 The main reason is the different effect of the Ice ages — while in North America, species were protected in more southern regions along north–south ranging mountains, this was not the case in Europe.Global warming and rising carbon dioxide levels in the atmosphere may delay the usual autumn spectacle of changing colors and falling leaves in northern hardwood forests in the future, and increase forest productivity.Experiments with poplar trees showed that they stayed greener longer with higher CO2 levels, independent of temperature changes.