Likens, G.E., Bormann, F.H., Johnson, N.M., Fisher, D.W. and R.S. Pierce. 1969. Effects
of forest cutting and herbicide treatment on nutrient budgets in the Hubbard
Brook Watershed-Ecosystem.
This experimental study was the first intentional large scale manipulation of a
forest ecosystem. The purpose of the study was to determine the importance of
vegetation in regulating biogeochemical cycles.
Question: What does deforestation do to watershed nutrient dynamics?
Approach: They cut down all vegetation from one of six watersheds, and suppressed new
growth with herbicide treatments. They didn’t clear out the dead stems. For the next few
years, they measured nutrient budgets: precipitation chemical input – output in stream
water.
Results: Vegetation is important for retaining nutrients and instrumental in regulating its
own environment. With these results, it is important to understand why the following
changes happened after deforestation:
As compared to stream water from the other 5 watersheds and the predicted outputs
modeled from a pretreatment calibration:
1. Nitrogen increased (NOx): This is probably the most interesting result. High
nitrogen outputs due to lowered demand by vegetation. Soil organisms converted
atmospheric N to nitrates just to make it easier for it to wash away. Nitrification is a
major controlling factor in determining the quality and quantity of dissolved material
flushed out of the watershed.
2. Water flow increased. There’s no plants to drink it up!
3. Ca, Mg and K increased 417%, 408% and 155% respectively. These cations
were released from the soil due to acidification of the soil (H+ ions have a higher affinity
for soil and thus replace the nutrient ions), erosion of bedrock and rapidly decomposing
organic matter.
4. Runoff, particulate matter, detritus and debris increased. Most particulate
matter was inorganic due to erosion in the stream. Lower amounts of roots and leaves to
dam up the streams led to higher erosion.
5. Acidity increased.
6. Stream water had higher dissolved O2: This is because there was turbulence in
the water.
7. SO4 decreased. This is due to the loss of sulfate generators (they weren’t quite
sure what happened: something with bacteria) and increased water flow.
8. Al and Si increase. Both due to increased weathering.
Conclusion: Conservation of nutrients within the ecosystem depends on a functional
balance within the intrasystem cycle.
Hubbard Brook: Second growth hardwood forest maintained by the Forest Service in the White Mountains
of New Hampshire. Supported by hardrock substrate which is impermeable to water (so you don’t have to
think much about groundwater when doing watershed analyses).
Likens: Cornell professor and director of the Institute of Ecosystems in Millbrook, NY. Ecosystem
scientist and first to link acid rain to fossil fuel combustion.
Bormann: Professor at Dartmouth while Likens was there: He came up with the project idea. Collaborated
with Likens in many other projects in Hubbard Brook.