Thank you all for your answers,
I'm not a big fan of selling things or economic reasoning. But I must admit, reading one of your posts, that assessing the value of things may give hints about feasability. For instance, if usable yew truncks are rare and need decades of growth, they will attract "looters" and it will be nearly impossible to proctect them for longer than a generation or two. (In our modern societies). And this type of reasoning may probably be applied - with some adaptation - to ancient societies and prehistoric/primitive contexts (my main interest).
The question I try to understand is related to the alleged devastation of yew "forests" in europe in medieval times, in response to the demand for bow staves. There seem to be no mention of the management of the population of yew trees, only savage exploitation. (not so extraordinary, if we compare with the modern, commercially motivated, devastation of the 3rd world's forests).
Alternatively, we can read that indian tribes (Nevada, ...) have managed the natural ressources they used for bows.
So, probably, repetitively in prehistory, during numbers of generations, there has been methods for cultivating and managing yew trees in order to obtain clear straight bow staves, continuously. This seems technically very feasible (according to the photos you linked to, and to some of your posts) : the trunks of a yew tree can be cut (not all at once) and the tree continues to produce new trunks, and in shady environnements they grow naturally knotfree and straight.
I've seen a documentary about a traditional way Japanese exploit the wood of Chestnut trees - not for bows, but for cultivating shiitake - : they cut some trunks one meter above the soil, letting each tree well alive and ready to produce more trunks.
So this attitude of not killing a tree, but "encouraging" it to produce new trunks is something that probably existed in numerous cultures. (totally the opposite of what we do nowadays with most species of trees : killing the trees, extracting and burning the roots, then replanting).