Seastedt T R, Suding K N
Institute of Arctic and Alpine Research, Department of Ecology and Evolutionary Biology, University of Colorado, Boulder, CO 80309-0450, USA.
Oecologia. 2007 Apr;151(4):626-36. doi: 10.1007/s00442-006-0620-z. Epub 2007 Jan 10.
Knapweeds (Centaurea spp.) are among the most invasive of non-indigenous plant species that have colonized western North America over the last century. We conducted a 4-year experiment in a reconstructed grassland to test hypotheses related to the ability of grasslands to resist the invasion of diffuse knapweed (C. diffusa). We experimentally invaded C. diffusa and three native species into areas where we manipulated soil nitrogen (N) and phosphorus (P) availability and removed extant grasses to reduce competition. We evaluated the growth response of these species to these resources and competitive manipulations. Of the native species that were experimentally added, only one species, Ratibida pinnata (prairie coneflower), established in any numbers. Establishment values in intact vegetation were low for both species, but establishment by C. diffusa (0.02%) clearly outperformed that of R. pinnata (0.001%). Under reduced grass competition, establishment was enhanced, but the values for C. diffusa (0.68%) were not statistically different from those of R. pinnata (0.57%). Neither species performed better under higher soil nutrients in the presence of competing grasses. In plots with both species, biomass of the two planted species was positively correlated, but the biomass of both species was negatively correlated with non-added weedy species. Subsequent harvests of C. diffusa indicated that establishment was enhanced in treatments with higher soil nutrients but that the biomass of these plants could only be enhanced when plant competition was also reduced. These results indicate that C. diffusa can establish in intact grasslands at rates higher than natives, but opportunism rather than competitive ability best describes the invasiveness of C. diffusa. Thus, the mechanisms contributing to the establishment of this knapweed species are different from factors identified as contributing to the dominance of this invader.
矢车菊（矢车菊属）是过去一个世纪在北美西部定居的最具入侵性的非本土植物物种之一。我们在一个重建的草原上进行了为期4年的实验，以检验与草原抵抗扩散矢车菊（C. diffusa）入侵能力相关的假设。我们通过实验将扩散矢车菊和三种本土物种引入到我们操纵土壤氮（N）和磷（P）有效性并去除现存草以减少竞争的区域。我们评估了这些物种对这些资源和竞争操纵的生长反应。在实验添加的本土物种中，只有一种物种，即松果菊（草原锥花），有一定数量的植株建立。在完整植被中，这两个物种的定植率都很低，但扩散矢车菊的定植率（0.02%）明显高于松果菊（0.001%）。在草竞争减少的情况下，定植率有所提高，但扩散矢车菊的值（0.68%）与松果菊的值（0.57%）在统计学上没有差异。在有竞争草的情况下，两种物种在较高土壤养分条件下都没有表现得更好。在两种物种都存在的地块中，两种种植物种的生物量呈正相关，但两种物种的生物量与未添加的杂草物种呈负相关。随后对扩散矢车菊的收获表明，在土壤养分较高的处理中定植率有所提高，但只有在植物竞争也减少时，这些植物的生物量才能增加。这些结果表明，扩散矢车菊能够以高于本土物种的速率在完整草原中定植，但机会主义而非竞争能力最能描述扩散矢车菊的入侵性。因此，促成这种矢车菊物种定植的机制与被认为促成这种入侵者优势的因素不同。
