Modelling the long-term effects of fire frequency, intensity and rainfall variability on tree-grass coexistence in a semi-arid savanna, Namibia

Background: The semi-arid savannas of Namibia are shaped by complex interactions among rainfall variability, fire regimes, and vegetation dynamics. However, the long-term consequences of these drivers for tree and grass coexistence remain poorly understood. We developed a simulation model in R to investigate the effects of fire frequency and intensity on grass and tree biomass in semi-arid Namibian savannas. The model incorporates rainfall variability, logistic vegetation growth, and stochastic fire events, capturing interactions between fuel availability, fire probability, and vegetation responses. Results: Grass biomass responded rapidly to wet-season rainfall pulses and showed resilience to fire, whereas tree biomass accumulated more slowly and was strongly suppressed by frequent, high-intensity fires. Low-frequency, mild fires promoted tree establishment and coexistence with grasses, while high-frequency, severe fires maintained grass-dominated states. Intermediate fire regimes fostered dynamic equilibria, with both vegetation types oscillating in response to rainfall and disturbance. Interannual rainfall variability amplified biomass fluctuations, occasionally enhancing tree recruitment in wet years and suppressing it in dry periods, underscoring the role of stochastic drivers in semi-arid ecosystems. Conclusion: Our results emphasize that adaptive fire management is crucial for sustaining productive and resilient savannas. Conducting controlled burns every 3 to 5 years helps reduce fuel loads, prevents excessive accumulation of grass biomass that can drive severe wildfires, and creates opportunities for tree recruitment. Ideally, burns should occur early in the dry season, when fuel moisture is higher, to limit fire intensity and reduce tree mortality. Late dry season fires should be minimized, as they tend to be hotter and more destructive. By integrating these practices with rainfall monitoring, for example scheduling burns after years of high grass growth, rangeland managers can balance forage availability, mitigate wildfire risk, and support the coexistence of trees and grasses in Namibia’s semi-arid savannas.