The main objective of the present thesis is to investigate the mechanisms underlying the age- and size-related decline observed in growth efficiency and in relative above ground mass growth rates in both Scots pine and poplar (clonal) chronosequences. Hence, I compared differentially aged Scots pine stands and trees growing at the same site, in contrast to most previous studies, isolating leaf- from stand-level responses. In addition, I used poplar clonal material to compare growth and morphological parameters of genetically identical cuttings taken from young, middle-age and old plants belonging to four clones and I carried out investigations both in the field on the original donor trees (trees of identical meristematic ages and different sizes) and on rooted cuttings (identical meristematic ages and same size). Scots pine study: Our results partially supported the hydraulic limitation hypothesis, but at the same time suggested that additional factors were also involved in the decline in growth efficiency with age. Such factors may involve decreased soil nutrient availability, increased below-ground allocation and reduced turgor pressure in tall trees. Poplar study: The observed decline in growth and carbon uptake in poplar donor trees in the field was related to the increases in tree size, the increased path length and possibly the architectural complexity, or in some cases to decreased nutrient availability in the soil but not to cellular senescence per se.

This study increases the body of knowledge on the drought response of plant-fungal associations. Hybrid black popular <i>Populus </i>x <i>canadensis</i>, a species commonly seen in shelter belts and small plantations, is tested on its drought response to mycorrhizal colonisation. It also suggests an alternative mechanism for the alteration of plant water relations by arbuscular mycorrhiza. The concept of chemical signalling compounds which control plant response to stress, including drought stress, is topical. This study aims to extend the concept of the plant-fungus symbiosis. The ability of the fungal hyphae to act as an integral part of the root system is tested in its contribution to signalling of drought stress to the host shoot. The response of mycorrhizal poplar to gradients in water availability was tested in two ways. First the effect of changing water availability over time, as drying and wetting cycles, was examined. Second, gradients in water availability across the root-fungal system were considered. The response of poplar to drought stress was tested when inoculated with one of four species of mycorrhizal fungi. These were <i>Glomus intraradices</i>, <i>Glomus mosseae</i>, <i>Gigaspora rosea </i>and <i>Gigaspora margarita</i>. There was no consistent improvement in plant response to drying. However there were variations in plant response over time, and with severity of drought conditions, particularly by plants colonised with <i>G. intraradices </i>and <i>Gi. rosea</i>. Changes in the host plant nutrient status were also found. The concept of hyphal to plant shoot signalling of drought was tested with poplar inoculated with <i>G. intraradices</i>, grown in a specially designed microcosm. Fungal hyphae were able to grow into a separate volume of soil from which plant roots were excluded. The soil water availability around plant roots and adjacent hyphae was altered independently. The plant response in terms of gas exchange was monitored under conditions of varying water availability in each section of the root-hyphal system. In general there were no consistent alterations in plant gas exchange with changing water availability. However during one experiment possible evidence for short-term hyphal signalling to host plants was noticed. This method is suggested as a new concept for further experimentation in plant-fungal water relations.

No description available.

The objectives of this research were to assess in different forest fragments the effects of disturbance produced by human activities on tree regeneration (by seedling and from the seed bank); to evaluate seedling survival and growth of native tree species in response to light availability in natural (by seedling transplant in the field) and controlled conditions (using shade houses); and to predict population viability in forest fragments of two of the selected species using a transition matrix model. The study was carried out in six forest fragments located in Veracruz, Mexico using <i>Carpinus caroliniana, Fagus grandifolia </i>var. <i>mexicana, Quercus acutifolia </i>and <i>Symplocos coccinea. </i>The least disturbed fragments (which were less accessible to local people) showed the highest density of seedling regeneration of shade-tolerant species, and the most disturbed fragments (near to human settlements) showed a high density of shade-intolerant tree species that were regenerating from the seed bank. The results of shade house and seedling transplant experiments suggest that <i>Fagus </i>and <i>Symplocos </i>(both native species with a restricted distribution) can be used in areas with no severe disturbance, whereas <i>Quercus </i>and <i>Carpinus </i>(considered on the basis of these results as a intermediate species) can be used in areas that need to be rehabilitated such as disturbed fragments or near the forest edge. In the fragments the effect of frequent tree removal harvesting on population viability will be greater in <i>Fagus </i>(the shade-tolerant species) than in <i>Carpinus </i>(the intermediate species). According to the results, even the smallest human disturbance in the forest fragments (such as the use of forest paths) can influence tree regeneration favouring the increase in density (seed or seedling) of shade-intolerant species. The understanding of the species response to irradiance provides for improved management of regeneration for forest rehabilitation.

No description available.

Some of the effects of wind upon young Acer pseudoplatanus L. tree seedlings were investigated, including the nature of wind damage to leaves and some of the effects of wind upon CO2 and H2O exchange at different levels of soil water. A controlled environment wind tunnel was used for much of the work but some of the observations of that damage involved outdoor plants. Brown lesions developed on leaves as a result of abrasion when they were exposed to wind. The zones of leaf that were readily damaged changed as the leaf expanded and its topography altered. Young, expanding leaves were more susceptible to damage than older leaves. In the wind tunnel, there was a linear relationship between % damage and windspeed and mean leaf relative growth rate appeared to be reduced by wind. Wind damage led to crushing of epidermal and mesophyll cells and disruption of the epicuticular waxes present on the abaxial leaf surface which was far more readily damaged than the adaxial surface despite apparently protective topographic features. The effects of wind upon CO2 and H2O exchange depended upon the state of leaf development and the availability of soil water. When leaves were fully expanded, day and night leaf conductance to water vapour were increased by wind over a period of several days but when leaves were still expanding, only night time conductance was affected. The effects were reduced at low levels of soil water. Where there were effects of wind they were always at least partially reversible in a subsequent calm period. Photosynthesis was only affected by wind at high levels of soil water, when the rate of photosynthesis of green areas of wind damaged leaves increased - thereby compensating for the loss of photosynthetically active area. There was a linear relationship between % macroscopic damage and cuticular conductance to water vapour. Mesophyll conductance and dark respiration were unaffected by wind and soil water.

No description available.

We tested two interlinked hypotheses concerning forest responses to hydraulic stress using data collected at a through-fall exclusion (artificial droughting) experiment at Caxiuana in Eastern Amazonia. First, we tested the ‘isohydric’ hypothesis, that stomatal conductance, in water stressed conditions, operates to maintain leaf water potential above a certain critical threshold limit. We used the soil-plant-atmosphere (SPA) model to predict the expected ecophysiological behaviour of the trees, and tested these predictions against intensive diurnal cycle measurements of leaf water potential, stomatal conductance, sap flow and stem water potential. The data and the model predictions were largely consistent, indicating that the isohydric stomatal control may be the prevailing mechanism controlling water use of rain forest trees in drought stressed conditions. The model was parameterised using independent measurements of ecosystem properties and as such required no fitted parameters. The implication of this is that the response of rain forest ecosystems to drought may be predicted using soil, rooting and vegetation properties. The SPA model is both computationally and data requirement intensive. We assimilated the model and produced a set of empirical equations which replicate the daily modelled gas exchange predictions from daily model inputs. We extrapolated this model 100 years into the future using the latest Amazonian climate predictions and found that after -50 years, a threshold was reached when all the rainfall falling on the plot was evaporated (by interception, soil surface evaporation or evapotranspiration) and none was drained away into stream flow. The response of the forest in the first 50 years was modulated strongly by soil hydraulic properties and rooting depth, and we conclude that increased field measurements of these properties are necessary if the response of Amazonian forests to anticipated drying is to be accurately predicted.

The objectives of this study were to estimate the population density and distribution of <I>Aquilaria </I>spp., and to evaluate the impact of trade in gaharu on populations of the species in natural forests. Investigations were made of factors influencing regeneration, reproductive ecology and stand structure of <I>Aquilaria,</I> and the status of the species was evaluated in relation to IUCN categories of threat and CITES criteria. Six species of <I>Aquilaria</I> are recorded in Indonesia; <I>A. beccariana, A. cumingiana, A. hirta, A. filaria, A. malaccensis</I> and <I>A. microcarpa. </I>Based on the national forest inventory (NFI), the density of <I>Aquilaria</I> spp. in Sumatra appears to be lower than in Kalimantan (0.4 ha<SUP>-1</SUP> and 0.36 ha<SUP>-1</SUP> and 0.83 ha<SUP>-1</SUP> and 1.17 ha<SUP>-1</SUP> in lowland and upland Sumatra and Kalimantan respectively). <I>A. malaccensis</I> has the highest germination probability and <I>A. filaria</I> the lowest. A positive relationship between light availability and seedling growth was recorded in the field for <I>A. malaccensis</I> and <I>A. microcarpa. </I>Mortality of these species in natural populations was found to vary with size class; highest mortalities were recorded for seedlings. These data were used to parameterize a matrix model to predict population dynamics under different harvesting regimes. Based on trade data and field analysis of harvesting, it was estimated that 59,000 trees of <I>Aquilaria</I> species were felled in East Sumatra in 1992 and 70,000 trees were felled in East Kalimantan in 1996. Matrix models predicted that harvesting of the species will be sustainable provided that the annual cutting level is set at a maximum of 30% of harvestable individuals with a minimum dbh > 10 cm. According to IUCN criteria, the species should be considered as vulnerable to extinction. The species also qualifies for listing under Appendix II of CITES.

Using Jamaica as a case study, this thesis investigates the role played by poverty and population in driving deforestation in the tropics. It argues that Jamaica provides a good middle ground or "meso-level" perspective from which to study these scarcity-forest interactions. Like many developing countries, Jamaica has experienced serious socio-economic dislocations since the late 1970s, which have had negative repercussions for the island's poor. Declining living standards and a deterioration in social welfare have in turn worsened long-standing problems of unemployment and production on the island, and thereby increased the immediate dependency of the poor on the natural resource base. Forest data for the quantitative analysis of these interactions is derived from an analysis of these interactions is derived from an analysis of Landsat MSS data from 1987 to 1992. Using a GIS (geographic information system) the study estimates that during this period, Jamaica experienced a national average deforestation rate of 3.9% per annum. Classification maps based on the original satellite images used to calculate this rate are combined with a political boundaries map of the island in a GIS to derive sub-national forest estimates at parish and constituency levels. The contributions of several scarcity-related land use and social variables to the calculated parish-level deforestation rates are presented and briefly discussed, before "going one level down" to the constituency unit. At this level, forest constituency data is used to quantitatively assess a conceptual model of scarcity-driven land use for the island. The model includes a variety of population/poverty measures reflecting key socio-economic and land-related features of the island. Simple correlation and OLS regression results for both parish- and constituency-levels support the importance of scarcity in driving the destruction of Jamaica's forests, and the relative contribution of its various population/poverty measures are noted and discussed.