Changes in root traits explain patterns of biomass allocation in response to N, CO2, and diversity

Background/Question/Methods

Roots are responsible for supplying where water and nutrients to aboveground tissues, and represent a large component of carbon inputs and storage in ecosystems. The amount of biomass plants allocate to this belowground interface responds to a range of conditions.  The ‘functional equilibrium’ hypothesis predicts that plant biomass is allocated to capture the most limiting resources.  Many studies have investigated biomass partitioning in relation abiotic and biotic factors, but these have mainly been short-term studies carried out on single plants or monocultures.  We used a long-term experiment manipulating N, CO₂, and diversity for 15 years in a mesic grassland to test the Functional Equilibrium hypothesis.  We measured aboveground and belowground standing biomass and a suite of root traits to investigate the mechanisms of biomass partitioning in response to important climate change variables.

Results/Conclusions

N addition, diversity, and their interaction all had significant effects on the amount of biomass allocated to the Root Mass Fraction (RMF).  At low diversity, plots exposed to elevated CO₂ had higher RMF than the other treatments, but at high diversity, the plots receiving additional N had the highest RMF compared to all other treatment and diversity levels.  N addition generally reduces the RMF in plants, and so the response of high diversity plots to N addition seems counterintuitive.  Here, we discuss changes in root morphology and physiology that help explain this apparent paradox.