Dune rehabilitation using a mechanical fixation technique

Transcription

1 EGU Vienna Université UAM de Niamey Dune rehabilitation using a mechanical fixation technique Effect on sediment fluxes and on the recovery of the herbaceous groundcover UCL Université catholique de Louvain A. Didier TIDJANI, Jean-Marie K. AMBOUTA, Charles L. BIELDERS, Bernard TYCHON Earth and Life Institute - Environmental Sciences

2 Location Tchago, Department of Gouré (Niger)

3 Semi-arid climate 300 mm annual rainfall (June - Sept.) Mixed crop-livestock system Cropland Fenced-off area Strong impact of human activities!

4 Problem statement Rain Dry Rain Dry Rain Dry Rain Sediment transport (kg/m) With grazing and harvesting Seasonal variation in calculated sediment mass transport on pasture (Tidjani, 2008)

5 Problem statement Rain Dry Rain Dry Rain Dry Rain Sediment transport (kg/m) Without grazing grazing and and harvesting harvesting Seasonal variation in calculated sediment mass transport on pasture (Tidjani, 2008)

6 Problem statement Dune reactivation : hectares in 30 years

7 Problem statement Conventional dune fixation technique : Mechanical fixation using low windbreaks made of dead branches (but rapid decay!) Planting of shrubs and trees (but slow!) Natural regeneration of grasses??

8 Objectives To evaluate the effect of a mechanical fixation technique : on sediment fluxes on soil fertility on the recovery of the grass cover, both quantitavely and qualitatively

9 Materials and methods Reactivated sand dune in the village of Tchago (Dept. of Gouré) Mechanical fixation : Branches of Leptadenia pyrotechnica, m high

10 Experimental dune : 2 x 20 plots (20 m x 10 m) 20 m 10 m Degraded Transition Undegraded Harmattan

11 Materials and methods Wind measurements : velocity, direction (2 m) Sediment flux measurements (Feb March) MWAC sediment catcher 60 cm 40 cm 25 cm 15 cm 5 cm

12 Materials and methods Harmattan Wind 1 st windbreak 2 nd windbreak Distance from 1 st windbreak (m) 8 Transects in total

13 Results Effect of windbreak on dry-season sediment fluxes Flux éolien sur la période de Sediment m esure flux (kg/m (kg/m) ) fev - 15 mars fev - 15 mars fev - 15 mars st windbreak 3 m au upwind vent de la 22m m sous downwind le vent de la 55m m sous downwind le vent de la 99m m sous downwind le vent de la 18m m sous downwind le vent de 1ère palissade 1ère palissade 1ère palissade 1ère palissade la 1ère palissade Emplacement des capteurs (MWAC) Wind

14 Results Changes in surface topography during a 1-year period 11/06/05-15/11/05 15/11/05-15/06/06 Dépôt/érosion (cm) Deposition (cm) Distance Position des downwind piquets par of rappor the 1 st à windbreak la palissade (m)

15 Results Effect of windbreak on soil texture (0-10 cm) % sable grossier ( µm) Coarse sand µm (%) Distance Position from/ palissade windbreak (m) (m) Année 2006 Année 2007

16 Results Effect of windbreak on soil texture (0-10 cm) 70 Fine sand µm (%) % sable fin ( µm) Distance Position from/ palissade windbreak (m) (m) Année 2006 Année 2007

17 2,500 Results Effect of windbreak on soil texture (0-10 cm) Organic Cot carbon (g/kg) (g/kg) 2,000 1,500 1,000 0,500 Année 2006 Année , Distance Position from/palissade windbreak (m) (m)

18 Results Recovery of herbaceous groundcover Upwind Windbreak Downwind

19 Results Herbaceous cover development Harmattan (NNE) Monsoon (SSO) Biomasse végétale sèche (Kg/ha) Biomass (kg/ha) Bande 0-5m 3 Bande 5-15m 4 Bande 15-20m 5 Bande 20-25m 6 Bande 25-35m Bande m8 mise Undegraded en défens Zone de mesure

20 Results Biodiversity of herbaceous ground cover Number of species (families) Location Outside / Inside 13 (8 families) 29 (18 families) 59 (20 families) Undegraded area 11 (8 families) 33 (18 families) 71 (23 families)

21 Conclusions The mechanical dune fixation technique was very effective at reducing sediment fluxes : 98% in the first year, and virtually 100% therefater. This is the result of the combined effect of the windbreak and the recovery of the herbaceous groundcover

22 Conclusions Immediately behind the windbreak, the soil is enriched in fine particles as well as Corg and Ntot (not shown). The latter may be both the cause and the result of better growth of the grass In the fenced-off area, grass biodiversity recovered to levels almost similar to the undegraded area

23 Recommendations The effectiveness of the technique strongly relies on grazing control As the windbreaks decay rapidly, the outer windbreaks must be maintained regularly in order to keep animals out. Inner windbreaks do not need to be maintained once the grass cover has established. Limited exploitation of the grasses for animal feed may be possible in good rainfall years Rapid grass recovery is not observed on all fixed dunes!

24 Thank you for your attention