Simplifying the savanna: the trajectory of fire-sensitive vegetation mosaics in northern Australia
Aim Fire is a key agent in savanna systems, yet the capacity to predict fine-grained population phenomena under variable fire regime conditions at landscape scales is a daunting challenge. Given mounting evidence for significant impacts of fire on vulnerable biodiversity elements in north Australian savannas over recent decades, we assess: (1) the trajectory of fire-sensitive vegetation elements within a particularly biodiverse savanna mosaic based on long-term monitoring and spatial modelling; (2) the broader implications for northern Australia; and (3) the applicability of the methodological approach to other fire-prone settings. Location Arnhem Plateau, northern Australia. Methods We apply data from long-term vegetation monitoring plots included within Kakadu National Park to derive statistical models describing the responses of structure and floristic attributes to 15 years of ambient (non-experimental) fire regime treatments. For a broader 28,000 km2 region, we apply significant models to spatial assessment of the effects of modern fire regimes (1995-2009) on diagnostic closed forest, savanna and shrubland heath attributes. Results Significant models included the effects of severe fires on large stems of the closed forest dominant Allosyncarpia ternata, stem densities of the widespread savanna coniferous obligate seeder Callitris intratropica, and fire frequency and related fire interval parameters on numbers of obligate seeder taxa characteristic of shrubland heaths. No significant relationships were observed between fire regime and eucalypt and non-eucalypt adult tree components of savanna. Spatial application of significant models illustrates that more than half of the regional closed forest perimeters, savanna and shrubland habitats experienced deleterious fire regimes over the study period, except in very dissected terrain. Main conclusions While north Australia's relatively unmodified mesic savannas may appear structurally intact and healthy, this study provides compelling evidence that fire-sensitive vegetation elements embedded within the savanna mosaic are in decline under present-day fire regimes. These observations have broader implications for analogous savanna mosaics across northern Australia, and support complementary findings of the contributory role of fire regimes in the demise of small mammal fauna. The methodological approach has application in other fire-prone settings, but is reliant on significant long-term infrastructure resourcing.