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From the artist's web page:
A tree's year rings are analysed for their strength, thickness and rate of growth. This data serves as basis for a generative process that outputs piano music. It is mapped to a scale which is again defined by the overall appearance of the wood (ranging from dark to light and from strong texture to light texture). The foundation for the music is certainly found in the defined ruleset of programming and hardware setup, but the data acquired from every tree interprets this ruleset very differently.
And that's how you get a tree to play it's song. Cool stuff!