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      Quantifying the effects of harvesting on carbon fluxes and stocks in northern temperate forests

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      Biogeosciences

      Copernicus GmbH

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          Abstract

          <p><strong>Abstract.</strong> Harvest disturbance has substantial impacts on forest carbon (C) fluxes and stocks. The quantification of these effects is essential for the better understanding of forest C dynamics and informing forest management in the context of global change. We used a process-based forest ecosystem model, PnET-CN, to evaluate how, and by what mechanisms, clear-cuts alter ecosystem C fluxes, aboveground C stocks (AGC), and leaf area index (LAI) in northern temperate forests. We compared C fluxes and stocks predicted by the model and observed at two chronosequences of eddy covariance flux sites for deciduous broadleaf forests (DBF) and evergreen needleleaf forests (ENF) in the Upper Midwest region of northern Wisconsin and Michigan, USA. The average normalized root mean square error (NRMSE) and the Willmott index of agreement (<i>d</i>) for carbon fluxes, LAI, and AGC in the two chronosequences were 20% and 0.90, respectively. Simulated gross primary productivity (GPP) increased with stand age, reaching a maximum (1200–1500 g C m<sup>−2</sup> yr<sup>−1</sup>) at 11–30 years of age, and leveled off thereafter (900–1000 g C m<sup>−2</sup> yr<sup>−1</sup>). Simulated ecosystem respiration (ER) for both plant functional types (PFTs) was initially as high as 700–1000 g C m<sup>−2</sup> yr<sup>−1</sup> in the first or second year after harvesting, decreased with age (400–800 g C m<sup>−2</sup> yr<sup>−1</sup>) before canopy closure at 10–25 years of age, and increased to 800–900 g C m<sup>−2</sup> yr<sup>−1</sup> with stand development after canopy recovery. Simulated net ecosystem productivity (NEP) for both PFTs was initially negative, with net C losses of 400–700 g C m<sup>−2</sup> yr<sup>−1</sup> for 6–17 years after clear-cuts, reaching peak values of 400–600 g C m<sup>−2</sup> yr<sup>−1</sup> at 14–29 years of age, and eventually stabilizing in mature forests (> 60 years old), with a weak C sink (100–200 g C m<sup>−2</sup> yr<sup>−1</sup>). The decline of NEP with age was caused by the relative flattening of GPP and gradual increase of ER. ENF recovered more slowly from a net C source to a net sink, and lost more C than DBF. This suggests that in general ENF may be slower to recover to full C assimilation capacity after stand-replacing harvests, arising from the slower development of photosynthesis with stand age. Our model results indicated that increased harvesting intensity would delay the recovery of NEP after clear-cuts, but this had little effect on C dynamics during late succession. Future modeling studies of disturbance effects will benefit from the incorporation of forest population dynamics (e.g., regeneration and mortality) and relationships between age-related model parameters and state variables (e.g., LAI) into the model.</p>

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          A global overview of drought and heat-induced tree mortality reveals emerging climate change risks for forests

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            A large and persistent carbon sink in the world's forests.

            The terrestrial carbon sink has been large in recent decades, but its size and location remain uncertain. Using forest inventory data and long-term ecosystem carbon studies, we estimate a total forest sink of 2.4 ± 0.4 petagrams of carbon per year (Pg C year(-1)) globally for 1990 to 2007. We also estimate a source of 1.3 ± 0.7 Pg C year(-1) from tropical land-use change, consisting of a gross tropical deforestation emission of 2.9 ± 0.5 Pg C year(-1) partially compensated by a carbon sink in tropical forest regrowth of 1.6 ± 0.5 Pg C year(-1). Together, the fluxes comprise a net global forest sink of 1.1 ± 0.8 Pg C year(-1), with tropical estimates having the largest uncertainties. Our total forest sink estimate is equivalent in magnitude to the terrestrial sink deduced from fossil fuel emissions and land-use change sources minus ocean and atmospheric sinks.
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              Nitrogen Cycles: Past, Present, and Future

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                Author and article information

                Journal
                Biogeosciences
                Biogeosciences
                Copernicus GmbH
                1726-4189
                2014
                December 04 2014
                : 11
                : 23
                : 6667-6682
                10.5194/bg-11-6667-2014
                © 2014

                https://creativecommons.org/licenses/by/3.0/

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