The mechanisms by which increases in the biomass of Panicum maximum might occur when growing under the canopy of mature Samanea saman trees were investigated in a pot trial. The subcanopy environment was simulated using soil collected from under, near and away from the canopy of trees and variable light levels imposed by shade cloths. The soil from under the trees had a greatly enhanced nitrogen level, and gave a higher total dry weight (TDW) at all levels of shade than soil from outside the canopy. The increase in biomass was affected by level of shading, with a 200% increase in full sunlight as compared with an increase of 100% at the deepest shade level (12% relative light). Deep shade augmented shoot nitrogen concentration, but caused only a slight increase in digestibility. The results show that, in the seasonal range of shade levels corresponding with those found beneath the canopy of S. saman (10-30% relative light), there will be a steep gradient in response in TDW. Consequently, factors affecting canopy light transmission - such as the extent of leafing out and canopy shape - will be critical in determining yield response.