Squirrels plant trees. ON ACCIDENT.
Despite the fact that they've yet to learn to look both ways before crossing the street, squirrels are actually pretty smart. They have an elaborate system for preserving their food. This includes burying nuts and acorns underground so that they can get to them much later. Sometimes if they don't get around to eating their buried treasure, the nuts grow into trees. This is actually really important because it allows for trees to grow farther away from other trees so that forests can spread.
Squirrels know exactly how well the nuts and acorns they find can stay preserved, which means that when they find nuts they know which ones to bury at which time and where. They also have a process to make acorns last longer: taking out the embryo!
They're also really sneaky. Give a squirrel some nuts and he'll dig several "dummy" holes for each actual nut that he buries. This throws off other animals watching them who would try to steal their food. Sometimes they even pretend to bury nuts to send their competition on a wild goose chase.