Despite the devastation caused when forestry slash and debris added to the flooding woes of Tolaga Bay, afforestation is fundamentally beneficial for erosion mitigation. Denis Hocking reports.

Denis Hocking.

Forestry is caught in both the limelight and the headlights – the billion trees versus a million tonnes of debris around Tolaga Bay. The slash and debris around Tolaga Bay, and earlier Nelson, is completely unacceptable and a very black mark against forestry. But – and there are always buts –we must be careful not to throw the baby out with all that bath water.

Although the forest debris seriously compounded the problems, even without it there would have been serious flooding, silting and debris.

While longer rotations have a cost, these forests serve more than just a wood production role and longer rotations do improve wood quality.

I well remember the pictures of Cyclone Bola’s devastation, and my understanding is that this time slipping was markedly less in the forested areas, including the cut-over areas, than on farmland, though apparently there is little pastoral hill country in this storm’s zone. The problem is the debris and I firmly believe the answer should be sought in the debris not the land use.

But first, let’s go back to fundamentals. The logic of afforestation, and the risks, for erodible hill country are very well covered in a series of papers in the August 2015 NZ Journal of Forestry.

To slow the greatly accelerated erosion of pasture land in such landforms, (generally about 10 times the rate under indigenous forest), you need trees that quickly develop a large subterranean root length and mass to reinforce the soil. Radiata pine is very effective though not as good early on as alders, willows and poplars. Sadly, manuka is not in the hunt, with toe toe being the best native listed, for early root development, though I am assured manuka roots are stronger.

In addition radiata is readily available, easily established, well understood and commercially valuable. But, it is a short-rotation species and the roots decay fast, leaving a four-to-five-year “window of vulnerability” after harvest and before replanted trees take over.

With radiata rotations being steadily reduced to the mid or low 20s, (I understand some problem forests for Tolaga Bay were harvested at 24 to 25 years), there is considerable “window” in each rotation. While longer rotations have a cost, these forests serve more than just a wood production role and longer rotations do improve wood quality.

We need a market that recognises this quality rather than the current undiscriminating Chinese vacuum cleaner. If the forests are not harvested, who pays the bills and how? Carbon?

The other priority must be to get that debris off the slopes at harvest. I gather this is not always easy with health and safety considerations and the National Environmental Standard often conflicting with the need to recover all the trees after felling.

Hopefully improving technology will help but don’t under-estimate the problems with haulers operating outside the line of sight and the need to keep logs suspended. I gather in some regions all accessible debris is pulled onto hauler sites and burnt, though some wonder why it can’t be taken away and burnt elsewhere – as boiler fuel.

People more knowledgeable than me also talk of designing more-effective riparian barriers.

One suggestion I don’t consider feasible is continuous cover forestry, as practised in parts of Europe. The costs of extraction on our steep hill country would kill such an option. Changing tree species and in particular moving to coppicing or suckering species that maintain a live root system is another suggestion.

Redwoods might be an option on wetter sites, and deserve testing. I know all about coppicing with eucalypts. It is very difficult to grow millable logs from coppice as they tend to detach from the stump. In addition, these high value end-use timbers aren’t worth much as logs. There is no infrastructure to handle them, though an export log trade might be possible.

So let’s recognise the benefits of tree cover and focus on solving the problems. Livestock was removed from these hills for good reason. But there is one other bit of advice – if severe storms are becoming more common* we must treat flood plains with great respect. They belong to rivers and rivers don’t respect infrastructure.

Scientists are reluctant to say we are seeing a long-term trend yet because the NZ data is variable from year to year. However, it is consistent with expectations.