Elevated atmospheric CO2 mitigates water shortage in Pinus only under severe drought stress

Background/Question/Methods

In the central European Alps, Pinus sylvestris as the dominant tree species of low elevation forests shows abnormal mortality due to climate change effects. To maintain ecosystem services of these forests on the long run, assisted migration of more drought adapted Mediterranean provenances or species is envisaged. In this context, we exposed seedlings of the three pine species Pinus sylvestris, Pinus nigra and Pinus halepensis to elevated atmospheric CO (eCO₂) and artificial drought in a common garden experiment located at the bottom of central Alpine Valais valley. We sowed seeds of pines originating from dry places in Central Europe and the Mediterranean in boxes and applied a fully crossed combination of ambient (control) and eCO₂(FACE) and wet (control), moderate and severe drought during the second growing season. We measured stomatal conductance during the course of one day in the most water-limited time of the year. After 2 years, we excavated the plants and measured root and shoot biomass. Non-structural carbohydrate (NSC) content of the seedlings before and after the second year's severe drought was measured and is quantified regarding the seedling's carbon limitation. 

Results/Conclusions

The second year's severe drought stress resulted in a significantly lower biomass (28% to 49%) of P. sylvestris and P. nigra, but not of P. halepensis. The latter was also able to maintain a significantly higher rate of transpiration under the severe drought treatment, compared to the other species. Elevated CO₂ partly mitigated severe effects of drought in P. sylvestris and P. nigra, with resulting higher shoot and root biomass of up to 140% compared to the control (p < 0.01). In contrast, P. halepensis did not profit from eCO₂. At moderate drought, eCO₂ had no significant effect on any of the species' biomass. By the end of the growing season, soil humidity in the boxes exposed to eCO₂ had increased slightly but significantly (0.015 cm³/cm³) in comparison to boxes with ambient conditions, an effect that we account for a better water use efficiency under eCO₂. NCS contents of the plants are in line with the findings above. We conclude that elevated CO₂ mitigates water shortage only in severely, and not in moderately drought stressed pines. Whereas young pines need to resist moderate water shortage by means growth reduction, eCO₂ protects from further desiccation under severe drought.