Response of ecosystem fine root respiration to soil warming in a sugar maple forest

Background/Question/Methods

Exponential increases in plant respiration could represent increased return of photosynthate to the atmosphere if acclimation does not occur, potentially negatively affecting NPP.  This study was conducted from 2009 to 2011 in a sugar maple dominated northern hardwood forest to determine the potential for sugar maple to metabolically acclimate fine root respiration to increased soil temperatures.  The experimental design consisted of twelve 10 x 10 m plots with four treatments replicated three times; control, heat (+4.5°C), water addition (+130% over control), and heat + water, with treatments initiated in September 2010. The water addition treatment was designed to maintain soil moisture in the heat + water treatment near control levels, helping separate effects of soil warming from the co-occurring soil drying that occurs in the heat only treatment.   Fine root respiration (<1 mm) was measured with an open-system infrared gas analyzer tri-weekly throughout the growing season.  Root biomass to a depth of 10 cm was determined in September 2011.  Our goal was to understand the effects of this experimental manipulations on ecosystem fine root respiration, the product of specific root respiration and root biomass, to improve our ability to model responses of ecosystem C cycling to climate change.  

 Results/Conclusions

Soil moisture greatly affected specific root respiration rates, but slight temperature acclimation was also apparent for treatments with heat addition (P=0.07).  These combined effects resulted in specific respiration for the heat and heat + water plots being 6 and 26% greater than the control, which is less than 54% increase that would be expected for a 4.5°C increase, with a Q₁₀ of 2.6.  Average fine root biomass was 367 g m^-2, with no significant treatment effects  (P=0.93).  Ecosystem fine root respiration, the product of specific root respiration and biomass, was 20 and 27% greater for the heat and heat + water treatments, respectively, than the controls.  Across a growing season of 122 days, ecosystem level fine root respiration was 157, 184, 188, and 200 g C m^-2 for the control, water, heat, and heat + water treatments respectively.  The increase in respiration for the heat and heat + water treatments indicates that sugar maple trees initially lose more C to root respiration in response to warmer soil conditions.  Potential longer-term adjustments that could alleviate this C loss, such as building new roots with a lower metabolic capacity or reducing root biomass, will be investigated during the 2012 growing season.