Importance of Recent Shifts in Soil Thermal Dynamics on Growing Season Length, Productivity, and Carbon

Sequestration in Terrestrial High-Latitude Ecosystems

E.S. Euskirchen, A.D. McGuire, D.W. Kicklighter, Q. Zhuang, J.S. Clein, R.J. Dargaville, D.G. Dye, J.S. Kimball,

K.C. McDonald, J.M. Melillo, V.E. Romanovsky, N.V. Smith

ICDC7, Broomfield, CO, September 2005

(Serreze et al., Climatic Change, 2000)

High Latitude Temperature

Trends

(1966-1995)

Annual data

°C per decade

Spring: beginning of the growing season:

Increasing temperature and light availability

The snow melts

Thawing of soil organic horizons

Onset of photosynthesis

Fall: end of growing season:

Temperatures and light availability decrease

Soils re-freeze

Photosynthesis slows or ceases

Net ecosystem productivity could increase or decrease in response to changes in

soil freeze-thaw regimes.

Increases would be due to a longer growing season.

However, enhanced productivity could be counter-balanced by increases in respiration from the soil

heterotrophs.

The recent availability of remotely sensed spatially

explicit data from high-latitudes provides an opportunity to

evaluate if a large-scale process-based model captures

changes in snow cover, soil freeze-thaw regimes, and growing season length.

Satellite detection of recent changes in timing of pan-arctic

spring thaw (K.C McDonald et al., Earth

Interactions, 2004) Earlier thaw Later thaw

Change in Day of Thaw (Days/Year)

-3 -2 -1 0 1 2 3

Pan-Arctic Growing Season Change

What are the implications of recent observed changes in snow cover, soil freeze-thaw regimes, and the

timing and length of the growing season on terrestrial carbon dynamics, both retrospectively (1960-2000) and prognostically (2001 –2100)?

Terrestrial Ecosystem Model couples biogeochemistry & soil thermal dynamics

Soil Thermal Model (STM)

Vegetation type; Snow pack; Soil moistureSoil temperature

RA RH

LC

LN

Soil

Temps.

at

Different

Depths

Upper Boundary Conditions

Snow Cover

Moss & litter

Frozen Ground

Thawed Ground

Frozen Ground

Lower Boundary Conditions

Heat Conduction

Moving phase plane

Organic Soil

Mineral Soil

Prescribed Temperature

Prescribed Temperature

Snow Depth

Moss Depth

Organic Soil Depth

Mineral Soil Depth

Moving phase plane

Heat balance surface

Lower boundary

Heat Conduction

Terrestrial Ecosystem Model (TEM)

TEM Simulations & Model Validation

-Conducted simulations focusing on terrestrial land areas above 30º N and retrospective decadal trends from the 1960s –2000

-Also conducted prognostic simulations focusing on 2001-2100 using interpolated climate data obtained from a two dimensional climate model (Sokolov and Stone, 1998)

-Performed simulations with transient CO2 and climate data

-Validated the TEM results with several remotely sensed datasets (Dye, 2002; McDonald et al., 2004; Smith et al., 2004)

8.0 –18.0 Weeks – Region 118.0 – 28.0 Weeks – Region 228.0 –37.0 Weeks – Region 3

Duration of Snow FreePeriod 1972-2000

Based on simulation of the TEM for

north of30o N 1972 1980 1990 2000

-3

-1

1

Sn

ow

Fre

e D

ura

tio

n A

no

ma

ly (

we

eks

)

-4

-2

0

2

4

-3

-1

1

3

Region 1

Region 2

Region 3

D. Dye = White lines TEM = Colored lines

Trends in the Duration of the Snow-Free Period

1972-2000 Anomaly (Weeks)Slope Intercept R2 Correlation

Region 1TEM 0.07 -1.05 0.14

0.36Dye* 0.03 -0.47 0.20

Region 2TEM 0.04 -0.64 0.12

0.73Dye 0.03 -0.70 0.23

Region 3TEM 0.03 -0.39 0.04

0.57Dye 0.01 -0.21 0.05

*D. Dye, Hydrological Processes, 2002

8.0 –18.0 Weeks – Region 118.0 – 28.0 Weeks – Region 228.0 –37.0 Weeks – Region 3

Duration of Snow FreePeriod 1972-2000

Growing season length (GSL)

change(days per year)

1960-2000 2001-2100

Shorter GSL Longer GSL

<-2 -1 -0.5 0 0.25 0.5 1 2 >3

Region

(Years)

Change in spring thaw (days earlier per year)

TEM McDonald et al.(1) Smith et al. (2)

North America

(1988 – 2000)0.22 0.92 0.09

Eurasia

(1988 – 2000)0.15 0.34 0.36

Pan-Arctic

(2001 – 2100)0.36

(1) Earth Interactions, 2004(2) Journal of Geophysical Research, 2004

Net primary productivity

Heterotrophic respiration

9.1 g C m-2 yr-1

day-1

3.8 g C m-2 yr-1

day-1

18.3 g C m-2 yr-1

day-1

8.8 g C m-2 yr-1

day-1

-250

0

250-550

0

550

-75

0

75-150

0

150

-8 -6 -4 -2 0 2 4 6 8 -30 -20 -10 0 10 20

Growing season length anomaly (days)

1960-2000 2001-2100A

no

mal

y (g

C m

-2 y

r-1)

[R2] = 0.40-0.87[p] < 0.0001

9.5 g C m-2 yr-1 day-1

Anomaly (g C m-2 yr-1)Soil C

Vegetation C

8.9 g C m-2 40 yr-1 33.8 g C m-2 100 yr-1

Growing season length anomaly (days)

-300

0

300

-75

0

75

[R2] = 0.30-0.88[p] < 0.0001

Net ecosystem productivity

-8.1 g C m-2 40 yr-1

-1000

0

1000

-30 -20 -10 0 10 20

-300

0

300-75

0

75 5.3 g C m-2 yr-1

day-1

-100

0

100

-8 -6 -4 -2 0 2 4 6 8

-13.2 g C m-

2 100 yr-1

22.2 g C m-2 100 yr-1

1960-2000 2001-2100

8.0 –18.0 Weeks – Region 118.0 – 28.0 Weeks – Region 228.0 –37.0 Weeks – Region 3

Trends in growing season length, productivity and respirationGreatest

increases in GSL. Smallest increases in

productivity and respiration

Similar increases in GSL to Region

2. Greatest overall increases in

productivity and respiration

Similar increases in GSL to Region 3. Intermediate

increases in productivity and

respiration.

Duration of snow-free period

1960197019801990

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

(c)

0

0.5

-0.5

-1

1

-1.5 2000 20102020 20302040 20502060 20702080 2090

(d)

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

2000 20102020 20302040 20502060 20702080 2090

1960197019801990

(a) (b)

Cu

mu

lati

ve N

EP

(P

g C

re

gio

n-1) 2

3

1

0

-1

-2

-3

4 1960-2000 2001-2100

J F M A M J J A S O N D J F M A M J J A S O N DMonth

Bo

real

& t

un

dra

re

gio

ns

(60

– 90

° N

)T

emp

erat

e re

gio

ns

(30

– 60

° N

)

Sou

rce

Sou

rce

Sin

kS

ink

Conclusions

Model simulations indicate strong connections between decreases in snow cover and changes in growing season length.

These dynamics substantially influence carbon fluxes, including enhanced respiration and productivity in our analyses.

Increases in productivity and respiration at high latitudes are not as large as those in lower latitudes.

It is important to improve our understanding of the relative responses of photosynthesis and respiration to changes in atmospheric CO2 and climate.

AcknowledgementsFunds were provided by:

The NSF for the Arctic Biota/Vegetation portion of the Climate of the Arctic: Modeling and Processes project within International Arctic Research Center at the University of Alaska Fairbanks

The USGS ‘Fate of Carbon in Alaska Landscapes’ project