Abstract
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 (WUEe; 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 WUEe 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 WUEe 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 WUEe. Two years following complete defoliation by gypsy moth, WUEe during the summer averaged 2.1 g C kg H2O−1, 80% of pre-disturbance values. WUEe was correlated with canopy N content only at the oak-dominated stand. Overall, our results indicate that WUEe during and following non-stand replacing disturbance is dependent on both the type and time since disturbance.
Highlights
Understanding the effects of disturbance and recovery on stand productivity and evapotranspiration (Et) is essential for accurate estimates of carbon storage and water yield in forest ecosystems
We asked (1) how do gross ecosystem productivity (GEP) and water use efficiency (WUEe) vary among oak and pine-dominated stands growing in the same climate and soil type before disturbance, and (2) how are leaf area index (LAI) and canopy N content linked to GEP and WUEe during recovery from non-stand replacing disturbances in these stands?
These calculations assume that λE, net ecosystem exchange of CO2 (NEE) and GEP measured at our sites during pre-disturbance periods characterize potential rates during later years in the absence of disturbance, they illustrate the magnitude of the impact that gypsy moth defoliation and prescribed burning can have on stand carbon dynamics, while having relatively little effect on Et and groundwater recharge (Schäfer et al, 2013)
Summary
Understanding the effects of disturbance and recovery on stand productivity and evapotranspiration (Et) is essential for accurate estimates of carbon storage and water yield in forest ecosystems. Fewer studies have estimated changes in GEP and Et following non-stand replacing disturbances such as insect defoliation or low intensity fires, limiting our understanding of patterns of forest productivity and water use during recovery. These events can reduce leaf area, alter forest floor mass, and affect the distribution of nutrients, but typically do not significantly reduce overall stand biomass (Lovett et al, 2006; Clark et al, 2010, 2012, 2014). We asked (1) how do GEP and WUEe vary among oak and pine-dominated stands growing in the same climate and soil type before disturbance, and (2) how are LAI and canopy N content linked to GEP and WUEe during recovery from non-stand replacing disturbances (gypsy moth defoliation and prescribed fire) in these stands?
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