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Topic 6: Assessment Of Possum Browse On Vegetation

YOUNG. I would be interested to know of people who have been able to evaluate the techniques of assessing plant damage by possums, particularly on native vegetation. Do we have people who are routinely determining the effect of different population levels on vegetation, either immediately or over the long-term? - I mean something on paper, not just a quick assessment of dead trees.

BROCKIE. In the Ecology Division of DSIR M.J. Meads has been looking closely at the effects of possums on northern rata trees and has devised a method for measuring the amount of damage and browse on them. He has published this information in the New Zealand Journal of Zoology ^*. He has attempted to apply this to other kinds of trees, but hasn't come up with the same degree of success there. It is very difficult to measure the effect on any kind of tree and put a solid figure on it.

FITZGERALD. Along with Mike Meads, I have been looking at browse on about 60 species of trees in the Orongorongo area; as Bob Brockie says, it is very difficult to try to make a quantitative assessment of damage that will stand up to the rigorous sort of statistics that seem to be required these days. Dr Ian Atkinson of Botany Division has been doing the same sort of thing on Kapiti Island and I think he and Mike Meads have used the same type of technique. Jim White has also done some research using the same techniques, so we do have some information, but apart from the Kapiti data, it all relates to the Orongorongo Valley. It probably is not all that relevant throughout New Zealand, but at least it gives you some ideas of techniques that can be used.

COLEMAN. We have done some browse work as well. In our case we were climbing some 100 rata trees for a year to mark some small branchlets. Needless to say the enthusiasm was not much initially and it waned as the study went on. I am not sure of the value of some of the information we gained.

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GREEN. I was particularly struck by the damage done by animals other than possums, such as invertebrates. I felt that it was extremely difficult, without much background knowledge, to differentiate possum chew from stick-insect chew, from larval chew and so on. Until someone can sort this problem out it will be difficult to correctly assess the extent to which possums contribute to observed damage.

FITZGERALD. Yes, I can substantiate that. For a long time I have called something possum damage when it was the decidious or semi-decidious nature of the particular plant species. It is just that easy to be wrong.

CLOUT. With the assistance of NZ Forest Products Ltd., we ran a trial on the effect of artificially browsing Pinus radiata seedings. The results will be appearing in my Ph.D. thesis^*.

KEBER. I have extended Mike Clout's work on newly-planted or very small trees and I have extended it to cover bark stripping of trees of up to say 10 years old and 20 m height. Besides the simulated damage work I have marked about 800 naturally possum-damaged trees, with measurements taken yearly, to see the effect on growth. At this stage it appears that for the damage to be important the tree has to be actually killed; trees can tolerate a large degree of damage which probably makes very little difference to the growth.