ricinodendron heudelotii</I> cuttings were obtained when treated with 40 μg IBA, inserted in sawdust, and with a leaf area above 80 cm<SUP>2</SUP>. Similarly, optimum treatments for rooting of the cuttings of <I>Irvingia gabonensis</I> were defined as 200 μg IBA, inserted in sawdust, and with a leaf area above 80cm<SUP>2</SUP>, and those for <I>Gnetum africanum</I> were 250 μg IBA, inserted in sawdust, and with a leaf area above 80 cm<SUP>2</SUP>. In the nursery stump height experiment, rooting of <I>Ricinodendron heudelotii</I> was found to decrease with increasing height, with an optimum height of 0.3 m.

To assess the effects of TCA on forest health, TCA uptake by Sitka spruce saplings (<i>Picea sitchensis</i>) via atmospheric and soil routes, and its effects on growth, enzyme activity and water retention of needles and visible damage were investigated. Mean TCA concentrations in soils from an upland forest site and a lowland agricultural site were 48 (±85, n=84) ng g^-1 and 26 (± 18, n = 5) ng g^-1 (fresh weight). TCA concentrations varied greatly between soil types and decreased significantly with soil depth. In spruce soils TCA concentration was negatively related to water content, and positively related to soil organic matter content. A strong positive relationship (R² = 0.9, P<0.001) observed between soil TCA concentration and soil microbial biomass carbon, in a variety of soil types, may be evidence for formation of TCA in soils by micro-organisms. In a 6-month field experiment, lysimeters (2.5 l) at forest and agricultural sites were dosed fortnightly with solutions containing 20 or 50 mg TCA, and soil leachates collected over 2 week periods. In all lysimeters less than 30% of TCA applied to soil was recovered in soil leachate. The “lost” TCA was not detected in the soil, indicating that the applied TCA was degraded, or bound to organic matter within the soil. In experiments where soil was spiked with radioactive [1,2-¹⁴C] TCA (»380 ng g^-1 (fwt)), ¹⁴CO₂ was produced, confirming that TCA in soil is rapidly degraded by micro-organisms, with a half-life of approximately 12 - 46 days depending on soil type. In a controlled greenhouse experiment 6-year old Sitka spruce saplings were exposed twice a week over two growing seasons, to 0, 10 or 100 mg l^-1 solutions of TCA via either the foliage or soil. TCA was taken up into tree needles via both routes (with foliage route being more significant than previously thought). There was no effect of different treatments on sapling growth, but increased activity of detoxification enzymes in saplings exposed to TCA via the foliage suggested that this route of uptake causes greater stress.

Results indicated that <I>Alstonia </I>growth was very satisfactory under pine overstorey where row thinning of pine trees had been done with considerable canopy opening. Mulch treatments significantly improved the height and diameter growth by 20% and 28% respectively while trenching and mycorrhiza application showed no response at this site. Total dry matter accumulation, Relative Growth Rate (RGR) and Net Assimilation Rate (NAR) were also significantly improved by mulch treatments. <I>Khaya </I>plants were grown under a row thinned pine stand which was a comparatively dry site and mulching improved growth rates of seedlings by 47% and 20% for height and diameter respectively. These growth increases were almost doubled when mulching was combined with trenching. Dry matter production, RGR and NAR were also improved with these treatments and root:shoot ratio was reduced. Mycorrhizal inoculation showed improvements of RGR, NAR and reduction in Leaf Area Ratio (LAR) of these plants. <I>Cedrela </I>which was grown under a uniformly thinned stand in a much wetter site did not perform well, and failed to show significant height and diameter growth improvements to any of the treatments. However, mulch treatments favoured its leaf growth compared to the control, showing higher leaf and Leaf Weight Ratios (LWR). <I>Swietenia </I>showed about 61% increase to the combined treatment of mulch, trench and mycorrhiza in terms of height growth during the shorter period of this experiment. Survival was very high in all species except <I>Cedrela. </I>Mulched conditions generally favoured the growth of all species particularly in the less wet sites. Observations suggested that it also improved soil micro-climate in terms of moisture, nutrients and aeration of topsoil. Different uniform thinning levels at higher elevations of pine from 30 to 10 m<SUP>2</SUP>basal area increased direct light level from 18% to 58%, and indirect light levels also followed the same pattern with slightly lower values as measured by hemispherical photographs. <I>Alstonia, Cedrela, Swietenia </I>and <I>Michelia </I>all responded positively to increasing light levels with growth being less in all species under 30 and 20 m<SUP>2</SUP> basal area. <I>Swietenia </I>and <I>Alstonia </I>tended to show much reduced growth with light levels below 50% full sun. Under mid elevations <I>Alstonia </I>indicated sufficient light conditions around 59% full sun with row thinning while <I>Swietenia </I>indicated about 53% or low light would be suitable for its height growth at mid elevations. <I>Khaya, </I>on the other hand, indicated light levels above 50% are beneficial for its growth although it can tolerate low light conditions.

An investigation of the impact of imposed drought on the physiology of 41 year old Scots pine (<I>Pinus sylvestris</I>) was made in southern Scotland. Measurements were made of the seasonal course of transpiration, canopy stomatal conductance, needle water potential, xylem water content, soil-to-needle hydraulic resistance, and growth. Two new techniques were developed to aid these measurements; (i) the use of time domain reflectometry with short probes to detect changes in xylem embolism, and (ii) the continuous measurement of bole water potential based on relating the elastic deformation of the xylem to the internal water tension. Comparison was made between droughted plots and those receiving average precipitation. Under drought, transpiration rate declined once a threshold soil moisture of 12 % volumetric moisture content (VMC) over the top of 20 cm of soil had been reached, and transpiration was thereafter a near linear function of soil moisture. As the drought became intense, hydraulic resistance between soil and needles increased by a factor of three as pre-dawn needle water potential declined from -0.54 MPa to -0.71 MPa. A small but significant increase in xylem embolism was detected in year-old shoots. Stomatal control of transpiration prevented needle water potential from declining below -1.5 MPa. Basal area, shoot and needle growth was significantly reduced in the droughted treatment. In the year following the drought, canopy stomatal conductance and soil-to-needle hydraulic resistance recovered. Current year needle extension recovered, but a significant reduction in basal area increment was evident. Multiplicative non-linear regression models for relating the canopy stomatal conductance to environmental variables were developed. It was found that a simple model that included the effects of air vapour pressure deficit and soil moisture deficit could explain 56 % of the measured canopy stomatal conductance. Overall results suggest that in response to soil drought mature Scots pine closes its stomata sufficiently to prevent the development of substantial xylem embolism. The reduced basal area growth in the year after the drought is not likely to be a result of residual embolism from the previously droughted period.

No description available.

The effects of nutrient and biomass distribution on growth and nutrition of an age series of young stands of Sitka spruce (<i>Picea sitchensis</i> (Bong.) Carr) prior to canopy closure have been studied to find out if the concept of steady-state nutrient status, pioneered by T. Ingestad, is valid under field conditions. Three similar experimental sites for this purpose were located at Manor Valley, Glentress and Innerleithen Forestry Commission Plantations, 40 to 50 km south of Edinburgh. Detailed quantitative analysis of the biomass and nutrient dynamics on tree growth have been made in relation to nitrogen uptake rate and nitrogen productivity. The results obtained showed that, leaves and branches constitute over 65% of the biomass production with over 75% of N, P and K nutrients. Significant allometric relationships were established between dry biomass, nutrient contents and tree dimensions for the difference component parts of the trees at the three ages (R<SUP>2 =</SUP> 0.66-0.99). Basal area provided a reliable estimation of leaf area up to canopy closure. Maximum leaf area density and leaf nitrogen mass density were determined with respect to leaf age and position of leaves within the tree crown. The distribution of leaf nitrogen on an area basis in older trees was consistent with an optimal distribution for photosynthetic activity with depth in the canopy. This may enhance whole plant carbon gain over that possible from a uniform distribution of fixed amounts of nitrogen among leaves of different age class in the canopy. A model was developed for specific leaf area distribution within the canopies and this indicated the potential of the photosynthetic apparatus to acclimate to low light conditions in dense stands compared to open stands. Particular attention has been given to biomass and nutrient productivity models to investigate ecophysiological processes and growth efficiency as a result of interactions between nutrition and stand structure, especially with respect to leaf age and the position of leaves within the tree crown. This showed that the comparatively stable canopy of older Sitka spruce trees seems to develop morphological and physiological gradients of leaf biomass and nitrogen distribution that parallel the gradient of decreasing light with increasing depth in the canopy. This thesis has demonstrated that the concept of relative addition rate, developed in the laboratory, was found to be consistent with exponential growth under field conditions and that a stable nitrogen status must have been achieved since the relative growth rate (R) and nitrogen uptake rate (R<SUB>N</SUB>) were found to agree very closely (R = 0.69 a<SUP>-1</SUP>, R<SUB>N =</SUB> 0.66 a<SUP>-1</SUP>). The efficiency of growth of young Sitka spruce was attributed to dynamic coupling between nitrogen content and growth rate, and it was concluded that higher productivity could be achieved by implementing fertilizer regimes that are designed to increase nitrogen uptake rate whilst maintaining a stable nitrogen status. The results in this thesis are relevant for estimating nutrient drain on forest sites and the prediction of the consequences of fertilizer application, and they also highlight important features of Sitka spruce for intensified management planning.

The thesis reports new studies on the factors affecting the rooting of single node leafy stem cuttings of <I>Shorea leprosula</I>, a Dipterocarp timber tree native to South East Asia. Several aspects of vegetative propagation were investigated including treatments of the stock plants from which the cuttings were taken, propagation systems and post-severance treatments to cuttings. Stock plants raised in 1 litre pots of forest top soil and sand (3:1), and fertilised every two weeks with 0.5 g per plant of NPK fertiliser (12%N:12%P<SUB>2</SUB>O<SUB>5</SUB>:17%K<SUB>2</SUB>O:2MgO + Trace elements) were suitable for production of cuttings. Cuttings from stock plants raised under low irradiance of 0 to 325 μmol photons m<SUP>-2</SUP>s<SUP>-1</SUP> (nominally 10% full sunlight) produced higher rooting and more roots than those from a high irradiance of 0 to 722 μmol photons m<SUP>-2</SUP>s<SUP>-1</SUP> (nominally 30% full sunlight). <I>S.leprosula</I> stem cuttings rooted equally well in the mist and non-mist propagation systems as long as a consistently low vapour pressure deficit (VPD) was maintained. A temporary increase in the VPD of more than 0.5 kPa at peak irradiance could be tolerated by <I>S.leprosula</I> cuttings. Cuttings also rooted equally well in media with either low or high water retaining capacity such as river sand, coconut fibre or a mixture of these two media. A diurnal irradiance of 0 to 360 μmol photons m<SUP>-2</SUP>s<SUP>-1</SUP> was adequate for rooting but 0 to 98 μmol photons m<SUP>-2</SUP>s<SUP>-1</SUP> resulted in low rates of net photosynthesis (<I>P<SUB>n</SUB></I>) and a much reduced rooting success. In the enclosed mist propagation system, misting every 1 hour with a 1 minute duration of spray, throughout the day and night, provided sufficient moisture to cuttings and maintained mean relative humidity of more than 90%. Cuttings planted in the same system with a 3 hour misting frequency tended to develop water deficit as indicated by low relative water content and stomatal conductance.

No description available.

Wind damage in British forests of Sitka spruce has been considered a serious commercial constraint and research has sought to predict and prevent such losses. However, developments in forest policy have reduced the focus on timber production. Not all stands are to be managed to economic rotations. Some stands are to be left in perpetuity to provide structural and biological diversity; others are being given extended rotations or managed by silvicultural systems that do not involve clearfelling. I consider that wind could become the determinant of stand structure, rather than simply an agent of damage, and in some cases lead to self-perpetuating stands of Sitka spruce through spontaneous regeneration. To examine the potential for self-perpetuation, I studied the frequency of strong winds in upland Britain, and examined the processes of gap formation, expansion and filling of Sitka spruce stands. Britain has an extreme wind climate, comparable with a few maritime locations in temperate areas of the World. This implies the need for caution in transference of results from elsewhere. Gaps of a range of size, including those apparently suitable for spruce regeneration, form during periods of very strong winds. In intervening periods, there is a slow expansion of gaps by attrition at the edges, but this does not produce the largest gaps found within stands. Gap filling is occurring through lateral closure of small gaps, and by seedling regeneration and epicormic sprouting within larger gaps. The mechanism of epicormic sprouting from fallen trees has not been emphasised in literature from the Pacific North West, and bears some resemblance to recovery of forests in hurricane-prone areas. A case study of one of the oldest stands of Sitka spruce in upland Britain, Birkley Wood, showed that the components can come together to produce a self-perpetuating stand. Gap area has increased from 4 to 30% over 16 years. The tallest regeneration has now reached 8 m height, close to cone-bearing size, and meeting one of the accepted definitions of gap filling. The results have practical application in the development of alternatives to clear-felling, the selection of spruce areas as 'natural reserves', and in the treatment on Britain's wind climate in models of windthrow risk.