Effects of water level on specific root respiration and biomass of Chamaedaphne calyculta, Larix laricina, and Picea mariana in a poor fen peatland

Background/Question/Methods

Peatlands comprise 3% of the Earth’s terrestrial surface yet account for one third of its soil carbon stock. They are largely considered sinks for atmospheric carbon dioxide, but some have become sources. Carbon flux from peatlands has been studied primarily on an ecosystem level, and less is known about the contributions of individual components on ecosystem carbon flux. It therefore can be difficult to determine if changes in peatland soil carbon efflux are due to changes in decomposition rates, root respiration of existing plants, or contributions from root biomass of encroaching woody species.  This study examined specific respiration rates for fine roots (<1 mm) of tamarack, black spruce, and leatherleaf in drained, wet and control areas of a poor fen peatland in the Upper Peninsula of Michigan. Fine root samples were excised from lateral roots of known individuals and measured for respiration rate using CIRAS IRGAs.  Additional roots were sorted by species from peat cores to a depth of 25 cm to assess biomass.  All root samples were returned to the laboratory for determination of dry weights and C and N concentrations.  Ecosystem root respiration rate was calculated as a product of measured specific root respiration and root biomass.

Results/Conclusions

Specific root respiration (nmol CO₂ g^-1 s^-1) was 31% greater for the wet plots than controls, and little changed in the drained plots.  Woody fine root biomass was 215, 182, and 131 g m^-2 for the drained, wet, and control plots respectively of which 16%, 5%, and 10% consisted of tree roots. Ecosystem root respiration was greater in both wet and drained treatments (0.31 and 0.33 μmol CO₂ m^-2 s^-1) than in the control (0.17 μmol CO₂ m^-2 s^-1).  All three species contributed to these increases in ecosystem root respiration.  This poor fen peatland clearly responded to changes in the water table, but, contrary to our hypothesis, both the drained and wet areas increased in total carbon efflux from roots.  If woody tree encroachment continues in the drained areas, contributions from tree root biomass may continue to increase in importance.  We suggest care in using smaller chambers to examine net ecosystem exchange in peatlands for which trees are encroaching, as chambers that do not fit over whole trees will capture root respiration from adjacent trees but not other components of the trees’ net carbon exchange.  This could lead to overestimates of the contribution of peat decomposition to chamber-measured NEE.