Response of Sahelian vegetation to climatic variability

Consequences for the Surface-Atmosphere interactions

Mougin E.1, Hiernaux P.1, Kergoat L.1, Seghieri J. 1, Lavenu F.1, Tracol Y.1, Guichard F. 2, Jarlan L.2, Diarra L.3, Dembélé F.3, Karembé M.3, Mougenot B.1, Timouk F.1, de Rosnay P.1, Le Dantec V.1, Baup F.1, Mangiarotti S1.

1 CESBIO, 18 avenue Edouard Belin 31405 Toulouse / France2 CNRM/Météo-France, 42 avenue G. Coriolis 31057 Toulouse / France3 IER Institut d’Economie Rurale, Bamako, Mali

Presented by L. Kergoat

Eco-climatic zones of WA

Sahel usually delimited by 100 - 600 mm annual rainfall

Characteristics of the Sahelian vegetation

TreesShrubs

Wood layer}

Perennial grasses Annual grasses

Grass layer

crops (millet, sorghum)

variability of rainfall over Sahel (10°N – 20°N)

High spatial variability (courtesy Lebel et coll.)

Rainfall

add soil type and land use,you get vegetation variabilitywhich is very high.

Hiernaux (ILCA) and Diarra (IER), 1984, Cesbio+IER from 1996 onward

Long term sites network established in 1984 (extrem drought)Long term sites network established in 1984 (extrem drought)

Grass biomasstree densityspecies% bare soil

sample 1 km transectconsistent with satellite

09/84

09/86

09/89

09/93

09/85

09/87 09/88

08/90

09/00

09/92

09/99

Phenology and growth of Sahelian vegetation

• annuals : direct response to rainfall (temporal distribution of rainfall events)

Seasonal and interannual dynamics

Model STEP vs data

Mougin et al

Biomass variability explained by climate+vegetation model

Observation period

Interannual herbaceous productivity

Large variability + non linearity (rainfall distribution, dry spells)

STEP modelMougin et al

AVHRR trend

Anyamba and Tucker 2005

J Arid Envir. ‘greening’ of Sahel special issue

Consequences for Surface / Atmosphere Interactions

1) Characterize time/space variability of vegetation and fluxes response at site, landscape, and West African gradient scales

AMMA will do it

Sahel vegetation : Large temporal variability

Grass and crop phenology responds quickly

non linearity

Sahelian ecosystems show SOME resilience propertiesview of Sahel drought has changed since the 70’ 80’

Gourma mesoscale Site (~300 x 100 km²)

Hombori Supersite (15.4°N, 1.6°W)

Agoufou local Site (15.3°N, 1.5°W)

Bamba local Site (17.1°N, 1.3°W)

Niger River

Rainfall 50 mm

350 mm

450 mm

300 mm

100 mm

See postersSeghieri et alLe Dantec et aland TT3

Albedo of Agoufou grassland

2002

2003

albedo

rainfall

2005

2004

dry

dry

wet

wet

Day of year Guichard et al

2002

2003

2004

2005

Surface Net Radiation for Agoufou grassland

higher

higher

dry

dry

wet

wet

Guichard et al

Radiative balance, AgoufouAugust 2002 to 2005

-600

-500

-400

-300

-200

-100

0

100

200

300

400

500

2002

2003

2004

2005

IR IR SW SW Rnet (W.m-2)

-40

-30

-20

-10

0

10

20

IR IR SW SW Rnet (W.m-2)-400 +460 -250 +50 -140

2002

2003

2004

2005

Wet (2003 & 2005) =

low SW downbut low albedo

and low IR up

result : more energy for sensible and latent heat flux(and thetae)

Rn - G = LE +H

Models probably able to simulate this (ask ALMIP)Guichard et al

Tree leaf phenology

Fluxes ? Models ?

HiernauxSeghieri

Interannuel effects …for SOME species or SOME trees/shrubs

Consequences for Surface / Atmosphere Interactions

2) Sahelian paradoxes

‘le diable est dans les détails’. What would be the typical time scale of return to pre-drought state ? Water routing versus vegetation recovery ? What about models ?

Changes in vegetation -but different cases - and soil surface.

Increasing water table (Niger) : more runoff (crops, fallows), more infiltration from ponds. Favreau et al

Increased river runoff in the Sahelian zone, as opposed to decreased in Soudanian zone Mahé et al 05

Tiger bush shrinks, increase of runoff Valentin et al 01, Wu et al 05

Mare dAgoufou : temporary pond before 1984 permanent pond after the 1984 drought. Less rain, more run on. Why ? (crops not responsible) Hiernaux pers obs

3) The Quest for Memory : time lags effects ?

Sahelian droughts are better simulated when droughts impact ecosystems on time > 1 year e.g. Zeng et al 99

Lag effects are present in rainfall and/or remote sensing datae.g. Philippon et al

Current year -> current growth, Rn, ETR of annual grasses OKTree : leaf phenology depends on yr, yr-1 depending on tree species

yr-1, yr-2 seed bank effect ?Wind erosion ?Litter/nutrient effect ?Species dynamic effect ?

Population dynamics of grass species and links to rainfall (Gourma 84-93) Hiernaux, Diarra

Asymetry wet/dryhysteresis

Impact on fluxes ?

Similar shemes for trees, crops/fallow

LSM, SVAT ….

SUMMARY

Sahelian ecosystems : high time and space variability some resilience ?

Need for long term sites, network of sites. (really)

Intra-seasonal : some confidence in vegetation/SVAT models

Trends : some confidence in models (rain driven)

Multi year effect : tree phenology more to learn but we already know something (LE flux ?) Sahelian paradoxes : challenging !Series of interactions vegetation/surface/runoffno idea of associated fluxes changes

Surface / atmosphere interactions