Insect invasion and forest resilience: Recovery of C and N dynamics following defoliation by gypsy moth (Lymantria dispar L.)

Background/Question/Methods

Invasive insects alter forest carbon (C) and nitrogen (N) cycles through their effects on leaf area and litterfall of high-N leaf fragments and frass during the growing season. Rates of recovery following infestations are not well characterized, but have important implications for assessing long-term dynamics and for the restoration of damaged ecosystems. We studied the recovery of C and N cycling following complete defoliation of the canopy and understory by gypsy moth (Lymantria dispar L.) during the summer months of 2007 in an oak-pine forest in the New Jersey Pine Barrens. We used eddy covariance to measure fluxes of CO₂, and biometric measurements to quantify litterfall and litter decomposition pre- and post-defoliation (2003-2008). Leaf, stem, litterfall, frass, litterbag, and soil samples were analyzed for N. We then tracked recovery of net ecosystem exchange (NEE), leaf area (LAI), and N pools and fluxes through 2008.

Results/Conclusions

Net ecosystem exchange of CO₂ was directly related to LAI, and rates were strongly reduced by defoliation, resulting in a net loss of 293 g C m^-2 yr^-1 in 2007. N flux in fine litterfall in 2007 was 41 kg N ha^-1 yr^-1, with 27 kg N ha^-1 yr^-1 falling in June. Annual N flux in litterfall in 2007 represented 158% of litterfall N in previous years (2003-2006). However, with the exception of extractable NH₄⁺ in soil during July, soil N pools and dynamics were largely unaffected in 2007. By July 2008, canopy oak mortality was 12% of stems and 25% of basal area. Large gaps in the canopy led to an increase in shrub LAI from 0.4 to 0.8, with total LAI in 2008 representing ca.74% of pre-defoliation values. Summertime (June 1- Aug. 31) maximum half-hourly NEE had recovered to -10 μmol CO₂ m^-2 s^-1 at 1500 μmol PAR m^-2 s^-1, approximately half of pre-defoliation values. N mass in foliage had recovered to 82% of pre-defoliation levels by 2008. Results indicate the importance of rapid increases in understory LAI and the stability of litter pools in the recovery of C and N cycling in disturbed forests.