Invasive thistles reduce the productivity of pastures and rangelands because their structural defenses make them unpalatable to grazing livestock. However, plants potentially alter their growth patterns, including their allocation of resources to defense, in response to the presence of competing vegetation. Understanding the effects of competition on the structural defense of problematic pasture weeds can inform management plans that reduce the economic harm caused by these pests. We grew musk thistle (Carduus nutans L., also commonly called nodding thistle) in a fully crossed factorial field experiment in a single abandoned pasture in 2017 with two experimental factors: the level of thistle infestation (1 thistle or 5, resulting in densities of 4 or 20 plants m−2) and the presence or absence of grazing (simulated by weekly trimming of competing vegetation). We assessed the effects of treatments on defense by counting prickles >3-mm long on leaves. Our analysis included leaf age and leaf size as covariates. Competition reduced the number of prickles present on leaves. Regression analysis showed that an increase from, for example, 50 g to 200 g of competing vegetation within 50 by 50 cm study plots reduced the expected average number of prickles on intermediate-aged leaves with average length 25.5 cm by 76.9 prickles per leaf, or 41%. This pattern was similar for leaves of all ages, although the oldest leaves generally had fewer prickles than younger leaves. We did not observe differences in defense structures between plants neighbored by conspecifics and those neighbored by other competitors. Carduus nutans has been previously managed using high densities of grazers, and this practice may be more likely to damage less-defended individuals such as those we observed in our treatments with competition. This finding suggests that maintaining competition in pastures may increase C. nutans vulnerability to grazing.

入侵蓟会降低牧场和牧场的生产力，因为它们的结构防御使放牧牲畜难以接受。然而，植物可能会改变它们的生长模式，包括它们的防御资源分配，以应对竞争植被的存在。了解竞争对有问题的牧场杂草的结构防御的影响可以为减少这些害虫造成的经济危害的管理计划提供信息。 2017 年，我们在一个废弃牧场进行了完全交叉因子田间实验，种植了麝香蓟（Carduus nutans L.，通常也称为点头蓟），其中有两个实验因素：蓟侵染水平（1 只蓟或 5 只蓟，导致密度4 或 20 株植物 m -2）以及是否存在放牧（通过每周修剪竞争植被来模拟）。我们通过计算叶子上>3毫米长的刺来评估处理对防御的影响。我们的分析包括叶龄和叶子大小作为协变量。竞争减少了叶子上的刺数量。回归分析表明，例如，在 50 x 50 厘米的研究地块内，竞争植被从 50 克增加到 200 克，平均长度为 25.5 厘米的中龄叶子上的预期平均刺数减少了每片叶子 76.9 个刺，或41%。这种模式对于所有年龄的叶子都是相似的，尽管最老的叶子​​通常比年轻的叶子有更少的刺。我们没有观察到同种植物相邻的植物和其他竞争者相邻的植物之间防御结构的差异。以前曾使用高密度的食草动物来管理秃鹫，这种做法可能更有可能伤害防御能力较差的个体，例如我们在竞争处理中观察到的个体。这一发现表明，维持牧场竞争可能会增加C. nutans 的放牧脆弱性。