Department of Biology, Wichita State University, Wichita, Kansas 67260, USA.
Ecology. 2010 Oct;91(10):3081-93. doi: 10.1890/09-1101.1.
Understanding spatial and temporal variation in factors influencing plant regeneration is critical to predicting plant population growth. We experimentally evaluated seed limitation, insect herbivory, and their interaction in the regeneration and density of tall thistle (Cirsium altissimum) across a topographic ecosystem productivity gradient in tallgrass prairie over two years. On ridges and in valleys, we used a factorial experiment manipulating seed availability and insect herbivory to quantify effects of: seed input on seedling density, insect herbivory on juvenile density, and cumulative impacts of both seed input and herbivory on reproductive adult density. Seed addition increased seedling densities at three of five sites in 2006 and all five sites in 2007. Insect herbivory reduced seedling survival across all sites in both years, as well as rosette survival from the previous year's seedlings. In both years, insecticide treatment of seed addition plots led to greater adult tall thistle densities in the following year, reflecting the increase in juvenile thistle densities in the experimental year. Seedling survival was not density dependent. Our analytical projection model predicts a significant long-term increase in adult densities from seed input, with a greater increase under experimentally reduced insect herbivory. While plant community biomass and water stress varied significantly between ridges and valleys, the effects of seed addition and insect herbivory did not vary with gradient position. These results support conceptual models that predict seedling and adult densities of short-lived monocarpic perennial plants should be seed limited. Further, the experiment demonstrates that even at high juvenile plant densities, at which density dependence potentially could have overridden herbivore effects on plant survival, insect herbivory strongly affected juvenile thistle performance and adult densities of this native prairie species.
理解影响植物再生的因素的时空变化对于预测植物种群的生长至关重要。我们在两年内通过在高草草原的地形生态系统生产力梯度上进行实验，评估了种子限制、昆虫食草性及其相互作用对高大蓟（Cirsium altissimum）再生和密度的影响。在山脊和山谷上，我们使用了一个因子实验来操纵种子的可利用性和昆虫的食草性，以量化：种子输入对幼苗密度的影响、昆虫食草性对幼体密度的影响，以及种子输入和食草性对生殖成体密度的累积影响。在 2006 年的三个和 2007 年的五个地点中，种子添加增加了幼苗密度。在两年中，昆虫食草性降低了所有地点的幼苗存活率，以及前一年幼苗的莲座叶存活率。在两年中，在添加种子的实验中使用杀虫剂处理导致下一年的成年高大蓟密度增加，反映了当年幼蓟密度的增加。幼苗存活率不受密度依赖性的影响。我们的分析预测模型预测，从种子输入中会出现成年密度的显著长期增加，而在实验减少的昆虫食草性下，增加幅度更大。虽然山脊和山谷之间的植物群落生物量和水分胁迫有显著差异，但种子添加和昆虫食草性的影响与梯度位置无关。这些结果支持预测短命单性结实多年生植物的幼苗和成年密度应受种子限制的概念模型。此外，该实验表明，即使在高幼体植物密度下，密度依赖性可能已经超过了食草动物对植物生存的影响，昆虫食草性仍然强烈影响这种本地草原物种的幼蓟表现和成年密度。
