Geoscience Reference
In-Depth Information
could enhance the rate and magnitude of climate warming. On the other hand, a large
release of inorganic nitrogen could also result in increased Net Primary Production
(NPP), which could buffer carbon loss from the soil or cause Arctic ecosystems to
act as a net sink for carbon dioxide (Clein et al.,
2000
).
NPP is Gross Primary Productivity (GPP) minus plant autotrophic respiration,
with a positive NPP representing net plant growth. GPP is the photosynthetic uptake
of carbon dioxide by plants and autotrophic respiration is the release of carbon diox-
ide by plants as they convert simple sugars, the raw products of photosynthesis, into
more complex organic matter, such as leaves, roots, and wood. This contrasts with
heterotrophic respiration (HR), which is the release of carbon dioxide by animals,
microbes, and other organisms that (like the authors of this text book) consume
organic matter rather than produce it themselves. Net Ecosystem Production (NEP)
is NPP minus HR and represents the net carbon balance of the ecosystem, with a
positive NEP representing a net uptake of carbon by the biosphere. Net Ecosystem
Exchange (NEE) is HR plus fire and other non-biological carbon dioxide emis-
sions minus NPP, with a positive NEE representing a net release of carbon into the
atmosphere. Ignoring ire and other emissions, NEE equals - NEP (see the topic by
Chapin, Matson, and Mooney,
2000
).
It should come as no surprise that modeling Arctic ecosystems and ecosys-
tem change is a vibrant research area. Ecosystem models vary widely in how they
represent physical and biological processes and their intended application. Some
models focus on agriculture ecosystems and crop yield, others on forest dynam-
ics and timber stocks, and still others estimate surface energy and mass fluxes in
fully coupled models of land-ocean-atmosphere dynamics (Earth System Models).
Biogeochemistry models represent ecosystem dynamics as the exchange of car-
bon and nutrients between various “pools,” such as wood, litter, and soil pools. In
addition to biogeochemistry, dynamic vegetation models also include competition
between plant species to allow simulated grasslands to evolve into forests and back
again as climate changes. Ground temperature, water availability, and other aspects
of the physical environment strongly influence ecosystem dynamics and many mod-
els include representations of soil thermodynamics, surface and soil hydrology, and
snow pack development. These models can now simulate how changing climate
influences ecosystem dynamics, and how changing ecosystem dynamics influence
climate on time scales of minutes to centuries.
These models must address important issues and challenges in the Arctic, such
as “woody encroachment” and the northward advance of the tree line into tundra
regions driven by higher temperatures and longer growing seasons. Another chal-
lenge is how warming will affect wetlands and lakes and the resulting changes in
carbon dioxide and methane fluxes. Permafrost or perennially frozen ground under-
lying much of the tundra in the Arctic is impermeable to water, resulting in innu-
merable lakes and wetlands. Warming will thaw some portion of the permafrost,
changing the number and distribution of lakes in the Arctic. The decay of organic
matter in these lakes and wetlands is often anaerobic, producing methane rather
than carbon dioxide fluxes to the atmosphere. Because methane is twenty-three
