Hubbard Brook Experimental Forest Data Analysis exercise
Ecosystems Ecology
Assignment is due on 9 April
Although you can work together in groups on forming hypotheses and making and discussing the
graphs, you will each need to hand in your own assignment that includes the graphs and your
own analysis and interpretation of the data.
Background information:
The USDA Forest Service established the Hubbard Brook Experimental Forest (HBEF)
in 1955 as a hydrologic research center. In the early years, scientists focused largely on assessing
the impacts of forest management on water yield, water quality, and floods. In 1960, additional
scientists become involved, and the Hubbard Brook Ecosystem Study (HBES) began. HBES
scientists developed the small watershed concept, which at the time was a new approach to
ecosystem science in which entire watersheds were studied and manipulated to learn more about
nitrogen, phosphorus, carbon, and other elemental cycling.
Around that time, the northeastern states were experiencing a drought and many
communities were suffering from water shortages. Knowing that plants (especially trees) take up
large volumes of water from the soil and convert it to water vapor, the forest scientists wondered
whether cutting down trees might increase water supplies. At the same time, the small watershed
scientists were interested in studying how entire watersheds respond to major disturbances.
Combining their interests, scientists designed an experiment in which all the trees in an
entire watershed would be cut down. The experiment had many different goals and objectives,
reflecting the diverse interests of the scientists involved. In this activity, you will only focus on
one of the major land management hypotheses: if trees were cut down and therefore not using as
much water, would more water flow into thereservoirs?
The Experiment:
The Hubbard Brook flows through New Hampshire's White Mountain National Forest and drains
a range of small mountains. The tributaries of Hubbard Brook form a set of discrete watersheds,
separated by mountain ridges. Because these watersheds share many characteristics in common
(for example, similar size and vegetation), they provide an ideal setting for conducting
ecosystem experiments. In the laboratory, scientists use controls to compare with results of
experiments. For example, a scientist studying the effect of salt on plants would expose some
plants to salt (treatment) and compare their growth to plants growing without salt (control).
Similarly, HBEF scientists devised an experimental treatment in which an entire watershed
would be manipulated. When scientists manipulate the world outside of the laboratory, they are
conducting a field experiment.
In the experimental watershed, researchers cut all the trees in the middle of winter and
left them lying on the snow so that the soil was not disturbed. A nearby watershed was left intact,
similar to a control. However, unlike laboratory studies, the ecosystem experiment did not have a
true control. Although the two watersheds were similar in size and vegetation, they were not
exact replicates. (It is virtually impossible to have true replicates or controls in nature because of
normal variability in soil, plants, etc.) Thus, the watershed that was left intact is referred to as the
"reference" rather than the control watershed.
To measure the changes in water flowing out of the watersheds, scientists installed
special gauges on forest streams, called "weirs" (see Figure 1). Weirs are permanent concrete
structures consisting of a large basin with a v-notch cut on the side of the downstream end. The
stream flows directly into the basin where it slows down and becomes still, and then flows out
over the v-notch. By constantly measuring the stream height as it passes over this v-notch, and
entering this height into a known formula, researchers can accurately determine streamflow
volume even when flow levels are very low.
In this activity, you will be looking at some of the original streamflow and water
chemistry data collected from the streams of two watersheds at Hubbard Brook to determine the
short-term and long-term results of a tree-cutting experiment. Watershed 2 is the treatment
watershed, and Watershed 3 is the reference watershed. All trees in Watershed 2 were cut in
December 1965 and left on top of the snow. In the summers of 1966, 1967, and 1968, two
herbicides were applied to the entire watershed to prevent the regrowth of any vegetation. The
herbicides were applied both to answer the water flow hypothesis discussed above as well as to
help collaborating scientists studying element cycling. Scientists believe that the herbicide was
added in such low concentrations that it did not negatively affect organisms in the watershed or
downstream.
The spreadsheet for this exercise includes the streamflow, precipitation, and water chemistry data
collected from Watersheds 2 and 3 at the Hubbard Brook Experimental Forest over a 30-year
period. The streamflow and precipitation data are on the first tab and the chemical data are on
the second tab.
1) Your first goal will be to examine the precipitation and stream flow data and how these
might be different as a result of the deforestation event. First, discuss with your group
what you expect might happen to stream flow as a result of deforestation, and then write
a testable hypothesis below about what you think will happen (5 points)
2) Now make two graphs – one that compares the precipitation and streamflow data over
time in watershed 2 and one that compares the precipitation and streamflow data over
time in watershed 3. Be sure to label all your axes with appropriate units and include a
descriptive title for your plot (10 pts). On the watershed two graph mark the time when
the deforestation occurred. You will need to print out these graphs and hand them in with
your individual assignments but it is OK if each member of the group hands in the same
set of graphs.
3) Discuss with your partners the patterns that you see and describe in the space below the
following: a) Change in precipitation over time for both the reference watershed and the
deforested watershed b) change in stream flow over time for both the reference watershed
and the deforested watershed (5 pts) c) similarity and differences in streamflow between
the two (5 points).
4) You may be able to pick out a subtle trend in the data, but it is hard to see because of the
natural year to year variability in the data. Next, in excel, calculate the differences in
stream flow between watershed 2 (the deforested creek) and watershed 3 (the reference
creek) and plot the difference (5 pts).
5) Discuss the pattern that you see with your group. What conclusions can you draw and
why (10 pts)
6) Now switch to the chemistry tab. On this tab you have data from 1965 to 2010 for
calcium, magnesium, potassium, sodium and pH for both the deforested and the reference
watersheds. Discuss with your group and formulate hypotheses about 1) How base
cations might change as a result of deforestation (5 pts) 2) how pH might change (5 pts).
7) Now make plots that show how all the base cations and pH change over time for both the
reference (5 pts) and the deforested (5 pts) watersheds change over time.
8) Discuss these patterns with your group and describe the trends that you see (10 pts). Begin by
describing the patterns in the reference watershed and then describe the patterns in the deforested
watershed and the differences between the two.
Analysis questions (to be answered individually though you can discuss them with your group).
1) Why is it inappropriate to refer to the non-deforested watershed as a control and why can
you not have a “true” replicate in nature? Do you think this is a serious limitation to the
field of ecosystems ecology, why or why not (5 pts)?
2) Describe the effect that deforestation had on the volume of streamflow from the
watershed. Was this the effect that you predicted? How did the effect of deforestation
change over time? What does this suggest for the capacity for ecosystems to respond to
disturbance (10 pts).
3) Describe the changes over time in the stream chemistry of the reference watershed. What
patterns do you see and what do they suggest about the condition of the system (5 pts)
4) Describe the effects of deforestation on stream chemistry. What did deforestation do to
the concentrations of base cations and pH in the stream? Was this what you predicted?
Explain the underlying geochemical relationships that caused the patterns that you see
(10 pts).