A warming climate could alter C balance within trees through a number of mechanisms, including potential increases in plant tissue respiration at higher temperatures and reduced C assimilation due to lower soil moisture availability in a warmer climate. A 4+ year soil warming experiment in a mature Michigan sugar maple forest was used to assess the degree to which root system respiration increased in response to warming and to examine the impacts of warming-induced soil moisture declines on growing season sap flux. The experiment utilized a factorial combination of increased soil temperature (+4 to +5 oC) and supplemental moisture additions (+30% of average ambient growing season precipitation). The moisture additions were intended to offset the effects of experimental warming on soil moisture availability, enabling separation of the influences of temperature and moisture on observed responses. Specific root respiration and root biomass were assessed for three different root diameter classes (<1, 1-2, and 2-10 mm) from three soil depths (0-10, 10-30, and 30-50 cm). Xylem sap flux was measured on 7 to 9 sugar maple trees per treatment continuously from mid-May to mid-October over three growing seasons using variable-length heat dissipation sap flux sensors based on the design of Granier.
Results/Conclusions
Specific root respiration increased with temperature, but partial acclimation of surface (0-10 cm) fine-root (< 1 mm) respiration occurred, causing the increase in respiration to be lower (14 to 26%) than would be expected for a 4 to 5 oC temperature increase (circa 55%). Acclimation was greatest when ambient soil temperature was warmer or soil moisture was low. There was no evidence for acclimation of fine root respiration at deeper soil depths, where warming caused respiration to more than double. There was evidence of acclimation for 1-2 mm roots at the 0-10 cm depth, but not for the larger diameter roots at any of the three soil depths. Root biomass was not altered by soil warming or moisture addition. Despite partial thermal acclimation in surface fine roots, there was an overall increase in the amount of C returned to the atmosphere by the root system, potentially reducing C availability for biomass production. Sugar maple transpiration decreased for the heated treatments in all years and increased with water additions. These responses were mediated by treatment effects on soil moisture. Reduced sap flux in trees growing in the experimentally warmed soils suggests that net C gain likely was reduced as well.