Root and mycorrhzial acclimation to increased temperature in experimentally warmed northern forests

Background/Question/Methods

In forests, a significant portion of assimilated C is allocated to the respiration of roots and their associated mycorrhizae.  If root system respiration does not acclimate to warmer soil temperatures, the amount of C respired could increase greatly, leaving less C for other uses, including aboveground growth.  We examined root and mycorrhizal respiration and biomass in northern forests with 0 to 20 years of experimental soil warming (+ 5 ^oC).   Our overall objective was to assess the degree to which temperature acclimation occurs in root systems and determine if such acclimation is a short-term, direct physiological adjustment to warmer temperatures (days to months) or a longer term response to changes in nutrient, moisture and C availability, and mycorrhizal status as the ecosystem adjusts to long-term warming (years).  Study sites included sugar maple forests in Michigan, with warming initiated in 2010, and mixed hardwoods at Harvard Forest that have been warmed since 1991, 2003 and 2006.  Measurements of specific root respiration were made periodically throughout the growing season on excised roots for three years at each location.  Fungal hyphal respiration was measured on hyphae collected using mesh ingrowth bags in 2011 and 2012.

Results/Conclusions

In Michigan sugar maple forests, partial temperature acclimation occurred for fine-root respiration in the first few years of warming.  Tests of potential mechanisms suggest that this was due, as least in part, to adenylate control.  Through this mechanism root respiration would be down-regulated to match the work required of the root system for nutrient uptake and assimilation.   Dry conditions created by soil warming also significantly affected root respiration.   Fine-root biomass was unchanged, as a result ecosystem fine-root respiration was slightly elevated.  In longer-term warming at Harvard Forest, fine-root specific respiration in warmed soil did not show evidence of thermal acclimation and was higher than that for the control.  However, overall carbon allocation to fine-root respiration in warmed soil was still constrained, in this case by a compensating decrease in fine root biomass.  At both study locations, mycorrhizal hyphal respiration and biomass tended to decrease with soil warming, leading to reduced overall mycorrhizal respiration in warmed soil.   Many models allow respiration to increase exponentially with temperature.  Based on our consistent findings of constraints on root/mycorrhizal system respiration, model overestimates of the C flux associated with root and mycorrhzial respiration are likely, which could lead to underestimates of future net primary productivity.