Alternative states defined by tree-canopy dominance result in different ecosystem functioning and shape habitat conditions for the understory vegetation. One example in the boreal forest is the alternation between broadleaf deciduous and coniferous forests. Disturbances related to natural fires and human land uses have produced changes in tree-canopy dominance in the boreal region where coniferous forests change to broadleaved forests, affecting understory community dynamics and their related ecosystem processes and functions. To analyze the factors driving changes in understory vegetation and the resistance of its vegetation to shifts between alternative states, we compared the effects of changes in the system between two contrasting boreal forest types (black spruce vs. trembling aspen) in adjacent stands with similar topoedaphic conditions. We performed a 5-year in situ experiment using alternative states as a theoretical framework including two approaches: (1) the ecosystem approach, manipulating environmental conditions of light, litter, and nutrients in each forest type to determine the main mechanisms associated with tree-canopy dominance that affect the diversity and composition of understory communities; and (2) the community approach, physically exchanging understory communities between alternative states, to determine their resistance under a new tree-canopy dominance through time, as well as the resilience of the forest understory after a small-scale disturbance. Results indicate that the understory vegetation of trembling aspen forests were resistant through time both after changes in local conditions in the ecosystem approach and in the new black spruce-dominated alternative state in the community approach. In contrast, mosses and ericaceous plants that typically dominate the forest floor of black spruce forests were negatively affected by the physical effect of broadleaf litter addition in our ecosystem approach and they were not resistant when transplanted to trembling aspen forests in the community approach, as they decreased in abundance and were invaded by aspen understory community species over time. The understory vegetation is a key forest ecosystem driver that can contribute to maintain the resilience of the boreal system and help to preserve their ecosystem services, which is a key aspect to consider in forest management faced with the effects of climate change.

中文翻译：

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针叶林和阔叶落叶替代州北方林下植被对比的驱动因素

由树冠优势定义的替代状态导致不同的生态系统功能并塑造林下植被的栖息地条件。北方森林的一个例子是阔叶落叶林和针叶林之间的交替。与自然火灾和人类土地利用有关的干扰导致北方地区树冠优势发生变化，针叶林转变为阔叶林，影响林下群落动态及其相关的生态系统过程和功能。为了分析驱动林下植被变化的因素及其植被对不同状态之间变化的抵抗力，我们比较了具有相似地形的相邻林分中两种对比鲜明的北方森林类型（黑云杉与颤抖白杨）之间系统变化的影响状况。我们使用替代状态作为理论框架进行了为期 5 年的原位实验，包括两种方法：（1）生态系统方法，操纵每种森林类型中的光、凋落物和养分的环境条件，以确定与树木相关的主要机制。影响林下群落多样性和组成的冠层优势；(2)群落方法，在替代状态之间物理交换林下群落，以确定它们在新的树冠优势下随时间的抵抗力，以及小规模扰动后森林林下植被的恢复力。结果表明，在生态系统方法中的局部条件发生变化以及群落方法中新的以黑云杉为主的替代状态下，颤抖白杨森林的林下植被随着时间的推移都具有抵抗力。相比之下，在我们的生态系统方法中，通常在黑云杉森林的森林地面上占主导地位的苔藓和杜鹃花植物受到了阔叶凋落物添加的物理效应的负面影响，并且当在群落方法中移植到颤抖的白杨森林时，它们没有抵抗力，因为它们随着时间的推移，数量减少，并被白杨林下群落物种入侵。林下植被是森林生态系统的关键驱动因素，有助于维持北方系统的恢复力并有助于保护其生态系统服务，