WINDBREAK-CROP INTERACTIONS IN THE SAHEL .2. GROWTH-RESPONSE OF MILLET IN SHELTER

Plant growth behind a windbreak varies considerably with changing macroclimate, species and windbreak type. This variability can often be explained in terms of the microclimate in the lee of a windbreak. This paper describes an investigation into the modification of microclimate by a windbreak and the influence of the windbreak on growth of a millet crop in its lee in Niger, West Africa. Growth of millet (Pennisetum typhoides) was reduced by shelter at the start of the season because of high soil surface temperatures, caused by reduced boundary-layer conductance, which caused a delay in germination and emergence. Towards the middle of the season, leaf temperatures were in general lower than at the start of the season, and higher temperatures in shelter increased the rate of leaf expansion and senescence above that in the unsheltered parts of the field. At the start of the season, air and leaf temperatures in shelter increased whereas ambient vapour pressures remained relatively constant. This led to higher vapour pressure deficits at the surface of the leaves (D-1) in shelter than in the unsheltered crop. In the middle of the growing season, vapour pressures increased in shelter so as to reduce D-1 and this increased the solar radiation conversion coefficient, probably because of increases in stomatal conductance. There was more transpiration in shelter as a result of both larger leaf area index in the middle and at the end of the growing season and higher transpiration per unit leaf area relative to the unsheltered crop. Transpiration per unit leaf area was higher in shelter because of higher leaf temperatures and stomatal conductances than in the unsheltered crop.