In 1998, we initiated
investigations of the ecosystem carbon budget of a
subalpine forest near Niwot Ridge, Colorado. Studying the carbon budgets of terrestrial
ecosystems is critical to improving our understanding of the global carbon budget.
Each year, over 8 billion tons of carbon dioxide is emitted to the atmosphere as the net
result of
various human activities on the earth (primarily fossil fuel combustion, industrial
activities and deforestation).
Approximately half of this carbon dioxide remains in the atmosphere and has the
potential to contribute to global warming. The other half is assimilated by the
chemical and biological processes in the oceans and the photosynthetic processes of
terrestrial ecosystems. Knowledge about the precise destination of this assimilated
carbon is crucial to understanding how the atmosphere and biosphere will respond to future
carbon dioxide emissions. On the basis of some past modeling studies, there is good
reason to
believe that much of this carbon dioxide is assimilated by the forests of the world,
especially those in the northern hemisphere. To gain better insight into this issue,
a series of tower flux sites have been established across the world, and organized into a
cohesive network, with the aim of measuring the capacity for terrestrial ecosystems to
assimilate carbon, and identifying the primary ecological and environmental controls over
net ecosystem carbon dioxide exchange (NEE). The network is called FluxNet. Our
tower site is part of this network.
To conduct the research at our site, a 30-m tower was
constructed in the forest, complete with electrical power and fiber optic links to
computers and instruments in a climate-controlled trailer located nearby. Continuous
measurements of net ecosystem fluxes of carbon dioxide, water vapor, and ozone are
being
conducted using eddy covariance techniques. In the past, we have also conducted
measurements of fluxes of the stable carbon and oxygen
isotopes in carbon dioxide using relaxed eddy accumulation techniques
in order to partition NEE into its photosynthetic and respiratory components.
Specific campaigns have been conducted to study fluxes of reactive hydrocarbon species
such as isoprene, monoterpenes, methylbutanol, and other oxygenated hydrocarbon
species. Finally, we have conducted studies to measure ammonia and nitrogen oxide
fluxes at the
site. The overall goals of these studies are to (1) determine the patterns and
capacity for carbon dioxide sequestration by this ecosystem, (2) identify possible
influences of
ozone and nitrogen oxide deposition from the nearby Denver metropolitan corridor on carbon
sequestration, (3) study seasonal and interannual variation in the components of NEE for
this ecosystem, and (4) study patterns of, and significant controls over, the emission of
biogenic hydrocarbons from this ecosystem. These studies are
being supported by the U.S. National Science Foundation and the U.S. Department of
Energy, and in particular by the DOE-funded National Institute for Climate Change Research.
One of the principal discoveries we have made in the
past nine years is
that mountain forested ecosystems in the western US are responsible for most of the regional
sequestration of carbon dioxide. Many of the ecosystems of the western US are arid or
semi-arid deserts, shrubland and grasslands. Carbon sequestration in these ecosystems
is
highly constrained by moisture. In forested mountain ecosystems, seasonal snow fall
provides a buffer against many of the drought effects seen for ecosystems at lower
elevations. Thus, to understand carbon sequestration patterns in the western US, we
must
study carbon dioxide uptake patterns in mountain ecosystems, and we must understand the
nature of the biogeochemical coupling between the water and carbon cycles in these
ecosystems.
Another major discovery we have made is that climate warming in the western U.S. over the past two decades has resulted in a longer
growing season
at our mountain site, particularly by accelerating the onset of warm weather in the spring. Our intuition would tell us that a longer growing should result in
more forest growth and thus more CO2 sequestered from the atmosphere each year. However, we have
observed just the opposite; a longer growing season has meant less forest growth and less CO2
sequestration from the atmosphere. We explain this counter-intuitive discovery by noting that
this forest is critically dependent on the wet, heavy snows that are received in the early spring. These spring snows
deposit a lot of water to the forest soils, and allow the forest to grow during the early dry
summer months that precede the late summer convective rain storms. An earlier spring warm up usually
occurs with less spring snow. Thus, the forest is left with less water for the early summer and becomes
stressed; this causes less CO2 uptake from the atmosphere.
Most recently, we have teamed with researchers at the National Center for
Atmsopheric Research and the US Geological Survey to study these issues. Our research
approach includes (1) studies of regional carbon exchange dynamics using aircraft sampling
of
carbon dioxide concentrations and fluxes and advanced modeling-data assimilation approaches,
(2) multiple towers located within a close distance at our study site to help us better
understand how carbon dioxide moves as it migrates downslope at night due to cold-air
drainage, and (3) the use of stable isotope analyses and monitoring of tree transpiration
patterns to determine the primary controls over carbon dioxide assimilation by snow-melt
water and mid-summer rain water.
(2) Research into the nature of forest hydrocarbon emissions and its
relationship to atmospheric chemistry
During the past decade we have been studying
the
influence of forests on atmospheric processes, especially the exchange of hydrocarbon
compounds and its influence on atmospheric chemistry. The studies have focused on two
types of non-methane hydrocarbons, isoprene and monoterpenes. These alkenes appear to have
very important roles in protecting trees from stress -- stress from high temperatures and
lack of water in the case of isoprene and stress from insect damage in the case of
monoterpenes. When plants produce these compounds there is an inevitable leak to the
atmosphere. In the atmosphere they are oxidized rapidly (within hours) primarily by
hydroxyl radical, a naturally formed oxidant in the atmosphere. Hydroxyl radical is the
dominant driver of daytime tropospheric chemistry over continental regions. It is
responsible for the oxidation of most organic compounds that enter the troposphere,
including carbon monoxide and methane. Thus, the emission of isoprene and monoterpenes
from forest ecosystems plays a dominant role in controlling the oxidative potential of the
troposphere.
The aim of our studies is to identify the
environmental and
biological determinants of isoprene and monoterpene emission at the leaf, canopy and
ecosystem levels. This includes measurements of leaf gas-exchange using special
chambers,
and canopy gas-exchange using instruments mounted at the top of towers that span the
vertical distance of forest canopies. Additionally, physiological and biochemical
models
have been developed and are being used to study processes at higher scales such as
emissions from canopies and ecosystems. To date, measurements have been made in the
Canadian boreal forest, the aspen groves and subalpine forests of Colorado, an oak-hickory
forest in Tenessee, and mixed deciduous woodlands in upstate Massachusetts.
Our most recent studies have focused on three
areas. In the
first area, attempts are being made to understand the relationships between carbon balance
and herbivory as controls over monoterpene biosynthesis and emission to the atmosphere.
Using these processes as a foundation, models are being constructed from existing
theory
on plant allocation and the co-evolution of plants and herbivores, to predict monoterpene
production and emission of these defensive compounds into the atmosphere. In the
second
area of research, measurements are being made to determine the magnitude of monoterpene
and isoprene emissions in a world of elevated carbon dioxide. This effort is important
to
improve our understanding how future changes in the carbon balance of the atmosphere may
affect the photochemical chemistry of the atmosphere. This research involves making
measurements on trees growing in simulated environments of elevated carbon dioxide from
various sites around the country, including pine trees in California, Douglas fir trees in
Oregon, sweetgum trees in Tennessee, oak trees in Florida, and pines and sweetgum in North
Carolina. The hypothesis being tested is that growth with excess carbon will result in
greater allocation to carbon-based compounds such as monoterpenes and isoprene. In the
third area of research, new methods and models are being developed with which to study
canopy-level isoprene emissions. This will lead to a better understanding of how
seasonal
development of the forest canopy microclimate influences seasonal emissions of isoprene.
To conduct these studies, new methods have been developed to measure the transfer of
isoprene by turbulent eddies. This has been accomplished with a new relaxed eddy
accumulation system and an ozone-induced chemiluminescene detector that is used for direct
eddy covariance measurements. Together, the studies described above have been funded
by
the National Science Foundation, the Environmental Protection Agency, the Department of Energy, and the National Aeronautics and Space
Administration's Mission to Planet Earth.
Most recently, we have focused on understanding the
biochemical mechanisms
that control isoprene emission in an atmosphere of elevated carbon dioxide. We have
discovered that the enzyme phosphoenolpyruvate (PEP) carboxylase, which is a cytosolic
enzyme, has
a primary role in regulating
the flux of carbon metabolites into the chloroplast and thus the flux of metabolites to the
production of isoprene. At elevated levels of atmospheric carbon dioxide, PEP
carboxylase
activity increases, and the rate of isoprene emission decreases. We have traced this
relationship to the effect of elevated PEP carboxylase activities on the transport of
isoprene precursors into the chloroplast. To further examine these relationships, we
are
studying the characteristics of the protein transporters that carry isoprene precursors into
the chloroplast. Our eventual aim is to develop computer models that will help us
predict
the effects of elevated carbon dioxide and climate change on the production of hydrocarbon
compounds from forests and their potential contribution to global air pollution.
(3) Research into the interactions between alpine and subalpine plants and
soil
microbes as they acquire nitrogen and produce carbon dioxide
A third line of research in my laboratory
involves the role of soil processes in the alpine and subalpine carbon and nitrogen
cycles. In the alpine ecosystems, we have been interested in how
plants obtain nitrogen given the
competitive pressures of soil microorganisms. According to our conventional
understanding
of the terrestrial nitrogen cycle, organic nitrogen that is deposited to the soil with
leaf and root litter is decomposed and mineralized by soil microorganisms to inorganic
compounds such as ammonium and nitrate. Plants are thought to "prefer" these
inorganic forms. Thus, the cycle is structured such that plants are "at the
mercy" of microbial mineralization. Recent measurements in the alpine ecosystem
of
Niwot Ridge, Colorado have shown that on an annual basis plants take up ten times more
nitrogen than is made available by microbial mineralization. How are these plants
obtaining their nitrogen, if as envisioned by the traditional nitrogen cycle, microbial
mineralization is required to provide inorganic substrates? It is our hypothesis that
the
plants are bypassing the microbial link of the nitrogen cycle and obtaining nitrogen
through direct uptake of organic forms -- primarily amino acids. Such a hypothesis has
also been proposed for the arctic tundra, and may be typical of ecosystems with cold, wet
soils in which microbial biomass and mineralization are strongly constrained. This
hypothesis is being examined through a series of studies on the capacity of
non-mycorrhizal roots to take up organic and inorganic forms of nitrogen, the role of
mycorrhizal fungi in facilitating organic nitrogen uptake, the constraints on microbial
utilization of organic nitrogen, and the reasons that some forms of organic nitrogen are
more prevalent in the alpine soil than other forms. This research is supported by
grants
from the National Science Foundation, and is part of the Niwot Ridge Long Term Ecological Research Program.
In the subalpine forest ecosystem, we have been studying
how certain
microbial communities influence soil respiration and forest carbon balance. We have
observed that microbial biomass in our forest soil exhibits two distinct seasonal maxima,
one in the winter and one in the summer. During the spring snow melt period there
appears to
be a turnover of these communities, just as in the alpine ecosystem. Mycorrhizal
fungi
appear to be a much more important component of the microbial biomass in this subalpine
ecosystem than in the alpine ecosystem. Our research is focused on understanding the
role
of mycorrhizal fungi in the seasonal turnover of microbial communities and its role in soil
respiration in general. We are conducting studies using tree girdling to alter the
flow of
carbon substrate to the mycorrhizal hyphae and we are studying the potential for mycorrhizal
hyphae to exude soil exoenzymes and take part in decomposition of soil organic matter when
the flow of carbon substrates from the trees is altered.