Introduction Page 9A


Watershed Experiments - The Details & Results

Before scientists conduct forest experiments, they first need to develop a good understanding of how the forest functions in its natural state - this is referred to as the "baseline." Once a sufficient amount of baseline data is collected, and the forest is well-understood, real experimentation can begin.

At the HBEF this means that once a watershed was marked out and its weir was installed (in the 1950s and 1960s for all of the watersheds except WS9), scientists studied the basic functioning - the plants, trees, soil, precipitation, streams, and animals - for many years, or even decades, before starting experiments.

As you may know, in an experiment it is necessary to compare the experimental treatment (for example, cutting down trees) to a reference (an undisturbed, comparable forest) or to a control. (Because the watersheds in the HBEF are all slightly different, they are not true controls.)

Watersheds 3 and 6 are reference watersheds, so scientists compare these to experimental watersheds. Scientists will never conduct experiments in Watersheds 3 and 6.

Because these two watersheds represent the natural state of the Hubbard Brook valley (after the early 1900s logging), a tremendous amount of research on northern hardwood forest processes has been conducted on them. If you would like to learn more about Watershed 6, click here. This tour was designed for a college-educated audience.

The 4 other south-facing watersheds have been experimentally manipulated. In contrast, no experiments have been conducted on the north-facing watersheds. Most likely scientists will use these watersheds for future experiments - perhaps to answer questions they have not even considered yet.


The Experiments

1. Watershed 2: The objective of this experiment was to dramatically alter the watershed to study biotic control over the abiotic environment in northern hardwood forest watersheds (i.e., how trees affect watershed processes). Scientists also wanted to determine how forests respond to deforestation and if the removal of vegetation would increase streamflow. This experiment was not a commercial logging experiment (future experiments in Watersheds 4 and 5 were conducted to look at more realistic logging practices).

Watershed 2 after it was cut

In December 1965, all the trees and shrubs on Watershed 2 were felled by chainsaw and left in place. The forest floor was virtually undisturbed, since the trees were felled on a snow surface of about 50 cm.

No products were removed and no vehicles were allowed in the area. Then the watershed was sprayed with herbicides during the growing seasons of 1966, 1967, and 1968 to greatly reduce transpiration.
Results:
  • Spring snowmelt began earlier.

  • Streamflow volume increased by 30 percent in the three years after the forest was cut, but dropped to below pre-experiment levels about 12 years after the cut. This means stream - and drinking water reservoir levels - could increase initially after a forest is cut, but could eventually drop in the decades following the cut.

  • Rapid growth of pin cherry trees and raspberry plants from seeds which had remained viable in the soil for decades; these "pioneer plants" conserve nutrients that otherwise might be leached from the site.
  • May - September nitrate (NO3-) concentrations in streamwater increased. Nitrate is an important plant nutrient and is also a human drinking water hazard at high levels.


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