Contrasting impacts of gypsy moth and southern pine beetle in oak and pine dominated forests in the mid-Atlantic region

Background/Question/Methods

Over the last decade, outbreaks of Gypsy moth (Lymantria dispar L.) in oak-dominated stands and Southern Pine Beetle (SPB; Dendroctonus frontalis Zimmermann) in pine-dominated stands have far exceeded the area impacted by wildfires or wind damage, previously the major disturbances in forests on the mid-Atlantic Coastal Plain.   How are these “new” disturbances altering forest structure and functioning?   How will they affect future forest composition?  We are using forest census techniques and eddy covariance measurements to understand the ecological consequences of invasive insects in upland forests in the New Jersey Pinelands.  Comparative tree, sapling, seedling and understory census plots based on USFS Forest Inventory and Analysis (FIA) protocols were installed in insect-impacted and control stands (n = 55) throughout the Pinelands National Reserve in New Jersey.  Eddy covariance measurements in oak- and pine- dominated stands made pre- and post-disturbance provide key information on carbon and hydrologic fluxes.  Foliage of pines, oaks and understory shrubs were analyzed for nutrient content, and litterfall was collected at representative oak- and pine-dominated stands.    

Results/Conclusions

In oak-dominated stands, defoliation by Gypsy moth and differential mortality has favored canopy trees with lower foliar nitrogen content such as chestnut oak (Q. prinus L.), the release and regeneration of pines in the understory, and an increase in understory shrub leaf area and productivity.  In denser pine-dominated stands, SPB infestations have resulted in > 90 % reduction in pine tree and sapling basal area and biomass, while having relatively little effect on oak and hardwood trees and saplings. Defoliation and mortality initially reduced the capacity for these forests to sequester CO₂.  Reduced leaf area resulted in lower CO₂ uptake during the growing season, and increased litter and coarse wood initially increased ecosystem respiration.   However, long term changes in species composition may have little impact on forest carbon dynamics; oak-dominated stands are more productive and sequester greater amounts of CO₂ during the growing season, but because pine-dominated stands retain a single cohort of needles through the winter and are productive earlier in the spring and later in the fall, annual rates of CO₂ sequestration are similar among forest types.  Because insect driven disturbances are both resetting and accelerating successional sequences, tree diversity is maintained at the landscape scale.