Another victim of possums

Forest and Bird, Issue 291, 1 February 1999, Page 10

Another victim of possums

—Geoff Rogers

ossums are to blame for the premature die-back and death of New Zealand’s mountain cedars, or pahautea (also known as kaikawaka). The majestic conical tree grows about the treeline of forests in our wetter, cloudier mountains. In places, such as Hihitahi, the Ruahine Range and on Mount Egmont/Taranaki, dead trees stand above the forest in considerable numbers. It is the scale of this decline that suggested to scientists that possums, and only to a lesser extent natural ageing, were to blame for the dead trees of all ages. Before this work, only two species of totara out of 23 native conifers were recognized as palatable to possums. For many decades forest ecologists

remarked on the poor health of the cedar in many catchments but, with no unifying or recognized threat, attributed the decline to natural ageing. A study of the advanced decline and eventual death of the trees led to close examination of their foliage, revealing clipped terminal shoots. Many trunks had typical signs of possum use. The clinching evidence against possums was not available for many years, however, because methods of diet analysis involved washing stomach contents through two-millime-tre mesh screens. Pahautea’s scale leaves, which are tiny, were mostly disappearing down the

sink! But a ‘pre-wash’ examination then revealed large amounts of pahautea in the possum stomachs. The research shows that possums browse cedar only in late spring-early summer, when the leaf buds burst and mature. While possums can be killed efficiently, conservation cost accountants now require measurable outcomes from their investments, and therein lies a difficulty. Early defoliation is difficult to detect in a tree with a tall conical profile and with branches attacked randomly, unlike the overall ‘hedging effect’ seen on the two totara species. Pahautea’s small leaves and sluggish growth rates make traditional methods of scoring tree

recovery rates unsuitable. A biological complication is that pahautea seems to have little or no capacity to resprout from damaged branchlets. Research shows trees with more than 75 percent of foliage damaged cannot recover. In contrast, Hall’s totara can recover with as little as five percent of its foliage remaining. One of the worst examples of accelerated die-back of cedar and Hall’s totara is in Hihitahi Forest Sanctuary, between Taihape and Waiouru. Ironically, this was set aside to protect one of the last remnants of a pahautea forest which once stretched from Waiouru to Ruahine Corner in the Ruahine Range.

The few trees on Banks Peninsula are in trouble. Many of those greeting visitors to Dunedin, on the northern motorway, are ‘unthrifty. In fact, the trees featured on the front cover of Wild Dunedin, the award-winning book by Neville Peat and Brian Patrick

sponsored by Forest and Bird, show classic symptoms of possum defoliation. It is not yet known whether pahautea’s cousin, the feathered cedar or kawaka of the northern North Island, is also preferred by possums.