Ecosystem root respiration in sugar maple forests increases with temperature within populations but not across populations: Results of a range-wide study

Background/Question/Methods

Common temperate tree species, such as sugar maple, have geographic ranges that encompass tremendous natural variation in climate.  Understanding the local adaptations that occur in response to these climatic differences could improve our understanding of potential large-scale responses to climatic warming.  Root respiration typically increases exponentially with temperature within a location and species, but there are little data regarding how this response may vary among populations and the mechanisms controlling population differences.  Our objective was to determine how carbon allocated to sugar maple root respiration varies with local climate at sixteen sugar maple dominated sites in the central U.S.  The sites span 10 degrees of latitude, across which mean annual temperature increases from 4 to 14 ^oC.  Fine-root (<1 mm) biomass and respiration at ambient soil temperature were measured during June, July, and September, 2014.  Ecosystem fine-root respiration was estimated as the product of specific fine-root respiration and biomass.  Models often allow plant tissue respiration to increase exponentially with temperature, with a Q₁₀ of 2.  This would imply 85% greater specific respiration rates at the southern sites than at the northern sites in July, when soil temperatures increased from 11.3 to 21.0 ^oC from north to south.

Results/Conclusions

Within each of the locations, root respiration rates were greater for sampling dates with warmer soil temperature. Across sites, however, root respiration did not increase from cooler to warmer locations for any of the three sampling periods, and were actually slightly lower at the southernmost sites (P = 0.01) in July, despite soil temperatures nearly 10 ^oC warmer than northern sites. This was associated with decreases in fine root N concentration (P = 0.003) and metabolic capacity (P < 0.001) from north to south. Fine-root biomass also decreased (P < 0.001) from north to south, which contributed to lower ecosystem fine root-respiration at the warmer, southern sites (P < 0.001).  It is not known if these differences among sites are a plastic response to environmental conditions that all sugar maple are capable of or if they are the result of genetically different ecotypes, tightly adapted to local climate, occurring at each site.  If the former is true, climatic warming will have little impact on belowground C allocation across sugar maple’s range.  If the latter is true, ecosystem root respiration would increase greatly at all locations, potentially altering ecosystem carbon balance and productivity.