# The Long-Term Consequences of Forest Fires on the Carbon Fluxes of a Tropical Forest in Africa
## 3 Results
### The Influence of Recurrent Fire Events on Forest Dynamics  
In order to estimate the consequences of regularly occurring fire events on the forest dynamics, numerous fires were simulated with the forest model FORMIND. On average, a fire of the size of 20% of the simulation area (9 hectares) was simulated every 4 years (cf. Table 1). This scenario corresponds to results of satellite data analysis on fire areas at the Kilimanjaro region. Compared with the undisturbed scenario without fire, the dynamics in the fire-disturbed forest are significantly altered (Figure 11). Total aboveground biomass is reduced by 80%; the forest thus has a deficit of 300 tons of biomass per hectare. The first species group consisting of large climax species has the largest reduction, as this species group loses 95% of its biomass. The pioneer species (PFT 4 and 6), on the other hand, can increase their biomass considerably. Both species groups account in total for 70% of the total biomass in the fire scenario, whereas they played no role in the undisturbed scenario. The disturbed forest does not reach a state of equilibrium in terms of species composition. The climax species can increase in biomass after a fire, but it loses a lot of biomass during the next fire event. Overall, the shade-tolerant tree species (PFT1 and 2) account for only 8% of the total biomass, which varies largely over time (1-45%). Figure 10: Simulated net ecosystem exchange after a fire event. Positive values correspond to net carbon sequestration in the forest. In the year 250, a large-scale fire was simulated, which led to a strong carbon source. (**a**) Annual carbon flux (NEE) and (**b**) cumulative carbon flux. The red area in (**b**) indicates the cumulative carbon flux before the forest fire. Due to the high carbon emissions during a fire event and the carbon sequestration in the regeneration phase after a fire, the variation of the carbon balance (NEE) is very high (Figure 12b). In the undisturbed forest, the standard deviation of carbon flux is 0.4 t\({}_{\text{C}}\) ha\({}^{-1}\) yr\({}^{-1}\), whereas in the scenario with regular forest fires, the standard deviation is 10 times higher. However, the mean carbon flux is balanced at zero, just as in the undisturbed scenario (Figure 12a,b). Considering the cumulative carbon balance of both scenarios (Figure 12c), both reach a state of equilibrium after about 100-150 years. However, the equilibrium state in the scenario without fire is 150 t\({}_{\text{C}}\) ha\({}^{-1}\) higher. Investigating the species composition with increasing fire intensity, the simulation results show the typical picture of a hump-shaped curve (Figure 13). If the ecosystem is undisturbed (i.e., fire frequency is zero), the normalized Shannon Index is low at 30% (= standard Shannon Index divided by ln(6)). If a fire occurs annually (equivalent to 10 fires per decade), the Shannon Index increases to 50%. Fires that occur less frequently than annually, however, produce an even higher species diversity (Figure 13). This results in a maximum of the Shannon Index for 2-3 fire events per decade. Analysis of satellite data for the Kilimanjaro region showed between one to four forest fires per decade. Thus, high species diversity can be expected in disturbed areas around Mt. Kilimanjaro. If future fire frequency increases due to higher temperatures and less precipitation [14; 57], there might be a reduction in functional diversity in the study area, according to the simulation results (Figure 13). Figure 11: Simulated aboveground biomass for each plant functional type (PFT; climax green, pioneer red, mid-tolerance purple) without fire events (**a**) and after numerous fire events (**b**). On average, there is a fire in this region every 4 years. (**c**) Comparison of the species composition for an undisturbed and disturbed forest (here, the biomass fraction of each plant functional type serves as a proxy for diversity). odm = organic dry matter.