Acacia is essentially a tropical genus with its greatest diversity of species in the tropical and subtropical regions where there is intense sunlight and a warm to hot climate for most of the year. Generally acacias require high light levels for seed germination and good growth. The evolutionary response of acacias to the low nutrient soils and poor water availability across much of Australia has resulted in a number of different strategies being evolved.
The root system of acacias has evolved to take advantage of available nutrient and moisture regimes over the various climatic ranges. Many of the species from areas with higher regular rainfall have shallow roots, and some may also develop cluster roots, usually near the litter-soil interface, to extract the maxium amount of nutrients from the decomposing litter. As well, this also enables the plant to take advantage of light rainfalls when moisture penetration is shallow, and may provide a competitive edge over plants with deeper root systems. However, such shallow rooted acacias also suffer in prolonged dry spells and drought, and many may be killed. These species rely on the soil seed store for regeneration. In arid areas root systems may be both deep and shallow, enabling the plant to tap the moisture and the accumulation of minerals held well below the surface, as well as taking advantage of lighter rainfalls and shallower nutrient supplies with the shallow root system.
Acacias, like most legumes, are well known as nitrogen fixing plants. With the aid of Rhizobium bacteria which form nodules on the roots they are able to extract atmospheric nitrogen from otherwise low nitrogen soils. A study of nutrient cycling in a stand of the tropical species Acacia holosericea showed that at least for this species (and probably many others) the roots were infected by two symbionts, the nitrogen fixing Rhizobium bacteria and a mycorrhizal fungi which was presumably implicated in extracting phosphorpus from the phosphorous and nitrogen deficient soil (Langkamp & Dalling 1982).