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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 Li DAR 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 Li DAR 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. [ABSTRACT FROM AUTHOR]

Factors controlling savanna woody vegetation structure vary at multiple spatial and temporal scales, and as a consequence, unraveling their combined effects has proven to be a classic challenge in savanna ecology. We used airborne LiDAR (light detection and ranging) to map three-dimensional woody vegetation structure throughout four savanna watersheds, each contrasting in geologic substrate and climate, in Kruger National Park, South Africa. By comparison of the four watersheds, we found that geologic substrate had a stronger effect than climate in determining watershed-scale differences in vegetation structural properties, including cover, height and crown density. Generalized Linear Models were used to assess the spatial distribution of woody vegetation structural properties, including cover, height and crown density, in relation to mapped hydrologic, topographic and fire history traits. For each substrate and climate combination, models incorporating topography, hydrology and fire history explained up to 30% of the remaining variation in woody canopy structure, but inclusion of a spatial autocovariate term further improved model performance. Both crown density and the cover of shorter woody canopies were determined more by unknown factors likely to be changing on smaller spatial scales, such as soil texture, herbivore abundance or fire behavior, than by our mapped regional-scale changes in topography and hydrology. We also detected patterns in spatial covariance at distances up to 50-450 m, depending on watershed and structural metric. Our results suggest that large-scale environmental factors play a smaller role than is often attributed to them in determining woody vegetation structure in southern African savannas. This highlights the need for more spatially-explicit, wide-area analyses using high resolution remote sensing techniques.