Geology and soil characteristicsGeology and soil characteristics

Geology and soil characteristics

The gorge walls are composed of Triassic sandstones from the Narrabeen Group. Soils are sandstone-derived boulder alluvium, with high organic matter and some shale component (Jones et al. 1995). The soil is very shallow. In some areas there is little or no soil layer. Roots of Wollemi Pine grow into rock fissures or extend for tens of metres away from the main groups of trunks. Nutrient levels are low and the soil is extremely acidic, often in the range pH 3–4, with low levels of most elements although high in aluminium, sulphate and iron (Offord et al. 2014; Zimmer et al. 2016 S2 Table). There are patches of highly saline soil (Offord et al. 1996; Offord unpubl. data).

Senescent Wollemi Pine branches fall and contribute substantially to the litter layer (Hill 1995). Decomposition of these fallen branches may contribute to the low pH of the soil. Acidity has not been found to inhibit growth of potted seedlings; indeed, potted plants exhibit higher growth rates in pH 4.5 soil compared to pH 6.5 soil (Offord et al. 2014).