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，以及该效应背后的潜在机制，迄今尚未得到充分关注。关于多样性对植物防御的影响，一种潜在机制为：高多样性生境中植物生长提升，可通过生长-防御权衡（growth-defence trade-offs）减少防御物质的投入。我们在墨西哥南部一处年轻的实验人工林中，对6种热带树木的2.5年生植株（共计N=597株）测定了树木生长量（胸径（diameter at breast height）），并采集叶片用于总酚类物质（total phenolics）的定量检测。所选植株被分配至23个样地（每个样地面积为21×21米），这些样地分为两类：单作样地（即单一物种的纯林，共N=13个，每个物种对应2-3个样地），以及混作样地（共N=10个，由6个物种中的4个物种混合配置而成）。研究发现，树木物种多样性对总酚类物质具有显著负向效应：混作样地的总酚类平均浓度较单作样地低13%。然而，不同树种在多样性对防御的影响上存在显著差异：部分物种在混交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。此外，我们未发现树木物种多样性对植株生长存在显著影响，结果表明，多样性对化学防御的负向效应并非通过生长-防御权衡所介导。本研究结果证实，树木多样性可改变长寿树种的化学防御物质投入，这对于预测植食动物消费者及生态系统功能的变化具有重要意义。