Microclimate and soil fertility were measured under and away from the canopies of small and large scattered trees of néré (Parkia biglobosa) and karité (Vitellaria paradoxa) to explain their influence on pearl millet (Pennisetum glaucum). During the rainy season, photosynthetically active radiation under both tree canopies was reduced to 24% of incident radiation but, surprisingly, millet biomass was not significantly different from that in the open. Millet seedlings under the tree canopies experienced only 1–9 h week−1 of supra-optimal temperatures (>40 °C) compared with 27 h week−1 in the open. Reduction in windspeed and soil evaporation was observed under the trees but was unlikely to have a significant influence on millet growth since water was not limiting (rainfall = 920 mm).

Exchangeable potassium, percentage carbon and pH were significantly higher and δ13C lower in soil under trees than in the open. Nitrogen levels were not significantly higher than in the open except for large néré trees. Carbon derived from C3 plants, calculated from δ13C, was significantly higher in soils under trees than in the open and is recommended as a good indicator of the influence of trees on soils. These results show that the combined beneficial effects of temperature modifications and soil fertility could exceed the negative effect of tree shade.