In this study, we found that it is possible for ABW to survive under various environment conditions with high relative dominance. Different vegetation types were observed depending on the sample location (Fig. 4), and the vegetation surrounding ABW tree location significantly influenced their growth. Nainokwe site was significantly different from the 2 other sites in terms of tree species composition and growth form (Fig. 4, Table 3). Nainokwe site is mainly covered by wooded grassland, while open woodland covers larger areas of Kikole . Although there has not yet been an official report, Nanjirinji site could also be categorized into mostly open woodland because of its statistical similarity to the parameters of Kikole site (Table 3, Figs. 4, 5 and 6).
Generally, there are many low trees with lower branch height in wooded grassland compared to open woodland  (Table 3, Fig. 6). In particular, some ABW trees in Nainokwe showed relatively small DBH in conjunction with tree height compared to those of other sites (Table 3, Fig. 5). This forest had many juvenile ABW trees with small DBH and low height (Figs. 5, 6). Considering the diagnostic parameters listed in Tables 3 and 6, it seems that environmental impacts from forest parameters continuously influenced growth conditions.
On the other hand, the DBH of all trees in Kikole forest were significantly bigger than those of the other 2 sites, with an intensive number of mid-sized ABW trees, quite different from the Nanjirinji forest (Table 3, Fig. 5). This suggests that there might be a relationship between forest density and ABW regeneration. ABW has been known as a light-demanding species; thus, it might not regenerate under heavy closed vegetation [6, 39, 40]. In cases where the forest density is lower, ABW trees can also become multi-stemmed with smaller DBH and lower height. This is generally known as a typical physiological response. Trees in dense forests must compete for light, which places a premium on height growth, meaning that trees grow tall . It was suggested that the significant difference of DBH distribution between Nainokwe and other sites was a result of the natural ABW habitat. Kikole forest apparently has the appropriate conditions under which ABW trees can coexist with other species because of both tree density and the number of individuals of each species (Table 3).
Furthermore, forest conditions including vegetation type generally depend on environmental factors such as topography, climate, and human activities. Tree growth can also be impacted by environmental factors such as topography, resource availability, and previous disturbance [31, 32]. The abundance, distribution, and diversity of vegetation tend to be strongly influenced by the qualities of the physical landscape, with plant species arising from both physical and chemical characteristics of the land . Luoga et al.  reported that harvesting activity significantly affects the vegetation structure of woodlands, and the specific distribution of aged trees might be the result of clear-cutting of such trees . Banda et al.  also reported that the gradient of land protection has been predicted to influence forest ecosystems in terms of growth form, regeneration, and species richness. As a result, some potential factors, including human activities such as fire and harvesting, have not yet been studied here. Further investigation should be conducted in terms of vegetation transition by human activities to clarify the specific distribution of ABW trees in natural forest.
Ilunga Muledi et al.  reported a variety of soil factors in a Miombo forest, and that vegetation was related to soil factors. In this study, we found a variety of soil types at the 3 sites: from sandy to clay, and with or without CaCO3 and/or Fe nodules (Tables 4, 5). However, the results clearly suggest that ABW can grow in a wide variety of soil types regardless of their properties. In addition, in this study, dark-colored soils from CL to C soil texture observed in some plots in the Nanjirinji (Table 5), which might have better physical (better drainage and water-retention) and better chemical (more nutrients) properties.
In general, soil color depends on major inorganic components and the amount of organic matter, which determines the physical properties of the top soil. High clay content results in a high capacity for stocking organic matter, so that soil color darkens. Heavier clayey alkali-soil with high CaCO3 content seems to affect root extension into deeper soil layers. In contrast, sandy soil (S), which was observed in Kikole, might have disadvantages for plant growth due to poor nutrients and low water holding capacity. The soils of Nainokwe were similar to those of Nanjirinji, although their vegetation obviously differed. We concluded that ABW trees could grow under a variety of soil types, and even where other plants cannot grow well. It has been suggested that rooting of ABW trees is not affected greatly by the soil condition due to their coexistence with mycorrhizal fungi, which fixes nitrogen and is commonly known to radiate out 30–50 m by root suckers [39, 43]. The survival of ABW was apparently the result of adaptation to a wide variety of soil conditions despite their less-competitive behavior in high-diversity dense forest.
Recently, studies of the relationship between tree growth and Vs have reported that velocity depends on planting density, which also influences tree-form properties such as bending, multi-stems, cracks, and decay. . A positive relationship was observed between MOE and Vs of the living coniferous tree, Hinoki (Chamaecyparis obtusa Endle.) [38, 44, 45], and another positive relationship between wood hardness and Vs has been observed by using a stress wave timer in some tropical hardwoods (Nectandra cuspidata, Mezilaurus itauba and Ocotea guianensis) . In addition, Vs, wood density and ultrasonic velocity which is another non-destructive measurement has also positively related to MOE of some planted hardwood trees (Melia azedarch, Shorea spp. and Maesopsis eminii) [47, 48]. Although wood density of the measured trees has not been evaluated in this study, the significant difference of wood density might result in the different Vs as shown in such current studies for other species. Evaluation of wood density thus should be needed for further discussing tree growth and wood quality. Vs is affected by defects such as cracks and pith including holes, because the stress-wave principally selects the shortest internal propagation route. Therefore, propagation time would be delayed by the existence of any serious defects between sensors. However, the physical quality of ABW was not significantly related to appearance conditions in this study, because there were only poor correlations between Vs and the appearance grades (Fig. 9), furthermore, Vs was also poorly correlated with appearance grades even in case of further analysis for only over middle grades.
African blackwood trees in Nainokwe site obviously had a worse appearance than those in the other 2 sites with the lower parameters in this study (Fig. 7, Table 3). This might have been due to the co-relationship between the environmental conditions and tree growth, although their growth rate have not completely evaluated yet. Trees on fertile, well-drained soils such as loam can grow rapidly, thus resulting in high density forest , but promoting fluting . Fluting severity has been positively correlated with tree growth and branch height in Western Hemlock trees (Tsuga heterophylla) . Furthermore, disturbances such as clear-cutting and mechanical stress can also induce more fluting . Karlinasari et al.  also showed that negative correlations were found between wood quality traits (wood density, dynamic MOE and ultrasonic velocity) and tree volume at the planting sites of same aged trees. Since the stress-wave velocities (Vs) were not significantly different among survey sites with a variety of soil/landscape conditions (Table 6, Figs. 8 and 9), our findings suggest that the dynamic physical properties of ABW trees are not related to growth conditions in the natural forest, which is generally influenced by various external factors.