That was exactly my thought: that the growing density of vegetation is a function of higher CO2 levels in the atmosphere. Like watering your plants a little extra.
Of course no one dares say this in the media or the scientific community. For them, increasing forest density is somehow an inexplicable mystery.
I did a quick search, and found a few articles, but that is not publicized much.
Global warming makes trees grow at fastest rate for 200 years
2 Feb 2010
Forests in the northern hemisphere could be growing faster now than they were 200 years ago as a result of climate change, according to a study of trees in eastern America.
Higher concentrations of carbon dioxide and extended growing seasons could be favourable for agriculture in some parts of the world, mainly in the northern hemisphere. The study in Maryland suggests that the extra growth in trees could help to act as a more efficient carbon “sink”, which could offset the carbon dioxide being added to the atmosphere by the burning of fossil fuels.
Iowa supposedly has more trees than before settlement, but I’d have to believe that has more to do with farmers needing wood and fruit, townspeople wanting shade, and the draining of the great potholes and marshes left by the Ice Age.