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

Kenneth Clark,Nicholas Skowronski,Karina Schäfer,Michael Gallagher,Heidi Renninger

doi:10.3390/f9030145

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 (Re) 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 Re 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 Re in long-term measurements of forest C dynamics following disturbance in intermediate-age forests.

Highlights

An understanding of processes underlying recovery following non-stand-replacing disturbance is essential for accurately predicting future carbon (C) dynamics in forest ecosystems of the eastern USA.Following extensive harvesting, conversion to agriculture or intensive forestry, and subsequent abandonment, large areas of forest throughout the northeastern USA are currently 80–140 years old [1,2]
Building upon two related studies that employed repeated light detection and ranging (LiDAR) acquisitions to demonstrate an increase in heterogeneity of canopy structure following defoliation by gypsy moth (Lymantria dispar L.) and subsequent tree mortality in an oak-dominated stand [33], and crown scorch and needle and stem consumption during a relatively intense prescribed fire in a pine-dominated stand [34,35], we evaluated two hypotheses: (1) Rapid recovery of foliage results in the maintenance of gross ecosystem productivity (GEP) and aboveground net primary productivity (ANPP) following non-stand-replacing disturbance; and (2) changes to detrital mass that occur during and following disturbance alter Rh and Re, and net ecosystem production (NEP)
Annual NEP averaged 169 and 173 g C m−2 year−1 at the oak and pine stands before disturbance, respectively (Table 1, Figure 1)

Summary

Introduction

Conversion to agriculture or intensive forestry, and subsequent abandonment, large areas of forest throughout the northeastern USA are currently 80–140 years old [1,2]. Since abandonment, these regenerating forests have been moderate to strong sinks for atmospheric carbon dioxide (CO2 ) [3,4,5,6,7]. These intermediate-age forests experience disturbance regimes dominated by wind and ice storms, insect damage, and managed wildland fire that differ in temporal and spatial scales and intensity compared to past stand-replacing disturbance such as intensive timber management or large wildfires [6,13,14,15,16]

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Results

Discussion

Conclusion