Minimum leaf water potential has been found to be rather constant in coniferous species over a range of environmental conditions and developmental stages. Such a functional homeostasis requires the balanced growth of transpiring foliage, absorbing roots and conductive sapwood, with profound implications for resource allocation, plant allometry and productivity. Although central to the maintenance of plant structure, the process of growth allocation is still poorly understood. The observation of a functional homeostasis in water transport has led to formulate a novel hypothesis of optimal plant growth under hydraulic constraints. The hypothesis has been tested against field and literature data of forest function and growth, choosing <I>Pinus sylvestris </I>as a model coniferous species. The newly developed hypothesis delineates a common framework that seems to explain conveniently changes in growth allocation both over the lifetime of the plant and in response to the environment, helping to explain the variability in forest growth observed at the regional scale as well as the age-related decline in forest productivity. A detailed process model of forest growth (HYDRALL) was developed, centered on the hypothesis of optimal carbon allocation under hydraulic constraints, and applied to the prediction of <I>P. sylvestris </I>growth patterns across Europe. The model was found to predict conveniently several of the growth patterns reported in the literature. Changes in carbon allocation were found to be most important under dry conditions. Information on root hydraulic characteristics under natural conditions is scarce. Part of the research effort was therefore devoted to the development of a new technique for the measurement of the hydraulic resistance of entire root systems of soil-grown plants, a parameter central to the newly developed model.

(1) Subjecting two year old Pinus contorta to high winds in a controlled environment wind tunnel, or to continuous shaking by a specially constructed shaking rig, caused a 20% reduction in extension growth of leader and lateral stems. Rates of needle extension were reduced 11% by shaking and 30% by exposure to high wind. Radial growth of the stem was not affected. (2) Lacroscopic investigation of cell size and number revealed that the reduced growth of leader stems was due primarily to a reduction in cell division. Cell extension was also slightly reduced. (3) The reduced extension growth caused by shaking was accompanied by large reductions in dry weight. Relative Growth Rate and Unit Leaf Rate were reduced, but Leaf Area Ratio was unaffected; suggesting that the reduced growth was due to a decrease in net photosynthesis, or to an increase in dark respiration. (4) Subjecting P. contorta to high winds had no effect on pet photosynthesis, determined with an Infra-Red Gas Analyser, but significantly increased dark respiration. (5) t'hole-plant and detached-needle transpiration rates were determined gravimetrically. High winds and shaking had no effect on stomatal or cuticular conductances. Total water potential, determined with a needle pressure-bomb, was slightly increased by wind and shaking. Solute and pressure potentials of individual needles, determined by the pressure-voltuae technique, were not affected. It is concluded that mechanical stress does not affect the growth of P. contorta via an effect on water relations. (6) It is postulated that mechanical stress causes an increase in 'maintenance respiration', with a resultant decrease in respiratory substrate for growth. The consequent reduction in cell division and extension leads to a decrease in extension growth and dry weight growth. It is accepted that the links between these various processes are unclear.

Generally, the growth of all forests accelerate as canopies develop as in young forests and declines substantially soon after maximum leaf area is attained. The causes of this decline trend are multiple. However, age and size are normally coupled growth. Therefore, an experimental manipulation has been done to separate the effects of size from those of age by using traditional grafting techniques. Genetically identical grafted seedlings were produced from scions taken from trees of four different age classes ranged from 4 to 162 years. The aim of this study is to investigate the effects of tree age and tree size on growth, physiology and water use of two broadleaf species by conducting three major experiments. Growth characteristics such as relative growth rate and growth efficiency were measured together with leaf-level gas exchanges and sap flow studies. Comparisons were established among results observed in the field with the ones obtained in the grafted seedlings. The results showed that relative growth rate and growth efficiency decreased substantially with increase in age of donor trees in the field. In contrast, these parameters seemed almost constant on grafted seedlings which are scions taken from different meristematic ages did not show the age-related trend after they were grafted onto the rootstocks. Similar patterns were also observed in net photosynthesis from leaf-level gas exchange and sap-flow-based parameters in both species. In general, these results suggested that size limitation to water and nutrient transport to the top of the canopy is a primary cause that triggered the declination in production of photosynthate and reduced growth of the trees, and/or increase in maintenance respiration with increasing in tree size rather than controlled by meristematic age.

South American savannas and seasonally dry topical forests (SDTFs) are two widespread and biologically important vegetation formations which can be highly species-rich (such as the cerrado tree savanna of Central Brazil) and which can show high degrees of endemism (such as the dry forests of northern Peru). Both savannas and SDTFs are highly threatened by agricultural development. Of critical importance in ensuring the protection of their biodiversity is the establishment of reserve areas. However, conservation planning is currently hindered by the lack of base-line floristic data. This work provides floristic data for three areas of priority conservation importance in the Neotropics and examines the patterns of diversity within them. The areas are (i) the cerrado biome of Brazil; (ii) the SDTFs of Peru; (iii) the savannas of Belize. For the cerrado biome, this study also investigates whether diversity patterns are similar to those recently proposed for Neotropical rainforests. A morphological and biogeographic study of the Acosmium dasycarpum complex was undertaken to resolve the taxonomic uncertainties relating to its putative subspecies.

<i>Cistus creticus </i>L is an ectomycorrhizal woody perennial that establishes after fires in Aleppo pine (<i>Pinus halepensis</i> Mill.) forests from small, hard-coated seeds that are stored in the soil. Field observations, and field and greenhouse bioassays were used to assess spatial and temporal variation in post fire colonization of <i>Cistus creticus</i> by ectomycorrhizal fungi and to investigate the potential of resprouting shrubs to act as refugia to ectomycorrhizal fungal inoculum. Four months after fire, ectomycorrhizal fungal inoculum was stratified both vertically and horizontally. <i>Cistus creticus</i> seedlings were predominantly colonized by E-strain and other ascomycete fungi that occurred mostly in top 3 cm of root systems throughout the forest. Basidiomycete fungi were rare and confined to lower parts of root systems in seedlings establishing close to both ectomycorrhizal and non-ectomycorrhizal resprouting shrubs. Few of the fungi colonizing naturally establishing seedlings appeared in the greenhouse bioassay of soils taken from the same site suggesting that most of these fungi colonize from mycelial inoculum attached to living or recently dead root systems of mature trees or shrubs. In the absence of competition from other EM fungi, the pot-grown seedlings were heavily dominated by one of the unidentified fungi observed in the top 3 cm of naturally establishing seedlings. This fungus formed a discontinuous Hartig net with or without a weak mantle in both short and long roots. Sequential observations of naturally establishing seedlings revealed that this pattern of colonization was common in naturally establishing seedlings during the first four months after germination. After six months, this fungus had been largely replaced on the short roots by other fungi but maintained its colonization of the long roots. Though the identity of this fungus remains unknown, its affinity to species of desert truffle (<i>Tirmania </i>and <i>Terfezia</i> spp.) is discussed. This unknown fungus and that forming the E-strain morphotype appear to provide an important initial source of inoculum in the immediate post-germination phase while other fungi are recovering from the fire. Interactions between plants and fungi are discussed in the context of nutrient acquisition and fungal regeneration strategies.

The success of reafforestation schemes depends largely on the silvicultural systems employed. An indication of the ecological sensitivity of such practices can be gauged from the extent to which the soils physical, chemical and microbiological properties are altered. Of particular interest is the impact on the vescicular-arbuscular mycorrhizal (VAM) fungi as they play an important role in ensuring that forest trees especially those growing on nutrient deficient soils acquire sufficient mineral nutrients for growth. This study therefore set out to examine the effects of different methods of site preparation (Manual 'recru', mechanical 'recru' and complete clearance) and the subsequent outplanting with <i>Terminalia ivorensis</i> on the VAM population dynamics in the Mbalmayo Forest in Cameroon. Observations made prior to site preparation indicated the presence of an array of 17 VAM fungi belonging to the generas <i>Acaulospora, Glomus, Sclerocystis</i> and <i>Scutellispora</i>. Of the 17 fungi, one, <i>Glomus etunicatum</i> always represented more than 50% of the spore population. The importance of the tree component of the forest vegetation as reservoirs of VAM inocula was evident from, a) the higher number of spores in association with <i>T. superba</i> compared with shrubs and b) the peak in spore density close to <i>T. superba</i> trees (2.5m). Seasonal effects were suspected as many VAM fungi sporulated more profusely in the dry season (February, 1987) compared to the two rainy seasons (August 1987 and August 1988). Site preparation led to a dramatic reduction in spore number with the completely cleared plot losing 65% of its initial spore population. One year after planting, however, mean spore numbers had increased dramatically in all cleared plots, a major cause of the increase being the increase in root densities from the planted <i>T. ivorensis</i> and the invasive ruderal <i>Eupatorium odoratum</i> and pioneer tree <i>Musangacecropioides</i>. In the mechanically and completely cleared plots the sharp rise in spore numbers was mainly by the fungal aggregate <i>G. occultum/A. scrobiculata</i> which sporulated profusely in the presence of the invasive ruderal <i>Eupatorium odoratum</i>. The amounts of infection within the roots of <i>T. ivorensis</i> 1.5 years after planting in the manual and mechanical 'recru' plots were significantly greater than observed within <i>T. ivorensis</i> roots in the completely cleared plot. These differences may be related to the initial drop in spore numbers following site preparation, the disruption of the VAM hyphal networks in soil and/or the increasing dominance of <i>G. occultum/A. scrobiculata</i>, a type believed to be more associated with <i>Eupatorium odoratum</i> and hence possibly less effective on <i>T. ivorensis</i>. The value of the silvicultural procedures that were less destructive appeared to be reflected in tree survival.

No description available.

Two field campaigns were carried out in which the wind and tree interaction was investigated. Wind profiles, turbulence, and turning moment at the tree base were measured with a high temporal resolution for a group of nine trees in both surveys. The first experiment took place in a mature even-aged Sitka spruce forest stand, which appeared to be more stable than model calculations anticipated. Differences in wind loading between the individual trees were calculated and related to tree properties. Absolute turning moments were positively correlated with tree properties such as diameter at breast height, tree height, and stem weight. The estimated turning moment for tree failure for the strongest tree in the sample is more than five times higher than the value for the weakest one. However, due to their dominance and their exposed position in the stand the biggest trees also experience higher wind drag. The results suggest that the balance between individual tree resistive moment and applied moments is such that the critical wind speeds for damage are very similar. This implies that the tress are adapted to their local wind climate. Post damage surveys from continental Europe suggest that irregular forests are more stable then regular ones. This hypothesis was tested in the second field campaign in which the wind and tree interaction were compared in two contrasting stands. One group of trees was located where an understorey was present, while two other trees were at a location with no understorey at all. The analysis suggests that the trees with an understorey benefit from the understorey in terms of wind loading and wind damage risk.

Attempts to restock large areas of exploited forests in Kenya have been hampered by inadequate understanding of the ecological requirements of indigenous tree species. In particular, very little is known about the responses of these species to different levels of light. This study, therefore, examined the significance of shade and light in regeneration of some important tree species. The species studied were <I>Cordia africana</I> Lam., <I>Vitex keniensis</I> Turill, <I>Markhamia lutea</I> (Benth.) K. Schum. and <I>Olea capensis</I> L. Artificial shading experiments were carried out in the nursery using seedlings of <I>C.africana</I> and <I>V.keniensis</I>. The effects of different shade levels were compared to full sunlight under low and high nutrient regimes. The response of seedlings transferred among different light environments was also examined. The effects of different levels of light quality (R:FR ratios) were tested. In a forest clearing, seedlings of the four species were grown under artificial shading and their responses were compared to those grown under full sunlight. The influence of deep canopy shade on survival and growth in seedlings of the four species was also determined. In the nursery, seedlings of <I>C.africana</I> and <I>V.keniensis</I> maintained positive relative growth rates at irradiance level of 19% of full sun and their growth was enhanced by increasing irradiance. The supply of nutrients at high level also enhanced the growth. Seedlings transferred between light environments acclimated within a month. Acclimation to increase in light availability was faster than acclimation to decrease. Although acclimation was mainly physiological, biomass was allocated in favour of shoots and roots when seedlings were shaded and exposed respectively. The supply of nutrients at higher level had no effects on the acclimation process.

This study reports the results of experiments on growth and physiology of an agricultural annual (French bean) and a young tree (poplar) in relation to limited soil water. Competition and complementarity between the species were evaluated in a model agroforestry experiment. The experiments were carried out in a greenhouse and growth cabinet with plants grown in pots containing sandy-loam compost. The species studied were <i>Phaseolus vulgaris</i> cv. Argus and <i>Populus trichocarpa</i> X <i>P. deltoides</i> cv. Raspalje. The main aim of the study was to characterise the responses of both species to different soil water supply regimes, shoot water supply by roots and chemical signalling from the roots in drying soil and to show how these responses could be used in the selection of suitable agroforestry for dry regions. Bean and poplar plants showed differential growth responses although both the species performed better in mixed stands than in monoculture when soil water was severely limited. Beanplants were more competitive than poplar, although both species showed complementarity in exploration for soil water. As the soil dried leaf water potential declined. Stomatal conductance of both species was more closely related to pre-dawn leaf water potential than to mid-day leaf water potential, indicating the importance of soil water status. Experiments with both vertically and horizontally divided root systems showed that bean and poplar plants with at least half of their root system in moist soil were able to maintain leaf water status as well as plants with all their root systems in moist soil. Furthermore, stomatal conductance and leaf expansion of both species were affected directly by soil drying independent of leaf water potential.