Pine plantations and water quality in central North Island lakes
John Quinn
New
Zealand Tree Grower November 2005
Increasing nutrient levels threaten the quality of many central North
Island lakes. A long term study showed that nutrients leaking from land
to water decreased markedly when a central North Island catchment was
converted from pasture to pine plantation. Logging and replanting
caused a short term increase in nutrient losses but these dropped to
low levels as active forest growth resumed.
Lakes and land use
Many of the lakes in the central North Island are suffering from
increased algal growth and reduced water clarity. These are symptoms of
nutrient enrichment by phosphorus and nitrogen that are contained in
the groundwater and surface run-off from the surrounding catchment and
wastewater inputs.
Replacing pasture with pines reduces this nutrient export, but what
happens when the pines are logged and the site replanted?
NIWA scientists have been studying this at the Purukohukohu
Experimental Basin, on the Paeroa Range, near Reporoa. This was
established in the 1960s as a long term research site on the effects of
land use on soil processes and streams. Although the streams drain into
the Waikato, they are similar to many that feed into the Rotorua lakes.
The area in pasture was cleared in the 1920s as part of Mangimingi
Station. The Puruki catchment was planted in radiata pine in 1973 then
logged and replanted in early 1997. Water studies have investigated the
effects of land use on water flows, quality and nutrient exports
downstream of pasture, pine and native forest by comparing nearby
paired catchments.
|
| Changes in annual water yield of
pasture and pine catchments before and
after logging and replanting pine forest. |
Water yield
About 30% less water flowed from the mature pine plantation than the
pasture as shown in the diagram. This is expected to be due to the
pines increasing the interception and evapotranspiration of rain. Flows
were higher from the pine than the pasture catchment for four years
from logging, but returned to pre-logging level by seven years after
logging.
Nutrients
The stream draining an established pine plantation, 9 to 12 years after
planting, exported less nitrogen and phosphorus than the adjacent
pasture stream and less than native forest. The lower catchment export
from pines compared to pasture probably results from the combined
effects of –
- lower fertiliser input
- removal of grazing stock whose urine patches create local hot
spots
of high nitrogen loading
- reduced stream bank and soil erosion and surface run-off
- uptake and retention of nutrients by the growing pine trees and
understorey vegetation
- inhibition of nitrification – transformation of ammonium-nitrogen
to
more readily leached nitrate – by mycorrhiza associated with
actively growing tree roots.
|
| Land use effects on average yields of
total phosphorus and nitrogen at
Purukohukohu. |
Nitrate dominates the nitrogen export from these catchments. The
long-term monitoring of nitrate concentration in the pine stream shows
that this declined over the four years after conversion from pasture to
pine plantation. However it increased somewhat late in the pine
rotation, suggesting mature forest was more ‘nitrogen leaky’.
Nitrate concentrations increased in the pine stream in the two years
after the time of logging, where these were similar to levels when the
land was in pasture. However, nitrate dropped from three years after
logging and replanting as the groundcover and the replanted pines
became established, and was very low after seven years.
Conclusions
These findings indicate that the increase in nutrient loss around
logging and replanting is short-lived. Over the whole cycle of pine
planting, forest growth, logging and replanting, pine plantations
export less much nutrient than pasture. Pine plantations within lake
catchments in the central North Island are likely to be at different
rotation phases. Therefore short-term increases in nutrient yield
around logging of one plantation forest patch will probably be more
than compensated for by the low yield from younger pine forests
elsewhere in the catchment.
The overall effect of afforestation of pasture on nutrient yields
remains beneficial. The higher nitrate yields from mature forest than
young forest, and lower yields a few years after logging than
immediately before, also suggest that logging may be beneficial for
long-term nutrient yield with a vigorous growth mode with high nutrient
retention.
This research was funded
by the NZ Foundation for Research Science and Technology through
Scion’s Centre for Sustainable Forestry Programme Protecting the
environment through forestry.
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