Managing gorse for cleaner lakes

For immediate release: 5 July, 2010

A plant pest which is widespread in the Rotorua area has also been found to be a significant contributor to nutrients polluting the lakes, an Environment Bay of Plenty study has found.

Gorse is New Zealand’s most widespread and well-known brush weed, reducing stock carrying

capacity on farms, competing with established trees in forests and providing cover for pests such as possums and rabbits.

A 2008 council-funded study by Scion indicated that gorse contributed to nitrate leaching to groundwater, and Environment Bay of Plenty has recently completed investigations to measure just how much nitrogen is leaching from gorse over the entire Lake Rotorua catchment. Sustainable Farming Advisor John Paterson said the aim was to measure how much gorse there was, how much nutrient was leaching from it and what that meant for the health of Lake Rotorua.

“We know we have gorse all over the catchment. Now we have mapped the gorse extent and categorised it by maturity and density. This showed that there are nearly 900 hectares of gorse in the Rotorua catchment, with most of it tall, mature gorse in dense stands.”

This gorse is leaching about 50 kg of nitrogen per hectare to groundwater, and that means that about 43 tonnes of nitrogen derived from gorse is being leached to groundwater in the Rotorua catchment each year. This will eventually enter Lake Rotorua and contribute to degrading water quality.

The amount of nitrogen derived from gorse has not been accounted for before as a nutrient-loss from land into the lake, but the study has shown that per hectare it is equivalent to some of the highest losses from pastoral farming. There is an opportunity here for a win-win with land owners that could return this gorse covered land to productive use as well as help reverse lake degradation. 

“If we can remove the gorse and replace with pine trees we can save about 41 tonnes a year going into the groundwater,” Mr Paterson said. 

The results of the change are not likely to be seen for some time though, as it can take anywhere from a few years to more than 100 years for groundwater to reach the lake, he said.

The report recommends that measures are taken to reduce or eliminate mature stands of gorse and encourage landowners to replace it with a low nitrate-leaching land use such as pine forest.

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