understanding deforestation

Scientists at Hubbard Brook were interested in the effects of logging operations on the surrounding ecosystems, and questioned how deforestation would impact biogeochemical cycles and hydrology. Hubbard Brook scientists therefore decided to conduct experiments of deforestation and tree cutting at the watershed level. In December 1965, Watershed 2 of Hubbard Brook was clear-cut and all felled trees were left in place. Herbicide was applied for a 3 year period to prevent regrowth of vegetation. A few years later, Watershed 4 was strip cut in three phases during 1970, 1972, and 1974, and the felled material was removed. Finally, whole-tree harvest was completed in Watershed 5 during 1983-1984, where all biomass was clear-cut and removed. Figure 1 shows an aerial view of some of these deforested or strip-cut watersheds.

FIGURE 1- From left to right, Watershed 5 is shown (whole-tree harvest in 1983-1984), then Watershed 4 (strip cut in 1970, 1972, and 1974) and Watershed 2 (devegetated ending in 1968). This picture can be retrieved from this link.

Scientists found that these deforestation experiments had a considerable impact on hydrology and biogeochemistry in Watershed 2, with similar but reduced impacts in Watersheds 4, 5, and 101. During the three years that Watershed 2 was devegetated, annual streamflow increased by 40, 28, and 26 percent. With the forest cut down, scientists explain that this is due to the lack of transpiration of water by vegetation, which increases the total amount of water available for streamflow. Scientists also found that about five months after the deforestation of Watershed 2, large increases in the stream water concentration of all major nutrients were observed, except for ammonium, sulfate, and bicarbonate. The heightened concentration of nitrate, in particular, exceeded the drinking water standard of 40 mg/L, which can have adverse health effects, especially for pregnant women and infants.

The increase in concentration of nutrients in stream water was a result of a disruption of the nitrogen cycle by deforestation. Normally, when organic matter falls to the forest floor, microbes decompose the nitrogen in this dead organic matter into ammonium and nitrify the ammonium into nitrate. When plants are present, they reabsorb the ammonium and nitrate through their roots. However, if trees are cut down, they cannot reabsorb these nitrogen compounds, and the compounds are instead released into stream water. The nitrate ions and hydrogen ions produced from nitrification, which aren’t absorbed by plants, further mobilize other ions through reactions with the soil complex. Figures 2 and 3 provide diagrams of the two nitrogen cycles.

FIGURE 2- Diagram of the nitrogen cycle in an undisturbed forested ecosystem

FIGURE 3- Diagram of the nitrogen cycle after deforestation in an ecosystem

Compared to the reference watershed, Watershed 6, Watershed 2 also showed acidification of stream water due to the altered nitrogen cycle caused by tree cutting, which released more hydrogen ions into streams.

What to look for in this data story

You can see that stream flow increased after deforestation/tree cutting events in the affected watersheds.

How does the stream flow compare to the inputted precipitation in these watersheds after deforestation or tree cutting events?

You can observe that concentrations of some nutrients increase in stream water after deforestation/tree cutting events.

How does the concentration of nutrients in stream water compare to the concentration of these nutrients in rainwater before and after deforestation or tree cutting events? Where are these nutrients in the stream water coming from?

Which nutrients increase in concentration in stream water after deforestation or tree cutting events? Which nutrients stay the same in concentration after these events? Why might that be?

RESOURCES TO LEARN MORE

Free internet resources

From Hubbard Brook Ecosystem Study, Experimental Watersheds

From Teaching Issues and Experiments in Ecology, Hubbard Brook Streamflow Response to Deforestation

Books

Holmes, R. T., & Likens, G. E. (2016). Hubbard Brook: The story of a forest ecosystem. New Haven: Yale University Press.

Key Papers

Likens, G. E., Bormann, F. H., Johnson, N. M., Fisher, D. W., & Pierce, R. S. (1970). Effects of Forest Cutting and Herbicide Treatment on Nutrient Budgets in the Hubbard Brook Watershed-Ecosystem. Ecological Monographs, 40(1).

Likens, Gene E, F. H Bormann, and N. M Johnson. 1969."Nitrification: Importance To Nutrient Losses From A Cutover Forested Ecosystem". Science 163: 1205-1206.

Pardo, Linda H, Charles T Driscoll, and Gene E Likens. 1995. "Patterns Of Nitrate Loss From A Chronosequence Of Clear-Cut Watersheds". Water, Air, and Soil Pollution 85: 1659-1664.