Land-Atmosphere CO2 Exchange

Many ecological, hydrological, and atmospheric processes are so intertwined that these cannot be considered separate disciplines. Successful modeling of net primary production, carbon storage, and trace gas emissions (e.g., methane, non-methane hydrocarbons, nitrous oxide) requires an accurate model of the micrometeorological and hydrological environments in addition to the traditional ecological emphasis on vegetation and biogeochemical controls. Successful modeling of latent and sensible heat fluxes requires an accurate description of the ecological state and biogeochemical controls in addition to the traditional emphasis on the physical environment.

The following studies document my long-standing interest in combining the relevant biophysical, biogeochemical, hydrologic, and ecosystem processes into a comprehensive model of land-atmosphere CO2 exchange that is physically and biologically realistic and also internally self-consistent.

Preliminary Work

The first model was a daily time step model of energy, water, and CO2 exchange for aspen, birch, balsam poplar, white spruce, and black spruce forests near Fairbanks, Alaska. It combined photosynthesis and respiration with surface energy exchange and whole-tree carbon allocation. The model was quite successful simulating CO2 uptake and loss during plant photosynthesis, plant respiration, and microbial respiration (as compared to annual net primary production and decomposition). The model also simulated the hydrologic and thermal (soil temperature) states of these forests fairly accurately.

• Bonan, G.B. 1991. Atmosphere-biosphere exchange of carbon dioxide in boreal forests. Journal of Geophysical Research 96:7301-7312.
  
  
• Bonan, G.B. 1991. A biophysical surface energy budget analysis of soil temperature in the boreal forests of interior Alaska. Water Resources Research 27:767-781.

Model applications:

• Seasonal CO2 fluxes . The increasing seasonal amplitude of atmospheric CO2 is thought to reflect increased photosynthetic uptake from higher CO2 concentrations or increased winter respiration loss due to warmer temperatures. Boreal forests have a large role in annual cycle of atmospheric CO2, and to test these hypotheses the model was driven with the observed annual atmospheric CO2 and observed daily air temperature and precipitation for the period 1974 to 1982. All other atmospheric variables were constant. During this period, the simulated seasonal amplitude in CO2 exchange for the boreal forests near Fairbanks increased by 0.52% per year. For comparison, the seasonal amplitude in atmospheric CO2 concentration increased by 0.4 to 0.8% per year. This shows that year-to-year differences in mean annual atmospheric CO2, daily air temperature, and daily precipitation can produce an increased seasonal amplitude in CO2 fluxes that is consistent with the observations. The increased amplitude was due to increased photosynthesis as atmospheric CO2 increased. However, the increase in annual aboveground tree biomass production was too small (3 g/m2/year/year) to be detectable given field sampling errors.
  
  Bonan, G.B. 1992. Comparison of atmospheric carbon dioxide concentration and metabolic activity in boreal forest ecosystems. Tellus 44B:173-185.
  
  
• Air temperature and tree growth. The model, applied to a single-tree was used to derive the relationship between air temperature and tree growth for black spruce. Physiological properties of black spruce growing near treeline in subarctic Quebec were used for the simulations. This physiology allowed black spruce to grow over a wider range of air temperatures (annual growing degree days) than is reflected in its geographic distribution. In particular, the northern limit to black spruce was not caused by the direct effects of cold growing season temperatures on tree growth; growth was optimal, with respect to temperature, at the southern range limit.
  
  Bonan, G.B, and Sirois, L. 1992. Air temperature, tree growth, and the northern and southern range limits to Picea mariana. Journal of Vegetation Science 3:495-506.

The model was substantially modified from this first version to study the environmental and physiological controls of forest production. The model was modified to include an updated photosynthesis parameterization and dynamic nitrogen limitation to tree growth. Photosynthesis was modeled using the Farquhar approach, with species-specific physiology. Decomposition and nitrogen mineralization were modeled using the Agren/Bosatta approach. The model was applied to aspen, birch, balsam poplar, white spruce, and black spruce forests near Fairbanks to examine the physiological controls of the carbon balance of boreal forests. Model analyses suggest that differences in the carbon balance of these forests can be explained by key physiological parameters that link photosynthesis, carbon allocation, nitrogen requirements, litter quality, and folige longevity. Simulations showed that coniferous and deciduous physiology maximizes annual tree production for coniferous and deciduous forests, respectively, giving a physiological basis for the evolution of their different life history characteristics.

• Bonan, G.B. 1993. Physiological controls of the carbon balance of boreal forest ecosystems. Canadian Journal of Forest Research 23:1453-1471.

Model applications:

• Leaf area index, forest type, and photosynthesis. The model was used to examine the relative importance of leaf area index and forest type, two key ecological variables that can be remotely-sensed, for land-atmosphere CO2 exchange in the boreal forests near Fairbanks. Model sensitivity studies showed than net canopy CO2 assimilation was as sensitivity to uncertainty in LAI as it was to uncertainty in species composition. The sensitivity to species composition was greater between needleleaf evergreen (black spruce, white spruce) and broadleaf deciduous (aspen, birch, balsam poplar) species than among species within a life form. These analyses highlight the importance of recognizing physiological differences among needleleaf evergreen and broadleaf deciduous forest types in addition to LAI when developing remotely-sensed based models of CO2 fluxes in boreal forests.
  
  Bonan, G.B. 1993. Importance of leaf area index and forest type when estimating photosynthesis in boreal forests. Remote Sensing of Environment 43:303-314.
  
  
• Net primary production and mean annual air temperature. The model was used to derive the observed relationship between NPP and MAAT. The model successfully reproduced this relationship and showed that it reflected, at least for boreal and temperate coniferous forests growing on moist soils, the length of the growing season, nitrogen limitation, and lower maintenance respiration rates in warm climates than cold climates. These analyses showed that simple physiological assumptions can result in reasonable predictions of NPP over a wide range of climates.
  
  Bonan, G.B. 1993. Physiological derivation of the observed relationship between net primary production and mean annual air temperature. Tellus 45B:397-408.

Current Work

These background studies were so successful (at least in my opinion) that I decided to abandon the boreal forest model and make a more general land-atmosphere model that would have a diurnal cycle and be applicable to all surface types. This would allow the model to be coupled to an atmospheric general circulation model. I call this new model the NCAR Land Surface Model.

• Preliminary simulations with version 0 of this model showed that simple physiological and ecological assumptions can result in reasonable simulation of land-atmosphere CO2 exchange, as compared to observed estimates of annual net primary production and annual microbial respiration.
  
  Bonan, G.B. 1995. Land-atmosphere CO2 exchange simulated by a land surface process model coupled to an atmospheric general circulation model. Journal of Geophysical Research 100:2817-2831.