Forbs comprise most of the plant diversity in North American tallgrass prairie and provide vital ecosystem services, but their abundance in prairie restorations is highly variable. Restoration practitioners typically sow C₄ grasses in high abundances because they are inexpensive, provide fuel for prescribed fires, can dominate reference sites, and suppress weeds that suppress sown forbs. However, C₄ grasses can also suppress sown forbs, calling this practice into question. We evaluated how C₄ grasses influence the abundance and diversity of sown forbs in 78 restored prairies across Illinois, Indiana, and Michigan. We found that the direct negative effects of C₄ grasses on sown forbs outweighed indirect positive effects that occurred as C₄ grasses suppressed nonsown species, which in turn suppressed sown forbs. This pattern was especially strong for the C₄ grass big bluestem (Andropogon gerardii). Therefore, strategies to promote big bluestem and other C₄ grasses would not promote sown forbs. Although C₄ grass cover was not strongly related to two hypothesized drivers (time since fire or site age), seeding density of C₄ grasses increased their cover. Sown forb cover also increased with forb seeding density, increased indirectly with fire (through its negative effect on nonsown species), and decreased indirectly with soil water‐holding capacity (through its positive effect on nonsown species). These results highlight the complex interplay of species groups during grassland restoration and show how managers can promote sown forbs in restored prairies: increasing forb seeding density and reducing time since fire and the abundance of C₄ grasses and weeds.

中文翻译：

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丰富的C4草在恢复的大草原中是好运

在北美高草草原上，Forbs构成了大多数植物的多样性，并提供了至关重要的生态系统服务，但是在草原恢复中它们的丰度变化很大。修复从业人员通常会大量播种C ₄草，因为它们价格便宜，可为规定的火提供燃料，可控制参考点并抑制杂草，从而抑制播种的草。但是，C ₄草还可以抑制播种的前叉，这使这种做法成为问题。我们评估了C ₄草如何影响伊利诺伊州，印第安纳州和密歇根州78个恢复的大草原中已播种的草的丰度和多样性。我们发现C ₄的直接负面影响播种过的草上的草大于C ₄草抑制了未播种的种而产生的间接积极作用，而后者又抑制了播种的草。对于C ₄草大蓝茎（Andropogon gerardii），这种模式尤为突出。因此，推广大蓝茎和其他C ₄草的策略不会促进播种的前生。尽管C _4的草覆盖与两个假想的驱动因素（自火灾或工龄以来的时间）没有密切关系，但C _4的播种密度草增加了它们的覆盖率。播种过的草覆盖率也随播种密度的增加而增加，随火间接增加（通过其对非播种物种的负面影响），并随着土壤持水量间接减少（通过其对非播种物种的积极影响）。这些结果凸显了草原恢复过程中物种群之间的复杂相互作用，并显示了管理者如何在恢复的草原上促进已播种的forbs：增加forb播种密度并减少自大火以来的时间以及丰富的C ₄草和杂草。