In this post Izak Smit discusses his recent paper ‘An examination of the efficacy of high-intensity fires for reversing woody encroachment in savannas‘.

Woody densification in savannas

Many studies have documented how grasslands and open savannas are being invaded by woody plants. This phenomenon is predicted to increase as atmospheric CO₂ levels increase, favouring woody plants at the expense of grasses. Woody encroachment can have many negative effects, including loss of agricultural production (↑cover ≈ ↓livestock), changed herbivore assemblages (↑cover ≈ ↓grazers) and reduced eco-tourism opportunities (↑cover ≈ ↓game viewing opportunities ≈↓tourism). Herbicides or mechanical clearing can reverse woody densification, but are expensive and impractical over large scales, and often have unintended and negative ecological consequences. Frequent fire can also be used to combat woody densification, but few studies have explored whether high intensity fire can be used as a large, infrequent disturbance to reverse densification. In our study we examined the effectiveness of high intensity fires for reducing woody cover in the Kruger National Park (South Africa) (Fig. 1).

Fire intensity effects on woody cover

We compared woody vegetation structure (cover and height) in areas exposed to repeated high intensity and low intensity fires, as well as protection from fire. We collected pre- and post-fire 3D information on woody vegetation using a LiDAR sensor mounted on an aeroplane (Fig. 2). We found that two successive high intensity fires opened up the landscape and reduced woody cover, as intended. In comparison the areas burnt by two successive low intensity fires became even denser over the 4 year study period (Fig. 3). The high intensity fire was successful, at least in the short term, in reducing woody cover, but we were surprised at the number of tall trees (>10m tall,  with a canopy radius of >3m) that collapsed following high intensity fires (Fig. 4). Tall trees are normally considered to be unaffected by fires because their canopies are beyond the flame zone, but more than a third of tall trees collapsed after two successive high intensity fires. This was much higher than the rate of collapse following low intensity fires (6%), or protection from fire (3%). Trees are apparently vulnerable to high-intensity fires because they have been compromised by having their bark removed by elephants (often also leading to wood borer infestation, Fig. 5).

We were hoping our results would indicate that high intensity fires could be used to reduce cover of encroaching shrubs. Although we confirmed this, at least in the short term, the accompanying damage to tall trees creates a management conundrum. High-intensity fires can be used to reduce bush encroachment, but only at considerable cost to the tall tree component. Considering that top-killed small shrubs can potentially recover rapidly after fires  (Fig. 6), and that recovery of tall trees will be slow (Fig. 4), it is clear that a regime of infrequent, high-intensity fires can simultaneously produce a positive and a negative outcome. Trees are important in the landscape, inter alia for nutrient cycling, shade for large mammals, habitat for small fauna and nesting sites for raptors. Therefore, managers will need to make trade-offs when contemplating the manipulation  of fire intensity to achieve specific goals. One 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. We predict that in areas without elephants or with lower elephant densities, high intensity fires may have less of an impact on tall trees.