Gorse conversion trial for Rotorua

Bay of Plenty Regional Council has approved funding for a trial to convert gorse-covered hills behind the Rotorua airport to pine forest in an effort to reduce the amount of nitrogen leaching into Lake Rotorua.

Mature gorse leaches nitrogen at a high rate into waterways, and with 864 ha of gorse known to be in the Rotorua catchment, about 43 tonnes of nitrogen enters the lake from mature gorse each year, contributing to the lake’s problems.

The council has reached an agreement with the Maori-owned block’s trustee to convert the 18.5 ha of land to pine forest for the trial – which could become a template for future nitrogen-saving conversions on other Maori trust blocks with significant gorse issues in the catchment.

Bay of Plenty Regional Council’s Sustainable Farming Advisor John Paterson said gorse was a well-known cause of nitrogen leaching into waterways. The 50 kg of nitrogen per hectare leaching from established gorse was comparable with the amount leached by intensive land use, such as dairying.

“Virgin bushland and pine forest has a very low leaching rate of about three to four kg per hectare, so this conversion could save about 90 percent of the nitrogen leaching. It is also much better than converting gorse back to pasture,” Mr Paterson said.

A Masters student will be working on the project, and also undertaking a three year study into the best ways of converting land from gorse into forestry. Test plots will determine the best methods of limiting the initial ‘spike’ of nitrogen released when the gorse is removed, so that the conversions will be as efficient as possible.

The $145,000 cost of the project is about a quarter of other nitrogen removal projects, Mr Paterson said.

“If this initial conversion is successful we will approach other owners of the largest blocks of gorse-covered land with similar plans. This project will also add to the existing scientific knowledge about nitrogen leaching from gorse, and establish best practice for future conversions.”

ENDS

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