Growing season dynamics in high-latitude ecosystems:

relations to soil thermal regimes, productivity, carbon sequestration, and

atmospheric heating

Bonanza Creek LTER Symposium February 2006

(Serreze et al., Climatic Change, 2000)

High Latitude Temperature

Trends

(1966-1995)

Annual data

°C per decade

Outline of this talk:

Part I: Examine changes in growing season length in

high-latitude ecosystems (Based on Euskirchen et al.,

in press, Global Change Biology)

Part II: Relate these changes to changes in energy

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 could 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

Slope = -0.25 days per year

23-Apr

28-Apr

3-May

8-May

13-May

18-May

23-May

28-May

1974 1976 1978 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998

Date of leaf-out in Fairbanks (Chena Ridge) 1974-1998

Data courtesy of J. Anderson

The increase in growing season length over the last 50 years averaged for 8 stations in Alaska having the longest and most consistent temperature records

(Keyser et al., 2000).

slope = 0.33 days/yr; p< 0.0001

Part I: 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

Boreal region

Boreal region

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

ma

ly (

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

Part I: 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.

Part II – What are the relative responses of changes in high-latitude carbon uptake due to growing season length increase versus changes in albedo on the climate system?

-1.5

-1

-0.5

0

0.5

1

1.5

2

1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

1.4

50-55 55-60 60-65 65-70

19401910

1970 - 20001970 2000

Air temperature anomaly (ºC, five year running mean)

Year

Air temperature anomaly (ºC)

Five-Degree Latitudinal band (ºN)

0.0078 – 0.0354 ºC year-1 0.0381 – 0.0388 ºC year-1

1910 - 1940

The snow – albedo feedback loop

Decreases in albedo

Decreases in snow cover

The greenhouse gas-ecosystem metabolism

feedback loop

Decreases in temperature

Enhancements in productivity greater than

enhancements in respiration

Increases in temperature

Increase in heat absorption

Increases in greenhouse

gases

Enhancements in respiration greater than

enhancements in productivity

Increases in growing season

length

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

1970-2000

1910-1940

Change in autumn snowfall

Change in springsnowmelt

Total change in snow cover duration

Days per year earlier (or shorter for the ‘total’)

Days per year later (or longer for the ‘total’)

<- 0.4 -0.4 -0.3 -0.2 -0.05 0.01 - 0.1 >0.1

Anomaly

-1.4 days

decade-1

-2.5 days

decade-1

SNOW:-Compiled seasonal data on surface energy balance (sensible-plus-latent heat flux to the atmosphere as a proportion of net radiation) by vegetation type.

-Calculated monthly mean incoming shortwave radiation in TEM.

-Estimated daily atmospheric heating depending on snow surface conditions.

-Multiplied changes in snow cover (days per year) by changes in atmospheric heating (Chapin et al. 2005).

ECOSYSTEM METABOLISM:A 4.4 W m-2 atmospheric heating change with a doubling of [CO2].

311 g C m2 increase/decrease for each 1 W m-2 increase/decrease

Translation of changes in snow cover and ecosystem metabolism to changes in radiative forcing

Houghton et al., 2001

springautumn spring & fallChanges in energy (W m-2) due to snow cover changes in:

1970-2000

1910-1940

< 0 0 - 0.5 0.5 - 1 1 - 2 2 - 3 ≥ 3 W m-2

Cooling Heating

< 0 0 - 0.5 0.5 - 1 1 - 2 2 - 3 ≥ 3 W m-2

Cooling Heating

1970-2000

1910-1940

Changes in energy (W m-2) due to changes in:

snow coverecosystem metabolism

For boreal forests,

changes in ecosystem metabolism(CO2 + climate): 1910-1940: 0.00 W m-2

1970-2000: -0.06 W m-2

changes in snow cover:1910-1940: +0.56 W m-2

1970-2000: +1.04 W m-2

Change in energy (W m-2)due to changes in:

Time period snow coverecosystem metabolism

1910-1940 1.1 ~ -0.05

1970-2000 1.9 ~ -0.10

(Negative sign represents negative feedback for the sink term, positive sign is positive feedback for a source term)

Changes in snow cover had a much greater effect on energy than did changes in ecosystem metabolism

Suggests the importance of considering other factors that may alter albedo.

Pan-Arctic: north of 50° N

Foley et al., 2003

Reduced growing season albedo and increased spring energy absorption

Part II Conclusions:

The effects of a longer snow-free season on atmospheric energy balances should considered in studies of climate change, particularly with respect to associated shifts in vegetation between forests, grasslands, and tundra.

We should also consider other factors that play an important role in altering surface albedo, such as changes in fire regime, insect defoliation, timber harvest, and conversion to/from agriculture.

And finally, how do these factors interact with changes in growing season length?

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