The neighbor's apple tree can't exactly dial 911 if its branches are pruned too short, but some plants can indicate when they are "under attack" or in distress by releasing a chemical compound. This chemical release lets other creatures and similar plants know if an insect or other unwanted intruder is attacking.
A study by German scientists showed that tobacco plants released a chemical compound when being eaten by tobacco hornworm caterpillars. The combination of the chemical mixing with the caterpillars' saliva caused nearby predatory bugs to respond by attacking the nibbling insects.
Similar studies have shown other plants, including maple saplings and bean plants, also give off an almost imperceptible scent when under attack from hungry bugs. But the attack doesn't always have to be insect-related; the smell of cut grass is actually a distress signal, too.