Decadal-Scale Reduction in Forest Net Ecosystem Production Following Insect Defoliation Contrasts with Short-Term Impacts of Prescribed Fires

Abstract

Understanding processes underlying forest carbon dynamics is essential for accurately predicting the outcomes of non-stand-replacing disturbance in intermediate-age forests. We quantified net ecosystem production (NEP), aboveground net primary production (ANPP), and the dynamics of major carbon (C) pools before and during the decade following invasive insect defoliation and prescribed fires in oak-and pine-dominated stands in the New Jersey Pinelands National Reserve, USA. Gross ecosystem production (GEP) recovered during the year following defoliation at the oak stand, but tree mortality increased standing dead and coarse woody debris, and ecosystem respiration (R e ) accounted for >97% of GEP. As a result, NEP averaged only 22% of pre-disturbance values during the decade following defoliation. At the pine stand, GEP also recovered to pre-disturbance values during the year following understory defoliation by gypsy moth and two prescribed fires, while R e was nearly unaffected. Overall, defoliation and tree mortality at the oak stand drove a decadal-scale reduction in NEP that was twofold greater in magnitude than C losses associated with prescribed fires at the pine stand. Our study documents the outcomes of different non-stand-replacing disturbances, and highlights the importance of detrital dynamics and increased R e in long-term measurements of forest C dynamics following disturbance in intermediate-age forests.

The Impact of Seasonal and Annual Climate Variations on the Carbon Uptake Capacity of a Deciduous Forest Within the Great Lakes Region of Canada

“…However, a major insect defoliation in 2007 resulted in the site being a large C source for 2 yr after the disturbance. During the postdisurbance years (2009–2016), mean annual NEP (37 ± 18 g C m −2 yr −1 ) was only 22% of the NEP observed during the predisturbance years (2004–2006) (Clark et al, 2018). Therefore, this site was excluded from the comparison.…”

Forest aging, disturbance and the carbon cycle

“…The principal substrate fueling this carbon loss is woody debris, which constitutes a particularly large pool in severely disturbed forests (Woodall et al ., ; Russell et al ., ). Following moderate disturbance, the influx of woody debris and corresponding release of carbon from decomposition are generally much less, and not sufficient to drive NEP below zero (Schmid et al ., ; Clark et al ., ). Instead, increases in carbon uptake by aging and moderately disturbed forests may fully offset or even outpace relatively small and drawn out increases in carbon losses from decomposition, a hypothesis grounded in theory (Harmon et al ., ), supported by limited observations (Clark et al ., ) and deserving of further investigation.…”

“…Canopies made more complex and physiologically efficient through periodic moderate disturbance may sustain NEP later into ecosystem development. (Schmid et al, 2016;Clark et al, 2018). Instead, increases in carbon uptake by aging and moderately disturbed forests may fully offset or even outpace relatively small and drawn out increases in carbon losses from decomposition, a hypothesis grounded in theory , supported by limited observations (Clark et al, 2018) and deserving of further investigation.…”

Temporal Dynamics of Aerodynamic Canopy Height Derived From Eddy Covariance Momentum Flux Data Across North American Flux Networks

“…The unaccounted changes in canopy structure (e.g., total leaf area, leaf area profile, stand density, gap fraction, and composition) are likely responsible for the unexpected interannual variation of h a at some forest sites, and for the difference between estimated trends from h a and h c (Aber, 1979;Maurer et al, 2013;. As shown in the known disturbed sites (Clark et al, 2018;Frank et al, 2014;Hardiman et al, 2013;Reed et al, 2014), the observed changes of h a are the consequence of changes in canopy structure (e.g., canopy height, stand density, gap fraction, and leaf area). Some forest sites may have undergone compositional changes (e.g., mortality and succession), which makes it challenging to delineate a physically meaningful trend from the year-to-year variation.…”