Effects of invasive insects and fire on forest energy exchange and evapotranspiration in the New Jersey pinelands

Abstract

We used eddy covariance and meteorological measurements to quantify energy exchange and evapotranspiration (Et) in three representative upland forest stands in the New Jersey Pinelands that were either defoliated by gypsy moth (Lymantria dispar L.) or burned in prescribed fires during the study period. Latent (λE) and sensible heat (H) fluxes were linear functions of available energy, and seasonality had a major effect on the partitioning of available energy into λE and H at each stand. Both defoliation and prescribed fire reduced leaf area, altered the partitioning of available energy, and reduced λE flux compared to undisturbed periods. Summer daily Et averaged 4.2±1.5, 3.3±1.2 and 3.9±1.3mm day−1 at the oak-, mixed, and pine-dominated stands during undisturbed periods, but only 2.4±0.9mmday−1 during defoliation at the oak stand in 2007, and 2.4±0.9 and 3.2±0.9mmday−1 following spring fires at the mixed and pine-dominated stands, respectively. For all years measured, seasonal maximum leaf area index (LAI) explained 82% of the variability in daily Et during the summer at the oak stand, and 80% of the variability at the mixed and pine-dominated stands. Annual Et averaged 614, 493, and 683mmyr−1 at the oak, mixed, and pine stands, respectively. When averaged across all stands and years, annual Et was 606mmyr−1, ca. 53.6% of incident precipitation, and similar to long-term averages reported in other studies in the Pinelands. Gypsy moth defoliation potentially reduced Et by ca. 31mmyr−1 across all upland forests in 2007, resulting in a 7.3% increase in groundwater recharge. Our research indicates that non-stand replacing disturbances can have significant effects on energy partitioning, and can reduce Et at the stand and landscape scales.