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Importance of soil

The particular groundcover species present in a woodland depend on the nature of the soil, and this depends on the underlying geology. Clay soils are generally derived from shales and basalts and are generally higher in nutrients than sandy soils. Grasses and small herbs, rather than shrubs, predominate in the groundcover, and the communities may be referred to as Grassy woodlands or Grassy forests. 

Grassy woodlands covered extensive areas of the NSW Tablelands and western slopes where rainfall ranges from 400-700 mm per year. Native grasses such as Kangaroo Grass, Themeda triandra, were common. These woodland areas were sought out by European settlers for grazing, and later cleared for crops or improved pasture, and have now been extensively cleared or degraded. In many areas only small remnants remain. Cumberland Plain Woodland in western Sydney is a good example of a grassy woodland.

By contrast, poor soils develop from sandstones and sands, and favour hard-leaved, often prickly-leaved (sclerophyllous) shrubs that dominate the woodland understories. These include many species of those familiar Australian plants BanksiaLeptospermum and Xanthorrhoea. Shrubby woodlands are characteristic of the sandstones of the Blue Mountains near Sydney, and mountain and alpine areas, as well as sandy coastal areas such as Myall Lakes, on the North Coast. These areas were not suitable for agriculture and much of the native woodland vegetation has survived. Many of these areas make up our main national parks.

Different vegetation and species however occur on the clay soils underlying Cumberland Plain Woodland.

Soil and topography influence floristic spatial patterns

Within the woodland community at the Australian Botanic Garden, Mount Annan, soil and topographical position are important factors in the ecology.

While some species are widely dispersed throughout the woodland area, the distribution patterns for other species  appears to relate to their topographic position. Moist slopes and watercourses are particular sites preferred by some species. Smaller scale microhabitat variation, such as locally moist sites due to subsurface moisture movement, and soil surface conditions e.g. hard compacted surfaces, and exposed subsoil surfaces may also show local variations in species composition.

Differences in species composition may also occur between sites under tree canopies, particularly large isolated trees, and open sites without a tree canopy. This may be partly due to an increased abundance of bird-dispersed species under tree canopies, possibly differing nutrient status under trees compared with more open areas, or the more shaded and therefore cooler conditions under the trees.

However though we can see that some species have preferences for particular condition such as moist or open sites, we don't know much about the ecological factors that allow species to establish in these sites and maintain their occupation in competition with other species.

Wildlife alter soil conditions

Wildlife may alter soil conditions and affect plant distributions. Areas that are cleared of surface vegetation, such as by rabbit diggings or camp sites of kangaroos, or bared by ant nests and their associated trails, may have higher subsoil temperatures. Bare soil areas and higher temperatures may influence soil seedbanks and may provide better sites for seedling germination for some species, with survival perhaps assisted by reduced competition from other plant species that can't tolerate the hotter conditions. Increased nutrients on the latrine mounds of rabbits may affect local species patterns. Apart from casual field observations however, we know very little about recruitment and competition of most species in the field.

Many soil holes are made by larvae of insects such as cicadas and swift moths, as well as by spiders, and these may allow deep, and relatively swift water penetration into the clay soils. In dry conditions cracks also develop in the soil allowing water to penetrate deeply, perhaps up to 30 cm.

Our observations are that rainfall on these clay soils tends to be readily absorbed by the dry surface, leading to relatively even penetration over the land surface, and little runoff except in extended wet periods. In contrast, rainfall on dry sandy soils on sandstone appears to be repelled by the soil surface, and tends to runoff rather than penetrate, leading to a greater concentration along watercourses, but less on upper hillsides.

Non-vascular soil surface organisms

The soil surface in the woodland may have open areas that may be colonised by mosses, lichens and algae. These organisms protect the soil from erosion and provide a microhabitat for germinating seedlings.

One common alga is Nostoc commune which forms a jelly substance on the surface during cool moist conditions, drying to a crust. It fixes nitrogen and picks up seeds and dust. As it dries out and breaks down, seeds caught may establish and grow in the higher nutrient conditions. Nostoc commune is a cosmopolitan alga found naturally in many places around the world.

Legume nodules

Legume species, plants like peas, beans, clover and wattles in the family Fabaceae, harbour nodules of nitrogen-fixing bacteria (Rhizobium spp.) in their roots. These bacteria convert free atmospheric nitrogen into compounds usable in plant metabolism and allow the plants to grow in low nitrogen soils. Leguminous species in our woodland include Desmodium varians, Desmodium brachypodum, Glycine tabacina, Lotus australis, Indigofera australis and Acacia implexa.

Past agricultural treatment

Past soil treatment also has an effect on species composition. Sites around the woodland which have been previously cleared, and possibly cultivated, have only a limited range of native species compared with woodland sites. Such sites have a high weed component possibly due to past fertiliser applications.