Increasing frequency of drought reduces carbon storage in an Eastern deciduous forest

Background/Question/Methods

Globally, deciduous forests in the Eastern US account for 15% of the terrestrial sink for atmospheric carbon dioxide (CO₂). These forests are predicted to experience increases in drought frequency over the next century that may decrease their ability to store carbon (C) despite other factors that may promote storage (i.e., elevated CO₂, longer growing seasons). At the Morgan Monroe State Forest AmeriFlux site in Indiana (MMSF), the net ecosystem production (NEP) of carbon has been measured using both eddy-covariance and inventory approaches since 1998.  We used the 15-year record of net ecosystem production coupled with a record of tree growth at MMSF to quantify the impacts of increasing drought frequency on C uptake and allocation. At the individual tree level, we determined the phenology of wood production and its impact on aboveground C allocation using bi-weekly diameter increment measurements on over 200 trees in the footprint of the eddy-covariance tower.

Results/Conclusions

Shifts in the water balance of the forest, particularly declines in soil moisture and increases in vapor pressure deficit decreased annual C uptake by 36% since 2004.  Across Eastern deciduous forests, we estimate that similar increases in drought frequency could decrease C storage by 12-36%.  Concurrent with the declines in NEP, we observed a shift in phenology of C allocation with declining soil moisture leading to an increasingly earlier end to the wood production season and less C allocated to wood.  Given that we observed no concomitant changes in the phenology of leaves over the same period, we speculate that the increased frequency of droughts over the past decade has increased the amount of C allocated belowground. This shift in allocation is likely to be consequential for ecosystem C balance, as belowground C pools turnover at rates an order of magnitude faster than C stored aboveground.  Collectively, our results suggest that enhanced drought frequency may alter the long-term storage of C in temperate forests.  Thus, increased drought frequency may lead to less C storage in forests by reducing both the annual uptake of C and the allocation of C to slow-turnover pools such as wood.