Leaf silicon concentration (leaf Si) varies widely among plants, and Si accumulation in certain plant species often acts as a physical defense against a range of antagonists, such as insect herbivores and microbial pathogens. While it is recognized that leaf Si is phylogenetically conserved, recent studies show large variation in leaf Si among congeneric plants in some groups (e.g., Cyperaceae, Dipterocarpaceae and Fabaceae). Thus, understanding the linkage between leaf Si and other leaf traits among congeneric species may provide better insight into the use of Si for plants. This study used 13 species in the genus of Piper, co-occurring in a lowland tropical forest on Barro Colorado Island, Panama. We collected intact mature leaves from the upper canopy of 6-17 individuals (< ca. 3 m height) per species, and measured leaf Si and other leaf traits: leaf area (LA), punch strength, leaf dry mass content (LDMC), leaf mass per area (LMA), total phenolic concentration (leaf phenolics) and leaf volume per area (LVA ≃ thickness). Species-mean leaf Si varied from 4.1 to 87.2 mg Si g-1. Leaf Si showed a significantly positive correlation with LDMC (r = 0.77, p <0.01) and LMA (r = 0.57, p <0.05) and a negative correlation with leaf phenolics (r = -0.57, p <0.05). In a principal component (PC) analysis, the first PC axis explained 48.6% of the variance, to which leaf Si, punch strength, LMA and LDMC contributed negatively while leaf phenolics contributed positively. The second PC axis explained additional 26.2% of the variance and was related strongly with LVA, while LA showed the largest contribution to the third PC axis, which accounted for 15.5% of the variance. Thus, the first PC axis suggests a physical and chemical defense trade-off (leaf structure, including silica deposits vs. phenolics), while the second and third PC axes highlight dimensions of leaf size and morphology. In conclusion, leaf Si is associated with both structural and chemical traits of leaves across the piper species, suggesting that Si potentially relates to physical and chemical defense of leaves against natural enemies.