Scientific Theme: Advanced Modeling and Observing Systems

Complementary Research: Faculty Fellows Research
Carbon Sequestration in a Front Range Forest
Russell K. Monson
Funding: DOE, NSF
For the past nine years, we have been monitoring the exchange of carbon dioxide from the
top of a 27-meter tower at the Niwot Ridge AmeriFlux site near the C-1 NOAA site. Our
aims are to understand the principal controls over, and magnitude of, the exchange of CO2
from the trees and soils of the ecosystem, and their response to interannual climate variation.
Our site is one of over 200 worldwide that functions in an integrated network known as
Fluxnet. The purpose of Fluxnet is to provide fundamental insight and data to modelers
using regional and global carbon budgets, particularly in response to future climate change.
We have particularly focused on the question of how this mountain forest ecosystem
responds to an acceleration of the onset of spring. In this forest, the transition from winter to spring has occurred 2-3
weeks earlier, on average, over the past five decades, compared to earlier decades. This acceleration is a systematic
component of climate change in the Colorado Front Range, and is expected to continue in the face of future
warming trends.
Niwot Ridge AmeriFlux Site. Rocky Mountains, Colorado, USA,
Subalpine forest. Lodgepole pine, subalpine fir, Engelmann spruce.
3050 m elevation.
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We have used a combination of direct
observations using our tower flux systems
and data assimilation into an ecosystem
process model known as the Simple
Photosynthesis and Evapotranspiration
(SIPNET) model to evaluate component
processes of the observed CO2 fluxes. The
results reveal a striking sensitivity of the
local carbon cycle to snow depth and the
timing of spring snowmelt. Unlike many
temperate- and northern-latitude forests that
appear to respond to earlier spring warming
through increased annual CO2 uptake, the
Niwot Ridge forest responds to earlier
spring warming through decreased annual
CO2 uptake. This is due to a high sensitivity
of the photosynthesis processes of this
forest to reduced soil water supply in the
face of earlier snowmelt and lower spring
snow depth, and to the acceleration of high
rates of soil respiration as the snow pack
recedes earlier in the summer. Thus, in the
face of earlier spring warming, the forest
takes less CO2 out of the atmosphere
through photosynthesis, and adds more CO2
to the atmosphere through respiration.
These results have led to a better
understanding of an important feedback
between climate warming and the forest
carbon cycle in mountain forests of the
western U.S.