Phenology of belowground carbon allocation in a mid-latitude forest

Background/Question/Methods

Annual forest productivity and carbon storage are affected by the amount and timing of carbon allocated belowground. Despite clear relationships between some climate factors (e.g. temperature) and NPP, there are still large gaps in our understanding of the partitioning between above and belowground C allocation. Researchers have long assumed that above and belowground phenology is synchronous, but recent studies show that there is wide variability.  Some phenological studies suggest that root production peaks are offset from shoot production, potentially affecting current estimates of belowground carbon storage.  We have collected measurements of root phenology for three monodominant stands at Harvard Forest in Petersham, MA: eastern hemlock (Tsuga canadensis), red oak (Quercus rubra), and white ash (Fraxinus americana). Monthly measurements of biomass growth, root respiration, and root total nonstructural carbohydrate content are used to determine when roots are receiving C from aboveground and patterns of C use.

Results/Conclusions

Monthly measurements of fine root biomass from 0-15cm of the mineral horizon during the 2011 growing season suggests that fine root biomass does not accumulate in a unimodal peak. Fine root biomass peaks in early April for F. americana and Q. rubra, but in both April and August for T. canadensis. In order to achieve finer temporal scale resolution and to calculate fine root growth and turnover rates we are installing a total of 12 root observation windows at Harvard Forest, as well as collecting images from 13 root viewing tubes using a BTC-100x high magnification minirhizotron camera system. Monthly measurements of root respiration made with a closed path infrared gas analyzer do not show a strong seasonal peak for any stand type in 2011, suggesting that some seasonal acclimation to temperature may be occurring at this site.