资料题：Low air pressure（　）。

Can trees talk? Yes - but not in words.Scientists have reason to believe that trees do communicate with each other.Not long ago, researchers learned some surprising things.First, a willow tree attacked in the woods by caterpillars changed the chemistry of its leaves and made them taste so awful that the caterpillars got disgusted and stopped eating them.Then, even more astonishing, the tree sent out special vapors - an alarm signal stimulating its neighbors to change the chemistry of their own leaves and make them less tasty.Communication, of course,doesn't need to be in words.We can talk to each other by a smile/frown, a shrug of shoulders, a gesture with our shoulders, or a gesture with our hands.We know that birds and animals use a whole vocabulary of songs, sounds, and movements.Bees dance their signals, flying in certain patterns that tell other bees where to find nectar for honey.So why shouldn't trees have ways of sending messages?