Effect of phloem sap deposition on properties of sapwood in Pterocarpus angolensis (Mukwa) following bark stripping by elephants in Chobe forests reserves in northern Botswana

Abstract

An investigation was carried out to assess properties of sapwood in Pterocarpus angolensis following debarking by elephants in Botswana. The objective of this study was to assess the effect of phloem sap deposition on properties of P. angolensis wood in northern Botswana. Wood samples were obtained from bark stripped trees in the Chobe district. Ten samples were obtained for the different woods types each for assessing density, compression and modulus of elasticity (MOE) and wood colour. Wood colour was evaluated using the Munsell Color chart Wood density was derived by dividing wood sample weight by its volume. Compression and elastic properties were determined using a Llyods intrumentslO kN materials testing system according to British Standard 373 for testing small clear wood samples. Debarked stems yielded red discoloured sapwood. The discoloured sapwood yielded the highest density at 537.7± 17.26 kg m’3 where the lowest density was recorded in the sapwood (F=25.98 and P<0.05). Compression stress was highest in phloem sap discoloured wood of 59.38±2.46 MPa, followed by the heartwood at 55.67±1.87 MPa and the lowest in sapwood at 39.66±l.61 MPa (F=30.64 and P<0.05). Modulus of elasticity showed no significant differences in between samples obtained from the different parts. The discoloured sapwood however yielded the higher MOE values. Overall, the wood with bark stripped by elephants and discolouration showed superior properties. It is therefore recommended that phloem sap discoloured wood be used for structural timber.