Surprise: logging can be good for water quality
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 which are contained in wastewater inputs and run-off from the surrounding catchment.
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, near Reporoa, which 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 typical of many streams that feed into the Rotorua lakes.
The results showed that when the pines were growing vigorously, little nutrient was exported in stream water. Nutrient levels increased as the pines reached maturity (just before logging) and peaked in the year of logging and the following year. After logging the catchment was replanted, and nutrient export was again low as the cycle of active forest growth resumed.
Our research shows that the mycorrhizal fungi associated with actively growing tree roots inhibit nitrification (the transforming of ammonium to nitrate) so that less nitrate makes its way into the streams. The increase in nitrate export as the forest matured, when root growth was less active, is consistent with this.
Over the long term, logging appears to help maintain low nutrient export from pine plantations. This is despite there being a short lived increase around the time of logging.