Changes in the seasonal activity of temperate and boreal vegetation

The critical role of Autumn temperatures.

Shilong Piao, Philippe Ciais,

Pierre Friedlingstein, Philippe Peylin, Nicolas Viovy and

Peter Rayner

LSCE, CEA-CNRS-UVSQGif sur Yvette, France

Carbon Fusion Meeting 9-11 May 2006

As temperature is rising, the length of growing season usually increases.

How will the net

Carbon Uptake Period

respond to the

warming ?

Jan DecJul Aug

earlier spring

delayed fall

Background

Global biospheric model ORCHIDEE

ORCHIDEESECHIBA

energy & water cyclephotosynthesis

t = 1 hour

LPJspatial

distributionof vegetation

(competition, fire,…)t = 1 year

STOMATEvegetation & soil carbon

cycle(phénologie, allocation,…)

t = 1 day

NPP, biomass,litterfall

vegetation types

LAI,roughness,albedo

soil water,surface temperature,

GPP

rain, température, humidity,incoming radiation, wind, CO2

meteorological forcing

sensible & latent heat fluxes, CO2 flux, net

radiation

output variables

prescribed vegetation

vegetation types

ORCHIDEE model simulations

1. Spin up (1000 y) using 1901 climate data, and 1850 CO2 concentration

2. Simulate from 1850 to 1900 using 1901-1910 climate data, and corresponding every year CO2 concentration.

3. Simulate from 1901 to 2002, using corresponding every year climate data and CO2 concentration. Save every day C flux from 1980 to 2002.

65

70

75

80

85

1901 1911 1921 1931 1941 1951 1961 1971 1981 1991 2001

Year

NPP, RH (Pg C)

-6

-3

0

3

6

NEP (Pg C)

NPP

RHNEP

Comparison of spring (AM) LAI

0

1

2

3

4

5

6

- 55 - 45 - 35 - 25 - 15 - 5 5 15 25 35 45 55 65 75

GIMMSPath finderORCHIDEE

Latitude (degree)

Comparison of autumn (SO) LAI

0

1

2

3

4

5

6

- 55 - 45 - 35 - 25 - 15 - 5 5 15 25 35 45 55 65 75

GIMMSPath finderORCHIDEE

Latitude (degree)

Interannual Variability in LAI

Spring

SDORCHIDEE = 0.04

SDGIMMS = 0.06

SDPAL = 0.19

Satellite sensor change

Autumn

SDORCHIDEE = 0.02

SDGIMMS = 0.07

SDPAL = 0.13

Define growing season and carbon uptake periods

A BC

D

A = growing season startB = growing season endAB = growing season length (GSL)

C = net carbon uptake startD = net carbon uptake endCD = Carbon Uptake Period (CUP)

1001 ×−+= ( )]AI t( )]/[LAI t L)[ (LAI t( )tLAIratio

Growing SeasonFrom rate of change of LAI

CUP

GSL

Carbon UptakeFrom NEP zero-crossing dates

Mapping the growing season and carbon uptake timing

Onset date increases with increasing latitude

CUP start occurs later than GS start (because of spring respiration)

Shortest GSL = Central Siberia near the Arctic coast (4 months).

Shortest CUP = Northern Eurasian forests and water limited steppes - also show the shortest GS length.

The distribution of End date in autumn is less uniform than in spring, (reflects vegetation type, as well as water / temperature limitations on plant growth).

Growing season(phenology)

Carbon Uptake

Start (day)

End (day)

Duration (days)

Derived from ORCHIDEE simulation

Trends GSL and CUP during 1980-2002

dGSLstart/dt = 0.16 days/yr

dCUPstart/dt = 0.19 days/yr

Same response of CUPstart and GSLstart to warming trend

RGSLstart-temp = -0.91 P<0.001

RCUPstart-temp = -0.62 P=0.002

dGSLend/dt = 0.14 days/yr

dCUPend/dt = -0.07 days/yr

Opposite response of CUPend and GSLend to warming trend !

RGSLend-temp = 0.71 P<0.001

RCUPend-temp = -0.51 P=0.01ORCHIDEE > 25°N

Spring

Autumn

Mapping the trends

More than 70% of the study region exhibits an advancing trend in the GSL start, especially in Eurasia.

In North America, large regions show delayed trends in the CUP start

GSL length : Trends to increasing GSL over high latitude regions, usually as a result of earlier beginning of growing season in Eurasia and later end of growing season in North America

CUP length : North America shows a trend to shorter CUP length, Eurasia has the opposite behaviour

GSL: most of northern North America shows a trend towards later GSL end, BUT there is a trend to earlier GSL end in temperate Western Eurasia (Europe).

CUP: 70% of the study region display a trend towards an earlier CUP end.

Growing seasonTrends

Carbon Uptake Trends

Beginning

End

Length

Derived from ORCHIDEE

(1) Period from 1980-2002; (2) Period from 1982-1998; (3) Period from 1988-2000

Comparison with satellite observation

Region Change in GSL start (days / year)

>0 = earlier ; <0 = later

Change in CUP

(days / year)ORCHIDEE(1) Zhou et al.(2)

Smith et al. (3)

North America

+0.04 -0.4 -0.08 0.12

Eurasia -0.32 -0.3 -0.39 -0.39

Region Change in GSL (days / year)

>0 = earlier ; <0 = later

Change in CUP

(days / year)ORCHIDEE(1) Zhou et al.(2)

Smith et al. (3)

North America

0.27 0.27 0.22 -0.11

Eurasia 0.11 0.21 -0.31 -0.07

Spring

Autumn

Atmosphere CO2 measurements

Although Keeling et al. (1996) found that there were no significant long-term changes in the upward zero crossing time at site Mauna Loa from mid-1970s to 1994, pronounced advancement at a rate of 0.77 days yr-1 (R=-0.65, P=0.001) is observed in the period of 1980-2002.

Temperature vs. Carbon Uptake Period

Spring

RBRW = -0.85, P<0.001

RMLO= -0.40, P=0.056

Autumn

RBRW = -0.60, P=0.003

RMLO= -0.59, P=0.005 (excluding 1992, 1993)

Differential response of gross C Fluxes to the warming trend in Northern Hemisphere (>25°N)

Spring: Warm temperatures accelerate growth more than soil decomposition. The annual relationship of NEP to temperature is positive=> Warming enhances carbon uptake

Autumn: Warm autumn accelerate growth less than soil decomposition. The annual relationship of flux to temperature is negative.=> Warming reduces carbon uptake

Derived from ORCHIDEE

Derived from ORCHIDEE

Autumn (SON) temperature vs. C Flux

Conclusion

Most of the study region exhibited extending of GSL, usually as a result of earlier vegetation green-up in Eurasia and later vegetation senescence in North America, which strongly supports a lengthening of growing season and greening trend at northern hemispheric observed in the past two decades.

Due to parallel stimulating soil carbon decomposition, increase in GSL does not necessarily lead to increase in CUP and eventually result in higher C net uptake.

Autumn warming does not benefit terrestrial net C uptake through postponing vegetation growing season end in the northern mid and high latitudes.

Relevance to IGCO

• Need for in situ phenological data• Need of long flux time series to confirm processe• Need for snow cover / frozen status of soil data• Long term satellite biophysical products (large

differences between sensors & data processing)• New CO2 column satellite obseravtions may allow

an unprecedented quantification of the spatial distribution in the CO2 seasonal cycle -> regional trends detection

• Integration of surface with atmospheric information

Thank you!