Temporal and spatial variation of fine roots in a northern Australian Eucalyptus tetrodonta savanna

Six rhizotrons in an Eucalyptus tetrodonta savanna revealed seasonal changes in the abundance of fine roots(≤5mm diameter). Fine roots were almost completely absent from the upper 1m of soil during the dry season, but proliferated after the onset ofwet-season rains.At peak abundance of 3.9 kgm⁻² soil surface, fine rootswere distributed relatively uniformly throughout 1mdepth, in contrast withmany tropical savannas and tropical dry forests in which fine roots are most abundant near the soil surface. After 98% of cumulative annual rainfall had been received, fine roots began to disappear rapidly, such that 76 d later, less than 5.8% of peak abundance remained. The scarcity of fine roots in the upper 1m of soil early in the dry season suggests that evergreen trees may be able to extract water from below 1m throughout the dry season. Persistent deep roots together with abundant fine roots in the upper 1m of soil during the wet season constitute a ‘dual’ root system. Deep roots might buffer atmospheric CO₂ against increase by sequestering carbon at depth in the soil.