Field measurements of root respiration indicate little to no seasonal temperature acclimation for sugar maple and red pine

Increasing global temperatures could potentially cause large increases in root respiration and associated soil CO2 efflux. However, if root respiration acclimates to higher temperatures, increases in soil CO2 efflux from this source would be much less. Throughout the snow-free season, we measured fine root respiration in the field at ambient soil temperature in a sugar maple (Acer saccharum Marsh.) forest and a red pine (Pinus resinosa Ait.) plantation in Michigan. The objectives were to determine effects of soil temperature, soil water availability and experimental N additions on root respiration rates, and to test for temperature acclimation in response to seasonal changes in soil temperature. Soil temperature and soil water availability were important predictors of root respiration and together explained 76% of the variation in root respiration rates in the red pine plantation and 71% of the variation in the sugar maple forest. Root N concentration explained an additional 6% of the variation in the sugar maple trees. Experimental N additions did not affect root respiration rates at either site. From April to November, root respiration rates measured in the field increased exponentially with increasing soil temperature. For sugar maple, long-term Q(10) values calculated from the field data were slightly, but not significantly, less than short-term Q(10) values determined for instantaneous temperature series conducted in the laboratory (2.4 versus 2.6-2.7). For red pine, long-term and short-term Q(10) values were similar (3.0 versus 3.0). Sugar maple root respiration rates at constant reference temperatures of 6, 18 and 24 degreesC were measured in the laboratory at various times during the year when field soil temperatures varied from 0.4 to 16.8 degreesC. No relationship existed between ambient soil temperature just before sampling and root respiration rates at 6 and 18 degreesC (P = 0.37 and 0.86, respectively), and only a very weak relationship was found between ambient soil temperature and root respiration at 24 degreesC (P = 0.08, slope = -0.09). We conclude that root respiration in these species undergoes little, if any, acclimation to seasonal changes in soil temperature.