Carbon and Forests

Introduction
The BC Ministry of Environment has recently released a draft Forest Carbon Offset Protocol for public review and comment. The deadline for comments is January 31, 2011.
The following analysis was developed in 2008 when carbon sequestration was becoming topical. The purpose was to illustrate that research can answer questions using simple information sources available on the web. The question addressed here was then being debated in the media.
Question: "Given an objective of using forests to sequester carbon, is that objective best achieved by managing forests on short rotations or by preserving old growth?"
Trees absorb carbon dioxide (CO2) from the atmosphere. Trees retain the carbon portion of the CO2 molecule to build their structures, and exhale most of the oxygen. Each tonne (1000 kg) of carbon stored in the forest removes 3.67 tonnes of CO2 from the atmosphere, and releases several tonnes of pure oxygen back into the atmosphere.
Governments around the world are seeking ways to slow down the rate of climate change. Climate warming is due, in part, to increasing accumulations of atmospheric CO2. In response, governments are creating legislation to reduce CO2 emissions, and to increase long-term storage of non-atmospheric carbon.
Forests are an ideal carbon storage medium. Forests absorb CO2 with little or no intervention required, and no expenditure of fossil fuels. There are two contending points of view regarding best forest management to maximize the storage of carbon over time.

Strategies to maximize carbon storage in forests
1. Preserve forests indefinitely near their biological maximum stored carbon content, protecting the forests from carbon-releasing agents such as fire, insects, disease and harvest.
2. Manage forests on a cutting cycle that maximizes growth rates, and store the wood from each harvest in structures protected from fire and decay.

Questions
A forester would wish to know:

How much carbon is sequestered over the long term under each strategy?
What is the magnitude of the difference between the two strategies?
Is there a compromise strategy that will best accomplish government's joint aims of maximizing carbon storage while maintaining the timber harvesting economy?

Solution
The following analysis seeks to briefly develop answers to the forester's three questions above.
Scientists measuring the carbon content of wood, across many species, find that wood is almost exactly 50% carbon. The distribution is 48% "stable" carbon and 2% "volatile" carbon. This analysis assumes that wood is 50% carbon by dry weight.
The table below lists wood densities (weight per volume) of common commercial tree species in the Campbell River area. A dry cubic metre of Douglas-fir weighs 510 kg, containg 255 kg of carbon, while a cubic metre of red cedar contains only 170 kg of carbon.
Table 1. Oven-dry wood density for BC coast species

The forester's three questions were investigated using the computer and a sample of a medium quality growing site (SI 27) stocked with stands of Douglas-fir planted at 1200 stems/ha in our local Campbell River area. The BC Forest Service growth and yield program TIPSY was used to describe development of the stands.
Figure 1 illustrates the accumulation of carbon by the sample stand in terms of merchantable wood volume. Carbon storage will also occur in bark, branches and tops, stump and roots, and in the forest soil. The impact on the solution of these additional factors is discussed later.
Figure 1. Accumulation of carbon in the sample stand

Only a portion of the round scaled log volume is sequestered as lumber. The carbon content for sawn lumber in Figure 1 includes adjustment for higher lumber recovery factors with larger piece sizes at older ages. In this stand, the lumber recovery factor is 50% at age 40, 60% at age 70, and 70% at age 170 years.
Maximizing MAI
The age at which a stand reaches maximum mean annual increment is known as "culmination age". This is the target rotation age that maximizes log volume production from an area of land over many crop rotations. The sample Douglas-fir stand has a culmination age of 70 years (MAI=8.67 m3/ha/yr). At 70 years of age the carbon content of the lumber cut from harvested logs is 92.8 tonnes/ha. This lumber may be stored under cover, while the land is regenerated to grow another stand. Following clearcut harvest of those logs, within 25 years much of the organic matter remaining on site will have decomposed, with the residual biomass carbon returned to the atmosphere as CO2, or absorbed by the subsequent crop.
Some of the residual chips and sawdust from processing the logs will also move into long-term storage as panel boards and paper products. Less than 100% of the lumber will move into long-term storage. More detailed estimates of the expected wood volume capable or likely to be placed in long-term storage (carbon sequestered in protected building structures) is a subject for further research.
Keeping it simple, the 70-year rotation strategy for this stand results in sequestration of 92.8 tonnes of carbon in structures each 70 years, or an average of 1.33 tonnes of carbon (which equates to 5 tonnes of CO2) per hectare per year. Figure 2 illustrates the average rate of carbon sequestration for different rotation ages.
Figure 2. Long-term average rate of carbon sequestration in wood and lumber as a function of rotation age.

The maximum rate of carbon storage as lumber occurs at 100 years, later than the culmination age of 70 years, because a larger percentage of stand volume is captured as lumber with older, larger logs.

Comparison with the no-harvest strategy
How does this compare with the no-harvest strategy? As long as trees remain healthy and alive, the stand is storing the round metric volume of wood on site. Reading the upper line in Figure 1, this would mean 331 tonnes of wood carbon present on site after 250 years, or storage at an average rate of 1.32 tonnes/ha/yr, almost identical to the lumber-only component in the 70-year rotation scenario. Both strategies score lower than a 100-year rotation, which achieves carbon storage as lumber of 1.38 tonnes/ha/yr (Figure 2, peak of the red line).

Issues for the forester to consider
1. No-harvest strategy: The non-merchantable portions of the trees, and carbon in the forest floor, should be included in the summation of total carbon storage. Environmental benefits of an intact forest may be added to the equation, and protection costs may be considered.
2. Short-rotation strategy: Products made from sawmill waste should be included in the long-term sequestration calculation. The social economic benefits of harvesting and manufacturing activity may be considered.
3. Tree species will make a difference. This example used Douglas-fir, a long-lived species. A short-lived species such as lodgepole pine makes fast early volume growth which tapers to very slow growth later in life. It is expected that lodgepole pine growth curves will favour the short-rotation strategy as the preferred means of maximizing carbon sequestration in that forest type.
4. There is potential to combine the two strategies using partial cutting methods that maintain much of the forest floor carbon on site rather than having it decompose into CO2 in the exposed conditions following clearcut.

Provincial-scale estimates
The analysis above enables thumbnail extrapolation to carbon balance estimates for the province of British Columbia. The example used a medium site coastal Douglas-fir forest stand. Forests in the interior of the province grow and sequester carbon at approximately half the coastal rate, and account for 70% of B.C.'s timber harvest.
These numbers suggest that the rate of carbon sequestration in the B.C. commercial forest (25 million ha) is approximately 22 million tonnes of carbon per year. Many refinements to this estimate are feasible to undertake. Currently in the province, harvesting removes approximately 70 million m3/yr, or 16 million tonnes of carbon in wood, much of which is sequestered longer-term as lumber and other products. Additional carbon is released to the atmophere through post-harvest burning and decay of litter, residues and the forest soil.
The difference between the annual commercial forest carbon sequestration (22 million tonnes) and the harvest (16 million tonnes) leave 6 million tonnes of net annual gain, less losses from decomposition directly to the atmosphere.
The crux of the equation is the post-harvest release of carbon due to decomposition of organic matter. The release is estimated at 5 to 12 million tonnes of carbon annually. If contained, it would be enough to compensate for the fossil-fuel emissions from all cars, trucks, busses and aircraft operating in B.C.

Further research
1. The Tree and Stand Simulator (TASS) could be used to develop approximations of tree bark, tops and branches, and stump and root volumes, and also the rate of litter input to the forest floor as a function of stand age.
2. The PrognosisBC model could be used to develop partial cutting strategies expected to yield reasonable timber volumes while maintaining significant carbon stores on site. In this example, if the stand can be maintained producing timber at effectively 70 - 120 years of age, with regeneration, the MAI over that age range exceeds 8 m3/ha/yr and the rate of carbon sequestration (using the lumber carbon line) rises to 1.39 tonnes/ha/yr with a hold on site of 100 tonnes/ha, the hold gradually increasing as litter and soil accumulate.
3. Strategies for different species on the same site might be compared.
4. Economic value increases with larger log sizes. In our example the 70-year rotation results in 4.5% high-quality (I-grade) logs, whereas a 100-year rotation yields 17.6% volume in this more valuable log grade. Research might compare forest value yield strategies with carbon balances.