Root growth and root respiration of a perennial grassland subjected to future climate change and extreme event

Future climate scenarios suggest not only an increase in atmospheric CO2 concentration and air temperature, but also more frequent and severe extremes events, such as summer heat wave and severe drought. An increase of atmospheric CO2 may improve plant growth through its primary effects on both leaf photosynthesis and stomatal conductance. The aim of this work was to test the hypothesis that elevated CO2, combined with air warming and severe soil drought, improves root growth and root respiration through its effects on leaf photosynthesis and plant transpiration. Forty-eight grassland monoliths coming from an upland site in France were divided in 12 replicate units and exposed, from May 2010 until December 2011, to a future scenario reproducing air temperature and precipitation expected for the period 2040-2060. Since January 2011 a CO2 enrichment of +180 ppm was applied on 6 units. In addition, during summer 2011 a heat wave and a severe drought stress were also applied to half of the units. Root growth rates and below-ground net primary production were estimated with the in-growth core method. Respiration of roots was measured before, during and after the extreme event with Licor6400-09 chamber system. Below-ground net primary production and root growth were significantly higher under elevated CO2 combined with the extreme summer drought but not under current water supply. In addition, root morphology and root respiration remained unchanged by the treatments. Our results show that elevated CO2 alleviate the effect of a severe summer extreme event and promote new root growth, essential for plant recovery.