Stem wood structure of four Ghanaian Khaya species

Abstract

Four closely related Ghanaian Khaya woods were studied for differences in anatomical, chemical and physical properties to enable clear identification o f the species. The species were Khaya cmthotheca (Ka), Khaya ivorensis (Ki), Khaya grandifoliola i (Kg) and Khaya senegalensis (Ks). Samples for the study were obtained from identified standing trees from eight Forest Districts in Ghana. Macroscopic, microscopic and ultrastructural features including physical and chemical features were examined qualitatively and quantitatively. Qualitative description of anatomical features indicated that the four species possess similar features with only a few differences in ray appearance of Ks and Ki. Ks rays were relatively more rounded and rich in gum deposits while Ki contained more o f smaller rays (uniseriate, biserite and triseriate) compared to the other species. Chemical test for colour reaction in sapwood and heartwood of the four species failed to show differences except with Bromcresol green. Ks heartwood at 12% m.c was stained yellowish-green by Bromcresol green whereas the other species were only stained by the green colour of the chemical. Some quantitative anatomical data further confirmed the close relationship of the species since reliable differences could hardly be found using features such as sapwood width and percentage; vessel diameter, length, and density. Also included were fibre length, diameter, lumen size and single wall thickness as well as ray frequency, height, width and height/width ratio. These features were either not significantly different with t-test of 95% confidence level, or varied considerably from juvenile to mature wood with extensive overlap between compared ranges. Hence these features were unsuitable for identification of the four species. However, fibre lumen/wall ratio, percentage multiseriate rays and relative density of the four species showed significant differences with negligible variation from juvenile to mature wood and no overlaps. It was found that the mean fibre lumen/wall ratio in Ks, Kg, Ki and Ka were 0.4, 2.4,4.5 and 4.8 respectively. Ki had the lowest o f percentage multiseriate rays averaging 64% while the other species ranged from 80-87%. Relative density range of Ka and Ki, i.e., 0.4-0.6 was classified as medium while that o f Kg and Ks, i.e., 0.6-0.8 was in the high density group. Therefore, a key for differentiating the four Ghanaian Khaya woods with a confidence level o f 99% was postulated based on fibre/lumen wall ratio, percentage multiseriate rays, relative density and ray appearance.