Publication
John Fryer
Senior Forestry Adviser
Forest Science Institute of Vietnam
The reputable journal Science (23 December 2005) contained a report written by 10 authors from seven institutions from USA, Australia, South Africa, and Argentina, with the following title and abstract:
‘Trading Water for Carbon with Biological Carbon Sequestration’
*Robert B. Jackson, Esteban G. Jobbagy, Roni Avissar, Somnath Baidya Roy, Damian J. Barrett, Charles W. Cook, Kathleen A. Farley, David C. le Maitre, Bruce A. McCarl and Brian C. Murray.
Abstract
Carbon sequestration strategies highlight tree plantations without considering their full environmental consequences. We combined field research, synthesis of more than 600 observations, and climate and economic modeling to document substantial losses in stream flow, and increased soil salinization and acidification, with afforestation. Plantations decreased stream flow by 227 millimeters per year globally (52%), with 13% of streams drying completely for at least 1 year. Regional modeling of U.S. plantation scenarios suggests that climate feedbacks are unlikely to offset such water losses and could exacerbate them. Plantations can help control groundwater recharge and upwelling but reduce stream flow and salinize and acidify some soils.
*Author for correspondence: E-mail: jackson@duke.edu
Conventional wisdom has it that if you have an environmental problem you should plant some trees and most times the problem will go away. This view has been challenged, of course, from within and from outside the forestry science profession, and rightly so. Trees are wonderful organisms but there are limits to the environmental damage they can repair and the benefits they can bestow. This report in the journal Science sounds a salutary note of caution that we should weigh all the environmental effects of tree planting before embarking on broad-scale afforestation.
The report examined data on the impacts of plantations on hydrology and soil chemistry on a very wide range of sites and discusses these in the context of potential benefits through biological carbon sequestration. It comes as no surprise that the examination showed that in general tree plantations used more water than other types of vegetation cover such as grass or shrubs that they replaced, or that some tree species used more water than others in plantations. Although the report does not specifically address this, in all likelihood the more productive species such as of Eucalyptus (in biomass terms) would be the highest water users. These effects will almost inevitably be evident in stream flow and surface and groundwater water availability, and this should be of vital concern in water scarce regions.
What did come as rather a surprise, at least to this reader, were the changes observed in soil chemistry under plantations compared with grasslands, shrublands and croplands. Significant increases in salinity and decreases in pH were observed under plantations on some, but not all, soil types. In case some are tempted to use these observations in the tired old debate about monocultures of exotic species the report noted that there appeared to be no differences in these effects between a number of different tree species examined.
Although comprehensive, this research was confined to more temperate climates, and appeared to be looking mostly at natural grasslands that had been planted to tree plantations. It would have been interesting to see similar comparisons between natural forests and land that has been converted from natural forest to grassland or cropland
What, if any, are the implications of these research findings to current reforestation efforts in Vietnam? In a situation of full research funding there could be similar comparisons here between plantations and other vegetation types but that seems to be unlikely in the present funding situation. But forest scientists should at least be aware that there could be environmental costs to plantation establishment, whether it is for carbon sequestration or other benefits.
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