According to a professor at Pretoria University, the Acacia trees emit ethylene into the air whenever antelopes start grazing on their leaves. This travels up to 50 yards, warning other trees that herbivores are in the area. Once the alarm signal is given, the trees start producing leaf tannin in quantities lethal to the antelopes.
The professor studying this phenomenon discovered it as he was looking into the death of around 3000 South African antelopes.
From the Upcoming ueue, submitted by milos87.
This eerily reminds me of a Sigourney Weaver film, any of them.