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To test the effects of shade on understorey herbaceous productivity in tropical savannas, water relations of important grass species that either grow primarily under tree crowns or in open grasslands were studied in Tsavo National Park, Kenya. The relationship of stomatal conductance to light intensity was investigated under well-watered conditions; and the relationship of stomatal conductance to light intensity, vapor pressure deficit, and leaf water potential was studied during a period of decreasing soil moisture. Below-crown species were more responsive to reductions in light intensity than open-grassland species, with stomatal conductance of below-crown species declining linearly with reductions in light intensity. Conductance of open-grassland species was relatively unchanged at all except the lowest light levels. Below-crown species therefore reduced their water loss when shaded, conserving soil water for later growth. Open-grassland species appeared to be more efficient than below-crown species at extracting water from dry soils under full-sun conditions.