The carbon crop: continued [Letter to the Editor]

With regard to the relentlessly positive ar-ticle by Anne Simon Moffat about forests ascarbon sinks (Research News, 18 July, p.315), it is worth noting several points, themost critical being the importance of differ-entiating between harvesting a forest standfor biomass energy and doing so for woodproducts. In the case of biomass energy,wood is a direct substitute for fossil fuel, andto the degree that the silvicultural practicesare sustainable over multiple rotations, therecycling of carbon through biomass energyis preferable to one-way emissions throughfossil fuel combustion. Harvesting for woodproducts, however, is certainly not a "one-
time, permanent movement of carbon fromthe air to the land surface." Particularly inthe case of converting native forests toplantations (1), but even in the case ofintensely managed forests, significant lossesof carbon are associated with harvesting. Atthe time of a clear-cut harvest, the noncom-mercial part of the trees (that is, branchesand roots) are burned as slash or left todecompose; other ecosystem componentssuch as the litter layer and understory arelikewise oxidized in one way or another,and a large fraction of the merchantablewood may go into products with lifetimes ofless than 5 years. In a case study in thePacific Northwest, only 23% of merchant-able wood harvested in this century wascurrently in long-term storage in landfills orwood structures (2).
With regard to the benefits of afforesta-tion as an emissions mitigation strategy, thearticle would have been more informativehad it told more about the relative magni-tudes involved. In the case of afforesting1000 hectares of land to offset "some" of thecarbon dioxide emissions from a proposedfossil fuel burning power plant in Oregon,this proportion over the lifetime of the pow-er plant amounts to about 1% of the plant'stotal carbon emissions. Afforestation on ascale to affect the global carbon cycle would
require many millions of hectares and mas-sive long-term commitments (3).
In relation to declining rates of defores-tation, the uncertainty about rates of defor-estation is such that it is difficult to specifyrecent trends. Globally, the rate of defores-tation in the 1990s is about the same as itwas in the 1980s (4). Intact forests are largereservoirs of, and in many cases sinks for,carbon. Thus, reducing deforestation andmaintaining the functional integrity of ex-tant forests are probably the biggest near- tomid-term contributions that forestry man-agement could make toward improving thecarbon balance of the terrestrial biosphere.