Factors influencing natural regeneration of Polyscias kikuyuensis summer in Nyamweru forest Kikuyu escapment-Kenya

Abstract

Polyscias kikuyuensis is a rare and endemic tree species to central Kenya highlands that shows poor ability to regenerating naturally. The causes of poor regeneration are not known and in addition there is widespread destruction of its natural habitat. The aim of this study was to determine some of the factors influencing the natural regeneration of P. kikuyuensis in order to recommend appropriate conservation measures. Population structure was studied from a sample of 33 plots of 0.09 ha each and the status of soil seed reserve from 66 plots of one m2 nested within the population structure sample plots. These studies were conducted independently in disturbed and undisturbed sites and results compared. The status of fresh fruits and seeds was studied from a sample of 25 trees in Nyamweru and from a similar sample size in Kinale forest. In order to confirm some of the population structure results, a glasshouse-based study on the effect of light and moisture on the seedlings growth was also conducted. There were 809.1 seedlings, 18.9 saplings, 12 mature trees and only one pole-sized tree, respectively ha-l in the undisturbed site. The corresponding figures for disturbed site were: 72.8 seedlings, 1.3 saplings, four mature trees and one pole-sized tree, respectively ha-l . This may suggest that the agent(s) hampering successful regeneration act to prevent development of seedlings into poles. Seedlings and saplings populations in the undisturbed site were greater than those in disturbed site. The soil seed reserves declined with soil depth in undisturbed and disturbed sites. In the litter layer the percentage of viable seeds were; 0.86 and 0.33 in the undisturbed and disturbed sites, respectively. Ninety three and 58.3 % of fresh seeds collected from Nyamweru and Kinale populations, respectively were empty. Germination of fresh seeds was also low « 1 %) in both Nyamweru and Kinale populations. The growth of seedlings under 30 % and 50 % of full sunlight and soil moisture at field capacity was the best and there was no mortality. The seedlings growing in the full sunlight irrespective of the soil moisture levels had stunted growth and about 40 % mortality. It therefore appears that P. kikuyuensis grow better under shade during early growth. There is therefore a need to set up a a field experiment to study on the role of light in development of seedlings into poles. It is also necessary to study seed physiology and dormancy and stop destruction of P. kikuyuensis natural habitat in order to conserve the existing population (s) in-situ. In addition development of protocols for raising P. kikuyuensis planting stock from seeds should be given priority to enhance ex-situ conservation.