Extension 17.7: Planting Trees

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Energy, Ch. 17, extension 7 <strong>Planting</strong> trees 6 “enhanced biomass accumulation in response to elevated levels of CO 2 or nitrogen, or their combination, is less in species-poor than in species-rich assemblages.” (739) The implication of this experiment is that a less diverse biome will be less efficient at carbon dioxide absorption than a more diverse biome. Another FACE experiment showed that estimates of forest productivity in a CO 2 - enhanced world might be too optimistic. Two sites were compared, one moderately nutrient rich, the other nutrient poor. Some biomass was added at the nutrient-rich site, but none on the nutrient-poor site. The added biomass disappeared after three years. (411) This suggests that fertility affects the response to added carbon dioxide, and that it is not clear that there will be added carbon sequestration in forests. Two different experiments shed some light on this question as well. In one, a group tried and failed to find indications of the “carbon dioxide fertilization” effect using inventory data from over 20,000 forest plots in five eastern states. (740) So, in this case, we would expect the forest sink eventually to disappear. Additionally, Cox et al. used a three-dimensional carbon-cycle model to find that “carbon-cycle feedbacks could significantly accelerate climate change.” They mean by this that in their model land became a source of carbon dioxide around 2050. (741) Adding carbon dioxide caused massive (threefold) gains in set seeds in one experiment. (742) This occurred in a stand of 19-year-old loblolly pines (Pinus taeda) after four years of enriched carbon dioxide. Since most loblolly pine stands are regenerated naturally, the spread may become faster than at present. They could take over from other species at elevated CO 2 levels. At a different site, increased CO 2 levels led to total net primary production gain of 25% for pine trees, implying that a lot of carbon could be sequestered. (743) In a study involving
Energy, Ch. 17, extension 7 <strong>Planting</strong> trees 7 a variety of forest types in Costa Rica, the upper and lower bounds on net primary productivity (NPP) varied greatly from site to site. (744) A study over 18 years of records from satellites and climate data indicates an increase in NPP. (745) However, additional research shows that this is limited to an approximately three-year timespan, after which storage decreases significantly. (746) Schlesinger and Lichter find that nearly half of the carbon uptake is allocated to short-lived tissues, for example, leaves or needles. When the leaves fall to the ground, they do not contribute much carbon to the underlying soil. (746) The authors say “Such findings call into question the role of soils as long-term carbon sinks, and show the need for a better understanding of carbon cycling in forest soils.” (746) Additionally, experiments in fungi indicate that the FACE approach—abrupt alteration in the concentration of carbon dioxide, which cannot occur in the “real world”—may not tell an accurate story. Fungi living under the condition of gradual increase of CO 2 showed little difference from control groups. Fungi exposed to abrupt shifts in concentration showed great differences. (b) This may mean that the FACE approach overstates the carbon dioxide fertilization effect. Lal has emphasized the role of soil carbon storage in agricultural soils. (747) In many regions of the world, agricultural soils have been degraded in carbon. Lal estimates the total soil carbon loss at 42 to 78 Gt of carbon. Restoring carbon to such degraded soils can enhance future crop production: “An increase of 1 ton of soil carbon pool of degraded cropland soils may increase crop yield by 20 to 40 kilograms per hectare (kg/ha) for wheat, 10 to 20 kg/ha for maize, and 0. 5 to 1 kg/ha for cowpeas.” Lal believes that this soil rehabilitation could take place over as little as two decades. (747) For carbon storage to take place, soil nitrogen must also be available. Hungate et al. have examined the possibilities using GCMs, and conclude that in scenarios with high nitrogen
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Extension 17.7: Planting Trees

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