Brush-control studies in the drier areas of Kenya. 4. Effects of controlled burning on secondary thicket in upland Acacia Woodland.
Two burning regimes (burning each year during 1958-60 and burning in 1958 and 1960 only) were compared for the clearance of thicket in Acacia woodland under medium rainfall/altitude conditions in Machakos District. The main species, Acacia brevispica, Acalypha fruticosa, Aspilia sp., Crotalaria saxatilis and Maytenus putterlickioid.es, appeared equally tolerant of one fire but showed differential susceptibility to subsequent fires. A. brevispica tolerated 3 burns but was only partially tolerant of the 2 burns. In contrast, Aspilia sp. tolerated 2 burns but suffered 40% mortality from 3 burns, while M. putterlickioides showed partial susceptibility (30% kill) to both burning regimes. Three burns resulted in 70-80% kill of A. fruticosa and C. saxatilis, but 2 burns gave 60% kill of the former and were ineffective against the latter. Plant size appeared to be associated with fire susceptibility only in the case of M. putterlickioides (all plants killed were > 8 ft high), but counts in supplementary quadrats of complete populations of Aspilia sp., in which 50% were < 1 ft high, indicated that seedlings of this species were susceptible to even one fire. Other factors influencing fire susceptibility were size of the root crown, number of shoots produced after burning and their rate of growth, and combustibility of the dead stems; in particular, if numerous shoots appeared after a first burn the species was likely to be sensitive to later burns.
The thicket association studied appeared to have become established at a time when fires were less frequent or lacking. To restore open woodland, frequent burning is necessary initially, with preliminary slashing in dense thicket. Under local conditions, this approach is safe and economical. F.s.-J.L.M.