| Modeling Studies for BOREAS |
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NASA-CASA Prediction for Northern Study Area Carbon Cycles |
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Top: Predicted net ecosystem exchange (NEP) of carbon (black line) and
average daily NEP fluxes measured by TF-3 in 1994 (OBS; triangle) and by
TGB-3 in 1996 (Fen; filled square-hollow, open square-hummock). The
midnight to midnight average of NEP at the OBS site was computed for days
during which no fewer than 44 half-hourly CO2 flux measurement
data points (i.e., >90%) were recorded. |
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Bottom: Predicted net primary production (NPP) for the respective sites
and years (black line-overstory, gray line-moss ground cover). |
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NASA-CASA Prediction for Annual NEP BOREAS Regional Area |
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Main Conclusions from BOREAS Follow-on Carbon Modeling Comparisons
(NSA - Old Black Spruce Tower Site) |
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Model-measurement and model-model discrepancies were observed among all the
BOREAS carbon models (with respect to eddy covariance and/or other site
measurements), although more detail (physiology, physics) in a subset of
models did tend to improve accuracy, as evaluated by RMSE analysis with
measured daily CO2 and ET fluxes.
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The BOREAS carbon models generally predict a small annual net CO2
sink for the site (1994-1996), whereas the tower data points to a small
source (at least more often a small sink). This is explained in part
because the models were parameterized for the most productive (well-drained,
tall tree) areas around the tower, and consequently their results may have
been biased to predict greater production fluxes than typically measured at
the NSA-OBS tower (whose footprint also included less productive, wetter
areas).
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Model sensitivity comparisons imply that past conditions of the ecosystem
(e.g., historical climate patterns and time since last major disturbance),
as represented in the models' initial standing wood and soil carbon
pools, can be as important as potential future climate changes in predicting
the annual model response for a net ecosystem carbon sink in the boreal
spruce forest.
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The BOREAS carbon models were observed to have similar responses of higher
plant respiration rates and higher ET fluxes in response to prescribed
sensitivity increase in air temperature. The models showed different
sensitivity responses in their predicted net primary production rates
to increased atmospheric CO2 levels, and in the responses of soil
microbial respiration to precipitation inputs and soil wetness. These
divergent responses represent some the the largest remaining uncertainties
in prediction of future carbon cycling patterns from the boreal forest models.
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