High-severity wildfires, which can homogenize floral communities, are becoming more common relative to historic mixed-severity fire regimes in the Northern Rockies of the U.S. High-severity wildfire could negatively affect bumble bees, which are typically diet generalists, if floral species of inadequate pollen quality dominate the landscape post-burn. High-severity wildfires often require more time to return to pre-burn vegetation composition, and thus, effects of high-severity burns may persist past initial impacts. We investigated how wildfire severity (mixed- vs. high-severity) and time-since-burn affected available floral pollen quality, corbicular pollen quality, and bumble bee nutrition using percent nitrogen as a proxy for pollen quality and bumble bee nutrition. We found that community-weighted mean floral pollen nitrogen, corbicular pollen nitrogen, and bumble bee nitrogen were greater on average by 0.82%N, 0.60%N, and 1.16%N, respectively, in mixed-severity burns. This pattern of enhanced floral pollen nitrogen in mixed-severity burns was likely driven by the floral community, as community-weighted mean floral pollen percent nitrogen explained 87.4% of deviance in floral community composition. Only bee percent nitrogen varied with time-since-burn, increasing by 0.33%N per year. If these patterns persist across systems, our findings suggest that although wildfire is an essential ecosystem process, there are negative early successional impacts of high-severity wildfires on bumble bees and potentially on other pollen-dependent organisms via reductions in available pollen quality and nutrition. This work examines a previously unexplored pathway for how disturbances can influence native bee success via altering the nutritional landscape of pollen.

中文翻译：

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与严重度不同的烧伤相比，高强度的野火限制了花卉花粉的质量和蜜蜂的营养。

相对于美国北部落基山脉历史上的混合严重火种制度，可以使花卉群落同质化的高强度野火正变得越来越普遍。花粉质量不足主宰着景观。高强度的野火通常需要更多的时间才能恢复燃烧前的植被成分，因此，高强度的火烧可能会持续到最初的影响。我们使用百分比氮作为花粉质量和大黄蜂营养的代用品，研究了野火严重程度（混合强度与高强度）和自烧以来如何影响可用的花粉质量，球状花粉质量和大黄蜂营养。我们发现，社区加权平均花卉花粉氮含量 在混合严重烧伤中，球状花粉氮和大黄蜂氮平均分别高出0.82％N，0.60％N和1.16％N。混合严重烧伤中花卉花粉氮增加的这种模式可能是由花卉群落驱动的，因为社区加权平均花卉花粉百分比氮解释了87.4％的花卉群落组成差异。自燃烧以来，仅蜜蜂百分比的氮含量变化，每年增加0.33％N。如果这些模式在整个系统中持续存在，我们的研究结果表明，尽管野火是必不可少的生态系统过程，但高强度野火会对大黄蜂以及可能通过降低可用花粉质量和营养而对其他依赖花粉的生物产生负面的连续演替影响。