High-intensity fires may have limited medium-term effectiveness for reversing woody plant encroachment in an African savanna

1. Woody thickening or “bush encroachment” is a growing concern in
savannas worldwide and can reportedly be reversed by applying
high-intensity fires. Preliminary findings following experimental fires in
2010 and 2013 indicated that woody plant cover declined one year after
high-intensity fires, but increased after low-intensity fires. However,
the longer-term outcomes of high-intensity fires are largely unknown. 2.
To establish longer-term outcomes, we re-assessed sites subjected to Low,
Medium and High-intensity fire treatments 10 years after the initial
experimental fires. We compared woody vegetation structure in 2010 with
that in 2020 using both ground surveys and airborne LiDAR. 3. Ground
surveys revealed increases in the number of stems and individual shrubs
(< 10 m tall) over 10 years, and decreases in shrub height, with no
significant differences between treatments. Large trees (≥ 10 m) declined
by about 65% in number due to ongoing high mortality across treatments
over 10 years. 4. LiDAR surveys revealed significant but very small
differences in woody plant height and cover between treatments after 10
years. Median height was around 2 m in all treatments, and 90th percentile
tree height was moderately taller in the Low fire treatment. Mean canopy
cover was about 55% in all treatments. The treatments, therefore, did not
result in a meaningful reversal of woody encroachment with no discernible
difference between the treatment sites after 10 years. 5. Synthesis and
applications. The application of high-intensity fires did not reverse
woody encroachment in the longer term. In addition, the application of
such treatments would be impractical at a large scale. Within a framework
of Strategic Adaptive Management, the next logical step would be to
attempt a different approach. In this case, it is intended to use early or
late wet season burns to increase the mortality of shrubs when they are in
a more vulnerable phenological state. This study illustrates the
importance of ongoing long-term monitoring, review and adaptation for
finding practical ways to achieve desired ecological outcomes.