Rain Forests Ameliorate Climate Change

Posted in Around the Garden on September 26 2011, by Scott Mori

Manisha Sashital, a student in Environmental Engineering and Environmental Policy at Carnegie Mellon University, worked on a botanical glossary under the supervision of Dr. Mori at the Garden this summer. As part of her internship she prepared a cartoon illustrating the relationship between photosynthesis and respiration.

Scott A. Mori has been studying New World rain forest plants for The New York Botanical Garden for over 35 years. His interest in tropical forests as carbon sinks have been stimulated by his studies of trees in old growth tropical  forests.

Global warming has become one of the planet’s deadliest threats. Since the Industrial Revolution, carbon dioxide concentrations have risen from 280 ppm to nearly 390 ppm, with the potential to reach 550 ppm by 2050 if carbon emissions from fossil fuel combustion are not controlled. The earth has experienced major warming three times; but the Cretaceous warming period took place over millions of years and the Paleocene/Eocene warming happened over thousands of years. In contrast, today’s temperature changes are happening over decades. As a result, many species, perhaps even humans, may not be able to adapt to such rapid and high increases in temperature. One concern that is generally unknown to the public is that photosynthesis, the source of energy for nearly all organisms on the planet, shuts down at around 104° F. Mankind’s extreme disruption of the carbon cycle is causing and will continue to cause serious consequences for life on earth.

Carbon dioxide levels contribute to global warming through the greenhouse effect. Greenhouse gases trap radiation from the sun in the atmosphere, which causes global temperatures to rise because the radiation is not reflected back out of the atmosphere. The reason for today’s increased atmospheric carbon levels can be attributed to the combustion of fuels used for the production of electricity and in transportation, both of which are essential to modern societies; as well as to cutting and burning forests throughout the world. Since there is no precedent for the rapidity of current temperature increases, it is impossible for humans to predict which areas of the world will be affected and at what magnitude. The unpredictability of global warming makes it an especially serious environmental problem.

Rain forests as well as other vegetation types play an important role in reducing the levels of carbon dioxide in the atmosphere. Annually, plants in tropical rain forests around the world take in millions of tons of carbon dioxide and release millions of tons of oxygen through photosynthesis, and this balances the respiration of microbes, plants, and animals, which take in oxygen and expel carbon dioxide. As seen in the accompanying cartoon, plants take in carbon dioxide and water and use the energy of the sun to create carbohydrates that are, in turn, oxidized to produce the energy needed for plants to sustain themselves. The carbohydrates are also the building blocks plants use to make leaves, stems, flowers, and fruits. Oxygen, the byproduct of respiration, is used by organisms to break down ingested carbohydrates to produce the energy needed for them to grow and reproduce. Mankind’s extreme disruption of the carbon cycle is and will continue to have serious consequences for life on earth.

Old-growth rain forests sequester more carbon than other vegetation types. Carbon is stored in above- and below-ground biomass, including standing timber, branches, foliage, roots; and dead biomass, such as wood and other litter that falls from trees and other plants. The reason that old growth rain forests are efficient at carbon sequestration is two-fold. First, they are stratified into the following layers: ground, shrub, small trees, trees that form the canopy, and trees that emerge above the canopy. In addition, plants, such as epiphytes and lianas, grow on or climb into the plants that form the structure of the layers. In short, all of the space in an old growth rain forest is occupied by plants and each of them sequesters carbon. In the second place, the trees of old growth forests have denser wood which requires more carbon to manufacture than does the less dense woods of new growth forests.

When an old growth forests is cut down, the new growth forest that replaces it is called a secondary forest–it differs from an old growth forest by possessing fewer strata, having fewer plants growing on the trees of its single stratum, and by having less dense wood. Since there is less vegetation to perform photosynthesis, a secondary forest sequesters less carbon than does an old growth forest of the same size. When an old growth forest is cut down, it takes hundreds of years for the biodiversity to return and the amount of sequestered carbon to reach the level of the original old growth forest.

When old growth forest is cleared by humans, the first step is burning the dead biomass to temporarily increase soil nutrients and make it easier to plant crops. This action releases large quantities of stored carbon into the atmosphere; in addition, carbon in the soil is oxidized and also escapes as carbon dioxide. The latest estimates of atmospheric carbon levels indicate that deforestation and forest degradation account for about one fifth of anthropogenic carbon emissions, which is about equal to the amount emitted by all of the motor vehicles in the world.

After several years of agriculture on a cleared old growth rain forest site, the soil becomes depleted and crops no longer give high enough yields to make their cultivation worthwhile; thus, the plots are often abandoned, secondary plant succession begins, and hundreds of year’s later old growth forests similar to the original forests are regenerated. By that time the climate may have become so warm that photosynthesis no longer functions efficiently, leaving only weedy secondary forests in the world. Not only will the planet be poorer because of the loss of biodiversity, but ecosystem services may become so modified that our current way of life may no longer be possible.