The Burden of Mt Burnett

Forest and Bird, Issue 302, 1 November 2001, Page 19

The Burden of Mt Burnett

DEAN BAIGENT-MERCER.

DEAN BAIGENT-MERCER

Miners and rare plants compete for the dolomite deposits of

Mt Burnett, according to

n summer from the southern summit of Mt Burnett you can see 4». the sweeping arch of Golden Bay, the picturesque Aorere Valley, a patchwork of farms and a winding river, and the velvety deep-red of flowering northern rata in Kahurangi National Park. Venture up Mt Burnett’s southern ramparts, however, and there is an opencast dolomite mine, climbing up through seven working ‘benches’ into the bush. The unusual dolomite baserock is both blessing and curse for Mt Burnett. It is a blessing because the Mt Burnett massif has numbers of unusual plants which have evolved specially to survive on its dolomite soils and rocky outcrops. It is a curse because Mt Burnett is the only commercial source for mining dolomite in New Zealand. Dolomite is essentially limestone enriched with magnesium carbonate. Like limestone, dolomite is formed on the seabed by layers of dead sea creatures, calcifying over time. Before quarrying began in 1959, the unique vegetation on Mt Burnett’s southern peak is estimated to have covered 3.7 hectares of shrublands and 5.3 hectares of forest. Mining has destroyed 3 hectares of the unique forest. Now around 40 percent of dolomite forest remains on the southern peak.

In July 1998, the owners of the mine notified the Department of Conservation of its intention to develop a seventh mining bench. They wanted access to more dolomite; also Occupational Safety and Health claimed quarrying had left over-steep contours which were becoming unstable and unsafe for workers. When DoC staff visited the area of the proposed seventh bench they found that "..two nationally threatened plant species endemic to Mt Burnett exist in high numbers’ Three other nationally rare species were also found, all of which have major populations centred on Mt Burnett. Since then, the seventh bench of the mine has been developed, with an application underway to extend the roading system into the adjacent forest park, to make a better turning elbow for machinery. The value of the area for plants has been recorded by Dr Geoff Rogers who says: ‘No other area in New Zealand is judged to have greater floristic endemism or biogeographic significance at such a restricted scale. Its lithological and botanical rarity is of the highest order. Apart from the [mining site] I conclude that the Mt Burnett dolomite-ecosystem is of the highest national significance’. On small areas where dolomite is

Fantastic

exposed, unusual plants are evolving. Mt Burnett has a high rainfall, big winds, with karrenfields of dolomite outcrops, and little soil. There are two obvious habitats for its dolomitic plants. The first is the steep, grey-finned dolomite outcrops, on the summit and ridge, which support shrublands. The second is the exposed upper slopes where a deeper soil has developed, which allows a low, windshorn forest to grow. Shrublands dominate the northern peak, while a mix of stunted forest and shrublands occur on the southern dolomite peak. Rock outcrops host mostly herbs and shrubs, which tolerate full exposure to the elements. Here there is a shrubby daisy (Brachyglottis laxifolia), an unusual form of flax with leaves only two centimetres wide and 40 centimetres long, a rare hebe (Hebe townsonii), an unnamed dolomite-loving aniseed (Gingidia aff. montana), a little unnamed yellow-flowered herb (Senecio aff. glaucophyllus), another handsome hebe (Hebe aff. glaucophylla subsp. "Burnett"), and a threatened sedge (Carex dolomitica). The last four species appear to exsist only on Mt Burnett. The combination of high light-levels, due to the relatively open forest canopy, and deeper soils has allowed a unique forest understorey to develop. You can’t help but notice the strange mix of trees

and shrubs, with small trunks appearing out of the severe fins of karrenfield, often with a carpet of an unnamed native iris (Libertia "marble" ) beneath. The small tree, Myrsine argentea, a relative of weeping matipo, is another plant specific to Mt Burnett and makes up a notable component of the canopy. Occasionally a rare carnivorous land snail Powelliphanta gilliesi gilliesi can be found amongst the leaf litter. Coprosma obconica, a threatened small-leaved shrub of two metres in height, appears in the stunted forest understorey and scrub. Mt Burnett is home to one of the largest populations of this plant. Coprosma obconica, Myrsine argentea, and Carex dolomitica are all listed as ‘vulnerable’ on the list of nationally threatened plants. They are likely to be moved into the ‘endangered’ category in the near future if factors causing their decline continue. Apart from habitat destruction, the other threat to these plants is weed invasion. Mexican daisy is the big threat for the shrubby daisy, aniseed, and endemic sedge, Carex dolomitica. Mining continually creates favourable habitat for Mexican daisy: full sun and fine rich gravels — the same habitat that rare herbs prefer. Some remedial work has been required of OMYA by DoC. Weed control has been contracted by the company against

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Mexican daisy, and this has had some success. Other weeds present are gorse and buddleia. Revegetation experiments are also being carried out. But will OMYA fund weed control and revegetation measures beyond the life of the mine?

OMYA already has within its license area all the best areas of dolomite forest and therefore the best places for their conservation.

is a freelance

writer about plants and wild places.