The results of the first set of 81 scenarios suggest that enhanced C sequestration brought about by a positive change in growth rates could compensate for the C losses resulting from an increase in disturbance rates of proportionally equal magnitude. For example, where only growth is adjusted (no corresponding change in decomposition rates), a 12% increase in growth rates would suffice to offset the C losses associated with a 20% increase in disturbance rates over 50 years. The results of the second set of scenarios suggest, however, that this compensation does not take place if decomposition rates increase in proportion to growth rate increases.
The scenario results are time sensitive, in particular for the second set of scenarios. The reason that the 50-year outcome differs from the longer-term outcomes in the second set of scenarios is that dead organic matter and soil C changes exhibit considerable time delays, even though we simulated sustained changes in decomposition rates. Rh is initially much higher in elevated decomposition rate scenarios, but stabilizes at levels more modestly elevated relative to base levels once the most dynamic dead organic matter C pools have reached their new levels (as illustrated in and d).
a,b shows the break-even domains for the two sets of scenarios. In a, the area above the break-even line represents C sinks and the area below the break-even line represents C sources. This configuration is reversed in b. These break-even lines can be thought of as ‘slices’ through the response surfaces shown in and , respectively. The opposite configuration of the sink and source regions relative to the break-even lines in a,b is a reflection of the different orientations of the response surfaces shown in and . In the first set of scenarios, where decomposition rates remained unchanged, increased growth rates tended to push the ecosystem towards C sink. In the second set of scenarios, where growth and decomposition rates were changed together, increased growth and decomposition rates tended to push the ecosystem towards C source because the impacts on Rh were greater than the impacts on NPP in the first 50 years.
Figure 8 Break-even domains extrapolated from the two sets of scenarios. The diagonal lines represent: (a) the change in growth or (b) growth and decomposition rates that are required to compensate for the C losses associated with the change in disturbance rates, (more ...)
Global change is projected to have greater impacts on disturbances and growth rates than we have simulated in any of our scenarios. For example, Chen et al. (2000)
suggest that an increase in NPP of the order of 30% could be anticipated in Canada's boreal forests, while Flannigan et al. (2005b)
project a doubling in the area burned annually by wildfire in Canada. Even if a 30% increase in NPP were to translate into a 30% increase in NEP, the star symbols in a
indicate that this would not be sufficient to offset the anticipated increases in fire disturbance rates. The response surfaces from the simulation experiments indicate that our hypothetical boreal landscape would be a net C source (). The source is smaller when decomposition rates are held constant (a
) and is larger when decomposition rates are adjusted with growth rates (b
). Moreover, a doubling of fire disturbance rates may bring about other complex changes that are not addressed here. For example, regeneration failures following fires in young stands with inadequate seed sources and other changes in successional dynamics would also have to be considered.
As described earlier, a +30% growth scenario simulated in the CBM-CFS3 would not equate to a 30% enhancement of NPP at the landscape scale. All stands in our hypothetical landscape are grown using the growth curve shown in , and changes in growth rates are simulated by scaling the curve upwards or downwards. The CBM-CFS3 uses growth increments derived from the growth curve to drive biomass C dynamics. Thus, in all of the growth scenarios simulated here, stands older than 175 years (17% of the area in the landscape at the start of the simulations) experience no impact from growth rate changes and no change in NPP. A net growth increment of zero increased by 20% is still a net growth increment of zero. Mature stands with no positive net growth still have high NPP in the CBM-CFS3 because the model simulates enough gross growth to balance biomass turnover in order to maintain zero net growth, when this is prescribed by the growth curve. However, a change in growth rates will have no impact on NPP of these stands until they are disturbed. In our experiments, old stands will be able to accumulate more C if they were established during enhanced growth scenarios. When an enhanced growth scenario is applied, only actively growing stands (positive net growth) experience growth enhancement in the model simulations, and only that portion of the landscape has enhanced NPP. This is consistent with the findings indicating that older stands do not necessarily exhibit enhanced C accumulation as a result of non-disturbance global change factors (Goulden et al. 1998
). In order for an entire landscape to have 30% increase in NPP, certain stands on that landscape would have to have much greater increase in NPP, which we did not simulate in our scenarios. Sustained increases in NPP of this magnitude over an entire forest landscape would not be consistent with observations that ecosystems often exhibit saturation behaviour under enriched CO2
and often are challenged by increased variability in weather conditions that result in reduced productivity (Canadell et al. 2007b
The dynamic (time-dependent) nature of NPP responses to changes in growth and disturbance scenarios in our model prevents us from rendering a response surface against an axis showing a change in NPP as one of the independent variables. In our modelling environment, NPP is not an independent variable—it is calculated and is sensitive to both changes in growth and disturbance rates (Li et al. 2003
). For example, an increase in disturbance rates results in a decrease in landscape-level NPP because larger areas of productive stands are killed, reducing the area of actively productive stands on the landscape. If it were possible to show response surfaces of the change in ecosystem C stocks after 50 years as a function of the change in NPP (and Rh in the case of the second surface) on the one axis and change in disturbance rates on the other, we might expect the result to appear less dramatic, but the essence of the findings would not change.
It is clear from a,b that very high increases in growth, sustained across the entire landscape, would be required to offset the increases in disturbance rates that are being projected for the North American boreal forest. Moreover, C losses associated with increased disturbance rates can only be offset if changes in NPP result in corresponding changes in NEP. Ecosystem responses to climate change will be complex and will probably not play out over short periods of time. If a simple, hypothetical landscape such as the one used in our simulations requires centuries to approach new equilibrium C stocks following one-time, stepwise ecological parameter changes, then we can certainly expect that real-world system responses to global change perturbations will involve long periods of adjustment with strong, temporally sensitive and synergistic behaviours.