Doctoral Degrees (Forest and Wood Science)

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Browsing Doctoral Degrees (Forest and Wood Science) by Author "Chanyenga, Tembo Faera"

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    Effect of population size on viable seed output, seed rand and natural regeneration pattern of a tropical conifer Widdringtonia whytei-Rendle in Malawi
    (Stellenbosch : Stellenbosch University, 2013-12) Chanyenga, Tembo Faera; Geldenhuys, Coert Johannes; Sacande, Moctar; Stellenbosch University. Faculty of AgriSciences. Dept. of Forest and Wood Science.
    ENGLISH ABSTRACT: Widdringtonia whytei is a tropical endemic, fire-adapted pioneer coniferous tree species within natural fire-fragmented Afromontane forest patches in a confined area on Mulanje Mountain in Malawi. Natural and anthropogenic fires within the surrounding fire-prone landscape, insect attacks, and uncontrolled harvesting of mature trees for timber threaten the survival of W. whytei. This study investigated the effects of population fragmentation on the reproductive biology of W. whytei, through four specific studies: effects of population size, tree size and crown position on viable seed output; seed rain variation among population sizes; effects of temperature and light on viability and germination of W. whytei seeds; and the influence of population size on natural regeneration patterns. The study was conducted during 2008 and 2009 on three sites using three W. whytei population sizes: small (≤10 reproductively mature cone-bearing W. whytei trees), medium (11-20 cone-bearing trees) and large (>20 cone-bearing trees). Data were collected using field and laboratory experiments and a review of secondary information. The effect of fragment size and crown position on viable seed output and seed rain pattern was tested with a nested linear mixed model. The influence of stem diameter (dbh) on viable seed output was tested using a separate data set. The variation in seed rain was explored with a generalised linear model (GLM) with a negative binomial link function. Variations in seed germination was analysed with a Weibull regression model. The influence of fragment size on seedling regeneration was tested with a GLM (binary logistic regression). Viable seed output from W. whytei cones was very low (23%) and was not affected by population size, tree diameter or crown position. W. whytei population size influenced seed rain with large fragments having higher seed rain densities. Number of dispersed seeds was generally very low and poorly dispersed. W. whytei cones are semiserotinous and depends on moderate to severe fires for heavy seedfall and wider dispersal. Such fires did not occur during the study period. Temperature is a critical factor for seed germination. Seeds germinate between 15 and 25°C with the optimum at ~ 20°C either under light or dark conditions. Regeneration density followed the seed rain pattern indicating that seed availability is a critical factor for regeneration. Regular cool fires outside the forest patches and along the edges during the hot dry season, and the thick litter layer and shady conditions inside forest patches, caused most of the seedling mortality. This study highlighted the difficulties W. whytei experiences with low output of viable seed, limited seed dispersal and low seedling regeneration in and around forest patches, i.e. factors important for conservation management of this species. It is recommended that gaps with diameter equal to canopy height and occasional spot fires would promote seed dispersal onto exposed mineral soil for rapid seedling establishment and subsequent population growth. Such natural fire disturbance events may show the true seed rain and seedling establishment patterns in this species.