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Russell K. Monson

Global Agriforest Expansion and Atmospheric Quality: A Glimpse at the Bad News and the Good News

Agricultural forests (or "agriforests") are being established worldwide at the rate of approximately 5 million acres per year. Most of these forests are intended for "short-rotation" schedules, whereby harvesting occurs every 10-20 years. The expansion of these forests is driven by the pressures of a human population exhibiting exponential growth and its demand for pulp, paper and wood products. Most of the tree species used in agriforests emit high rates of volatile organic compounds (VOCs), which are oxidized by other components of the atmosphere and in the process, produce pollutants such as tropospheric ozone and organic nitrate compounds. Additionally, global atmospheric chemistry models predict that the emission of one of the most abundant VOC’s, isoprene, from the entire world’s forests causes a 20% increase in the lifetime of methane, a potent greenhouse gas that contributes to global warming. Thus, the global expansion of agriforests is expected to cause a significant deterioration of atmospheric quality and enhance the potential for global warming.

In studies of a model poplar agriforest ecosystem in Columbia University’s Biosphere 2 Center near Oracle, Arizona, we discovered that elevated atmospheric CO2 concentrations cause a reduction in ecosystem isoprene emissions. This reduction is due to a unique biochemical process in the leaves of the trees that utilizes CO2 but at the expense of the compounds required for isoprene biosynthesis. The results of these studies provide some glimmer of good news in the overall expectation that agriforests will deteriorate air quality – basically one form of pollution (the emission of carbon dioxide into the atmosphere from automobile exhaust and factories) will work against another form of pollution (the production of tropospheric ozone and organic nitrate compounds from the isoprene emitted by agriforests). It is doubtful that this glimmer of good news (which operates at the level of a unit acre of forest) will outweigh the overall burden of the bad news (which operates at the level of total acres of forest), as agriforests expand in coverage. However, it does illustrate the point that environmental science does not always bring forth the bad news!