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

<p>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.</p> <p>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.</p> <p>3. Ground surveys revealed increases in the number of stems and individual shrubs (&lt; 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.</p> <p>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 90^th 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.</p> <p>5. <em>Synthesis and applications</em>. 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.</p>