I believe that this speaks to the subject of "knocking on wood" as a protection from bad things happening:

Janheinz John's book "Muntu" {USA, Faber & Faber, 1961 American edition; pps 99-102) speaks about the high regard traditional African philosophy has for trees {wood}.

In summary, John describes four categories that the Bantu use:
I.   Muntu: forces endowed with intelligence, including the Supreme
            God, spirits, living people, deceased people,and trees
            {plural Bantu}
II. Kintu: thing, including plants, animals, minerals, tools, etc.
            {plural: Bintu}
III. Hantu: place & time
IV. Kuntu: modality {forces such as Joy, Sorrow, Beauty}

"All being, all essence, in whatever form it is conceived, can be subsumed under one of these categories. Nothing can be conceived outside of them...The relationship of all these forces is expressed by their very names, for if we remove the determinative the stem NTU is the same for all the categories....

NTU is the universal force as such which, however, never occurs apart from its manifestations: Muntu, Kintu, Hantu,and Kuntu....

..plants, animals, minerals etc are all Bintu, as the plural of Kintu is expressed. None of these bintu have any will of their own, unless, like animals, they are given a drive by the command of the Bon Dieu. The bintu are 'frozen' forces, which await the command of a Muntu. They stand at the disposal of muntu, or 'at hand' for him. The only exceptions are certain trees,which like poteau-mitan in Voodoo are the 'sreets of the loas' *. In them the water of the depths, the primal Nommo, the word of the ancestors, surges up spontaneously; they are the road traveled by the dead, the loas, to living men; they are the repository of the deified. In many Bantu languages, therefore, trees belong, linguistically, in the Muntu class. Yet when a sacrifice is made to a 'tree', it is never the plant for whom the sarifice is meant, but the loas or ancestors, that is,the Muntu forces, that are journeying along it. As part of such trees, the wood from which sculptures are carved also have a special quality; the Nommo of the ancestors has given it a special consecration."
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* "loa"=spirit; also same as "orisa" {orisha}

Note also that musical instruments made out of wood such as drums, marimbas {thumb pianos} and balafons {wooden xylophones} are traditionally also considered sacred,living beings, and roads to the spirits.