An examination of the potential efficacy of high-intensity fires for reversing woody encroachment in savannas.
Abstract
Frequent fires are often proposed as a way of preventing woody encroachment in savannas, yet few studies have examined whether high-intensity fires can effectively reverse woody encroachment. We applied successive fire treatments to examine the effect of fire intensity on woody vegetation structure. The treatments included early dry season, low-intensity fires; late dry season, higher-intensity fires; and an unburnt control. We used pre- and post-fire airborne LiDAR to compare vegetation structural changes brought about by fires of different intensity. Early dry season fires were of lower intensity (1400-2100 kW m-1) than late dry season fires (2500-4300 kW m-1). The two treatments also differed in terms of fuel consumed, scorch heights and char heights, indicating that clear differences in fire intensity and severity were achieved. After 4 years and two fire applications, relative woody cover increased by between 20 and 110% in different height categories following low-intensity and control treatments and declined by between 3 and 70% following high-intensity fire treatments. Declines were markedly higher following two repeated high-intensity fires than following a high and then a moderate-intensity fire. Because woody shrubs in lower height classes can recover rapidly, repeated high-intensity fires would be needed to maintain lower cover. Tall trees are often assumed to be unaffected by fires. However, we found that the rate of tree loss was directly related to fire intensity, where 36% of trees were lost following repeated high-intensity fires, compared to 22% after a high- and then a moderate-intensity fire and 6% after two low-intensity fires (3% without fire). Synthesis and applications. Using LiDAR data we show that high-intensity fires can, at least in the short term, significantly reduce woody cover in South African savannas. The use of repeated high-intensity fires simultaneously causes both a positive (reduction in cover of short shrubs) and a negative (loss of tall trees) outcome, and managers need to make trade-offs when contemplating the use of fire intensity to achieve specific goals. One potential solution may be to repeatedly apply high-intensity treatments to some areas, and not to others. This could generate a heterogeneous landscape where grasses become dominant and tall trees become scarce in some places, but in others, tall trees persist (or at least decline at slower rates), and shorter woody shrubs increase in dominance. Whether this would be acceptable, or practical, remains to be tested.