The Donald Creek site in Tasman District provides a real-time, and scale, view on sediment impact from forestry.
The impact of forest felling on waterway sediment levels will be better understood in coming years, thanks to Tasman district forestry company OneFortyOne securing funding that turns one of its forest sites into a seven-year large-scale experiment. Richard Rennie reports.
A large-scale trial to help foresters and councils better understand the best approaches to felling timber and managing sediment loss has been given a shot in the arm through a recent 50:50 funding agreement between the Ministry for Primary Industries (MPI) and company OneFortyOne.
Each is providing $1.37 million to a trial now in its second year that will deliver long-term monitoring and assessment on two different approaches to felling management, with the ability to compare the outcomes to each other and to a non-felled “control” site, all within the same forest area.
Located on the company’s Donald Creek Forest near Tadmor in the Tasman district, the project’s study area comprises a significant 600ha chunk of the company’s 2200ha block and provides researchers with a rare opportunity to study how land responds to second rotation forest felling.
“The three treatment sites are all very similar within the forest, and the timing of felling works well too, with the first treatment site due for felling now, with the second treatment site coming up in year seven, with the control are harvested two years after that,” OneFortyOne environmental officer Jo Field said.
It is a country she describes as “nothing like Gisborne, but steeper than Hawke’s Bay”, comprising small pockets of granite country that is highly erodible, which she likens to Wairoa area soils.
With the industry sometimes on the receiving end of claims about invoking high sediment losses in some catchments, Field says the trial is a chance to determine with more scientific analysis just how great those losses are and whether they would have occurred with or without forestry activity.
“Sediment loss is a natural process in any water system. When you get a flood event you will get waterways running black or brown as a natural part of water flushing through systems and often that includes sediment already present along the sides of those waterways,” she said.
“The challenge is what is actually going through streams naturally, versus what is being picked up due to forestry erosion losses.”
The first treatment site takes a ‘business as usual’ approach to felling and will measure erosion losses against OneFortyOne methods, which Field says will sit a level above industry best practice. Such practices include sediment traps and weirs placed around sites to reduce runoff.
The second treatment site involves the construction of a large sediment retention pond on the felling site, designed along lines similar to those seen on large road work projects and in civil building subdivision work.
“No one really knows if this technique will deliver a better outcome, so we can study it. But it is one of those ideas that are taken from civil engineering, to see if it could apply effectively out here,” she said.
Given it is a real-world harvesting site, the researchers are able to incorporate and calculate the cost of the different approach into final felling economics. Base line sediment levels have been calculated by scientists from supporting partners the Cawthron Institute, Manaaki Whenua- Landcare Research and Envirolink.
Further analysis and drone tracking after rainfall events will help with comparisons between sites.
MPI director of investment programmes Steve Penno says this was the first project to focus specifically on forest sedimentation controls. He says the project will determine how well a range of techniques perform in order to help foresters meet national freshwater standards.
In a case of being careful what one wishes for, Field says researchers are guardedly praying for some high rainfall events to provide the desired flushing effect and put momentum behind any potentially erosion-prone events on the sites.
Field says the Tasman District Council has proven to be highly supportive of the project, working alongside the sector to gain a better understanding of runoff to aid in framing standards and regulations within the catchment.
The industry will not have to wait the full seven years for full results, with an interim report due out after the first zone has been harvested.
Field says the ability to study similar catchments is not a new one.
The industry’s seminal study is the Pakuratahi land use study, a 12-year paired catchment study comparing pine plantation to pastoral farming on coastal Hawke’s Bay north of Napier.
“This study will do much to build on that one, but that one was a study on two different land uses. This one will bring a lot of information for the industry about methods and approaches to harvesting, most of which can be applied across a number of catchments,” she said.
