Bush-Control Studies in the Drier Areas of Kenya. IV. Effects of Controlled Burning on Secondary Thicket in Upland Acacia Woodland

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Abstract
Observations on 2 burning regimes extending over 3 years showed that burning can be a practical method of clearing thicket species in Acacia woodland under medium rainfall/altitude conditions in Kenya and that the susceptibility of plants to fire depends as much on burning regime as on species. The 5 main constituent species, each represented by about 20 selected bushes, were equally tolerant of 1 fire, but differed in susceptibility to subsequent fires. A. brevispica was tolerant of 3 annual burns but only semi-tolerant of burning in 1958 and 1960; whereas Aspilia sp. was tolerant of the latter treatment but sensitive (40% kill) to the former, and Maytenus putterlickioides was semi-tolerant (30% kill) of both treatments. Both Acalypha fruticosa and Crotalaria saxatilis were intolerant of 3 annual burns (70-80% kill), but Acalypha was sensitive to the 2 burns treatment (60% kill) while Crotalaria was tolerant. Only in Maytenus are plant size and fire susceptibility associated (all plants killed were over 8 ft tall), but counts in supplementary quadrats of complete populations of Aspilia, in which half were less than 1 ft. tall, indicate that seedling plants of this species are intolerant of even one fire. Other factors controlling fire susceptibility are the size of the root crown, the numbers of shoots produced after burning and their rate of growth, and the combustibility of the dead stems: in particular, if numerous shoots appear after a 1st burn the species is likely to be sensitive to later burns. The thicket association studied became 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 economic.

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