Effect of population size on viable seed output, seed rand and natural regeneration pattern of a tropical conifer Widdringtonia whytei-Rendle in Malawi

[摘要] ENGLISH ABSTRACT: Widdringtonia whytei is a tropical endemic, fire-adapted pioneer coniferous tree specieswithin natural fire-fragmented Afromontane forest patches in a confined area on MulanjeMountain in Malawi. Natural and anthropogenic fires within the surrounding fire-pronelandscape, insect attacks, and uncontrolled harvesting of mature trees for timberthreaten the survival of W. whytei. This study investigated the effects of populationfragmentation 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 rainvariation among population sizes; effects of temperature and light on viability andgermination of W. whytei seeds; and the influence of population size on naturalregeneration patterns.The study was conducted during 2008 and 2009 on three sites using three W. whyteipopulation sizes: small (≤10 reproductively mature cone-bearing W. whytei trees),medium (11-20 cone-bearing trees) and large (>20 cone-bearing trees). Data werecollected 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 rainpattern 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 seedrain was explored with a generalised linear model (GLM) with a negative binomial linkfunction. Variations in seed germination was analysed with a Weibull regression model.The influence of fragment size on seedling regeneration was tested with a GLM (binarylogistic regression).Viable seed output from W. whytei cones was very low (23%) and was not affected bypopulation size, tree diameter or crown position. W. whytei population size influencedseed rain with large fragments having higher seed rain densities. Number of dispersedseeds was generally very low and poorly dispersed. W. whytei cones are semiserotinousand depends on moderate to severe fires for heavy seedfall and widerdispersal. Such fires did not occur during the study period. Temperature is a criticalfactor 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 seedrain pattern indicating that seed availability is a critical factor for regeneration. Regularcool fires outside the forest patches and along the edges during the hot dry season, andthe thick litter layer and shady conditions inside forest patches, caused most of theseedling mortality.This study highlighted the difficulties W. whytei experiences with low output of viableseed, limited seed dispersal and low seedling regeneration in and around forestpatches, i.e. factors important for conservation management of this species. It isrecommended that gaps with diameter equal to canopy height and occasional spot fireswould promote seed dispersal onto exposed mineral soil for rapid seedlingestablishment and subsequent population growth. Such natural fire disturbance eventsmay show the true seed rain and seedling establishment patterns in this species.

[发布机构] Stellenbosch University