Spatial and temporal patterns of growth in Ghanaian tropical rain forest
This thesis tests the hypothesis that variation in water supply, nutrient availability and irradiance determined variation in tree growth along local and regional gradients of resource availability, and over time, in Ghanaian forests. Regional variation in soil water availability determined seasonal patterns of diameter change of Celtis mildbraedii and Strombosia glaucescens in semi-deciduous and evergreen forest, over two years. However, in a semi-deciduous forest, annualized diameter increment of Celtis mildbraedii was higher in summit and slope, compared to valley, positions after two years, even though trees in valley positions experienced a shorter effective dry season. Growth was also greater in the semi-deciduous than the evergreen forest in the second year of the study. These patterns suggest that concentrations of N in soil over topographic gradients, and concentrations of available P and the base cations over regional scales, may be important determinants of growth. Dry season stem shrinkage in semi-deciduous forest can comprise up to 0.5% of tree diameter, and varies between years. Re-enumeration of forest plots in seasonal climates should be carried out over whole year intervals, during the wet season, to minimise bias derived from variation in tree water status. In a semi-deciduous forest, no relationship was found between topography and six year growth rates of two common species or of six functional types defined on the basis of regeneration strategy and regional distribution pattern. However, within this forest, and in a comparison within five different forests across the regional gradient of rainfall and soil fertility, pioneer species with distributions biased towards drier forests had significantly higher growth rates than pioneer species associated with wetter forests. Variation in growth of dry forest pioneer species explained more than half the total variation in stand-level growth rates, demonstrating that it is the presence of abundant, potentially large, fast growing pioneer species in more seasonal forest types that generate regional scale variation in forest growth. These results indicate that the environmental variables found to determine growth are dependent on the scale of the study and the magnitude of the gradient in resources being compared. Variation in soil fertility over regional rainfall gradients in tropical forests has a significant impact on variation in tree growth, within and between species, and at the stand-level.