Spatio-temporal trends in tree cover of a tropical mesic savanna are driven by landscape disturbance.
Abstract
How tree cover in tropical savannas changes through time and space is a major unresolved issue in ecology due to the complexity of these systems where fire, grazing, climatic variability and landscape variation interact, determining the spatial patterns of biomass. We aimed to assess the relative importance of fire and grazing in determining multi-decadal and landscape-scale patterns of savanna tree cover and biomass in a mesic savanna. Assessment of digitized aerial photography, from 1964, 1984, 1991 and 2004, was undertaken for 40 sites at the Kapalga fire experimental area (∼1470 mm mean annual rainfall) in Kakadu National Park in monsoonal northern Australia. Spatio-temporal changes in tree cover were analysed using linear mixed effects modelling, with multi-model inference in an information theoretic framework. Savanna tree cover fluctuated greatly over the 40-year period, and varied spatially amongst the 40 sites, by 20-30% in a given time period. Tree cover was most likely to decline when initially high, and with a high frequency of fire, especially late dry season fire. The presence of a feral grazer had little effect other than through reducing fire frequency. Our results are consistent with an overall increase in tree cover in these mesic savannas during drier-than-average periods. Ground-based measurements of change in stand basal area under experimental burning regimes agreed with the estimated effects of fire on tree cover at the same site based on our analyses of aerial photography. Synthesis and applications. The dynamism of these mesic savannas is driven by variation in fire frequency and severity, which is influenced by feral grazers, creating a mosaic of tree cover that shifts over time in response to localized variation in disturbance. These findings are relevant to the management of this biome. We show that changing disturbance regimes alter the dynamics of these systems. Fire management incorporating a variety of fire regimes will promote the persistence of a savanna patch mosaic.