Defoliation by eastern spruce budworm is one of the most important natural disturbances in Canadian boreal and hemi‐boreal forests with annual area affected surpassing that of fire and harvest combined, and its impacts are projected to increase in frequency, severity, and range under future climate scenarios. Deciding on an active management strategy to control outbreaks and minimize broader economic, ecological, and social impacts is becoming increasingly important. These strategies differ in the degree to which defoliation is suppressed, but little is known about the downstream consequences of defoliation and, thus, the implications of management. Given the disproportionate role of headwater streams and their microbiomes on net riverine productivity across forested landscapes, we investigated the effects of defoliation by spruce budworm on headwater stream habitat and microbiome structure and function to inform management decisions. We experimentally manipulated a gradient of defoliation among 12 watersheds during a spruce budworm outbreak in the Gaspésie Peninsula, Québec, Canada. From May through October of 2019–2021, stream habitat (flow rates, dissolved organic matter [DOM], water chemistry, and nutrients), algal biomass, and water temperatures were assessed. Bacterial and fungal biofilm communities were examined by incubating six leaf packs for five weeks (mid‐August to late September) in one stream reach per watershed. Microbiome community structure was determined using metabarcoding of 16S and ITS rRNA genes, and community functions were examined using extracellular enzyme assays, leaf litter decomposition rates, and taxonomic functional assignments. We found that cumulative defoliation was correlated with increased streamflow rates and temperatures, and more aromatic DOM (measured as specific ultraviolet absorbance at 254 nm), but was not correlated to nutrient concentrations. Cumulative defoliation was also associated with altered microbial community composition, an increase in carbohydrate biosynthesis, and a reduction in aromatic compound degradation, suggesting that microbes are shifting to the preferential use of simple carbohydrates rather than more complex aromatic compounds. These results demonstrate that high levels of defoliation can affect headwater stream microbiomes to the point of altering stream ecosystem productivity and carbon cycling potential, highlighting the importance of incorporating broader ecological processes into spruce budworm management decisions.

中文翻译：

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溪流栖息地和微生物组对云杉芽虫落叶的响应：疫情管理的新考虑因素


东部云杉芽虫的落叶是加拿大北方和半北方森林中最重要的自然干扰之一，年受影响的面积超过了火灾和采伐的总和，预计在未来气候情景下，其影响的频率、严重程度和范围将增加。决定采取积极的管理策略来控制疫情并最大限度地减少更广泛的经济、生态和社会影响变得越来越重要。这些策略在抑制落叶的程度上有所不同，但人们对落叶的下游后果以及管理的影响知之甚少。鉴于源头溪流及其微生物组对森林景观中河流净生产力的不成比例的作用，我们调查了云杉芽虫落叶对源头溪流栖息地和微生物组结构和功能的影响，以告知管理决策。我们在加拿大魁北克省加斯佩西半岛的云杉芽虫爆发期间，实验操纵了 12 个流域之间的落叶梯度。从 2019 年 5 月到 2021 年 10 月，评估了溪流栖息地（流速、溶解有机物 [DOM]、水化学和营养物质）、藻类生物量和水温。通过在每个流域的一个溪流中孵育六个叶包为期五周（8 月中旬至 9 月下旬）来检查细菌和真菌生物膜群落。使用 16S 和 ITS rRNA 基因的元条形码确定微生物组群落结构，并使用细胞外酶测定、落叶分解速率和分类功能分配检查群落功能。 我们发现累积落叶与增加的流速和温度以及更多的芳香 DOM（以 254 nm 处的特异性紫外线吸光度测量）相关，但与养分浓度无关。累积落叶还与微生物群落组成的改变、碳水化合物生物合成的增加和芳香族化合物降解的减少有关，这表明微生物正在转向优先使用简单的碳水化合物而不是更复杂的芳香族化合物。这些结果表明，高水平的落叶会影响源头溪流微生物组，以至于改变溪流生态系统的生产力和碳循环潜力，突出了将更广泛的生态过程纳入云杉芽虫管理决策的重要性。