The six forests straddle a period in Earth history 306 million years ago that saw a rapid shift from an icehouse climate with big polar ice caps to a greenhouse climate in which the ice caps would have melted.
http://news.bbc.co.uk/2/hi/science/nature/7604721.stm

"The fascinating thing we've discovered is that the rainforests dramatically collapse approximately coincident with the greenhouse warming," explained Dr Falcon-Lang.

"Long-lived forests dominated by giant club moss trees almost overnight (in a geological sense) are replaced by rather weedy fern vegetation."

The next stage of the research is to try to refine the timings of events all those years ago, and work out the exact environmental conditions that existed. The thresholds that triggered the ancient collapse can then be compared with modern circumstances.

So global warming is a naturally occurring process?