Egeria densa and Elodea canadensis are two common invasive submerged macrophytes in streams and rivers worldwide. We conducted a six-week growth experiment with monocultures of E. densa and E. canadensis and mixed communities of the two species under five different shade levels (35%, 63%, 79%, 90% and 95%). Our aim was to test the effect of shade and competition on colonization of vegetative propagules of these species in flowing waters. We found that biomass accrual in both species was greatly reduced at high shade (≥ 79%) and that E. densa individuals accrued more biomass than E. canadensis individuals at all shade levels. At the least shade level (35%), interspecific competition with E. densa stimulated biomass accrual of E. canadensis individuals, compared to those within an E. canadensis monoculture. At higher shade (≥ 63%), this facilitation response disappeared, although E. canadensis plants continued to have longer stem length and a greater degree of branching than E. densa plants at ≤ 79% shade. At the highest shade level, competition with E. canadensis reduced biomass accrual of E. densa. Hence, shade influenced the competitive interactions between these two species. At low shade, the competitiveness of E. canadensis was enhanced, but in the field E. densa appears to be the dominant species in streams, especially those with higher levels of shade and less frequent disturbance by flushing flow events.

中文翻译：

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遮荫可能会改变两种入侵性水生植物物种之间的定居模式和优势

Egeria densa和Elodea canadensis是全世界溪流和河流中的两种常见的浸没性大型植物。我们进行了为期六周的生长实验，在五个不同的阴影水平（35％，63％，79％，90％和95％）下，对E. densa和E.canadensis进行了单培养，并采用了两种物种的混合群落。我们的目的是测试遮荫和竞争对这些物种的营养繁殖体在流水中定植的影响。我们发现在这两个物种生物量权责发生制在高遮荫（≥79％）大大降低，并且密花香薷个人累计超过生物质E.加拿大一枝黄花在所有各级荫个人。最低阴影等级（35％），种间竞争与E. canadensis单培养中的相比，E。densa刺激了E. canadensis个体的生物量累积。在较高的阴影下（≥63％），尽管在≤79％的阴影下加拿大E. canadensis植物比E. densa植物继续具有更长的茎长和更大的分支程度，但这种促进反应消失了。在最高的阴影水平下，与加拿大E.canadensis的竞争降低了E. densa的生物量积累。因此，阴影影响了这两个物种之间的竞争相互作用。在低阴影下，加拿大E.ensis的竞争力增强，但在田间E. densa 似乎是河流中的主要物种，尤其是那些阴影较高且冲刷水流事件干扰较少的物种。