I know of Wade Davis's work only at second hand. My completely untutored guess is that detailed plant lore (also called wortcunning) such as Wade Davis studies can take generations of trial and error to develop, so some of it, in some societies, might resemble the "ancient" orally transmitted traditions that MudPuppy wanted to study. Archaeological study of pollen samples can be used to establish a chronology for the use of certain plants; other archaeology might find circumstantial evidence for rituals involved with the use of plants. Still other archaeology might provide evidence that the people using the plants today are the cultural successors of people who used the same plants in the same way in the past. So it might be possible, though not easy, for an interdisciplinary study to trace the non-literary transmission of wortcunning traditions through the ages. But this is, I repeat, just a guess.