Carbon balance of coniferous forests in response to different harvesting strategies : a model based analysis
Sammanfattning: The aim of this study was ultimately to suggest a harvesting strategy for northern coniferous forests that would be the best choice of carbon management for optimising carbon storage regarding the concerns of both wood production and carbon stored in the forest pools not harvested.For two climatic diverse sites in Sweden, Asa (57°08'N, 14°45'E) and Flakaliden (64°07' N19°27'E), a comparison has been made between three different forest management scenarios by use of a process-based modelling approach. The three scenarios addressed were a control scenario, a clear-cutting scenario and a single-tree selection scenario. The focus was on the carbon balance of the sites and how this was affected by the different ways of management.For the simulation of the trees in the forest the daily time-step based model BIOMASS was used. This model runs on daily meteorological inputs and consists of a large amount of sub-models interacting with each other. For the heterotrophic part of the forest, the Q-model was applied. The Q-model is a model simulating decomposition of organic matter. The BIOMASS model could successfully be parameterised in such a way that it was able to reproduce sufficiently both the CO2 - fluxes and evapotranspiration fluxes of the target forest, but also the realistic development of a forest at the specific site.All scenario runs were run for 300 years on the climatic data of one-year repeated climate, to exclude effects of changing climate. The control scenario was run until it reached an equilibrium point, the clear-cutting scenario was run until the final harvest of 100 years and then repeated 2 times. The single-tree scenario was run in small time intervals and always the same small proportion of harvest was taken out. Harvesting residues were decomposed, but the harvested stems were considered to be taken away after the harvests and did not contribute to the heterotrophic respiration.Concerning the carbon pools, the scenarios followed the expected behaviour, the control and the single-tree selection balanced out, whilst the run on Asa went much faster to an equilibrium state than the run in Flakaliden. The carbon pool of the clear-cutting scenario was rising fast, even if some thinnings had been applied it was rising until the trees were harvested fully. The control scenario stored the most amount of wood. The highest accumulated net ecosystem exchange (NEE) for Asa and for Flakaliden was in the single-tree selection, followed by the clear cut scenario and the control scenario. A comparison between the two different harvesting strategies concerning the amount of wood which could be harvested resulted in the outcome that both harvesting strategies would result in approximately the same amount of wood. The clear-cutting management was a little more effective at the Northern site, whilst the single-tree selection seemed to work better in the South. This first attempt of a modelling comparison of single-tree selection management with traditional harvesting strategies indicate that the single-tree selection was the best managing form regarding total carbon accumulation and under certain circumstances may even be interesting for economic reasons because of the continuous income from harvests. Single-tree selection should be considered a serious alternative to traditional harvesting management.
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