The effects of a changing climate on root respiration of woody plants in sugar maple forests and northern peatlands

Climate change will potentially impact C cycling in terrestrial ecosystems during the next century. Plant respiration uses a significant portion of CO2 fixed during photosynthesis, and predicted warmer future temperatures could result in an exponential increase in plant respiration, increasing the amount of photosynthate returned to the atmosphere as new CO2, and decreasing the amount of C sequestered in new plant biomass. One way a plant may counteract this C loss is through metabolic acclimation, a physiological downregulation of respiration at increased temperature. This study examined root respiration in an experimentally warmed sugar maple dominated northern hardwood forest in the Upper Peninsula of Michigan, United States. The objective was to determine if fine roots of these trees had the capacity to acclimate to warmer soil temperatures (+4 °C) and minimize the C loss from the ecosystem. This study was conducted from 2009-2011, and included a pre-treatment period from May 2009 through June 2010, with the initiation of treatments during late summer of 2010 and continuing throughout the growing season of 2011. Root respiration was measured biweekly throughout the growing season, at both ambient soil temperature for the sample date and at a reference temperature of 18°C. The pre-treatment period found no inherent differences between any of the future treatment 15 plots. Part of the experimental design consisted of an additional treatment of heat and water (ambient +30%), intended to maintain adequate soil moisture content for heated soil experiencing increased evaporative demand. The heat + water treatment allowed us to assess whether apparent acclimation due to soil warming was due to increased temperature or simply a reduction in respiration associated with drier soil conditions. During the treatment period we found down-regulation of metabolic capacity (respiration rate at the 18°C reference temperature) for the plots receiving the heat treatment. Much of this was due to drier soil conditions caused by heating, but when soil moisture effects were accounted for, there was still down-regulation of root respiration with heating, indicating a slight degree of acclimation. The combined effects of dry soil conditions and acclimation resulted in average root respiration for the heat and heat + water treatments being 6 and 26% greater, respectively, than in the control, which is far less than the 48% increase that would have resulted if a simple exponential increase had occurred for the 4°C increase in soil temperature.