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Patch Experiment

Xstrata Coal Mount Owen Complex

The Forest Topsoil Transfer  experiment showed that forest topsoil could successfully be transferred to pasture land to assist in reconstructing native vegetation communities. Because forest topsoil is such a valuable and limited resource, it has to be used to maximum effect. A large number of small patches in the landscape, acting as sources for native seed dispersal, could potentially have a greater impact on the surrounding pastureland than a few large patches of topsoil. Small patches may however, be more vulnerable to weed and grass invasion.

To determine whether topsoil patch size affects native plant sustainability and dispersal capacity.

Summary of Results:
Six blocks of two topsoil patch sizes were set up in an offset area of Mount Owen Mine (Hunter Valley, NSW, Australia), one four times as big as the other (see diagram below). Half of each patch was seeded with canopy species.

After one year, surveys indicated that 57 different native species had emerged from the topsoil. These ranged from the seeded canopy species Corymbia maculata to herb species.

Native plant density of the shrubs and trees increased over time, with no statistical difference between the plot sizes at either time point (see figure below).

Some Eucalypts emerged out of the un-seeded plots indicating some provenance seed bank and numbers also increased over the two years since the start of the experiment. The system is dominated by the upper middle storey Acacias and Pultenaea retusa in the sub-shrub layer. There are signs of reproductive potential on the site, including flowers, seed pods and seedlings. The plots appear to be healthy and sustainable at this point.

One of the primary questions of this experiment is whether the small topsoil patches will be more rapidly invaded by the surrounding grass and weed species, which will compete with the native plant population. Surveys showed an average of 60% grass and weed cover, with no significant difference between the big and small plots after two years. If the plots were being colonised from the outside inwards, there would be a gradient of ground cover from the edge to the centre of the plots. Analysis of the edges versus the inside areas of the big plots however, revealed no significant difference. Instead, it seems as though the grasses are emerging from beneath the topsoil layer, with more colonisation occurring where the topsoil was spread less thickly. Scalping the grassland topsoil layer prior to spreading the forest topsoil would have reflected colonisation from the outside more accurately.

To date, the two patch sizes are not significantly different from one another. The native plants appear to be reproducing successfully and their dispersal into the surrounding pastureland will be followed. This will determine how successful they are as sources for revegetating the entire area over time.