SPECIAL FEATURE: ROADING FOR WOODLOTS

How much forest roading do we need for woodlots?

Story: Professor Rien Visser

AS AN INDUSTRY, WE WILL BE HARVESTING

a lot more woodlots – and harvesting woodlot is typically more expensive than working in our larger plantation forests. One potentially big cost is building infrastructure to get the job done. But do we need more or less forest roads when harvesting woodlots compared to our larger estates? That was the research question Jacob Allum worked on for his final year Forest Engineering project.

Some basic forest road numbers have already been published. As an industry we are building somewhere between 1600 and 2000km of new forest roads each year, and we will be expected to do so for another 10 years – as long as we are still harvesting many first rotation forests. And many of our woodlots coming up for harvest are areas with limited infrastructure.

We also know that building a road can cost anywhere from $30,000 up to $180,000 per km – with a national average of $90,000 per km. While access to quality aggregate is one of the big cost factors, another is the slope of the terrain.

More roads means easier access and shorter extraction distances for our harvesting systems. But roading costs are often paid for ahead of harvesting, so smallscale forest owners can be apprehensive about justifying such investments.

In terms of the overall harvesting cost we are often looking for an optimum roading density. In terms of our larger-scale forests, a typical roading density is about 22 to 28m per hectare. What that means is that if you have a 100 hectare forest, you might expect 2.2 to 2.8km of forest roads to be built to give you access from the forest gate to all the landings. In comparison, our colleagues in the European mountains are often looking to build at least 40m/ha or more. This is because their cable yarding systems are only really cost-effective with extraction distances less than 250m, but our New Zealand yarders are larger and can pull over 400m.

To get a measure of actual roading density, Jacob sampled 96 randomly selected woodlots from across New Zealand. For each woodlot he measured not only the roading density, but also factors such as woodlot size, average slope, length-to-width ratio, and recorded soil type. The average woodlot size in his sample was 67ha., average length to width ratio was 2.4, and average terrain slope was 41%.

An early and interesting finding was that 15 of the 96 randomly selected woodlots had no roading infrastructure inside the forest boundary. So either the landings were placed on the farmland outside the woodlot, or the landing was just inside the boundary but the access road still outside.

For the woodlots with roading infrastructure, the average roading density was 29.9m/ha. So overall just slightly more than we would expect in our larger plantation estate. 24 of the woodlots had roading densities higher than 44m/ha. Typically that is not done to help the harvesting systems reduce extraction distance, but simply a lot of woodlots are complex in shape and you need more roads just to reach all the areas. Hence the volume harvested to each landing is less, and the average harvest area per landing area is 12.8ha. As a square block, that is only 350m x 350m in size.

Landing size was also measured in Google Earth and averaged 3000m2. A previous study in larger commercial forests showed landing size averaged 4900m2, so as expected woodlot landings are smaller on average.

However, crew daily production has always been a design factor in landing size. Given that woodlot crews have, on average, lower daily targets, smaller landings can readily be explained.

In more detailed analyses, increasing (a) terrain slope, (b) length to width ratio, and (c) boundary complexity all increased the road requirements. This was also evident when comparing ground-based and cable yarder harvested woodlots. Road density for ground-based sites averaged 27.4m/ha., while yarder sites were 33.4m/ha. Because it is so hard to define a meaningful difference in soil type, no correlation was found between soil type and roading density.

Variability in the data was high, meaning it is hard to predict exactly how much road you are likely to need just by knowing the size, slope and shape of the woodlot. However a basic equation that a landowner, and or harvest planner, can use is:

Road density (m/ha.) = 7.8 + 0.24 x Ave. Slope (%) + 5.5 x LengthWidthRatio.

Where average slope is in % and length and width ratio is simply dividing the longest distance across the woodlot by the perpendicular measure. With this information Jacob has established a point of reference for our forest industry with regard to roading needs in woodlots.

Note: During his studies Jacob was awarded the NZIF Jon Dey Award to support his research. His complete dissertation can be downloaded here: http://forestengineering.org • Jacob Allum is now a Forester with the China Forestry Group in their Wellington office, and Professor Rien Visser oversees the Forest Engineering programme at the University of Canterbury. NZL Above: Jacob Allum, now working for China Forestry Group, inspecting a forest road. Opposite page: A Google Earth image showing how the boundary of the woodlot was mapped, roads and landings identified and measured. Below: A Google Earth image showing a more complex woodlot that required quite a high density of road infrastructure.