Abstract
AbstractProcess‐based models are effective tools to synthesize and/or extrapolate measured carbon (C) exchanges from individual sites to large scales. In this study, we used a C‐ and nitrogen (N)‐cycle coupled ecosystem model named CN‐CLASS (Carbon Nitrogen‐Canadian Land Surface Scheme) to study the role of primary climatic controls and site‐specific C stocks on the net ecosystem productivity (NEP) of seven intermediate‐aged to mature coniferous forest sites across an east–west continental transect in Canada. The model was parameterized using a common set of parameters, except for two used in empirical canopy conductance–assimilation, and leaf area–sapwood relationships, and then validated using observed eddy covariance flux data. Leaf Rubisco‐N dynamics that are associated with soil–plant N cycling, and depend on canopy temperature, enabled the model to simulate site‐specific gross ecosystem productivity (GEP) reasonably well for all seven sites. Overall GEP simulations had relatively smaller differences compared with observations vs. ecosystem respiration (RE), which was the sum of many plant and soil components with larger variability and/or uncertainty associated with them. Both observed and simulated data showed that, on an annual basis, boreal forest sites were either carbon‐neutral or a weak C sink, ranging from 30 to 180 g C m−2 yr−1; while temperate forests were either a medium or strong C sink, ranging from 150 to 500 g C m−2 yr−1, depending on forest age and climatic regime. Model sensitivity tests illustrated that air temperature, among climate variables, and aboveground biomass, among major C stocks, were dominant factors impacting annual NEP. Vegetation biomass effects on annual GEP, RE and NEP showed similar patterns of variability at four boreal and three temperate forests. Air temperature showed different impacts on GEP and RE, and the response varied considerably from site to site. Higher solar radiation enhanced GEP, while precipitation differences had a minor effect. Magnitude of forest litter content and soil organic matter (SOM) affected RE. SOM also affected GEP, but only at low levels of SOM, because of low N mineralization that limited soil nutrient (N) availability. The results of this study will help to evaluate the impact of future climatic changes and/or forest C stock variations on C uptake and loss in forest ecosystems growing in diverse environments.
Highlights
Eddy covariance flux data from tower networks (e.g. AmeriFlux, EuroFlux, AsiaFlux and Fluxnet-Canada, presently known as the Canadian Carbon Program) are available for a number of sites worldwide, monitoring the dynamics of major terrestrial ecosystems (Aubinet et al, 2000; Baldocchi et al, 2001; Gu & Baldocchi, 2002; Margolis et al, 2006; Yu et al, 2006)
It was possible to use a single set of photosynthesis parameters to simulate half-hourly net ecosystem productivity (NEP) values, with acceptable accuracy limits, at all seven forest sites growing in different environments
Analysis of the initial model testing results revealed that at least three groups of parameters appeared to be crucial for successful simulation of C uptakes at all seven forests, i.e. (1) the maximum carboxylation rate of Rubisco in leaves, Vcmax and the potential rate of wholechain electron transport, Jmax, (2) temperature criteria for Vcmax and Jmax and (3) empirical parameters controlling the relationship between the canopy conductance for CO2, Gc and net CO2 fixation, Anet [Eqn (A4)]
Summary
Eddy covariance flux data from tower networks (e.g. AmeriFlux, EuroFlux, AsiaFlux and Fluxnet-Canada, presently known as the Canadian Carbon Program) are available for a number of sites worldwide, monitoring the dynamics of major terrestrial ecosystems (Aubinet et al, 2000; Baldocchi et al, 2001; Gu & Baldocchi, 2002; Margolis et al, 2006; Yu et al, 2006). Various model inter-comparison studies have shown that even for the same site, various models used different values of the same parameters to simulate ecosystem processes relevant to carbon dioxide (CO2) and water vapor fluxes (e.g. Grant et al, 2005, 2006) These site-specific model parameters may be compared and/or classified into certain categories depending on biome type, age, climate or geographical regions, etc., to conduct regional and/or perhaps global C cycle modeling studies. Because of the continuing accumulation of flux data from multiple sites across the world and the development of coupling of physical, physiological, biogeochemical and other relevant processes in terrestrial ecosystem models, it may be possible to generalize process-based model parameters at larger scales This would challenge model parameterization and development, but use of a common set of model parameters for key processes for multiple sites would help to assess the ability of the model to simulate correctly seasonal and annual C cycles in diverse climatic regions and to identify sources of errors for conducting these simulations at larger spatial scales
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