Tree species diversity interacts with elevated CO2 to induce a greater root system response

Area of Science:

• Forest ecology
• Global change biology
• Biogeochemical cycling

Background:

• Rising atmospheric CO2 from land-use change and fossil fuels impacts forest carbon cycles.
• Previous studies show elevated CO2 increases fine root biomass in single tree species.

Purpose of the Study:

• Investigate the interaction between tree species mixture and elevated CO2 on root biomass and morphology.
• Assess how elevated CO2 affects forest ecosystems composed of multiple species.

Main Methods:

• A free-air CO2 enrichment (FACE) study exposed Alnus glutinosa, Betula pendula, and Fagus sylvatica to ambient or elevated CO2 (580 μmol mol⁻¹) for 4 years.
• Trees were grown in monocultures and a three-species polyculture.
• Fine and coarse root biomass, fine root turnover, and fine root morphology were measured at various soil depths.

Main Results:

• Elevated CO2 differentially affected fine root biomass and morphology by soil depth in monocultures.
• In polyculture, elevated CO2 enhanced coarse root biomass (to 20 cm) and fine root area index (to 30 cm).
• Total fine root biomass increased with elevated CO2, but species diversity did not significantly influence this response.

Conclusions:

• Forests show a positive response in total fine root biomass to elevated CO2, independent of species diversity.
• Current biogeochemical models may underestimate belowground responses to global change if based solely on monoculture data.
• Understanding mixed-species forest responses is critical for accurate carbon cycle modeling.