About 5800 hectares of land has been converted from farming to forestry in the Lake Taupo catchment in a move to protect the quality of the lake’s waters.

A Waikato Regional Council staff member measuring dissolved oxygen levels in the Waitahanui Stream, which flows into Lake Taupo. Photo by John Barran.

The conversions are a result of Waikato Regional Council enforcing its Variation No. 5 - Lake Taupo Catchment rule in 2011.

A further 2000ha could be converted as a result of a proposed land use change in the catchment.

Council’s new policy and rules aim to manage land use in the catchment, with some farming practices controlled or requiring consents. It also contains tighter controls for new urban development in the Lake Taupo catchment. However, the extensive change of land use from farming to forestry hasn’t seen an improvement in water quality.

In fact, there’s been an increase in nitrogen levels – but council’s water scientist Bill Vant says these are likely to be “legacy” effects rather than a result of recent land use changes.

“When Variation 5 was developed it was anticipated that, despite capping, the loads of nitrogen entering the lake in its inflows would continue to increase until the new measures began to take effect.

“It was expected it would take several decades or more before the full effects of intervention would be seen in the lake.”

Scientific evidence
This underlines Variation 5 as no “quick fix” but a long-term investment in the health of the lake, one with measures underpinned by sound scientific evidence, says Bill.

However, Bill admits some of the changes in water quality are somewhat unexpected.

“In particular, more than half of the overall increase in nitrogen between 2002 and 2011 was carried by four inflows, which drain undeveloped and pine catchments on the eastern side of the lake, the Hinemaiaia, Tauranga-Taupo, Waimarino and Waitahanui rivers.

“When the variation to the Waikato Regional Plan was being developed, it was assumed the nitrogen loads from undeveloped and forested catchments would remain stable.

“Recently, we commissioned a preliminary analysis of historic 1958-1965 aerial photographs of the south-eastern part of the Taupo catchment including part of the Tauranga-Taupo sub-catchment.

“The area photographed currently contains about 159km2 pine plantation, or about one-third of the combined area of plantation in this part of the lake’s catchment.

“Of this area of pine forest, nearly half is growing on land where pasture was present from 1958-1965. It is probable the pre-plantation history of land use in this area is at least partly responsible for recently-observed increases in nitrogen concentration in the streams that drain from it.”

Pollen’s impact
Farming and forestry in the lake catchment is now under strict caps on nitrogen leeching, with forestry deemed to leech less nitrogen to ground water than farming.

However, during Environment Court hearings into council’s plans to limit nitrogen leeching some of the arguments related to the impact of nitrogen from pine pollen on water quality.

In written evidence to the court, Bill said: “The main difficulty with estimating the load of nitrogen to Lake Taupo that is associated with pine pollen is knowing clearly what proportion, on average, of the pollen from pine forests in the catchment [and elsewhere] actually falls into the lake”.

The effects of pine pollen on the quality of fresh water in the Northern Hemisphere is the subject of a report called ‘Effects of pollen leaching and microbial degradation on organic carbon and nutrient availability in lake water’ by scientists Stefan Rosel, Anna Rychła, Christian Wurzbacher and Hans-Peter Grossart.

Their report says: “The role of pollen for biogeochemical cycling in aquatic systems seems to be greatly underrated”.

The report goes on to say: “The potentially large amount of organic carbon and nutrients introduced by pollen can fertilise freshwater ecosystems”.

Relatively minor
In relation to Lake Taupo, Bill says he made some calculations for pollen, based on estimates from several sources, and these suggest to him any extra nutrients from pine pollen due to more planting would be a “relatively minor matter”.

However, Bill says he can’t be certain about how realistic his calculations are, as his expertise is in aquatic ecology rather than forest ecology.

Some idea of nitrogen loads from pollen entering the lake have been gained from calculations about how “atmospheric deposition” – mostly rainfall, but including “dry” deposition – added to nitrogen in the water.

Bill says these figures suggest the total load of nitrogen from pollen getting into the lake from existing and new plantings would be relatively minor in the overall scheme of things.

Essentially, Bill feels there’s not really enough information available to form a firm conclusion on whether replacing pastoral farming with forestry may negatively impact on lake quality because of production of pollen from forestry.

“In particular, estimates of the loads of nitrogen and phosphorous delivered to the lake in deposited pollen are imprecise.

“What is clear is capping farming activity on land around the lake will cut nutrient loads from this source; and this action is expected to help protect lake health over time, even if there is more forestry.”  

There was also debate, during Environment Court hearings, about different calculations over how much nitrogen would leach to the lake from areas of scrub on undeveloped land. Bill’s evidence to the court was his estimates were nitrogen from gorse and broom areas would be about 12kg per hectare per year. Another witness told the court they thought the figure would be 23kg/ha/year.

A scientist’s group asked by the court to look at these types of unresolved issues concluded Bill’s 12kg figure was the better figure.

The group also considered any wide proliferation of gorse and broom could further threaten lake water quality.

Bill says this implies it will be important for people doing conversions to keep on top of wild broom and gorse to help protect the lake.