Archaeopteris, an extinct tree that made up most of the forests across the earth in the late Devonian period, is now considered by scientists to be the first modern tree. According to a report in the journal Nature, earth's first modern tree and forests emerged around 370 million years ago. Ancient plants made it out of water 130 million years earlier but none were considered "true" trees.
These "true" trees developed certain characteristics that took them far beyond all other common plants in the Devonian. These trees developed rings which supported increased height and weight. The new trees developed bark that protected water and nutrient conducting cells and a supportive collar of extra wood that surround the bases of each branch.
Archaeopteris Illustration by UC Berkley - Museum of Paleontology