November 22, 2012
Aaron Thornell
201102614
Lab 6-8
Changes in stream water chemistry and
transport in Watershed 2 of the
Hubbard’s Brook Experiment Forest
following harvesting
Abstract
This report examines the effect that deforestation has on stream dissolved and particulate loading by
using data from the Hubbard Brook Experimental Forest from 1964 to 2007. It also looks at the nitrogen
transport in this ecosystem, and the effect deforestation has on water pH levels. The study revealed
that deforestation dramatically increased dissolved particulate load, and to a lesser extent, particulate
load in the stream. It also showed that nitrogen levels increased greatly, in both nitrogen species
examined, nitrate and ammonium. Finally, it also increased the pH of rainwater by about one pH level
after coming in contact with the soil of the deforested land.
Introduction
This study focuses on the effect of deforestation on stream dissolved and particulate loading, as well as
nitrogen transport. It also looks at trends in pH of both stream water and precipitation. The data for
this experiment originates from the Hubbard’s Brook Experimental Forest in New Hampshire, conducted
between 1964 and 2007. The forest is found in the southern portion of White Mountain National
Forest. The stream itself is usually characterized by low levels of concentrations of suspended materials,
trends in pH of both stream water and precipitation in this area, which was clear cut in 1965.
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Aaron Thornell
201102614
Materials and Methods
This examination of the effects of deforestation on stream dissolved and particulate loading was
performed using data from the Hubbard’s Brook Experimental Forest, as mentioned. This data was put
into an Excel spreadsheet where I was able to be manipulated. The data provided includes the amounts
(in mg/l) of many different dissolved materials, which are: calcium (Ca 2+), magnesium (Mg 2+), potassium
(K +), sodium (Na +), ammonium (NH4 +), sulfate (SO4 2-), nitrate (NO3 -), chlorine (Cl -) and silicon dioxide
(SiO2). It also detailed the stream flow, in mm per month, the pH levels in the stream, as well as the pH
of rainfall of the area. Finally, it contained the annual values of particulate load from 1964 to 1998. All
these, save for the pH of the rainfall, were gathered on a monthly basis from 1964 through to 2007. The
measured values of dissolved materials were first all taken and tabulated, to create annual values in kg
per hectare for all dissolved materials, and these values were graphed against particulate stream loads.
This involved converting the loads from their measured unit, mg/l, to kg/ha. Following this, two 3-year
periods were isolated, in which it appeared that deforestation had altered dissolved and particulate
loads in different ways, and again, the two loads were compared. Later, the focus turned specifically to
how deforestation affected nitrogen species in the stream. This was done by calculating annual loads
(again, in kg/ha) for NO3 – and NH4 +, and the two were graphed so that they could be compared. This
was followed by another closer look at these two species, by selecting a 2-year period of “peak data”
and a 2-year period of “recovery” data within the annual loads. Finally, rainwater pH was examined,
looking at the differences in pH levels prior and after coming in contact with the soil of the clear cut
land.
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Aaron Thornell
201102614
Results
The results of this experiment showed many things. Beginning with the first exercise, it showed that the
annual loads of dissolved stream load was much higher than those of particulate loads, as is shown
below (Figure 1).
Annual Trends: Particulate vs. Total Dissolved
Stream Loads
Stream Load (kg/ha)
1000
800
600
Total Annual Dissolved Stream
Load (kg/ha)
400
Particulate Load (kg/ha)
200
0
1960
1970
1980
1990
2000
2010
Year
Figure 1
This graph shows that these two loads experience similar peaks in concentration, however the annual
dissolved stream is substantially greater than that of the particulate concentration. It is clear that in the
early years of the experiment, just after the deforestation, both loads experience a great spike in
concentration. This is followed by several more mild spikes, approximately every ten years. The
particulate load data ends in 1998.
When looking a specific 3-year period of data that is representative of deforestation altering dissolved
and particulate loads in the stream, I selected the years which, on the graph, appeared to be the most
irregular. This was the period from 1968 to 1970, whose values are represented in Table 1, below.
Table 1
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Aaron Thornell
201102614
Year
1968
1969
1970
Annual Dissolved
Stream Load (kg/ha)
698.942348
829.769346
429.909032
Particulate Load
(kg/ha)
91.9
194.7
365.3
This made for an average of approximately 652.8 kg/ha of dissolved material, and 217.3 kg/ha of
particulate. Dissolved material comprised around 75% of the stream load, while the particulate load
made up 25%. This exercise was then repeated for a different 3-year period, which contrasted the first
period. For this I chose the years 1983 to 1985, when the stream was in a recovery state (Table 2).
Table 2
Year
1983
1984
1985
Annual Dissolved
Stream Load (kg/ha)
124.693524
129.452288
86.84367
Particulate Load
(kg/ha)
3.3
39.0
6.3
The average of dissolved material for these years was approximately 113.7 kg/ha, and for particulate
load it was 48.6, making the total composition approximately 70% dissolved and 30% particulate.
The next experiment performed was looking solely at dissolved nitrogen species. First, a comparison
between measurements of NO3 – and NH4 +, and these were plotted (Figure 2).
November 22, 2012
Aaron Thornell
201102614
700
1.6
600
1.4
1.2
500
1
400
0.8
300
0.6
200
0.4
100
0
1960
0.2
1970
1980
1990
2000
0
2010
NH4 + Dissolved Particulate (kg/ha)
NO3 - Dissolved Particulate (kg/ha)
Annual Trends in Stream Measurements:
NO3 – vs. NH4 +
Total Annual Dissolved
Particulate - NO3 - (kg/ha)
Total Annual Dissolved
Particulate - NH4+ (kg/ha)
Year
Figure 2
It is important to note that NO3 – values are on the vertical axis on the left, and the NH4 + values on the
right. With that in mind, one can see that there are substantially higher levels of NO3 –, with very low
levels of NH4 +. One can also see that they follow similar trends seen in the total dissolved material plot
(Figure 1). The next thing examined was again a more concentrated look at levels of nitrogen species.
The first 2-year period was one during the peak period, just after deforestation (Table 3).
Table 3
Year
1967
1968
NO3– Dissolved
Particulate (kg/ha)
602.931833
464.207133
NH4+ Dissolved
Particulate (kg/ha)
0.590393
0.395235
These are the peak values for NO3 – and among the highest for NH4 +. The same concentrated look was
done for a “recovery” 2-year period, and the years 2003 and 2004 were selected (Table 4. These two
years were chosen because they bear the greatest similarity to the levels prior to the clear-cut, and also
one which is similar to the values of years before and after them.
November 22, 2012
Aaron Thornell
201102614
Table 4
Year
2003
2004
NO3– Dissolved
Particulate (kg/ha)
4.500209
3.447679
NH4+ Dissolved
Particulate (kg/ha)
0.06521
0.047551
In the final exercise, the focus was on the change in pH levels between rain water and water after
coming in contact with the soil in the clear-cut region. We see an overall increase in pH of
approximately 1.0 pH level guarantee following the water’s integration with the soil and stream water,
as shown in Figure 3.
pH Level
Rainwater pH vs. Stream Water pH
6
5.8
5.6
5.4
5.2
5
4.8
4.6
4.4
4.2
4
3.8
3.6
3.4
3.2
3
2.8
2.6
2.4
2.2
2
1.8
1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
0
1970
Annual averal stream water
pH
Annual average rainwater pH
1980
1990
2000
Year
Figure 3
2010
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Aaron Thornell
201102614
Discussion
It is obvious from examining Figure 1 that total dissolved material increases dramatically following
deforestation. This is in all likelihood due to the loss of vegetation and the structure it provides against
erosion. Without root systems there is nothing to hold the soil together, and nutrients dissolved and
are transported downstream (Likens et al., 1970). Another contributing factor to this increase stems
from the alteration of the ecosystem’s nitrogen cycle. The greatest increase of any dissolved nutrient
was that of nitrogen, primarily in the form of nitrate. This ties in with the second experiment, in which a
massive increase in dissolved NO3- was observed (Fig. 2). The cause for this increase is the disconnect in
nitrogen cycle caused by deforestation. In forested areas, nitrogen is cycled between living and
decaying organic bodies. However, in a deforested area such as the one examined here, nitrate is still
produced, through microbial nitrification from decaying organisms, but it is washed away in the stream
because it is unused (Likens et al., 1970). Returning to the second portion of the first exercise, we
examine a representative 3-year period when the effects of deforestation previously described are
evident. We see a similar, yet lesser increase in particulate load within the stream. As is shown in Table
1, dissolved material makes up 75% of total stream load from the years 1968 to 1970. Following this,
another 3-year period was chosen, this one contrasting the former 3-year period (Table 2). Dissolved
mater still made up 70% of stream load, it appears that while dissolved loads remained high, still due to
the fact that NO3- is being flushed out upon introduction to the system. Particulate material, on the
other hand, has returned to lower levels. It was likely higher immediately following deforestation due
to the extreme disturbance in land caused by the clear-cutting, as well as the loss of soil stability and
structure. This trend, especially of increased dissolved materials, can and has been observed in other
similar ecosystems. The nitrogen cycle does not change very much from ecosystem to ecosystem, and
similar increases have been explored in prior studies (Thomas et al., 2004).
November 22, 2012
Aaron Thornell
201102614
Focusing now solely on nitrogen species, it is important to reiterate the prior point that nitrogen levels
increase dramatically due to the interruption in the nitrogen cycle brought on by deforestation. When
looking more closely, however, it is clear that there is a drastic increase in nitrate, while there is a similar
increase in ammonium, seen in Figure 2. This increase in ammonium is on a much smaller scale, as
ammonium levels begin at a substantially lower concentration level than that of nitrate. As we see in
tables 3 and 4, there are substantial changes in levels for both nitrate and ammonium. While it seems
the levels have “recovered”, it is difficult to describe them as “normal” due to the lack of forest in the
area now (Likens et al. 1974). If they were truly to return to normal, there would need to be forest in
the area as well.
Finally, this study examined the pH change of rainwater after coming in contact with the clear-cut
forest’s ground. We see an increase in pH after contact with the ground and integrating with the soil.
This stems from the increase in nitrate, an anion, which would cause this increase in the water’s pH
level.
References
Bormann, H., Likens, G., Siccama, T., Pierce, R., Eaton, J. 1974. The export of nutrients and recovery of
stable conditions following deforestation at Hubbard Brook. Ecological Monographs 44: 255-277.
Likens, G., Bormann, H., Johnson, N., Fisher, D., Pierce, R. 1970. Effects of forest cutting and herbicide
treatment on Nutrient Budgets in the Hubbard Brook watershed-ecosystem. Ecological Monographs 40:
23-47.
Thomas, S., Neill, C., Deegan, L., Krusche, A., Ballester, V., Reynaldo, V. 2004 Influences of land use and
stream size on particulate and dissolved materials in a small Amazonian stream network.
Biogeochemistry 68: 135-151.
Unknown author. 2012. Hubbard Brook Ecosystem Study: Overview. National Science Foundation.
Retrieved from http://www.hubbardbrook.org/overview/site_description.htm.
November 22, 2012
Aaron Thornell
201102614