Climate warming can induce the encroachment of shrubs and may trigger treeline dynamics. However, the responses of shrubs and trees to climate change may be modulated by other environmental drivers such as land-use change and biological interactions. The European Alps are one of the three areas experiencing the most intense warming globally in the twentieth century. We analyse, through a multidisciplinary approach, the shrub and tree encroachment at the Stelvio Pass (Italian Alps) focusing on three target species (Rhododendron ferrugineum, Larix decidua, Pinus mugo) to reconstruct their dynamics and assess which drivers (climate change, land-use change, biological interactions provided by shrub facilitation) promoted their ingression. Shrub colonization started in 1867, in coincidence with the end of the Little Ice Age. Tree recruitment started since 1960 for P. mugo and 1972 for L. decidua and correlated strongly with air warming and shortening of the snow cover duration. Climate (air temperature, snow cover) exhibited the highest correlation with shrub and tree dynamics both during the period of recruitment and in the following and preceding 5-year period. Air warming appeared to be crucial for tree ingression and persistence. Land-use change was not related to shrub encroachment, and only weakly to tree recruitment. Both the correlation analysis as well as the patterns of recruitment highlighted that shrubs were characterized by different ecological requirements from trees. There was not a “nurse effect” of shrubs on trees, and this biotic interaction changed with the developmental stage of the involved species, being lowest for seedlings and highest for adults, requiring further investigations. Future scenarios of climate change indicate a further and intense warming, and our data show that it is likely that shrub and tree encroachment will proceed, with relevant consequences on the extremely vulnerable high-elevation alpine ecosystems.

气候变暖会引起灌木丛的侵蚀，并可能触发树木动态。但是，灌木和树木对气候变化的响应可能会受到其他环境驱动因素（如土地利用变化和生物相互作用）的调节。欧洲阿尔卑斯山是二十世纪全球变暖最剧烈的三个地区之一。我们通过多学科方法分析了Stelvio Pass（意大利阿尔卑斯山）上的灌木和树木侵犯情况，重点研究了三个目标物种（杜鹃花杜鹃，落叶松，落叶松）以重建其动力学并评估哪些驱动因素（气候变化，土地利用变化，灌木促进提供的生物相互作用）促进了它们的入侵。灌木丛殖民化始于1867年，与小冰河时代的结束相吻合。自1960年开始为P. mugo和1972年为L. decidua招募树木与气候变暖和积雪持续时间的缩短密切相关。在招募期间以及随后的和之前的5年期间，气候（气温，积雪）与灌木和树木动态的相关性最高。空气变暖对于树木入侵和持久性似乎至关重要。土地利用的变化与灌木丛的侵占无关，而与树木的吸收无关。相关性分析和招聘模式都强调灌木具有与树木不同的生态需求。灌木对树木没有“护理作用”，并且这种生物相互作用随所涉物种的发育阶段而变化，对于幼苗最低，对成年人最高，需要进一步研究。