THIS ARTICLE APPEARED IN THE DECEMBER 1997 TREE LIFE ISSUE – NO.214
Some time back an extract from the publication Beyond Supernature, by Lyall Watson, appeared in Tree Life. This extract gave an account of how southern African bushveld trees respond rapidly to heavy browsing pressure by producing extra tannins in their foliage as a defence mechanism. The tannins not only make the browse temporarily unpalatable, but also can even make it lethal. A bizarre twist to this research finding was the discovery that untouched trees close by also increase their tannin content within a very short time, apparently in sympathy with their damaged neighbours.
An even more bizarre discovery was reported in the National Geographic magazine of December 1989. Scientists have found that some plants emit ultrasonic acoustic emissions (ultrasonic chirps) when they are affected by drought stress, and the chirps increase as stress intensifies. Further experimental probing showed that bark beetles evidently recognize the distress signals and home in on the affected plants, while tending to leave “chirpless” plants alone.