skidder

Skidding logs near Grant Lake on southern Vancouver Island.

Welcome to the second post in our “Harvesting Planning” series.

The first post talked about harvesting as one of the most important phases in the forest management cycle because it sets the stage for the creation of a new forest.

Choosing when to harvest is just one of a number of considerations you’ll make when you’re planning your harvesting schedule.

When to harvest depends on a number of factors. The most important factor is how the proposed harvesting fits with your management objectives. Other factors that affect the harvest date include:

  • Age of the stand
  • Rate of growth
  • Financial needs of the operation
  • Unplanned interference like insects, disease or fire

The concept of stand maturity is a useful indicator of when to harvest. The biological maturity of a stand is the age at which the stand has reached its maximum rate of volume production, or when its average annual growth is greatest.

If you harvest before the stand reaches maturity you potentially lose significant volume increase and value. If you delay harvest beyond stand maturity you retain a stand whose annual rate of growth is slowing down, but not stopping altogether.

Widespread insect or disease infestation can trigger pathological maturity. This means the volume increment of your stand is very slow or even negative.

Foresters refer to the point of optimum volume production as the maximum mean annual increment (MAI). The MAI is the average annual rate at which the stand has grown over its lifetime. It’s usually expressed in cubic metres per hectare per year.

The figure below depicts the growth of a stand over time, showing the total volume production and also the trend in MAI as the stand grows. The biological maturity is the point at which the MAI is greatest. This is also called the culmination point of MAI.

graph showing volume production in a stand

The culmination of MAI is comparable to the point at which a teenager peaks in the rapid growth spurt that often characterizes puberty. Past this point both the teenager and the tree keep growing, but at a slower rate.

The total volume line in the figure is known as a ‘volume-over-age-curve’ (VAC). This curve represents the VAC for coastal Douglas-fir on a good site in the Vancouver Forest Region. VACs are produced by forest cover type, for different sites in forest regions throughout the province and are available from the Forest Analyis & Inventory Branch of the Ministry of Forests, Lands and Natural Resource Operations.

By drawing a straight line from the origin (0,0) on the graph, to just touch the edge of the VAC1 it is possible to estimate the culmination point of the MAI. The age of

the stand at this point is the biological maturity of that species on that site. To obtain the actual value of the MAI at this point you would divide the stand volume by the age. In this case the stand volume of 650 cubic metres divided by 70 years gives a MAI of
9.3 cubic metres per hectare per year.

Foresters have traditionally used the biological maturity of trees as the minimum harvesting age for planning harvesting schedules in the province.

graph depicting biological and financial maturity

Long-lived tree species such as ponderosa pine, Douglas-fir or redcedar have not maximized their economical value at the time of biological maturity. The market value generally increases with log size so that additional value gains can be made at the expense of some volume losses (lower allowable annual cut), if the stand rotation would be extended beyond the age of biological maturity.

In practice, you will likely base the decision of when to harvest on a combination of factors, some biological and some financial. It will depend on weighing the costs and benefits of cutting now, against cutting later.

Ultimately, when to harvest is influenced by your cash flow situation, and how badly money is needed, since the risks associated with holding onto the trees for a little longer are relatively small. Fire and a change in the market prices for logs and other products are the major risks.

Generally speaking, the stand becomes more valuable as it gets older, so you can afford to delay the harvest if it is not costing you money to hold the stand, and you have no financial constraints that make it necessary to harvest immediately.

Selective cutting of some commercially valuable trees can be carried out to provide cash flow, while allowing the stand as a whole to continue to grow to greater value.

Pleas stayed tuned for future posts in our “Harvesting Planning” series. You can find the first post, Harvesting: So Much More Than Cutting Trees, by following the link.

If you can’t wait for the next post, you can find all the information excerpted above, and more, in the complete online document: A Non-forester’s Guide to Small-scale Forestry in British Columbia.

As always, a big thanks to the authors for letting us share the information.