Context. Integrated crop-livestock-forestry systems can be an alternative to monoculture pastures; however, the effect of the arboreal component on tillering and tussock dynamics of understorey grass canopy is not well understood.

Aims. Our objective was to evaluate the patterns of tillering dynamics, population stability and tussock distribution of Piatã palisadegrass (Urochloa brizantha) as affected by contrasting shading regimes in the Brazilian Amazon biome.

Methods. The following three shading regimes were assessed: pasture with no trees (no shading, NS), moderate shading (MS, 338 trees ha^-1), and intense shading (IS, 714 trees ha^-1). Pastures were rotationally grazed by dairy heifers. Paddocks from MS and IS were stratified into three sampling strips, including two closer to the tree rows (i.e. lateral) and one between the two lateral strips (i.e. central). Two methods for monitoring sward canopy light interception (LI) were used for IS regime, namely, LI taken under the tree canopy (i.e. inside) and LI was taken above the tree canopy (i.e. outside).

Key results. Relative to NS, there was a reduction in photosynthetically active radiation transmittance of 18.1% and 37.1% on MS and 49.9% and 52.0% on IS for central and lateral shading strips respectively. Overall, the increase in shading level was associated with a decrease in average tussock perimeter, tiller population density, and an increase in the frequency of bare ground.

Conclusions. Our findings clearly indicate the key role of the light environment as a determinant of Piatã palisadegrass horizontal structure.

Implications. It highlights the importance of planning the levels of tree density and layout in areas of crop-livestock-forestry integration.