Data from: Tree species diversity alters plant defence investment in an experimental forest plantation in Southern Mexico

The effects of plant species diversity on plant traits conferring herbivore resistance (e.g., chemical defences), as well as the mechanisms underlying such effects, have received little attention. One potential mechanism for diversity effects on plant defences is that increased plant growth at high diversity could lead to reduced investment in defences via growth-defence trade-offs. We measured tree growth (diameter at breast height) and collected leaves for quantification of total phenolics on 2.5-year old plants of six tropical tree species (N = 597 plants) in a young experimental plantation in southern Mexico. Selected plants were distributed across 23 plots (21 x 21 m each) classified as monocultures of one species (N = 13, 2-3 plots per species) or polycultures (N = 10) represented by mixtures of four out of the six species. We found a significant negative effect of tree species diversity on total phenolics, where polycultures exhibited a 13 percent lower mean concentration than monocultures. However, there was marked variation among tree species in diversity effects on defences, where some species exhibited strong reductions in phenolics in mixtures, whereas others were unresponsive. Further, we found no effect of tree species diversity on growth and results indicated that the negative effect of diversity on chemical defences was not mediated by a growth-defence trade-off. These results demonstrate that tree diversity can alter investment in chemical defences in long-lived tree species, which has important implications for predicting effects on consumers and ecosystem function