Wallaby browsing in the Mt Cole State forest – a coupe-level analysis.  J. Di Stefano.  February 2002.  35pp. (unpubl.)

SUMMARY

Research during the last four decades has shown that browsing on vegetation by native and introduced species of mammal can have a substantial impact on the survival, growth rates and form of trees in both native forests and plantations. Recent surveys (Wallace and Fagg, 1999; Sebire, 2001) show that browsing on Victorian native forest coupes is a widespread and serious problem. Much anecdotal evidence points to swamp wallabies (Wallabia bicolor) as a species that causes major browsing damage.

The primary aim of this research was to test the hypothesis that browsing intensity on native forest coupes in the first 12 months after eucalypt germination will be lower near mature burnt forest than near mature unburnt forest. Coupe-level data were also collected about a number of other browsing-related variables. These are:

1. The regeneration of understorey vegetation after fire.
2. Changes in wallaby abundance over time.
3. The spatial distribution of browsing damage and wallabies.
4. Comparisons between the use of eucalypt seedlings by browsing animals and the availability of these seedlings.
5. The relationship between seedling density and browsing intensity.

Understorey vegetation (mainly bracken) burnt by the March 1999 fire took between six and nine months to recover in the mature burnt forest. On the coupes, vegetative cover (mainly bracken, silver wattle and eucalypts) rose to a level equivalent to the cover in the mature unburnt forest by December 1999 - nine months after the fire and about six months after eucalypt germination. The observed pattern of vegetation change meant that the coupe areas contained both food and shelter for swamp wallabies at this time.

The browsing assessment showed that most seedling mortality occurred in the first six months after eucalypt germination. Browsing damage was greatest six months after germination, although much variability existed between coupes. A moderate (but acceptable) level of browsing was observed in the second year after germination. Variation in browsing damage also existed within coupes, particularly six months after eucalypt germination. At that time, browsing damage was significantly greater at the coupe edges adjacent to the mature unburnt forest than at the coupe edges adjacent to the mature burnt forest. Six months after eucalypt germination, the majority of browsed seedlings had some of their main stem removed.

The growth rate of seedlings varied between species, and in time and space. Overall, blue gum (Eucalyptus globulus) grew faster than messmate (E. obliqua). On the two coupes affected by the wildfire, the growth rate of both species six months after germination was greater at the coupe edge adjacent to the mature burnt forest than at the coupe edge adjacent to the mature unburnt forest. A strong negative correlation between browsing damage and growth rate at this time suggests that browsing intensity explained much of the variation in growth rate. Measurements of growth rate from October 1999 to June 2000 showed that in all but one instance, the growth rate of seedlings that had experienced substantial browsing during this period was significantly less than seedlings experiencing little or no browsing.

Swamp wallaby abundance (measured as scats/m²/day) remained constant in the mature unburnt forest throughout the duration of the study. In the burnt forest, abundance was close to zero until January 2000 after which it rose to levels comparable to the unburnt forest. Swamp wallaby abundance on the coupes was close to zero until December 1999 after which it rose constantly.

By the end of the monitoring period (June 2000) wallaby abundance on the coupes was about four times as great as wallaby abundance in the unburnt forest.

In general, the distance from monitoring plots to shelter vegetation explained little of the variation in either mean browse score or swamp wallaby abundance. For each of the independent variables (mean browse score and abundance), the relationship was only significant in one out of three cases. This suggests that variables other than distance from shelter were responsible for most of the variance in mean browse score and swamp wallaby abundance at the study coupes.

Analysis of use and availability data for messmate, blue gum and manna gum (E. viminalis) seedlings showed that messmate was browsed more than expected by chance while manna gum was browsed less than expected. Further analysis indicated that messmate seedlings were browsed more heavily than the other two species.

A significant positive relationship between seedling density and mean browse score was observed at an infra-coupe scale. The form of the relationship suggested that browsing intensity would be low at very high seedling densities (e.g. > 100 000 seedlings per hectare). At seedling densities, however, that forest managers can hope to achieve as part of normal regeneration operations, browsing intensity was variable.

There are a number of management implications that emerge from this work:

1. If possible, seedlings should be protected from browsing as early as possible, as most browsing occurs prior to six months of age. Nevertheless, protecting seedlings 12 months after germination may be acceptable.
2. High initial seedling growth rates are likely to mitigate the effects of browsing.
3. Wallabies at relatively low abundance have the capacity to cause substantial browsing damage. As a consequence, browsing control strategies that aim to reduce population size may not be effective.
4. Increasing the size of native forest coupes up to 40 ha is unlikely to reduce browsing severity.
5. Manipulating the mix of eucalypt species regenerating at a site may reduce overall browsing damage.
6. Resource manipulation may not be an effective way to reduce browsing by swamp wallabies on native forest coupes. This is because the two most critical resources for swamp wallabies (food and shelter) occupy the same space as the crop (eucalypt seedlings) shortly after harvesting.

Browsing damage to eucalypt seedlings in native forest coupes involves complex interactions between many factors including wallaby abundance, seedling density, seedling growth rate and the availability of different food species. Manipulating these factors to reduce browsing damage will be a difficult task. Wallaby exclusion (fencing) is, at present, the only effective browsing reduction strategy.