As discussed in previous articles, old growth forests can be considered in two categories — old forests in protected areas and those as set aside in the working forest (timber harvesting and non-timber harvesting areas).While many would challenge the use of just age to classify old forests it has been accepted as a way to inventory and classify old growth forests for planning activities. Early harvesting practices concentrated on fibre production with the conversion of older so called decadent stands into young faster growing forests.
Forests grown on optimal short rotations (120 years) could produce more fibre compared to natural disturbance cycles with older portions over hundreds of years. As harvesting increased substantially throughout the province it was apparent that harvesting (in particular clear cutting) had a major impact on other values like recreation, fish and wildlife as well as hydrological impacts.
Because old growth forests were often harvested first it soon became apparent their decline may impact not only non-timber values but potential lumber values in the future. The size of old growth trees at a given age is influenced by a number of factors including site index and species as described in Status of B.C.s old forests. The article’s author, Cam Brown, also describes how the amount of old growth by site class changed when he used the site index values derived from ecosystem/climate correlations rather than photo interpreted stand heights and ages (ages are difficult to get from overhead photos). The point being that VRI site index values are underestimated at older ages and more accurate at younger ages – leading to inconsistent measures of site productivity and an underestimate of old forest on productive sites.
Bottom line – the high productivity sites have been more heavily logged than the lower productivity sites, but the amount of old growth remaining on good sites is not as low as ‘The Last Stand for Biodiversity,’ the report suggested.
Another valuable tool for understanding old growth on the landbase is to track the amount of forest in each age class. Figure 17 is a good example and shown (with permission ) in this article.
The light green on the bottom of the figure is the contributing timber harvesting land base (THLB) that contributes to the annual allowable cut (AAC) for the entire province. The dark green represents the non-contributing THLB and consists of lower site indexes, very steep slopes (uneconomical to harvest), as well as productive forests that are considered more valuable for purposes other than for lumber production (i.e caribou, fish habitat around rivers and lakes.) The 2.5 million hectares in the first decade is mostly from the harvesting of the old mature stands where as the two million hectares of non THLB would be from natural disasters (wildfires) and natural processes. The THLB area in younger age classes reflects the amount of area harvested, burnt, or killed by MPB in the last 60 years. The last 20 years has seen a lot of mortality and salvage harvest in the B.C. Interior. While I think Figure 17 is also useful at demonstrating how rare forests are in the THLB beyond 340 years old but it is difficult to see where these stands are since it is for the entire province.
As described in the report, provincial datasets can be helpful to identify large scale trends and potential issues of concern, but are not appropriate to establish detailed forest management direction.
In future articles I would like to look at detailed assessments of old growth conditions on a region or watershed basis. Examples of this work are already occurring in the province’s forest landscape planning pilot projects which include local experts along with First Nations in order to ensure sufficient context is available.
Jim Hilton is a professional agrologist and forester who has lived and worked in the Cariboo Chilcotin for the past 40 years. Now retired, Hilton still volunteers his skills with local community forest organizations.
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