Evaluating the impacts of climate variability and disturbance regimes on the historic carbon budget of a forest landscape
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Landscape-level understanding of forest carbon (C) dynamics is required to quantify the net contribution of forest biomes to the global C cycle and to help forest managers to understand the impacts of forest management activities on the C sequestration in forests. In this study, the effects of interannual climate variability, carbon dioxide (CO2) fertilization, and disturbance regimes on the C dynamics of an old-growth Pacific Northwest temperate conifer forest landscape (2500ha) were studied from 1920 to 2005, using a process-based land surface model, known as the Carbon and Nitrogen coupled Canadian Land Surface Scheme (CN-CLASS). The model was parameterized with ecological, forest inventory and historical land-use data, and run using historical meteorological observations. Before performing landscape level simulations, model results were evaluated against eddy covariance flux tower observations. Simulated mean annual net ecosystem productivity (NEP) over the flux tower footprint area was 340gCm−2yr−1 from 1998 to 2005, while the measured value was 293±20.5gCm−2yr−1. When two anomalous weather years, corresponding to El Niño (1998) and La Niña (1999) events, were excluded while performing statistical analysis, measured and simulated fluxes were highly, but negatively, correlated to both annual mean air temperature and annual precipitation (R2=0.69 and 0.60, respectively). Interannual variability of simulated and measured NEP over the flux footprint area, calculated as deviations of the respective annual NEP values, was 143gCm−2yr−1 and 61gCm−2yr−1, respectively. On the landscape-level, prior to disturbance in 1920, simulated C fluxes indicated that the forest landscape was close to C neutral, with annual net biome productivity (NBP) of 0.8gCm−2yr−1. However, during the intense disturbance period from 1938 to 1944, landscape-level NBP reached about −5083gCm−2yr−1. Then from 1951 to 1997, when there were no major disturbance events, NBP gradually recovered to about 365gCm−2yr−1. At the end of the study period, in 2005, the landscape again became C source, due to harvesting of second growth stands that occurred from late 1990s to 2000s.The regression of age-detrended variations in the simulated annual C fluxes to mean daily maximum air temperature over the peak growing season (July–September), during an undisturbed period from 1963 to 1984, indicated that summer temperature was the dominant climatic control on landscape-level C fluxes. Higher temperatures caused a decrease in gross primary productivity at almost twice the rate of increase in ecosystem respiration (i.e. 27gCm−2yr−1C−1 versus 15.7gCm−2yr−1C−1, respectively). A sensitivity analysis to evaluate the impacts of climate variability and disturbances showed that the relative effect of disturbance on carbon stocks was greater than the effect on carbon fluxes. Overall CO2 fertilization effects were minor. Disturbance type and severity, represented by the standard deviation in NBP, as described in the model, determined the magnitude of the simulated C losses to the atmosphere. This study enhances our understanding of the impacts of future climate change and forest management on landscape-level C dynamics in forests.
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