Carly Apple
Classic Study paper
Due Nov. 18, 2005
Hubbard Brook and its Contribution to our Understanding of
Nutrient Dynamics
In the field of ecology, experimental studies (as opposed to observational methods)
allow us to determine “causal relationships that generate ecological pattern” (Odum,
2005). Ecology is unique in that it requires an understanding of several processes and
systems happening at once. This is difficult to simulate in a laboratory. Before Gene E.
Likens performed his classic study of the Hubbard Brook Ecosystem in New Hampshire,
no one had created a controlled study of ecosystem manipulation on a large scale.
According to ecologist Eugene Odum, “The Likens-Bormann experiments represent one
of the first planned manipulations of an ecosystem to ascertain the importance of
vegetation in regulating biogeochemical cycles” (Odum, 2005). Using Likens’ classic
study on the “Effects of Forest Cutting and Herbicide Treatment on Nutrient Budgets in
the Hubbard Brook Watershed Ecosystem,” I will trace the history of research of largescale ecosystem manipulations on our knowledge of nutrient budgeting/dynamics in
disturbed ecosystems. This knowledge has important implications for future research on
the effects of climate change on nutrient dynamics.
Likens’ original experiment was relatively simple. During November and
December of 1965, all vegetation in Watershed 2 of Hubbard Brook Experimental Forest
in New Hampshire was cut. The area was a second-growth hardwood forest with a
hardrock substrate. No vegetation was removed, and herbicide was applied over the entire
area periodically for two years to inhibit any regrowth. The purpose of this experiment
was to measure the changes happening in the dynamics of a forested ecosystem after a
clearcutting event (Likens et al., 1970).
Likens and Bormann recorded changes in many aspects of the forest and stream.
They looked at water conductivity, temperature, particulate matter, acidity, stream flow,
ion concentrations, and annual runoff. Their results show empirically what ecologists had
been trying to prove for years: vegetation is vital for “processes governing the retention
of essential nutrients in forest ecosystems” (Odum, 2005). Some of the results from this
study were surprising. For example, the synergy between nitrification and pH seemed to
lead to higher stream water concentrations than expected for almost every compound
studied. This shows that decomposition plays only part of the role in increasing
concentrations of compounds in stream water after deforestation, and left new questions
for other ecologists to answer. His research had an immediate impact on forest
management, and continues to be useful in management for both disturbed and
undisturbed ecosystems of all types.
Since Likens' classic study, many other ecologists have used his results, his
experiment design, and, notably, the Hubbard Brook watershed itself to base later
research. Likens and Bormann performed a follow-up study in 1974 on the longer-term
effects of the deforestation at Hubbard Brook after herbicide treatment had ended.
Specifically, they measured “the effects of deforestation on the export of particulate
matter, erodibility of the ecosystem, and the relative importance of dissolved substances
and particulate matter in exported materials” (Bormann et al., 1974). From their previous
study, they knew that deforestation increases erosion by about 15 times compared to
mature healthy forests, but here, they found that the rate of erosion was not constant
through time. In fact, Bormann et al. found that “the increase in export was exponential
with rather minor increases in the first 2 years after cutting and a sharp increase in the 3rd
year” (Bormann et al., 1974). They believed that the sharp increase of particulate matter
in the stream during the third year was due to the eventual inability of the ecosystem to
control erodability after 2 years of zero primary productivity. There was a dramatic
increase in erosion after two years because the natural “biotic control of erodibility
weakens, while dissolved substance export declines, probably because of diminution of
readily available nutrients stored within the system” (Bormann et al., 1974). Their
findings suggest that there is a certain level of homeostasis that regulates erosion in an
ecosystem for up to two years after a disturbance, after which time basic biotic resources
begin to fail, and the system will fall apart (Bormann et al., 1974).
In 1982, Peter Vitousek built on the idea of nutrient retention and homeostasis
from the 1974 study, and set out to understand the most important processes preventing
or delaying nitrogen loss from disturbed forests. Through a combination of laboratory
and field experiments, he found a relationship between the amount of nitrogen in litterfall
and the amount mineralized on the forest floor. He concluded that low net nitrogen
mineralization and lags in nitrification were the most influential factor in nitrogen
mobility (Vitousek, 1982). Furthermore, “these patterns suggest that nitrogen retention
within disturbed forest ecosystems can be caused by low nitrogen availability prior to
disturbance” (Vitousek, 1982). In other words, the more scarce resources are before a
disturbance, the more nutrients are retained after one has taken place. Vitousek built on
research by Likens and Bormann to show that nutrient availability after a disturbance
differs dramatically depending on the condition of an ecosystem before a disturbance.
In 1985, D.W. Schindler used the model of Hubbard Brook to study large-scale
implications of acidification of lakes on trophic structure and species composition
(Schindler et al., 1985). For eight years, he and his colleagues not only examined
different effects of acidification on the small experimental lake, but also compared their
results with results that would have been found “if simpler methods were used, such as
the laboratory toxicological or physiological tests that usually form the basis for
regulating water quality standards” (Schindler et al., 1985). The longer-term study
produced results that were more dramatic than expected. This happened because
Schindler looked at indicator species that were more sensitive than traditional species
used for this type of survey, and because “the magnitude of food-web disruption which
occurred early in the acidification of Lake 223 could not have been predicted from smallscale studies” (Schindler et al., 1985). These results further prove that a thorough
understanding of natural systems after a disturbance depends on large-scale experiments
due to the complex nature of nutrient cycles.
Schindler’s study further emphasizes that large-scale ecosystem manipulation is
difficult due to factors like the high cost, time, and site limitations (Odum, 2005). In
order for these studies to be useful, they must be able to be compared to plots in different
geographical areas with different environments. In 1989, Stephen Carpenter set out to
find a statistical solution to this problem using randomized intervention analysis and time
series analysis in his study “Replication and treatment strength in whole-lake
experiments” (Carpenter, 1989). His findings suggest that, “a series of unreplicated
paired-system experiments (one reference and one experimental system)… staggered in
time and performed in many locations, will provide more ecological insight than a
replicated experiment in a single region” (Carpenter, 1989). For the first time,
sophisticated statistical modeling was used to temper the effects of variable differences in
large-scale studies. He later incorporated these methods while looking at predator-prey
interactions in manipulated lake ecosystems in 1998 and 2001, furthering the body of
knowledge on size distributions and trophic cascades (Pace, 1998; Havlicek et al., 2001).
In 1999, Donald R. Zak performed an experiment that expanded on the research
of Vitousek, but this time looked for possible differences in leaching after clearcutting
between forest ecosystems with different net nitrification rates. Zak followed Schindler’s
research methods and used several control plots in different forests. He looked at two
general types of forests, clearcut them, and followed their progress over five years. Those
with the lowest previous rates of nitrification and aboveground biomass accumulation (in
this case, sugar maple – red oak forests) leached NO3 faster than forests with higher
aboveground biomass accumulation (sugar maple – basswood forests) following
clearcutting. In other words, “ecosystem-specific patterns of biomass accumulation
appear to control rates of NO3- leaching” (Zak, 1999). This study has direct implications
in forest management, showing that the most unproductive forests are the most
vulnerable to a disturbance event like clearcutting.
Peter Vitousek (1997) looked at nitrogen as a pollutant in forest and riparian
ecosystems. Previous studies had focused on nitrogen as a limiting nutrient. Here, he
collected all knowledge generated up to this point on the effects of fertilizers, fossil fuel
combustion, and nitrogen-fixing crops on the worldwide nitrogen cycle, and tried to
predict possible consequences. He concluded that human activity has doubled the amount
of nitrogen in the atmosphere, and suggested strategies to slow anthropocentric nitrogen
additions down. To support his research, he incorporated previously mentioned largescale ecosystem manipulation experiments, including his own and those of Schindler,
Likens and Bormann.
Currently, intensive research in the watershed of Hubbard Brook continues. Some
studies are narrow in their scope, but research at the site continues to focus on nutrient
budgets in whole-ecosystem manipulation. General topics include the cycling of nitrogen,
sulfur, phosphorus, mercury, calcium, and carbon, and the effects of pollution on flux of
these and other minerals (Stream Ecosystem Research, 05/28/04). Likens himself is
working with other ecologists to understand the effects of forest age in stream ecosystems.
This study looks for correlations between forest canopy and stream shade with nutrient
budgets and overall health of streams (Stream Ecosystem Research, 05/28/04). Mean
ages of forests are dropping in this area due to tree harvesting, so the results will have
important implications for all streams located in threatened forests. These large-scale
studies touch on the important emerging focus in science: understanding the effects of
anthropocentric changes that are increasing in frequency and severity all over the world.
Recent research on large scale experiments outside of the Hubbard Brook
ecosystem emphasizes long term manipulation or observation of manipulation by humans.
Some research topics include monitoring invasive species populations, or carbon dioxide
increases on top of Mauna Loa (Hobbie et al, 2003). Gene Likens has recently completed
a study at Hubbard Brook to look at biogeochemical changes as products of emerging
environmental problems of local to global concern, and is promoting the “small
watershed approach” in order to study human alterations of the environment in other
parts of the country (Likens et al., 2003). Only large-scale ecosystem studies are able to
model the effects of large-scale problems. According to the US Long Term Ecological
Research Program, it is impossible to study many current scientific areas of focus in a
laboratory with short-term goals. “Environmental problems often develop slowly and are
difficult to identify and solve without a long-term baseline... [E]ach year is different, and
multiyear observations expand our perspective. For example, we may realize that our 1year study took place during a decadal drought, or that we need 50 years of lake ice data
to detect the pattern of recurring El Niño climate disturbance” (Hobbie et al., 2003).
Laboratory or microcosm studies provide more hurdles in this type of research, because
they “cannot predict declines or disappearances that result from the interaction of
multiple stresses” (Schindler et al., 1985).
The limitations to current approaches of short-term, microcosm or laboratory
research are prohibitive when researching anthropocentric impacts on earth. The hot
button issue right now is climate change, and this is unlikely to change in the near future.
As of now, studies have been mostly limited to glaciers, coast lines, sea surface
temperature, or other areas that have easily measurable changes attributable to global
warming. Climate change will have huge impacts on the nutrient budgets of ecosystems,
but effects are difficult to measure because they are sometimes unexpected and come
from multiple sources. Hubbard Brook is an obvious choice to be among the first study
sites because year-to-year variation in water and air temperature, nutrient budgeting,
rainfall, and disturbances have been recorded for decades. This watershed will most
likely provide important answers the the many questions surrounding how we as humans
are affecting our planet.
References
Bormann, F. H., G. E. Likens, T. G. Siccama, R. S. Pierce, and J. S. Eaton. Summer,
1974. The export of nutrients and recovery of stable conditions following
deforestation at Hubbard Brook. Ecological Monographs 44:255-277.
Carpenter, S.R. 1989. Replication and treatment strength in whole-lake experiments.
Ecology 70:453-463.
Havlicek, T.D., and S.R. Carpenter. 2001. Pelagic species size distributions in lakes: Are
they discontinuous? Limnology and Oceanography 46:1021-1033.
Hobbie, J.E., S.R. Carpenter, N.B. Grimm, J.R. Gosz, and T.R. Seastedt. 2003. The US
long term ecological research program. BioScience 53:21-32.
Likens, G.E., F.H. Bormann, and N.M. Johnson. 1970. Effects of Forest Cutting and
Herbicide Treatment on Nutrient Budgets in the Hubbard Brook
Watershed-Ecosystem. Ecological Monographs 40:23-47.
Likens, G.E. 2003. Use of long-term data, mass balances and stable isotopes in watershed
biogeochemistry: The Hubbard Brook model. Gayana Botanica 60:3-7.
Odum, E.P., and G.W. Barrett. 2005. Fundamentals of Ecology. Thomson Brooks/Cole,
Belmont, CA.
Pace, M.L., J.J. Cole, and S.R. Carpenter. 1987. Trophic Cascades and Compensation:
Differential Responses of Microzooplankton in Whole-Lake Experiments.
Ecology 79:138-152.
Schindler, D.W. et al. 1985. Long-Term Ecosystem Stress: The effects of years of
experimental acidification on a small lake. Science 228:1395-1401.
“Stream Ecosystem Research” Hubbard Brook Ecosystem Study 28 May, 2004. 13
November, 2005
<http://www.hubbardbrook.org/research/current/projects/streams/stream_9
9.htm>.
Vitousek, P.M., et al. 1982. A comparative analysis of potential nitrification and nitrate
mobility in forest ecosystems. Ecological Monographs 52:155-177.
Vitousek, P.M., et al. 1997. Human alteration of the global nitrogen cycle: sources and
consequences. Ecological Applications 7:737-750.
Zak, D.R., T.M. Iseman, W.E. Holmes, and A.G. Merill. 1999. Revegetation and nitrate
leaching from lake states northern hardwood forests following harvest.
Soil Science Society of America Journal 63:1424-1429.
Good discussion of contribution of classic and its impact on ecology. But heavily
dependent on direct quotations.
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