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.植物物种多样性对赋予植食动物抗性的植物性状（例如化学防御（chemical defences））的影响，以及此类效应背后的作用机制，迄今仍鲜有研究关注。物种多样性对植物防御影响的潜在机制之一是，高多样性生境下植物生长的增强，可能通过生长-防御权衡（growth-defence trade-offs）降低防御物质的投入。我们在墨西哥南部一处年轻的实验人工林内，对6种热带树木的2.5年生植株（共计597株）开展了研究：测量了植株的胸径（diameter at breast height）并采集叶片，以定量总酚类物质（total phenolics）的含量。所选植株分布于23块样地（每块面积为21×21米），这些样地分为两类：单优栽培样地（每个样地仅种植单一树种，共13块，每个物种对应2~3块样地），以及多混栽培样地（共10块，由6个物种中的4个物种混合构建而成）。我们发现树木物种多样性对总酚类物质含量存在显著负向影响：多混栽培样地的总酚类平均浓度较单优栽培样地低13%。不过，不同树种对多样性介导的防御效应的响应存在显著差异：部分物种在混栽生境中酚类物质含量大幅降低，而另一些物种则无明显响应。进一步分析表明，树木物种多样性对植株生长并无显著影响，且化学防御的负向效应并非通过生长-防御权衡所介导。本研究结果表明，树木多样性可改变长寿树种的化学防御物质投入，这对于预测其对消费者及生态系统功能的影响具有重要意义。