Phenology of belowground carbon allocation in a mid-latitude forest

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. It is generally assumed that above and belowground phenology is synchronous, but a number of recent studies show that there is wide variability. Some phenological studies suggest that root production peaks are offset from leaf out and shoot elongation. Related belowground processes such as root respiration and nonstructural carbohydrate accumulation may also be offset from root or shoot production as a result of tradeoffs in C allocation. Due to uncertainties in the seasonal pattern and magnitude of allocation to roots, we have collected measurements of root phenology for three temperate tree species at Harvard Forest in Petersham, MA: eastern hemlock (Tsuga canadensis), red oak (Quercus rubra), and white ash (Fraxinus americana). Bi-weekly to monthly measurements of root production, root respiration, and root nonstructural carbohydrate content are used to determine when roots are receiving C from aboveground and patterns of C use.

Minirhizotron and soil core data suggest that fine root biomass does not accumulate in a unimodal peak. In T.canadensis stands, fine root production peaks in late May, coinciding with green up and shoot elongation. In Q.rubra stands, fine root production begins in early June, about 3 weeks after leaf out and continues throughout the season in oscillating peaks. Average turnover times for Q.rubra and T.canadensis were 3.76 years and 6.83 years respectively. Standing root biomass for all stands fluctuates seasonally but with high spatial variability, with live fine root biomass averaging 210 ± 75.2 gC m^-2 in F.americana stands, 554 ± 241 gC m^-2 in Q.rubra, and 449 ± 172 gC m^-2 in T.canadensis. Root respiration for all stands increases with temperature from early May to mid-June, with an average seasonal Q₁₀ of 2.1. Nonstructural carbohydrates in F.americana and Q.rubra roots experience a slight drawdown from mid-May to late-June, coinciding with increases in root production and respiration. Nonstructural carbohydrates in all stands accumulate after June. The lack of synchronicity of these processes suggests that trade-offs in C allocation, in addition to abiotic factors such as temperature and soil water content, govern the timing of belowground processes.