September 8, 2010

What’s So Bad About Planting Trees?

Print More

From 2000 to 2005, the rate of forestland depletion slowed from 8.9 to 7.3 million hectares per year. This net increase in forestland occurred as deforestation continued to accelerate but was outpaced by “afforestation,” the conversion of land to trees. While afforestation seems to demonstrate a positive ecological trend, research indicated significant consequences for soils.

On Sept. 3, postdoctoral associate Sean Berthrong, horticulture, presented his findings on afforestation’s effects on soil microbial structure and biogeochemical functions, as part of the Biogeochemistry & Environmental Biocomplexity seminar series.

Afforestation is a rapidly growing phenomenon, contributing to seven percent of the world’s forestland. Berthrong conducted research in the eucalyptus forests of the Rio de la Plata Grasslands, which stretch from northeastern Argentina across Uruguay to southern Brazil.

According to Berthrong, the eucalyptus trees “grow like wildfire,” and can be harvested every 10 years. Their high productivity allows for the preservation of primary forests, which have remained largely undisturbed by human activities.

Another beneficial use of afforested trees is their ability to offset carbon emissions through carbon sequestration.

From there, Berthrong said, the benefits diminish. The commonly afforested blue gum eucalyptus trees are native to Australia, not to the Rio de la Plata grasslands, where the natural vegetation consists of grasses and woody shrubs.  The forest of Rio de la Plata have not grown since the last glacial period.

The fast-growing eucalyptus trees, while beneficial in some respects, also have “particularly blunt and quick” effects on grassland soils, Berthrong stated.

For instance, Berthrong referenced a difference between the native and afforested species’ interactions with water.  Whereas water is able to move down through the loose native grasses, eucalyptus trees have a deeper wood structure.

The highly productive eucalyptus trees also have higher water demands, and as its roots draw saline groundwater up to the surface, vital freshwater sources become contaminated.

Research has even questioned afforestation’s touted productivity. According to Berthrong, data from China indicates that afforestation productivity derives from soil, and as soil is depleted through subsequent rotations, productivity eventually declines to one-half that of the first rotation. Berthrong’s genetic analysis of soil from afforested tracts in Argentina and Uruguay reveals a profile change in microbial DNA.

To put it plainly, “microbes are really, really important,” Berthrong said.

Microbial structure and their associated enzymes comprise the organic material of soils, and microbes complete a myriad of processes, including carbon and nitrogen cycling. Berthrong’s findings on gene abundance and the activity of common biogeochemical cycles supported the conclusion that microbial function has decreased.

Berthron emphasized the need for a regional analysis of afforestation due to the precipitation gradient across the Rio de la Plata grasslands. Dry (Argentina) and wet (Uruguay and Brazil) systems exhibit drastically different behavior. With respect to carbon sequestration, a dry system gains carbon over time, whereas a wet system loses carbon.

Further research may refine scientists’ understanding of afforestation. According to Berthrong’s research, its environmental and economic benefits remain outweighed by its detrimental effects on the surrounding environment, particularly soils.

Original Author: Jing Jin