Abstract
Stand-replacing disturbances including harvests have substantial impacts on forest carbon (C) fluxes and stocks. The quantification and simulation of these effects is essential for better understanding forest C dynamics and informing forest management 5 in the context of global change. We evaluated the process-based forest ecosystem model, PnET-CN, for how well and by what mechanisms changes of ecosystem C fluxes, aboveground C stocks (AGC), and leaf area index (LAI) arise after clearcuts. We compared the effects of stand-replacing harvesting on C fluxes and stocks usingtwo chronosequences of eddy covariance flux sites for deciduous broadleaf forests 10 (DBF) and evergreen needleleaf forests (ENF) in the Upper Midwest region of northern Wisconsin and Michigan, USA. The average values of normalized root mean square error (NRMSE) and the Willmott index of agreement (d) between simulated and inferred from observation variables including gross primary productivity (GPP), ecosystem respiration (ER), net ecosystem productivity (NEP), LAI, and AGC in the two chronose15 quences were 20% and 0.90, respectively. Simulated GPP increased with stand age, reaching a maximum (1200–1500 gCm−2 yr−1) at 11–30 years of age, and leveled off thereafter (900–1000 gCm−2 yr−1). Simulated ER for both forest types was initiallyas high as 700–1000 gCm−2 yr−1 in the first or second year after clearcuts, decreased with age (400–800 gCm−2 yr−1) before canopy closure at 10–25 years of 20 age, and increased to 800–900 gCm−2 yr−1 with stand development after canopy recovery. Simulated NEP for both forest types was initially negative with the net C losses of 400–700 gCm−2 yr−1 for 6–17 years after harvesting, reached the peak values of 400–600 gCm−2 yr−1 at 14–29 years of age, and became stable and a weak C sink (100–200 gCm−2 yr−1) in mature forests (>60 years old). The decline of NEP with 25 age was caused by the relative flatting of GPP and gradual increasing of ER. ENF recovered slower from net C source to net sink and lost more C than DBF, suggesting ENF are likely slower to recover C assimilation capacity after stand-replacing harvests due to slower development of photosynthesis with stand age. Model results indicatedthat increasing harvesting intensity would delay recovery of NEP after clearing, but had little effect on C dynamics during late succession. Further improvements in numerical process-based forest population dynamic models for predicting the effects of climate change and forest harvests are considered.
Citation
Wang, W.; Xiao, J.; Ollinger, S. V., Desai, A. R.; Chen, J.; Noormets, A. 2014. Quantifying legacies of clearcut on carbon fluxes and biomass carbon stock in northern temperate forests. Biogeosciences Discuss., 11, 8789–8828, 2014. 40 p.