Silicon (Si) is a beneficial element for many higher plants, and plant species vary widely in degrees of Si uptake and accumulation. Many aspects of ecosystem Si cycling remain unexplored in tropical forests. We aimed to assess the spatial variation of water-extractable soil Si and local variation in leaf Si concentrations within tropical forests, using four large-scale forest dynamics plots. The four forests differed in overall water-extractable Si as well as its within-plot spatial patterns. These among-plot differences may reflect differences in geological substrates, climate and plant community composition. Leaf Si concentration varied greatly among species, but the four forests did not differ greatly in species-mean leaf Si concentration. Our results suggest that leaf Si concentrations, which differ widely among tropical tree species, are relevant for ecosystem Si cycling in tropical forests. Plants, rather than soil Si availability, may regulate Si uptake, thereby modulating ecosystem Si cycling.