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Abstract:
Spatial variability in the effects of fire on savanna vegetation structure is seldom
considered in ecology, despite the inherent heterogeneity of savanna landscapes. Much has been
learned about the effects of fire on vegetation structure from long-term field experiments, but
these are often of limited spatial extent and do not encompass different hillslope catena elements.
We mapped vegetation three-dimensional (3-D) structure over 21 000 ha in nine savanna
landscapes (six on granite, three on basalt), each with contrasting long-term fire histories (higher
and lower fire frequency), as defined from a combination of satellite imagery and 67 years of
management records. Higher fire frequency areas contained less woody canopy cover than their
lower fire frequency counterparts in all landscapes, and woody cover reduction increased linearly
with increasing difference in fire frequency (r2¼0.58, P¼0.004). Vegetation height displayed a
more heterogeneous response to difference in fire frequency, with taller canopies present in the
higher fire frequency areas of the wetter sites. Vegetation 3-Dstructural differences between areas
of higher and lower fire frequency differed between geological substrates and varied spatially
across hillslopes. Fire had the greatest relative impact on vegetation structure on nutrient-rich
basalt substrates, and it imparted different structural responses upon vegetation in upland,
midslope, and lowland topographic positions. These results highlight the complexity of fire–
vegetation relationships in savanna systems, and they suggest that underlying landscape heterogeneity needs more explicit incorporation into fire management policies
