Integrated Terrestrial Ecosystem Carbon model (InTEC)

InTEC, an Integrated Terrestrial Ecosystem C-budget model, was developed to estimate the carbon budget of Canada's forests by a government-industry scientific team from CCRS and Intermap Technologies Ltd. (Chen et al., 1998a). The InTEC model estimates the carbon budgets of forests from atmospheric, climatic and biotic changes since the pre-industrial period (Figure 1). The atmospheric changes include the increases in carbon dioxide concentration and nitrogen deposition. The climatic changes include inter-annual variations in temperature and precipitation. The biotic changes include forest area and age structural changes due to disturbances caused by forest fires, insects and timber harvest. Based on these changes, the effects of various biological and physical processes on the carbon budget of forest ecosystems are modeled. Processes considered in the model include nitrogen deposition, fixation and deposition, carbon release due to disturbances, forest regrowth, change in the length of the growing season, CO2 fertilization, soil respiration, and changes in carbon/nitrogen ratios of different components in biomass and soil carbon pools.

Figure 1 Structure of the Integrated Terrestrial Ecosystem C-budget model (InTEC) which synthesizes the interactive effects of disturbances, nitrogen deposition, climate change, and CO2 fertilization on the carbon budget of boreal forests. Dashed arrows indicate influences, and solid arrows show C-N flows.

The main characteristic of the model is the mechanistic integration of two widely used models: the Century model for carbon and nitrogen cycles in soils, and Farquhar's biochemical model of leaf photosynthesis. The integration is implemented through new spatial and temporal scaling algorithms developed in this study. The spatial scaling from leaf to canopy is done with a canopy radiation model with sunlit/shaded leaf separation. The temporal scaling from minutes to a year is conducted using statistics of variance and covariance of various meteorological and biological factors involved in photosynthesis. The statistics are obtained from the measurements of two boreal species from 1994-1996 (Chen et al., 1998b; Goulden, et al., 1998). This temporal scheme allows the estimation of annual total photosynthesis using annual mean meteorological measurements such as temperature and precipitation and yet is able to take into account the effects of diurnal and seasonal variability of the photosynthetic processes on the integrated carbon budget. A similar temporal scaling method was used for inter-annual variations in NPP and in other factors such as nitrogen deposition, climate, CO2 and disturbance rates. Disturbances are considered explicitly in this model as processes that directly release carbon and nitrogen into the atmosphere, and modify the total NPP by changing forest area and age class. The carbon cycle in forest products is also included as a process which stores a fraction of harvested wood in products that slowly oxidize. Sub-models of respiration and canopy photosynthesis are verified and parameterized using BOREAS data at the Old Black Spruce (1995 and 1996) and Old Aspen (1994 and 1996) sites. In particular, we determine the time fraction of photosynthesis dominated by the electron transport control or Rubisco activities after separating a canopy into sunlit and shaded leaf groups, thus avoiding a large uncertainty in using Farquhar's model to determine canopy photosynthesis and inter annual variation in NPP. Remote sensing derived data of leaf area and NPP, network measurements of N deposition, and historical data of climate and disturbance rates are used in the InTEC model.

InTEC results indicate that in the past 100 years, Canada's forests as a whole were a small carbon source of about 30 Mt C y-1 in the period 1895-1905 due to large disturbances near the end of the 19th century; a large carbon sink of about 170 Mt C y-1 (1930-1970) due to forest regrowth in previously burnt areas; and moderate carbon sinks of about 50 Mt C y-1(1980-1996) (Figure 2). The 1980-1996 sink is a net balance between the negative effects of increased disturbances and the positive effects of other non-disturbance factors. In order of importance, the non-disturbance factors, were found to be

  1. atmospheric nitrogen deposition measured by a national monitoring network,
  2. net nitrogen mineralization and fixation estimated from temperature and precipitation records,
  3. CO2 fertilization estimated from CO2 records using a leaf level photosynthesis model,
  4. growing season length increase estimated from spring air temperature records.

Figure 2 Carbon budget of Canada's forests from 1895 to 1996. Five-year means are plotted, the last point being only the average from 1995 to 1996. Also included is the record of Canada's green-house gas carbon emission rate

The positive effect of these non-disturbance factors is experimentally supported by the recent BOREAS measurements. Increased disturbances (mostly fire and insects) in recent decades caused a loss of about 60 Mt C y-1 from the forests in 1980-1996. If disturbance rates had remained unchanged, Canada's forests in 1980-1996 would have been a sink of about 150 Mt C y-1 in size. The large amount of carbon accumulated during 1930-1970 with low disturbance rates caused an additional loss of about 40 Mt C y-1 through decomposition in 1980-1996. From the modeling perspective, it is evident that:

  • attention must be given to nutrient cycles as they are particularly closely coupled with carbon cycle in the boreal environment,
  • estimating the carbon budget from historical changes in nitrogen and carbon cycle components is the preferred approach since the outcome is not sensitive to inaccuracies in estimates of soil respiration andNPP
  • the absolute magnitude of current sinks per unit area is small (about 0.35 t C/ha or 13% of current NPP) and exceedingly difficult to measure directly.

To reduce the uncertainties in our estimates, improvements are therefore needed in the comprehensive modeling of all processes involved in the terrestrial nutrient and carbon cycles using biological and physical principles which can be experimentally verified.

© Revised: Mar., 2005