When I first became involved in forestry in the 1980s there was a debate over choosing a younger harvest age which would maximize fibre production rather than produce larger dimension more valuable lumber from older forests.
The rapid growth rates of young forests produces more fibre per hectare which could be reconstituted (glued back together) to produce better more acceptable products than dimension lumber.
This approach had less likely hood of loss to fire, insects and other diseases and was supported by an emerging industry that was using small dimension lumber to produce products like wood trusses, finger jointed lumber and a variety of pressed wood products.
Some 30 years later, lumber production (including large dimension lumber) is still considered the best use of the logs with chips as a by product.
There is still an ongoing debate over rotation ages but a few new considerations have been added.
The mountain pine beetle, global markets and concerns about climate change and green house gas (GHG) emissions are all new things to consider when maximum harvest age is being considered. For example, a study mentioned in Ben Parfit’s article (Managing BC Forests for a Cooler Climate) shows harvest age has a considerable impact on GHG.
“Upon being logged, lands denuded of trees are significant CO2 sources, as the carbon stored in the exposed soil releases. How much so, surprised even the scientists themselves. The youngest stand of managed forest, dominated by Douglas fir trees, and logged three years earlier, released 22 tonnes of CO2 per hectare annually, making it “the largest carbon source ever measured for a terrestrial ecosystem.”
Another stand, logged 12 years prior, was also a sizeable source of CO2 emissions, albeit much less so than the first site. There, the CO2 emissions averaged five tonnes per hectare per annum. The picture was decidedly different, however, when a nearby tract of forest logged 53 years earlier was examined. On that site, the forest sequestered (stored) an average of nine tonnes of CO2 per hectare per year.
What this means is that if some stands have a very short rotation (less than 50 years) they could be a net producer of GHG rather than storing any carbon relative to what they release.
“The team concluded that by extending the rotation age from a “business as usual” scenario where trees were cropped every 59 years, to a rotation age of 80 years, an additional three tonnes of carbon were stored per hectare, or nearly 1.3 million tonnes overall. However, this carbon benefit could be maintained only if the overall volume of timber logged did not increase substantially after the rotation age was increased to 80 years. Thus, a longer rotation age of 80 years, combined with reasonable controls over the number of trees subsequently logged, appeared optimal if increased carbon storage was the goal.”
It was also pointed out that when the wood products were used in well-built long lasting wooden structures, carbon sequestration was maximized.