The persistence and distribution of species under changing climates can be affected by both direct effects of the environment and indirect effects via biotic interactions. However, the relative importance of direct and indirect climate effects on recruitment stages is poorly understood. We conducted a manipulative experiment to test the multiway interaction of direct and competition-mediated effects of climate change on vegetation dynamics. Following stand-replacing fire in California mixed-conifer forest, we seeded two conifer species, Pinus ponderosa and Abies concolor, in two consecutive years, one relatively normal and the other with an unusually wet and snowy winter followed by a hot summer. We additionally manipulated snow amount and competitive environment for both years. We found the effects of the snowpack treatment were contingent upon other abiotic factors (year of seeding) and biotic factors (shrub competition). Under ambient snowpack, shrubs reduced recruitment of P. ponderosa seedlings, but this negative effect disappeared with reduced snowpack. Additionally, the effects of shrubs on seedlings differed between cohorts and by life stage. In a warmer future, decreased snowpack may increase seedling emergence, but hotter and drier summers will decrease seedling survival; the effects of shrubs on conifers may become less negative as temperatures increase.

中文翻译：

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积雪减少和温度升高降低了种间竞争者对再生针叶树的负面影响。

在气候变化的影响下，物种的持久性和分布既会受到环境的直接影响，也可能受到生物相互作用的间接影响。但是，人们对直接和间接气候影响对征聘阶段的相对重要性了解甚少。我们进行了一个操作性实验，以测试气候变化对植被动态的直接影响和竞争介导作用的多方面相互作用。在加利福尼亚的混合针叶林中，用替换林的火烧了之后，我们连续两年播种了两种针叶树种，即黄松和冷杉木，一种相对正常，另一种具有异常潮湿多雪的冬天，然后是炎热的夏天。两年来，我们还额外控制了降雪量和竞争环境。我们发现积雪处理的效果取决于其他非生物因素（播种年份）和生物因素（灌木竞争）。在周围积雪下，灌木减少了美国黄杨幼苗的吸收，但是随着积雪减少，这种负面影响消失了。此外，灌木对幼苗的影响在不同群体之间以及在生命阶段中也不同。在更暖和的未来，积雪减少可能会增加幼苗出苗，但夏天更热，更干燥会降低幼苗的存活。温度升高，灌木对针叶树的负面影响可能会减小。灌木丛对幼苗的影响在不同群体之间以及在生命阶段中都不同。在更暖和的未来，积雪减少可能会增加幼苗出苗，但夏天更热，更干燥会降低幼苗的存活。温度升高，灌木对针叶树的负面影响可能会减小。灌木丛对幼苗的影响在不同群体之间以及在生命阶段中都不同。在更暖和的未来，积雪减少可能会增加幼苗出苗，但夏天更热，更干燥会降低幼苗的存活。温度升高，灌木对针叶树的负面影响可能会减小。