The importance of animals in the survival of a canopy tree, Pod mahogany (Afzelia quanzensis WELW) in the arabuko - sokoke forest, Kenya

Abstract

Field research was conducted between September 1990 and July 1991 in the Arabuko-Sokoke forest. The study involved a detailed investigation of the importance of animals for the survival of the pod mahogany, Afzelia guanzensis (Welw). Four mammals, one bird, two insects and fungi were observed to have strong relationships with this tree at various stages of its fruit development. The red-legged sun squirrel Heliosciurus rufobrachuim (Waterhouse), yellow baboon Papjo cyanocephalus (Linn), a beetle Caenocara weisii and an unidentified weevil consumed the seeds at the immature stage. Primates were found to be the major seed dispersers with the white throated monkey Cercopithecus mitis albogularis (Schwarz) and yellow baboon dispersing the seeds. Ad ~. data collection revealed that the crowned hornbill Tockus alboterminatus was a rare and unreliable disperser. A nocturnal rodent, the giant Gambian rat, Cricetomys gambianus (Ogilby) was found to be the sole seed predator after dispersal. Observations and seed removal experiments revealed seed survival to be low in the range of ~ gambjanus. Seedling density was observed to be highest below and around adult trees. There was variation in the density of seedlings between habitats with the open Brachystegia woodland having a higher density than the thick forest. The vigour of seedlings increased with distance from conspecific adults. All seedlings found 7m and beyond the adults exhibited good growth which was even up to 100% survival. The adult A. guanzensis population structure exhibits an inverse J-curve. This was analysed by use of belt transects and measuring diameter at breast height (dbh). There was a decline in the number of stems with 55cm dbh which corresponds with the stem size desired by loggers. The tree is still logged illegally and forms a source of timber with 12.4 % of the adults having been cut. The adult tree population shows a regular dispersion pattern as shown by the nearest neighbour technique used to test for nonrandom dispersion. A. guanzensis dominates the thick forest range. Th~ data obtained from converting diameter at breast height (dbh) to cross sectional area at breast height (CABH) and importance value index (IVI) were used, and 16 canopy trees associated with Afzelia guanzensis were enumerated. The vertical stratification of the canopy was analysed by sketching profile diagrams. The forest exhibits layering with distinct upper, middle and lower canopies.