Environmental and demographic correlates of tree recruitment and mortality in north Australian savannas

Tropical savannas cover approximately 20% of the earth's land area, and therefore represent an important carbon store. Under scenarios of future climate change it is thus important to understand the demographic processes determining tree cover, namely tree recruitment, growth and mortality. This study measured tree recruitment and mortality in 123 (0.08 h) plots in Kakadu, Nitmiluk and Litchfield National Parks, in the Australian monsoonal tropics, over two consecutive 5-year intervals. Plots were located in two important habitats, both dominated by eucalyptus-lowland savanna and savanna growing on sandstone plateaux. All trees with diameter at breast height (DBH) 5 cm were tagged and identified. Recruitment was calculated as the proportion of tagged trees present at the end of an interval that were not present at the beginning. There were a total of 6666 and 6571 tree-intervals for mortality and recruitment, respectively. We used Akaike Information Criterion (AIC)-based model selection and multi-model inference to relate tree mortality and recruitment to fire frequency, mean annual rainfall (MAR), stand basal area, tree density and eco-taxonomic group. Recruitment decreased with tree density in both savanna types, and in lowland savanna, with the frequency of fires. In sandstone savanna, recruitment increased with MAR. Effects of fire on recruitment were better explained by season than severity of fire, while fire severity had a stronger influence on mortality. Mortality decreased with tree size up to about 25 cm DBH, but increased sharply when DBH exceeded 50 cm. Mortality increased with stand basal area, and increased with the frequency of late dry season fires in lowland savanna only. There was little evidence that mortality was affected by the frequency of early dry season fires or MAR. Both recruitment and mortality rates were higher for Acacia and Proteaceae species than for pantropical or Myrtaceae (including Eucalyptus) species. We identified several negative feedbacks,mediated by changes in tree density and stand basal area that help confer long-term stability to savanna tree cover. Nonetheless, changes such as a long-term increase in MAR or an increase in frequency or severity of fires are likely to result in changes in tree density, stand basal area and therefore carbon storage potential of savannas.