Terrestrial Plants and Carbon Interactions

Terrestrial plants consume carbon dioxide during the daytime period and respire some of this CO₂ back to the atmosphere at night. The accompanying sketch (Figure 17) shows typical levels of CO₂ within a vigorously growing plant canopy at different times of day and night. At night, plants tend to respire or give up carbon dioxide, raising the ambient level well above the global mean (which was about 330 ppm when this sketch was printed). During the day however, if the plants are not limited by lack of water or nutrients, photosynthesis may draw down the amount of CO₂ to approximately 300 parts per million. The carbon taken up by the plants is converted to plant carbon and remains in this form until the decay process begins. For typical agricultural plants, the biomass is broken down during the dormant period and is converted to soil carbon and atmospheric carbon dioxide at rates that depend on temperature and moisture conditions.

Woody plants play a special role in the capture (sequestration) of carbon from the atmosphere. Trees lose their leaves (deciduous) and some or all needles (conifers) every year and return a portion of the season's carbon capture back to the atmosphere. The woody parts of the tree, however, may persist as plant carbon for several decades until the tree dies and decay begins. Trees, therefore, like annual plants, participate in the rapid part of the carbon cycle by cycling CO₂ back to the atmosphere in 1-3 years, but also store some carbon that is not recycled to the atmosphere for 50 to 100 years. Tropical rain forests have very large amounts of carbon sequestered in the trunks of trees as a part of this longer term cycle. Deforestation interrupts this natural long term cycle and puts carbon from woody material back into the atmosphere before the natural decay process would recycle it into CO₂ .

The "greenness index" of the planet, shown in Figure 18 for July and August shows very rich terrestrial biological activity in the Northern Hemisphere continental areas, particularly in the boreal forests of northern Russia and Canada. At this time of year, the Southern Hemisphere, of course, is experiencing the biologically dormant winter period. Major desert regions, located approximately 30^o north and south of the equator, show lack of vegetation in both hemispheres (both summer and winter). During the Northern Hemisphere winter, shown in Figure 19, the boreal forest regions are seen to have shut down their biological production. In the Southern Hemisphere, the smaller amount of land mass does not reveal the dramatic seasonal variation of the Northern Hemisphere. This suggests that global seasonal cycles of atmospheric carbon dioxide will be dominated by the seasonal cycle in the Northern Hemisphere.

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