Influence of root biomass and specific respiration rates on variation in ecosystem level fine root respiration among forest types

Background/Question/Methods

Fine root respiration utilizes a significant proportion of belowground C allocation and thus also contributes greatly to ecosystem soil CO₂ efflux. We sought to better understand the factors controlling variability among ecosystems in this important C flux by examining relationships of ecosystem fine root respiration to specific root respiration rates, fine root biomass, and fine root N. Specific root respiration rates were measured periodically during the growing season of 2010 on excised root samples from seven common Lake States forest ecosystem types (sugar maple, red pine, red oak (moraine and outwash soils), trembling aspen, eastern hemlock and jack pine). Measurements were made at both ambient soil temperature and a reference temperature (18° C) using an open system infrared gas analyzer. Specific root respiration rates (nmol CO₂ g^-1 sec^-1) were then extrapolated to an ecosystem respiration level (µmol CO₂ m^-2 sec^-1) based on estimates of fine root biomass from the samples. Root N was measured for all samples using an elemental analyzer.  

Results/Conclusions

Sugar maple (Acer saccharum) on a productive moraine demonstrated the highest ecosystem fine root respiration flux, while jack pine (Pinus banksiana) on a glacial outwash plain had the lowest ecosystem fine root respiration. Patterns among ecosystems in fine root respiratory CO₂ efflux were dependent on both specific respiration rates and biomass. Sugar maple had the highest root biomass, which contributed to it also having the highest ecosystem fine root respiratory flux. Trembling aspen had the overall highest specific root respiration rates, but its ecosystem root respiration was lower than that for sugar maple due to a lower root biomass. Jack pine had the lowest specific root respiration rate and also the lowest root biomass. Across ecosystems, specific root respiration rates were significantly correlated with fine root N concentration (P = 0.002). The most productive ecosystems (sugar maple, aspen and eastern hemlock) tended to have higher ecosystem fine root respiration while the less productive ecosystem had lower ecosystem fine root respiration. Further measurements in 2011 will allow for a cross-site comparison of ecosystem respiration to nutrients required for annual foliage production.