Jose Lopez, Tom Ryan, Nathan Turk
FOR 332
LAB #5: Nutrient Cycling
Harvesting Affects Stream Chemistry in Hubbard Brook Watersheds
Abstract
Less nutrient uptake corresponds with greater nutrient output in stream flow. We
expected concentrations of Ca, NO3 and NH4 to change in conjunction with the severity
of disturbances at Hubbard Brook Experimental Forest, near New Woodstock, NH.
Stream chemistry analyses were taken following human disturbances such as clear
felling, herbicide application, strip cutting and whole tree harvesting, to test for impacts
on calcium (Ca), nitrate (NO3) and ammonium (NH4) concentrations at various
watersheds. The watershed that was clear-felled and then herbicide-treated had the
greatest concentration of Ca and NO3 in stream flow following disturbance. The
watershed that was whole-tree harvested showed smaller increases; the strip-cut
watershed, the least; and the control watershed, none except for those caused by natural
occurrences. Strip-cut and control watersheds showed little variation in NH4, NO3 and
Ca concentrations, although harvesting in general promoted higher nutrient
concentrations in stream water. Nutrient concentrations have important implications for
watershed management endeavors.
Introduction
Uptake by vegetation of macronutrients, which include nitrogen (N), phosphorous (P),
potassium (K), magnesium (Mg), calcium (Ca), and sulfur (S), decreases following
natural disturbances (Barnes et al, 1998) as well as human disturbances such as strip
cutting, whole tree harvesting and clearcutting with herbicide treatments. Less uptake
corresponds with greater nutrient output in stream flow. Our hypothesis is that the
concentrations in stream water (mg/L) of NO3 (nitrate), NH4 (ammonium) and Ca
(calcium) were higher after harvesting operations at Hubbard Brook Experimental Forest,
near New Woodstock, New Hampshire, than before harvesting. We expected to find the
greatest increase in concentrations of these compounds in years following clear cutting,
and the smallest increases following strip-cutting. We also expect to find the control
watershed to have very little change in the nutrient concentration over time since there
was no disturbance applied in this watershed. However, we also expected to find
fluctuations in nutrient concentrations in periods of non-disturbance. The objective of this
study was to evaluate the impact of harvesting on site productivity, assuming that less
available nutrients leads to less productive sites.
Methods
At Hubbard Brook Experimental Forest, stream chemistry analyses were taken following
tree harvests of different intensities at various watersheds. The amount of nutrient export
was reported in grams of nutrient in the water per hectare of land per month (g/ha-mo).
Daily export values were derived by multiplying daily stream flow amounts (in mm) for
each watershed, by the concentration of solutes (in mg/L) in the numbered watershed
sample taken. Measured and estimated daily values were summed to produce monthly
export, and export divided by total monthly stream flow to produce volume-weighted,
average monthly concentrations for stream water at Hubbard Brook Experimental Forest
numbered watersheds. Data was compiled by nutrient for this report in figures 1-3. The
presence of nitrate (No3), ammonium (NH4) and calcium (Ca) was recorded in each
watershed for each year of the study between 1963 and 1998.
Results
The clear-cut and herbicide-treated watershed had the greatest initial concentrations of Ca
and NO3 in stream flow. Levels of Ca and NO3 runoff in the stream water gradually
decreased after the herbicide treatment. The NH4 runoff of the clear-cut watershed was
higher than all others in 1969 due to 1967 and 1968 herbicide applications. The strip-cut
watershed had the highest average level of Ca annual runoff from 1973 to 1998, with the
exception of 1984, ’85 and ’86, during which the whole-tree harvested watershed had the
greatest. Concentrations of NH4 were least affected by human disturbance, although the
1983-’84 strip cuttings caused a slight increase in relation to the control (fig. 2). The
undisturbed watershed showed the lowest concentrations of Ca in all years; of NH4 from
1963-’77; and of NO3 in 1964.
Discussion
Team Gray Van found significant increases in average NO3 concentration in yearly
stream flow between 1963 and 1965 in the undisturbed watershed. Our data also showed
fluctuations in NO3 over an even longer period in this watershed. These fluctuations
support our hypothesis that natural occurrences also affect nutrient concentrations.
Team Xtreme found consistent concentrations of Ca and NO3 in stream exports between
1990 and 1996 of around 0.02 mg/L, and Ca of around 0.08 mg/L. The N03 and Ca
concentrations we measured were similar to Team Xtreme’s after 1987. However, after
1983-’84 whole-tree harvesting, NO3 concentrations returned to pre-harvest levels while
Ca levels leveled off at a slightly higher concentration. Team Xtreme’s consistent No3
and Ca concentrations support our thesis that watersheds will eventually recover from
harvesting in terms of exhibiting stable nutrient concentrations, although Ca and NO3
levels might still lower as vegetation grows.
Ammonium (NH4) concentrations are more erratic on our graph (fig. 2) than on Team
Xtreme’s. Disturbances such as clear-felling and whole-tree harvesting may have lasting
effects on stream water deposits that only dissipate over long periods, such as the two
decades between the whole-tree harvesting and the beginning of Team Xtreme’s data.
Team Xtreme found greater concentrations of Ca (average around 0.8 mg/L) than NH4
(approximately 0.02 mg/L) and NO3 (as low as 0.03 mg/L). Our findings also showed
smaller concentrations of NH4 and NO3 than Ca, except when NO3 values spiked (fig. 3)
due to clear-cutting and whole-tree harvesting. Because NH4 and NO3 contain the
limiting macronutrient nitrogen, they were utilized more efficiently by plants and are
were less present in stream runoff than non-limiting Ca.
To better gauge whether human disturbances affect ammonium concentrations, watershed
analyses following other types of disturbance would have to be performed. Additionally,
data regarding concentrations of Ca, NH4 and NO3 prior to 1963 might lend toward a
better picture whether the clear-felled watershed recovered to pre-harvesting conditions.
Conclusions
Harvesting impacted site productivity by promoting higher stream water concentrations
of NH4, NO3 and Ca in relation to control values. Our findings showed that the 1966
clear-felling had the greatest impact on Ca, NH4 and NO3 concentrations in stream
water. Whole-tree harvesting also increased watershed nutrient concentrations
significantly; strip-cutting raised Ca, NH4 and NO3 concentrations slightly over the years
it occurred (’70-’74). Concentrations of NO3, NH4 and Ca generally leveled off within a
decade of respective harvesting methods. Our hypothesis that concentrations eventually
return to pre-harvesting states was supported. However, slight fluctuations in
concentrations in years following disturbance recovery indicate environmental factors
such as nitrification, denitrification, mineralization and population growth characteristics
might also affect the presence of Ca, NH4 and NO3. Knowing effects of different human
disturbances on nutrient concentration can help us in watershed management endeavors.
For example, streams’ reception of excess nutrients can lead to eutrophication, which
deprives oxygen from stream water. Additionally, effects of harvesting on nutrient
concentrations can help us allow watersheds enough time to recover.
References
Likens, Gene E. Institute of Ecosystem Studies, Robert S. Pierce Ecosystem Laboratory,
Hubbard Brook Experimental Forest, W. Thornton, NH . 28 Oct. 2005
<http://www.hubbardbrook.org/research/data/stream/stm_chem/w6-stm.htm>.
Martin, C. W., R. S. Pierce, G. E. Likens and F. H. Bormann. 1986. Clear cutting Affects
Stream Chemistry in the White Mountains of New Hampshire. USDA Forest Service
Research Paper NE-579. 12 pp.
fig. 1
Average Annual Calcium (Ca) Concentration vs time at Each Watershed
9
Average annual Ca concentration (mg/L)
8
7
6
clear-felled (66) and
herbicided (67, 68)
5
strip-cut (70, 72, 74)
4
whole-tree harvested
(83, 84)
3
control
2
1
0
1963 1966 1969 1972 1975 1978 1981 1984 1987 1990 1993 1996
Year
fig. 2
Average annual Ammonium (NH4) Concentration vs Time at Each Watershed
0.14
Annual ammonium concentration (mg/L)
0.12
0.1
0.08
clear-felled (66) and
herbicided (67, 68)
strip-cut (70, 72, 74)
0.06
whole-tree harvested
(83, 84)
control
0.04
0.02
0
1963
1966
1969
1972
1975
1978
1981
1984
1987
1990
1993
1996
Year
fig. 3
Average Annual NO3 (Nitrate) concentration vs Time at Each Watershed
60
concentration of NO3 (mg/L)
50
40
clear-felled (66) and
herbicided (67, 68)
strip-cut (70, 72, 74)
30
whole-tree harvested
(83, 84)
control
20
10
0
1963
1966
1969
1972
1975
1978
1981
Year
1984
1987
1990
1993
1996
OUTTAKES
Time was related to NH4 concentration with reverse-J curves as represented in fig. XXX;
to NO3, in generally linear fashions (fig. XXX); and to Ca, with gently negatively sloped
curves with the exception of the clear-cut stand’s period of herbicide application (fig.
XXX).
One reason why the NO3 concentration were lowest in the clear cut watershed compared
to concentrations in the control, strip-cut and whole-tree watersheds by 1972 may be that
lack of herbs following the 1967-’68 herbicide treatments allowed more ground-level
sunlight to reach the soil and enhance germination. Herbs and pioneer species were able
to reestablish vigorously without competition from residual vegetation, thus encouraging
greater NO3 uptake.
concentrations in the clear-cut watershed immediately after harvesting, with additional
increases after the 1967-’68 herbicide treatments. We expected concentrations of NO3,
Ca and NH4 to be slightly less at the whole-tree harvest watershed, followed by the stripcut watershed, with the control watershed having the least nutrient concentrations.
Stream Export of Macronutrients
1.35
Amount Exported (g/ha/mo)
1.15
0.95
Calcium
0.75
Ammonium
Nitrate
Phosphate
0.55
0.35
0.15
-0.05
1
2
3
4
5
6
7
8
Month
January
February
Ca Avg
0.8344
0.8806
NH4
Avg
0.02475
0.02225
NO3
0.8638
0.9062
PO4
0.00332
0.0032
9
10
11
12
March
April
May
June
July
August
September
October
November
December
0.8412
0.7522
0.7892
0.8146
0.7976
0.8182
0.8066
0.821
0.8514
0.8192
0.025
0.01675
0.0185
0.01925
0.01625
0.021
0.017
0.021
0.01175
0.01925
1.404
0.872
0.365
0.1002
0.1448
0.0366
0.0404
0.1904
0.3768
0.687
0.00348
0.00324
0.0022
0.0037
0.00578
0.00426
0.00254
0.00228
0.00296
0.0032
Absence of NH4 and NO3 data in Hubbard Brook’s tables for the control and clear cut
watersheds until mid-1964, in the strip cut stand until mid-1965 and in the whole-tree
harvest watershed until mid-1971 and made it hard to compare the effects on nutrient
concentration immediately following harvesting.
Some data regarding concentrations of Ca, NO3 and NH4 was hard to reconcile with our
hypothesis. Calcium concentrations in mg/L were consistently the highest at the strip-cut
watershed, although the highest periodic Ca concentrations were measured in the four
years following the 1967 herbicide treatments on the clearcut stand. Nitrate (NO3) also
spiked considerably after the herbicide treatment, although the concentrations were
generally less than at the strip-cut, whole tree harvest and control watersheds after 1972.
Ammonium (NH3) concentrations showed the least increase in the years following the
herbicide treatments. By 1999, the NH4 and NO3 concentrations varied little between
watersheds, while the calcium concentration was greatest at the strip-cut watershed,
followed by the clearcut stand. The control and whole-tree harvest watersheds were
nearly tied in terms of the lowest calcium concentration.
However, concentrations of NH4 and Ca were the greatest in the ten years following
clearcutting, while significant surges occurred in years corresponding with herbicide
treatment (1967-’68), as depicted in FIG. XXX and FIG. XXX. In contrast, NO3
concentrations were the greatest following the 1967-’68 herbicide treatments on the
clear-cut stand.
, although data is missing data about NH4 and NO3 concentrations in the years
immediately following some of the cuttings. Figures concerning concentrations during
these years might paint a better picture of how NH4, NO3 and Ca are affected by
harvesting. Additionally, data from years prior to harvesting might be helpful in showing
exactly how much concentrations of the element and compounds were affected by the
harvests. Team Xtreme’s erratic values for NO3 (0.04 to 1.4 mg/L), as well as our huge
range of NO3 concentrations from 0.05 mg/L in 1990 to 18.8 in 1985, show how NO3
concentrations are sensitive to factors besides plant uptake. Factors affecting NO3
concentration might include rates at which denitrification returns it to the atmosphere and
at which mineralization by organic matter and nitrification of NH4 deposit it into the soil
solution.
Introduction
All plant life on earth is composed of three elements, carbon hydrogen and
oxygen, but these two elements alone cannot create life, a series of other elements or
nutrients are needed. Plants are composed of 14 nutrients, which are separated into two
categories, macronutrients and micronutrients. The The micronutrients are all those
nutrients that are needed in very little quantities by plants, like iron, zinc, and other 6
nutrients. Nutrients fallow particular cycles which involve biogeochemical and
geochemical processes that control the supply and uptake of nutrients by plants (Barnes
et al., 1998). In this lab we studied how nutrient flux, hydrologic and cycling of nutrients
all relate in response to human and natural activities. We expect to find how the
watershed that was clear cut and then applied with herbicide will have the greatest impact
on the nutrient concentration, followed by the watershed that was strip cut and then by
the one that was whole tree harvesting.
This increase corresponds with the spike in NO3 concentrations we observed (fig. 3)
following clear-felling, as well as the high concentrations maintained by the herbicide
treatments. Team Gray Van observed a sharp increase in NO3 concentration in 1972, the
same years we documented an increasing concentration trend caused by the strip cuttings
that began in 1970.
The concentrations of NH4 and NO3 in stream water followed similar trajectories in
measured years, although NH4 increased more in