The fate of sub-adult trees trapped in the demographic bottleneck of mesic savannas: Interaction of size, fire season, and grassy understorey determine tree survival, growth, and transition to canopy and ultimately savanna physiognomy

Background/Question/Methods

Tree populations in mesic (>650 mm ppt yr^–1) savannas of the world have strong demographic bottlenecks to the transition of sub-adults to the canopy layer. Such bottlenecks are a major determinant of savanna physiognomy, yet factors that allow sub-adults to traverse the bottleneck are little studied. A conceptual framework was developed, linking landscape-scale variables in savannas (eg, seasonal fires) to the probability of a sub-adult tree reaching the canopy, considering local environments (eg, understorey) and tree attributes (eg, size, seasonal phenology which varies with life history stage). In a 32,400 m2 field experiment, 2,405 juveniles (<150 cm height) and saplings (150–499 cm) of the eucalypt canopy species were individually marked and measured the year prior to fires set in 3 different seasons and again at the end of the following growing season (without fires) and assessed for survival, resprouting, and height growth. All fire treatments were repeated in plots dominated by the most common understorey, a native annual grass (sorghum) and in plots dominated by perennial native species; these produce different fuels for fires and vary in seasonal phenologies and competitive regimes. Trees in unburnt plots served as controls, as did wet season fires which naturally eliminate sorghum.

Results/Conclusions

Fires topkilled most juveniles, but almost all were persistent and few genets died. Genet mortality of juveniles depended on fire season and of saplings to understorey. Fire season x understorey interactions were important for growth of large juveniles and the transition of juveniles to sapling stages. After early dry-season fires, large juveniles and small saplings grew faster, especially those in sorghum. After late dry or wet season fires, however, all of the small saplings (<299 cm height) were reduced to “juveniles” and did not recover pre-fire heights but, produced many new basal (coppice) stems. Initial size was an important explanatory variable for all demographic and growth responses by juveniles and small saplings, generally positively, but not for large saplings. For example, after late dry-season fires, >80% of large saplings (300–499 cm) were reduced to juvenile size in sorghum understorey, whereas in non-sorghum understorey, 60% of large saplings grew to poles (500–999 cm). Unburnt large saplings were more likely to die and transition probabilities to larger sizes decreased, the larger their size, especially in non-sorghum. The key to understanding and managing sustainable woody cover in mesic savannas lies in the complex interactions of sub-adult trees with local environments.