Impacts of the emerald ash borer (Agrilus planipennis) on forest carbon stocks, tree physiology, and forest ecosystems

Background/Question/Methods

The accidental importation of the invasive emerald ash borer (EAB) (Agrilus planipennis) into the United States in the 1990’s has already impacted millions of Fraxinus trees in the Great Lakes region. To date no broadly effective management strategy has been identified. Continued EAB spread poses a threat to the local and regional abundance of Fraxinus trees and associated temperate forest ecosystems. Despite this threat, the impacts of EAB on ecosystems are not well understood. Here we examined the effect of EAB on nutrient cycling at regional, local, and individual tree scales. We used the USDA Forest Inventory and Analysis database to quantify the potential impacts of EAB on Fraxinus spp. carbon stocks in the US. To investigate the impacts of EAB on tree physiology, 29 Fraxinus trees from a single stand were identified, canopy health was rated, and leaf tissue was excised from the canopy and analyzed for carbon and nitrogen isotopic composition. Trees were felled, bark peeled and EAB larval gallery cover, an assessment of larval activity and density, was quantified. Finally, to investigate the local feedbacks of EAB induced tree mortality on soils; we assessed soil biogeochemical properties in 28 permanent plots established in 6 mixed deciduous forests.

Results/Conclusions

Our results highlight the potential consequences of EAB on forest carbon stocks, tree physiology and soil biogeochemical properties in temperate forest ecosystems. Fraxinus constitutes >5 % of the standing tree carbon mass in the Great Lakes region and is an important component of the carbon storage and accrual of these temperate forest ecosystems. Forests in the US contain over 303 TgC in standing aboveground Fraxinus trees, a quantity roughly equivalent to ~2 times the quantity of carbon annually sequestered by forest vegetation in the US. Reductions in leaf area associated with tree mortality will lead to temporary declines in regional net primary productivity. We found significant positive correlations among the carbon isotopic composition of leaf tissue, EAB larval gallery cover, and tree canopy health, which suggests that the mechanism by which EAB feeding behavior causes tree mortality is by inducing chronic water stress, and by increasing fine root mortality. Results from the chronosequence study investigating soil biogeochemical properties will also be discussed. Impacts of widespread tree mortality on soil biogeochemistry may have lasting impacts on forest successional dynamics and productivity at the patch and landscape scales.