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 S 1. 1 CESBIO, 18 avenue Edouard Belin Toulouse / France 2 CNRM/Météo-France, 42 avenue G. Coriolis Toulouse / France 3 IER Institut d’Economie Rurale, Bamako, Mali Presented by L. Kergoat

Eco-climatic zones of WA Sahel usually delimited by mm annual rainfall

Characteristics of the Sahelian vegetation Trees Shrubs 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 variability which is very high.

Hiernaux (ILCA) and Diarra (IER), 1984, Cesbio+IER from 1996 onward Long term sites network established in 1984 (extrem drought) Grass biomass tree density species % bare soil sample 1 km transect consistent with satellite

09/84 09/86 09/89 09/93 09/85 09/8709/88 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 model Mougin 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 properties view 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 posters Seghieri et al Le Dantec et al and TT3

Albedo of Agoufou grassland albedo rainfall dry wet Day of year Guichard et al

Surface Net Radiation for Agoufou grassland higher dry wet Guichard et al

Radiative balance, Agoufou August 2002 to IR  IR  SW  SW  Rnet (W.m -2 ) IR  IR  SW  SW  Rnet (W.m -2 ) Wet (2003 & 2005) = low SW down but 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 ? Hiernaux Seghieri 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 data e.g. Philippon et al Current year -> current growth, Rn, ETR of annual grasses OK Tree : 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/dry hysteresis 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/runoff no idea of associated fluxes changes Surface / atmosphere interactions