According to this article, the Acacia tree has a chemical defence system which leads to the release of ethylene in the surroundings when a herbivore grazes on it. This leads to an increase in tannin concentration in the leaves not only in the plant where the herbivore is grazing but also in the acacias surrounding 45m.
The question is, what prevents acacias to store tannins beforehand? Wouldn't the giraffes totally avoid them then? Tannins aren't poisonous by themselves (correct me if wrong).