TY - JOUR
T1 - An examination of the potential efficacy of high-intensity fires for reversing woody encroachment in savannas
AU - Smit, Izak P.J.
AU - Asner, Gregory P.
AU - Govender, Navashni
AU - Vaughn, Nicholas R.
AU - van Wilgen, Brian W.
N1 - Funding Information:
We thank D. Knapp, C. Anderson and R. Martin for contributions as members of the Carnegie Airborne Observatory (CAO). Application of the CAO data was supported by the Andrew Mellon Foundation. The CAO was supported by the Avatar Alliance Foundation, John D. and Catherine T. MacArthur Foundation, Andrew Mellon Foundation, Mary Anne Nyburg Baker and G. Leonard Baker Jr., and William R. Hearst III. Working on Fire and SANParks rangers are thanked for assistance with fire treatments. BWvW thanks the DST-NRF Centre of Excellence for Invasion Biology and the National Research Foundation (grant 87550) for funding. We thank Chris Jones and an anonymous reviewer for comments that have significantly improved the manuscript.
Publisher Copyright:
© 2016 The Authors. Journal of Applied Ecology © 2016 British Ecological Society
PY - 2016/10/1
Y1 - 2016/10/1
N2 - 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.
AB - 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.
KW - Kruger National Park
KW - LiDAR
KW - bush encroachment
KW - fire frequency
KW - fire intensity
KW - fire regime
KW - fire trap
KW - global change
KW - trees
KW - woody densification
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U2 - 10.1111/1365-2664.12738
DO - 10.1111/1365-2664.12738
M3 - Article
AN - SCOPUS:84979537100
SN - 0021-8901
VL - 53
SP - 1623
EP - 1633
JO - Journal of Applied Ecology
JF - Journal of Applied Ecology
IS - 5
ER -