SOILS, CARBON CYCLES, AND MASS TIMBER CONSTRUCTION
Independent Study, Spring 2019
• PLANTS •
Plant life holds less than 2% of surface carbon at any one time, and yet plays an integral role in carbon cycles. Made up of various carbon compounds, plants use CO2 during photosynthesis and absorb more carbon out of the atmosphere than any other part of the global carbon cycle (Schlesinger and Bernhardt). Photosynthesis occurs in the chlorophyll of the plant, and essentially creates carbohydrates from water, sunlight, minerals, and CO2. Photosynthesis also involves some respiration of CO2 back into the atmosphere, but only half of what was absorbed initially, so the process results in a net capture of atmospheric carbon (Schlesinger and Bernhardt). The majority of that carbon stays in the plant tissues until it is eaten or dies.
Dead plants generally decompose and release carbon back into the atmosphere. Organisms like fungi, insects, and microbes metabolize the plant tissue and respire most of its carbon. If the tissues happen to fall into a hypoxic bog where decomposition is slow to impossible, or if they are gathered and used, the carbon may stay out of the atmosphere. Whether the carbon stays terrestrial after harvest depends on how the plant tissues are used. Wood used in construction sequesters carbon for the lifetime of the building. But if animals or people consume plants, we then respire most of the carbon into the atmosphere. If plant tissues burn, like wood chips at a biomass power plant, the carbon is chemically released into the atmosphere (Schlesinger and Bernhardt, Wiloso et al). Slash and burn land clearing practices, for example, cause large quantities of carbon emissions (Davidson et al).
Theoretically any emissions resulting from the digestion or combustion of plants can be balanced by the regrowth of an equally large plant, but there is an issue with timing. If we burn a tree that took 50 years to grow, it would take 50 years for an equally carbon rich tree to re-sequester the carbon. This means that atmospheric levels of CO2 will be higher in the short-term, even compared to burning coal. Wood is an inefficient fuel source and releases more CO2 per joule of energy obtained than coal does. The global greenhouse effect will be magnified in the short-term, amplifying the effects of climate change (Wiloso et al). Also, while most forestry practices try to ensure healthy re-growth of harvested areas, supply systems for biomass electrical generation plants are too complex and energy intensive to guarantee that the trees will immediately grow back and recapture all of the carbon emissions (Nian).
Terrestrial ecosystems intake the most carbon from the atmosphere, more than oceans absorb, representing the largest volume flow in the carbon cycle (Schlesinger and Bernhardt). Forests in North America are important carbon sinks, sequestering approximately 17% of anthropogenic emissions every year (Martin et al). These temperate forests in the northern hemisphere are partially responsible for the oscillations seen in the “Keeling Curve”, which reports atmospheric CO2 concentrations in the northern hemisphere from 1959 to the present (Schlesinger and Bernhardt). Although the curve shows that atmospheric CO2 is steadily increasing over time, each year it shows a significant oscillation that correlates with the growing season in the northern hemisphere. Less atmospheric CO2 is present during the spring and summer than is measured in the fall when most plants die and decompose. This data shows how important plant life is to global carbon flows.
Recently there has been amplified oscillation in the Keeling Curve, likely due to the carbon dioxide fertilization effect: carbon dioxide concentration in the atmosphere can change the rate of photosynthesis (Graven). Higher levels of CO2 make it easier for a plant to absorb and use CO2. Normally there is a tradeoff between water retention and interstitial CO2 because a plant must open the stomas on their leaves to absorb CO2. The wider the stomas open, the more CO2 available for photosynthesis; but also more water is lost from the plant’s cells though evapotranspiration. When there is a high concentration of CO2 in the air, stomas can use smaller apertures to absorb the same amount of CO2. The result is that plants become more water efficient, making more photosynthesis possible at a given moisture level. This CO2 fertilization effect is a negative feedback loop in the issue of anthropogenic climate change because increased atmospheric CO2 also results in increased Net Primary Productivity (NPP) and carbon capture by vegetation (Schlesinger and Bernhardt). However, because many ecosystems are more limited by soil nutrients like nitrogen than they are by water, the effects of the negative feedback loop may also be limited (O’Sullivan et al).
It has been proposed that forest management can help mitigate climate change. Because forests are such important carbon sinks already, some ask what we can do to manipulate them to capture even more carbon. Forests managed for resource development already follow strict logging cycles based on market demand, prices, and harvest costs. It may be possible to plan for maximum carbon capture, although there are conflicting ideas about how to do that. It seems very important to make these decisions based on specific locations because different forests in different climates function differently. Warm, fertile plantation forests like those in the southeastern United States may tend to sequester more carbon during the early years of accelerated growth, and the argument has been made that shorter logging rotations could be better for carbon capture. In colder climates it has been argued that longer rotation times might allow for more complete renewal of soil and plant carbon stocks (Dean et al, Pukkala). A significant root issue seems to be found in carbon accounting methods: while the rate of carbon uptake in a young forest may be higher, the previously stable forest biomass removed during harvest was likely released to the atmosphere, possibly resulting in a net deficit of terrestrial carbon until the young forest reaches a certain level of maturity (Martin et al).
The question of whether logging can help capture additional carbon also depends heavily on what happens to the wood after harvest, as mentioned briefly above. Short-term use products like paper are not a good method of sequestration due to the timeline of use and the energy inputs in processing. Biomass electricity generation seems to create such an emissions spike in the short-term that the timeline of re-capture is impractical. Durable wooden goods like furniture and buildings may be successful in carbon sequestration but there are a number of variables (Lippke et al). This issue is addressed in more detail Here.
References:
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