Contrasting effects of invasive insects and fire on ecosystem water use efficiency

Abstract. We used eddy covariance and meteorological measurements to estimate net ecosystem exchange of CO₂ (NEE), gross ecosystem production (GEP), evapotranspiration (Et), and ecosystem water use efficiency (WUE_e; calculated as GEP / Et during dry canopy conditions) in three upland forests in the New Jersey Pinelands, USA, that were defoliated by gypsy moth (Lymantria dispar L.) or burned using prescribed fire. Before disturbance, half-hourly daytime NEE during full sunlight conditions, daily GEP, and daily WUE_e during the summer months were greater at the oak-dominated stand compared to the mixed or pine-dominated stands. Both defoliation by gypsy moth and prescribed burning reduced stand leaf area and nitrogen mass in foliage. During complete defoliation in 2007 at the oak stand, NEE during full sunlight conditions and daily GEP during the summer averaged only 14 and 35% of pre-disturbance values. Midday NEE and daily GEP then averaged 58 and 85%, and 71 and 78% of pre-defoliation values 1 and 2 years following complete defoliation, respectively. Prescribed fires conducted in the dormant season at the mixed and pine-dominated stands reduced NEE during full sunlight conditions and daily GEP during the following summer to 57 and 68%, and 79 and 82% of pre-disturbance values, respectively. Daily GEP during the summer was a strong function of N mass in foliage at the oak and mixed stands, but a weaker function of N in foliage at the pine-dominated stand. Ecosystem WUE_e during the summer at the oak and mixed stands during defoliation by gypsy moth averaged 1.6 and 1.1 g C kg H₂O⁻¹, representing 60 and 46% of pre-disturbance values. In contrast, prescribed fires at the mixed and pine-dominated stands had little effect on WUE_e. Two years following complete defoliation by gypsy moth, WUE_e during the summer averaged 2.1 g C kg H₂O⁻¹, 80% of pre-disturbance values. WUE_e was correlated with canopy N content only at the oak-dominated stand. Overall, our results indicate that WUE_e during and following non-stand replacing disturbance is dependent on both the type and time since disturbance.