Extension 17.7: Planting Trees

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Energy, Ch. 17, extension 7 Planting trees 4 Forest showed, on the other hand, that the forest there was a net sink of 2.0 ± 0.4 tonnes of carbon per hectare per year. (726) This study over nine years used eddy covariance to measure the rate of uptake. (726) So from this conflicting evidence, it is inconclusive whether forests will be a source or a sink of carbon in the north temperate zone. China’s forests are apparently storing carbon at present. (727) Tropical forests are seen as a net source of carbon (from deforestation), (724) but measurements in mature tropical forests in South America show that tree growth exceeded loss in 38 out of 50 sites monitored. (728) Another study in Brazil showed that even with deforestation, land use change and deforestation together nearly balance the carbon (the Amazon was a source of 200 Mt/yr of carbon over the years of the study). (729) And a computer simulation shows that even if carbon dioxide stopped growing, the momentum of increased growth in topical forests could continue for a century more. (730) There are indications that the warming occurring in the Arctic regions is already leading to ecosystem changes there. Permafrost is melting at a rapid rate (in the region of tundra studied, 18% had melted in 1959 and only 13% remained unmelted in 2003). (731) By 2100, central Canada is predicted to have no regions of permafrost left. (a) The number of shrubs is growing. (732) In the high north, the boreal forest is expected to be a large sink of carbon under warming, but evidence shows that for Alaskan boreal forests, droughts coming from changing weather stresses the trees and has not lead to enhanced sequestration. (733) A study of many European forests at all latitudes showed that uptake increased as latitude decreased, not as latitude increased. (734) Further, it may be that afforestation in boreal regions will backfire. The surface albedo changes when the tundra is replaced by trees, and this could more than compensate for the carbon sequestration, according to Reference 413 and Reference 735. Experimental evidence is mixed; a 1993
Energy, Ch. 17, extension 7 Planting trees 5 study showed that Arctic soils had become drier and become sources of carbon dioxide, (736) while a more recent study by the same group found that even though the warming has increased, some regions are sinks for carbon part of the time. (737) Many people had expected a “carbon dioxide fertilization” effect, in that plants are limited in the amount of carbon they can fix by the low concentration of atmospheric carbon dioxide in the atmosphere. Experiments are under way to learn about this as well as about how plants adapt to a future world with doubled CO 2 concentration, known as FACE experiments (FACE stands for Free Air CO 2 Enrichment). In these experiments, pipes arranged in a circle (Fig. E17.7.2) emit carbon dioxide at some height above the surface (CO 2 is heavier than air and so tends to sink) in a locale and the effects are compared with a comparable nearby plot not enriched in CO 2 . FACE studies, which originated at Brookhaven National Laboratory, are taking place in seven locations in the United States, in Switzerland, and in Panama. (738) Fig. E17.7.2 Views of several FACE experimental sites. FACE plots are up to 30 m in diameter and 20 m high. (Brookhaven National Laboratory) A group led by Peter Reich at the University of Minnesota studied how elevated CO 2 affected grassland in one FACE experiment, 6 native grasses and herbaceous plants in various combinations on six plots in Cedar Creek Natural History Area and found that
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Extension 17.7: Planting Trees

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