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Forest Topsoil Transfer Experiment

Funding:
Xstrata Coal Mount Owen Complex

Background:
In spoil rehabilitation areas, the spreading of forest topsoil has proven to be the best way of reconstructing a native forest or woodland ecosystem. This is primarily due to the contribution of the provenance seed bank. In revegetating old pastureland, direct seeding is not possible due to very high grass and weed competition. An alternative approach might be to spread a layer of forest topsoil on the pasture, thereby supplying seed while suppressing the competitors.
 
Aim:
To determine whether transferring forest topsoil to abandoned pasture land is a successful method for reconstructing a native forest ecosystem, and whether scraping the ground prior to and, or, ripping the site after topsoil application is more effective.

Summary of Results:
Four different site preparation treatments were tested: simply transferring the topsoil onto unprepared pasture area (un-scalped, un-ripped); double ripping the soil once it had been spread (un-scalped, ripped); scalping the top 10 cm of pasture topsoil prior to spreading forest topsoil (scalped, un-ripped); and both scalping the pasture topsoil prior to, and ripping the forest topsoil after spreading (scalped, ripped).

Native plant density increased with time particularly in the un-ripped plots (see figure below). Partitioning the variance showed that ripping significantly decreased the number of native plants that emerged and survived to 13 months.

The greatest negative effect of ripping was on native shrubs, mainly Pultenaea retusa, while scalping increased their number. Scalping significantly decreased percentage grass cover after 13 months, (see photos below), thereby reducing competition for the native plants. This was not however, reflected in an increase in plant growth of Acacia falcata, which grew slowest in the scalped, un-ripped treatment, although the effect was not statistically significant.

Left - Un-scalped, Right - Scalped

Overall, 44 native species were found in the forest topsoil areas, 26 of which were not present in the untreated control areas. Some uncommon species, such as Murdannia graminea and Templetonia stenophylla were also found in the experimental plots, emphasizing the value of the forest topsoil transfer strategy. There was no significant difference in average species richness between the different treatments after 13 months. No canopy species emerged from the topsoil seed bank.

Conclusions:
Forest topsoil successfully transferred a number of species to the pasture area. These include species that would have been very difficult to obtain as seedlings to plant. After 13 months, the best strategy appeared to be scalping the pasture topsoil away first, then spreading the forest topsoil and not ripping. This produced the highest native plant density, and lowest cover of weeds and grasses, even though plant growth was a bit slower than in the other treatments. This experiment will continue to be surveyed to observe any changing trends with time. We will also determine if these treatments affected the availability of nutrient-acquiring root-microbe associations