Plants respond to their outside environment much more than people usually think. Day to day they can modulate a variety of variables from their leaf shape to the angle at which their stem is bent. Over the course of the year however much more is modulated, changing enough about the behavior of the plant to distinguish a discrete "Growing Season" during spring and summer and relative hibernation in the other half of the year. These changes are controlled by a variety of signals, but a paper published in Nature describes the significant effect that increasing CO2 levels have on the growing season.
Elevated CO2 levels have been attributed with a slight lengthening of the growing season already, and the experiment detailed in this paper hopes to determine what the growing season will look like in the future. Since a lot of water is lost by plants during CO2 uptake, increased CO2 concentrations allow the plants to lose less water per unit of CO2 taken in. Desiccation is a major threat to plants, so they will be more effective at growing as well as growing for longer. In plots of plants with raised CO2 levels, growing season was lengthened by 6.2 days. However, growing season is controlled by more than CO2 levels. The other main factor is temperature, and when the experimenters changed this as well the plot growing season lengthened by 14.2 days. Since temperatures are poised to rise as well as CO2 levels, this is an important metric.
While increased growing season may sound like a good thing at first, it can have very disruptive effects on the distribution and biodiversity of an area. In areas where freezing resistance is important, disruption of growing season can mean plants stay vulnerable for too long and end up taking freezing damage. Drastic changes in competitive advantage mean some plants will outcompete others quickly, decreasing biodiversity and increasing ecosystem fragility later.