Effects of Future Warming and Fire Regime Change on Boreal Soil Organic Horizons and Permafrost Dynamics in Interior Alaska

• Comparison of Historical and Predicted Fire Regimes in the AKYRB

Climate warming in northern high latitude regions, e.g., in Alaska, (Fig. 1), has the potential to change terrestrial ecosystem structure and function through altering disturbance regimes such as fire. Fire has the potential to remove the surface organic layer and expose the underlying permafrost to degradation, which can expose previously frozen soil organic matter to decomposition. Thus, it’s important for ecosystem models to represent how changes in fire regime will affect soil organic horizons and corresponding soil thermal properties which influence Arctic and boreal permafrost. In this study, we present a preliminary results on the effects of warming and fire regime changes on boreal soil organic horizons and permafrost dynamics, by coupling a landscape model of fire (ALFRESCO) with an ecosystem model that represents the dynamics of soil organic horizons (DOS-TEM). The modeling analysis was conducted in the Alaska portion of the Yukon River Basin (AKYRB) (Fig. 2).

The initial results of the model coupling between ALFRESCO and TEM in this study indicate that both future warming and associated changes in the fire regimes will be very important to the dynamics of organic soil horizons and the distribution of permafrost in Interior Alaska.

This study indicates that there is a need to couple models of fire dynamics and ecosystem dynamics to make projections of changes in ecosystem structure and function that are relevant to conservation and natural resource management.

Our next steps are (1) to synchronously coupled ALFRESCO, TEM and GIPL in the AIEM so that there is a two-way exchange of output data between the models in the AIEM framework, and (2) to apply for the AIEM framework to the entire Alaska region.

Acknowledgements: This study was partial results (Phase I) of the Alaska Integrated Ecosystem Model Project (AIEM) jointly funded by the Department of Interior Alaska Climate Science Center, U.S. Geological Survey and U.S. Fish and Wildlife Service. F.-M. Yuan’s work is partially sponsored by CCSI at ORNL.This poster is presented at the 2011 Fall Meeting of American Geophysical Union (AGU) held in The Moscone Convention Center, San Francisco, CA , December 5 – 9, 2011.

F.-M. Yuan1*, A. D. McGuire2, S.-H. Yi3, E. S. Euskirchen1, T. S. Rupp4, A. L. Breen4, T. Kurkowski4, E. S. Kasischke5 and J. W. Harden6

• Impacts on permafrost dynamics

1 Institute of Arctic Biology, University of Alaska Fairbanks, Fairbanks, AK 99775; 2 U.S. Geological Survey, Alaska Cooperative Fish and Wildlife Research Unit, University of Alaska Fairbanks, Fairbanks, AK99775; 3 State Key Laboratory of Cryosphere Sciences, Cold and Arid Regional Environmental and Engineering Research Institute, CAS, Lanzhou, Gansu, China;4 Scenarios Network for Alaska and Arctic Planning (SNAP), University of Alaska Fairbanks, Fairbanks, AK 99775; Department of Geography, University of Maryland, College Park, MD; U.S. Geological Survey, Menlo Park, CA* Current address: Climate Change Science Institute, Oak Ridge National Laboratory (ORNL), Oak Ridge, TN 37831. Email: yuanf@ornl.gov

Fig. 5 Area burned (left panel ) and fire size frequency (right panel) for historical data and ALFRESCO simulations in the AKYRB region. Note that there are two ALFRESCO fire projections for the future driven by GCM outputs of CCCMA-CGCM3.1 (“cccma”) and MPI ECHAM5 models (“echam5”) for the A1B scenario.

Fig. 3 The Alaska Integrated Ecosystem Model (AIEM) framework linking simulations of ecosystem structures and functions affecting natural resources in Alaska.

Figure 5 summarizes and compares historical fire occurrences (1950 – 2006) and future fire occurrences (2007 – 2099) simulated by ALFRESCO for downscaled GCM climate outputs from CCCMA-CGCM3.1 (referred to as “cccma”) and MPI ECHAM5 (“echam5”) models for an A1B emissions scenario at 1km x 1 km resolution for AKYRB. ALFRESCO predicts that fires, which have been increasing since the 1990s, will continue to become more frequent through the middle of the 21st Century, when fire activity would revert to pre-1990 levels because of a shift in forest composition to a greater fraction of deciduous forest . ALFRESCO predicts that the fire size distribution pattern will shift (the right panel of Fig. 5) to more intermediate fire years (0.5 – 1.0% burned area) and fewer small fire years (<0.5% burned area), with small changes in the frequency of larger fire years (>1.5% burned area), except for the much fewer extra-large fires (>2.0% burned area) simulated by “cccma”.

Ret

entio

n Fr

actio

n of

Dep

ositi

on N

Year Forest floor SOM Vegetation

Yea

rly

peak

N c

once

ntra

tion

in s

trea

m f

low

s (m

gN L

-1

)

N deposition rates (kgN ha-1 yr-1)

N deposition rates (kgN ha-1 yr-1)

(a) CP (normal)(b) BF (normal)(c) CP (under reduced PPT)(d) BF (under increased PPT)

Yea

rly

peak

N c

once

ntra

tion

in s

trea

m f

low

s (m

gN L

-1

)

N deposition rates (kgN ha-1 yr-1)

N deposition rates (kgN ha-1 yr-1)

(a) CP (normal)(b) BF (normal)(c) CP (under reduced PPT)(d) BF (under increased PPT)

Fig. 6 The TEM simulated soil organic thickness dynamics over the AKYRB driven by the historical (1950 - 2006) and future (2007 - 2099) climate change and fire occurrence (historical and ALFRESCO predicted).

GC43C-0947

• Soil organic horizon dynamics

Fig. 2 Vegetation distribution in the Alaska portion of the Yukon River Basin (AKYRB).

Both ALFRESCO and TEM are components of the part of Alaska Integrated Ecosystem Model (AIEM) framework (see Fig. 3). The primary goal of the AIEM framework is to project how interactions among disturbance regimes, permafrost dynamics, hydrology, and vegetation succession/migration affect natural resources in Alaska. The framework is currently coupling (1) a model of disturbance dynamics and species establishment (the Alaska Frame-Based Ecosystem Code, ALFRESCO), (2) a model of soil dynamics, hydrology, vegetation succession, and ecosystem biogeochemistry (the dynamic organic soil/dynamic vegetation model version of the Terrestrial Ecosystem Model, TEM), and (3) a model of permafrost dynamics (the Geophysical Institute Permafrost Lab model, GIPL). The AIEM will provide an integrated framework for natural resource managers and decision makers to improve understanding of the potential response of ecosystems due to a changing climate and to provide more accurate projections of key ecological variables of interest (e.g., wildlife habitat conditions).

Fig. 7 (Left) Active layer depth (ALD) and (Right) fractional area of shallow permafrost, i.e., within top 5.4 m depth (the max. soil depth of DOS-TEM), in AKYRB as simulated by DOS-TEM driven by the projected climate changes and fire occurrences used in this study.

Fig. 8 TEM simulated change of shallow permafrost (within top 5.4m depth) (dark blue – shallow permafrost, orange – non-permafrost or deepen permafrost) in the AKYRB under future climate warming and associated predictions of fire occurrence.

The DOS-TEM simulations indicate that permafrost currently exists in eastern two-thirds of AKYRB (dark blue area in Fig. 8). Under future climate warming and predicted fire regime changes, areas underlain by not-shallow permafrost would expand eastward and northward rather rapidly by 2050. The pattern of changes before 2050 are not that different between two climates, but the pattern of changes after 2050 are more dramatic for the warmer “echam5” projection.

1900 1920 1940 1960 1980 2000 2020 2040 2060 2080 2100-10

-7.5

-5

-2.5

0

2.5

5

YEAR

MAT (o

C)

CRUcccmaecham5

1940 1960 1980 2000 2020 2040 2060 2080 21000

1

2

3

4

5

6

Year

Perc

enta

ge o

f Bur

ned

Are

a

ALFRESCO - cccmaALFRESCO - echam51950 - 2006

< 0.5 0.5 - 1.0 1.00 - 1.50 1.5 - 2.0 > 2.00

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Percentage of Total Burned Area in a Year

Frac

tion

of Y

ear N

umbe

rs

1950 - 2006cccmaecham5

Fig. 1 Historical and projected climate warming in the Alaska portion of Yukon River Basin (AKYRB).

Fig. 4 Coupling of ALFRESCO and TEM in this study.

Figure 4 shows how we asynchronously coupled ALFRESCO and TEM in this study. This coupling transfers information from ALFRESCO to TEM annually. Eventually, the AIEM project will synchronously couple the models so that the changes in ecosystem structure and function simulated by TEM can influence ALFRESCO’s simulation of fire dynamics.

In this study, the historical fire database and outputs of ALFRESCO were used to drive the dynamic organic soil version of the Terrestrial Ecological Model (DOS-TEM), which was specifically designed to evaluate changes in organic soil dynamics for the northern high latitude boreal and arctic ecosystems (see Fig. 4). The TEM simulations indicate that the shallow fibrous organic horizon thickness in the AKYRB region tended to increase in response to warming and rising CO2, but dramatically decreased during the strong active fire period that occurred in 2000s for one to several decades depending on the climate projection, and then continued to increase through most of the 21st Century (the left panel of Fig. 6). Fire size and related burn severity were the primary factors for the differences between “cccma” and “echam5” as warming in the two climates did not diverge until around 2040 (see Fig. 1). The thickness of the deep amorphous organic horizon (the right panel of Fig. 6) varied only within about 0.5 cm between 1950 and 2100. In general, the thickness decreased after large fire years until the upper fibrous horizon thickness became thick enough to transfer carbon to the amorphous horizon or until fire caused sudden addition of burned residue and dead roots to the amorphous horizon.

Figure 7 indicates that the active layer of permafrost in the AKYRB area will deepen across the basin from about 1 m to between 1.6 (“cccma”) and 1.8 m (“echam5”) in average during the first decades of the 21st Century, but will then return to current depth by the end of the 21st Century. However, the simulation also indicates that area of the AKYRB underlain by permafrost will decrease from the currently estimated 68% to between 20% (“echam5”) and 30% (“cccma”).

Alaska CSC