Phenology - the timing of seasonal life cycle events such as flowering or leaf opening - is widely recognised as one of the most sensitive indicators of ecosystem response to environmental change. Numerous previous studies have show that growing season length (the period between leaf opening and leaf browning) has been extending in recent decades, a phenomenon linked to changes in climate, particularly spring warming.

This new study is interesting as it shows that in addition to temperate, CO2 itself can increase growing season length. In water-limited ecosystem the end of the growing season is often determined by drought stress - i.e. when the soil water runs out. Increased CO2 increases the "water use efficiency" of plants, which can help to reduce ecosystem water use and consequently extend the growing season.

The study has important implications for the response of ecosystems to future climate change and increased atmospheric CO2. However, it is probably only relevant in water-limited ecosystems, which are less important to the global carbon cycle than tropical or boreal ecosystems.