Andrew Richardson of Northern Arizona University set up the experiment in bubble-like chambers in Northern Minnesota which mimic a range of possible future conditions on Earth. The chambers are heated and exposed to elevated carbon dioxide, but are still open to rain and snow.
Richardson set up digital cameras inside the chambers to monitor the greenness of the trees. He says, "What we could see was that in the warmest chambers things were starting to get green earlier in spring, and they stayed green later in the autumn," stretching the growing season by nearly a week for every degree Celsius of warming.