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A Dynamic Soil Layer Model for Assessing the
Effects of Wildfire on High Latitude Terrestrial
Ecosystem Dynamics Yi, S1, A.D. McGuire1,2, J.
Harden2, E. Kasischke3, K. Manies2, L. Hinzman1,
A. Liljedahl1, J. Randerson4, H. Liu5, V.
Romanovsky1, and S. Marchenko11University of
Alaska Fairbanks, 2USGS, 3University of Maryland,
College Park, 4 University of California, Irvine,
5Jackson State University
Organic matter removal and regrowth TEM was first
run to an equilibrium state for lowland black
spruce at Delta Junction, Alaska, and then run
with fire disturbance and organic layer regrowth
for 900 years using the monthly driving of period
1901-1930 periodically. Here, outputs from two
simulations are presented, one with one burn at
year 100 and the other with a burn every 100
years (Figure 14, 15, and 16).
INTRODUCTION
The Environmental Module The radiation and water
fluxes among the atmosphere, vegetation canopy,
snow and soil, and soil moisture and temperature
are updated at daily time step (Figure 3). A
Two-Directional Stefan Algorithm is used to
predict the positions of freezing/thawing fronts.
The temperatures of layers above first front and
below last front are updated separately by
solving differential equations, and for
temperatures of layers between first and last
fronts are assumed to be 0 oC. Soil moisture is
updated for unfrozen portions of the soil layer.
The thermal properties of the soil at a
particular depth are affected by the water
content at that depth.
RESULTS
Six sites in Alaska were used for evaluating EnvM
in TEM, including a tussock tundra site in
Kougarok, Seward Peninsula (K2) (65o25N,
164o38W) and an aspen site (DF87), two black
spruce control sites (DFTC and DFCC), and two
black spruce burn sites (DFTB and DFCB) located
in Delta Junction (63o54N, 145o40W).
Wildfire is considered an important disturbance
to boreal and arctic ecosystems. It can affect
high latitude carbon dynamics directly through
combustion emissions, and indirectly through
vegetation succession and removal of the surface
organic layer, which might accelerate the
degradation of permafrost and hence the release
of soil carbon. At the regional scale, the direct
effects of fire have received a lot of attention,
but the evaluation of the indirect effects has
been more limited because the appropriate tools
have not yet developed for application at the
regional scale.
Burn at year 100
Burn every 100 years
The K2 site was burned in 2002, DF87 was burned
in 1987, and DFTB and DFCB were both burned in
1999. The surface organic layer thicknesses are
shown in Figure 9.
The Ecological Module The carbon and nitrogen
fluxes among the atmosphere, vegetation, and
soil, and the carbon and nitrogen pools of
vegetation and soil are updated at monthly time
step (Figure 4). Soil carbon at each depth is
divided into reactive and non-reaction carbon.
Fluxes of carbon into and out of soil organic
matter sub-layers are explicitly considered for
each depth of the sub-layers.
Figure 9. Surface organic layer thicknesses
OBJECTIVES
Evapotranspiration Measured evapotranspiration
(ET) was available at DF87, DFTC and DFTB for
2002-2004. The simulated ET explained 82, 86, and
92 of the variability of measured ET for DF87,
DFTC, and DFTB, respectively (Figure 10).

• The overall goal of this study is to develop a
  dynamic soil layer model used in Terrestrial
  Ecosystem Model (TEM) to investigate the effects
  of changes of surface organic layer on soil
  temperature, moisture, and carbon dynamics. More
  specifically, our objectives are to
• implement stable and efficient soil thermal and
  hydrological algorithms
• dynamically remove part of the soil organic
  layer based on fire severity
• dynamically grow the soil organic layer based on
  accumulated soil carbon

Figure 14. Active layer depth and water table
depth
The Fire Disturbance Module Fires can occur
annually and are currently implemented in July,
at which time C and N of the vegetation (above-
and below-ground) and soil organic sub-layers are
removed (Figure 5). Currently, 23 of
above-ground C and N are combusted and 76 of
above-ground C and N remain in standing dead. The
amount of C of below-ground biomass burned
depends on the burn depth. 1 of C and N of both
above- and below-ground biomass are reserved for
vegetation regrowth. The lost of soil organic C
and N depends on the burn depth, which is based
on active layer depth, water table depth, and
maximum possible burn thickness. 85 of combusted
N is retained and input into soil N pool. The 15
of N lost from the ecosystem is reintroduced into
soil N pool evenly in years after a fire based on
the estimated fire return interval (FRI).
Figure 10. Comparisons of evapotranspiration
between measurements (black dot) and simulation
(red line)
Soil Temperature Simulated soil temperatures
generally compared well with measurements, except
DFTC (Figure 11). Simulated soil temperature
explained the 89, 86, 79, 90, 79, and 76 of
variability in the measurements at K2, DF87,
DFTC, DFTB, DFCC, and DFCB, respectively.
MODEL DESCRIPTION
Overview The modified TEM consists of four
interacting modules an environmental module
(EnvM), an ecological module (EcoM), a fire
disturbance module (FDM), and a dynamic soil
layer module (DSLM) (Figure 1).
Figure 15. Thickness of moss, shallow organic,
deep organic, burn
Figure 11. Comparisons of soil temperatures
The Dynamic Soil Layer Module At a fire event,
the surface organic matter is burned, and after
fire event, surface organic matter regrows at the
beginning of each year. Regrowth of the moss
sub-layer is based on year since last fire
(Figure 6). The maximum thickness of moss is
specified for each ecosystem, e.g. 5 cm for
lowland black spruce. The thickness of shallow
organic matter is determined by the relationship
between soil carbon and the depth below moss
(Figure 7). The thickness of deep organic layer
is determined by the relationship between soil
carbon and the height above mineral (Figure 8).
It is assumed that the carbon density of shallow
organic ranges from 10000 gC/m3 to 35000 gC/m3,
and that of deep organic ranges from 35000 gC/m3
to 70000 gC/m3. When a layers thickness is
beyond the specified maximum and minimum
thickness, a layer will either be divided into
two layers or combined into adjacent layer to
keep the calculation of soil temperature and
moisture stable and efficient.
Soil Moisture The performance of the model in
simulating near-surface soil moisture was not
quite as good as for soil temperature. However,
the model did capture seasonal variation of soil
moisture dynamics and simulated soil moisture of
deeper organic layers quite well (e.g., at 25 cm)
(Figure 12).
Figure 1. Overall structure of TEM
Figure 2. Processes applied to each type of layer
The TEM has a flexible ground structure, which
consists of a hierarchy of layers. There are
three layers snow, soil, and bedrock. The soil
layer consists for four sub-layers representing
moss, shallow organic matter, deep organic
matter, and mineral soil. Snow layers are
subjected to accumulation and melt. Moisture is
only be updated for unfrozen portions of the soil
layer. Temperature is updated for all layers of
the ground structure. Moss and portions of the
organic matter sub-layers can be removed by
wildfire and can regrow after fire disturbance.
All sub-layers of the soil layer are considered
in calculating C balance (Figure 2).
Figure 16. Vegetation and soil carbon
CONCLUSIONS
Figure 12. Comparisons of soil moisture
(volumetric water content) between measurements
(black dot) and simulation (red line) at 25 cm.

• The EnvM accurately simulates the soil water and
  temperature dynamics of the ground layers of
  high-latitude ecosystems.
• The FDM and DSLM accurately simulates the
  temporal dynamics organic soil matter after fire
  disturbance.
• The simulated active layer depth is quite
  sensitive to the thickness of organic matter
  after fire disturbance.

Soil Freezing and Thawing Fronts The soil
freezing and thawing fronts were explicitly
simulated in TEM using a Two-Directional Stefan
algorithm. The active layer depths, i.e., the
maximum depth of thawing fronts in a year, of
burned sites were significant deeper than the
corresponding unburned sites (Figure 13).
ACKNOWLEDGEMENTS

• Dr. Dan Hayes for providing NCEP reanalysis
  datasets
• Dr. Xingang Fan, IARC, for providing bias
  corrected station data
• NASA (North American Carbon Program)
• NSF-LTER (Bonanza Creek LTER)
• NSF-OPP (Arctic System Science Program
  International Arctic Research Center)
• USGS (Fate of Carbon in Alaskan Landscapes)
• ATLAS (Arctic Transitions in the Land-Atmosphere
  System)

Figure 13. Simulated freezing and thawing fronts
of control sites (left panel) and burned sites
(right panel). Blue dots top-down freezing
fronts, red dots top-down thawing fronts, and
green dots bottom-up freezing fronts
Figure 5. Fire disturbance
Figure 7. Shallow organic growth
Figure 8. Deep organic growth
Figure 3. Environmental processes
Figure 4. Ecological processes
Figure 6. Moss growth