Reduced non-structural carbohydrate concentration and temperature acclimation in sugar maple roots following experimental warming

Background/Question/Methods

Temperature acclimation of fine root respiration in a sugar maple dominated northern hardwood forest was found to occur during the first 1.5 years of experimental soil warming.  The mechanisms underlying the observed acclimation could include both substrate limitation and adenylate control.  Understanding the mechanisms involved and the time steps over which they operate could improve our ability to accurately model belowground C allocation under future climatic conditions.  The objective of this study was to examine total non-structural carbohydrates (TNC) and respiration in fine roots (<1 mm) from a natural sugar maple forest that has been experimentally warmed since 2010 to assess whether substrate limitation contributed to respiratory acclimation. The fully factorial design consists of control, heat +4°C, water +120% ambient, and heat + water with added water used to offset drought effects. Fine root respiration was measured at a reference temperature of 18°C as a measure of the degree of acclimation present in the samples.  Total sugars were measured colorimetrically using a phenol-sulphuric acid method after sugars were extracted with hot ethanol.  Remaining starch was then enzymatically converted to glucose and measured colorimetrically.  Decreased TNC in roots from the heated plots would indicate that substrate limitation might be contributing to acclimation. 

 Results/Conclusions

Fine root respiration at the reference temperature was 27% and 39% lower for the heat + water and heat treatments, respectively, than for the control, indicating temperature acclimation had occurred.  Similarly starch (P = 0.002) and TNC (P = 0.076) were significantly lower in treatments receiving soil warming.  These data indicate that the reduced fine root respiration that is occurring on the heated plots may be explained by decreases in TNC.  Acclimation may allow the amount of C used by the root system to remain in balance with the work (nutrient uptake, assimilation, and transport) required of fine roots to maintain an optimal balance of above- and belowground resource acquisition.  At the ecosystem level, fine root respiration remained similar to that in the control treatment, in contrast to what would be predicted by the application of exponential temperature response functions to root respiration.