Lawrence Berkeley National Laboratory

Abstract Root growth, respiration, and exudation are important components of biogeochemical cycles, yet data on the timing and partitioning of C to these processes are rare. As a result, it is unclear how the seasonal timing, or phenology, of root C allocation is affected by the phenology of its component processes: growth of root tissue, respiration, mycorrhizal allocation, and exudation of labile C. The objective of this study was to estimate the phenology and partitioning of C belowground across the growing season in a midlatitude forest located in central Massachusetts. Fine and coarse root production, respiration, and exudation were summed to estimate a monthly total belowground C flux ( TBCF ) in two hardwood stands dominated by Quercus rubra and Fraxinus americana , respectively, and one conifer stand dominated by Tsuga canadensis . We observed significant stand‐level differences in belowground C flux and the partitioning of C to root growth, mycorrhizal fungi, exudation, and respiration. The deciduous hardwood stands allocated C belowground earlier in the season compared to the conifer‐dominated stand. The deciduous stands also allocated a greater proportion of TBCF to root growth compared to the conifer‐dominated hemlock ( T. canadensis ) stand. Of the three stands, red oak partitioned the greatest proportion of TBCF (~50%) to root growth, and hemlock the least. Low root growth rates in hemlock may be related to the arrival and spread of the invasive pest, hemlock wooly adelgid ( Adelges tsugae ), during the study period. Ongoing research in the eastern hemlock stand may yet determine how whole tree allocation and partitioning change as a result of this infestation.