Soil Phosphate and Its Role in Molding Segments of the Australian Flora and Vegetation, with Special Reference to Xeromorphy and Sclerophylly

Abstract

It is postulated that the Australian angiosperm flora ins derived from a sample of a one—time world flora of about 180 families that entered Australia by a land bridge from Asia. Many of the genera that entered still persist. New genera evolved in all habitats, many from lineages which were adapted to soils of low fertility determined primarily by phosphate level. Soil parent materials with the highest phosphate content are derived mainly from the Tertiary basalts in the east and support rainforests (except in cold regions). Soil phosphate levels decrease from east to west across the continent. The number of rainforest genera in any community is usually correlated with soil P. The converse holds for xeromorphic genera. Some rainforest genera therefore are more readily adapted to aridity than to low fertility conditions. Expansion and contraction of the rainforest is determined partly by climate and partly by soil fertility levels. A sudden expansion of the rainforest in the Oligocene occurred when the basalt flows covered part of eastern Australia. Since that time the rainforests have contracted, partly as a result of the removal of basaltic material and partly because of the development of colder climates in the ranges. The absence of rainforest and the paucity of rainforest genera in southwest Western Australia are accounted for by low fertility levels rather than by a decimation of rainforest genera by a past dry climate. Xermorphy, a result of high lignification, heavy cutinization, silicification, or a combination of these may occur in rainforest species. Adaptation to low fertility accentuates xeromorphic characters through a reduction in leaf size. The low fertility xeromorphs are not xerophytes. Anatomical features have little significance in their survival, which is determined by the ability of the plants to withstand long periods of mineral starvation. Experimental work indicates that the degree of xeromorphy can be reduced in many taxa by the addition of phosphorus and nitrate; four different types of leaf response have been identified.