ABSTRACT Background: Climate change is projected to alter the elevation and latitude of treeline globally. Seed germination and seedling survival are critical controls on treeline expansion. Neighbouring alpine plants, either through competition for resources or through altered microclimate, also affect seedlings emerging in the alpine zone. With warming, alpine plant species may interact with each other more or less strongly. Aims: To determine whether establishing tree seedlings and an alpine herb are similarly sensitive to alpine plant neighbours under ambient and altered climate. Methods: We imposed active heating, watering, and removed all plants adjacent to emerging conifer seedlings and an alpine herb. Results: Picea engelmannii seedlings showed lower survival compared with Pinus flexilis 3 weeks following neighbour removal, and after 1 year only survived in watered plots. Pinus seedlings responded to neighbour removal by lowering the quantum yield of photosynthesis (ϕPSII). Contrary to expectations from the stress gradient hypothesis, survival was reduced without neighbours near the low-elevation range limit of Chionophila jamesii. Conclusions: Pinus flexilis has higher expansion potential into the alpine, while Picea engelmannii requires moist conditions that could be facilitated by neighbours to expand its range. This implies likely range expansion by P. flexilis with consequences for alpine plant diversity and ecosystem function.

中文翻译：

---

树木对高山苔原的殖民化：高山邻居有助于晚浆果但不利于早浆果针叶树幼苗

摘要背景：预计气候变化将改变全球林线的海拔和纬度。种子发芽和幼苗成活是林线扩张的关键控制因素。邻近的高山植物，无论是通过资源竞争还是通过改变的小气候，也会影响高山地区的幼苗。随着变暖，高山植物物种可能或多或少地相互影响。目的：确定树木幼苗和高山草本植物在环境和气候变化下是否对高山植物邻居同样敏感。方法：我们主动加热、浇水，并移除了与新兴针叶树幼苗和高山草本植物相邻的所有植物。结果：在去除邻居后 3 周，与松树相比，恩格尔曼云杉幼苗的存活率较低，1年后只能在浇水的地块中存活。松树幼苗通过降低光合作用的量子产率 (φPSII) 来响应邻居的移除。与应力梯度假设的预期相反，在 Chionophila jamesii 的低海拔范围限制附近没有邻居的情况下，存活率会降低。结论：Pinus flexilis 具有更高的向高山扩张的潜力，而 Picea engelmannii 需要潮湿的条件，邻居可以促进其扩大其范围。这意味着 P. flexilis 可能扩大范围，对高山植物多样性和生态系统功能产生影响。在 Chionophila jamesii 的低海拔范围限制附近没有邻居的情况下，存活率会降低。结论：Pinus flexilis 具有更高的向高山扩张的潜力，而 Picea engelmannii 需要潮湿的条件，邻居可以促进其扩大其范围。这意味着 P. flexilis 可能扩大范围，对高山植物多样性和生态系统功能产生影响。在 Chionophila jamesii 的低海拔范围限制附近没有邻居的情况下，存活率会降低。结论：Pinus flexilis 具有更高的向高山扩张的潜力，而 Picea engelmannii 需要潮湿的条件，邻居可以促进其扩大其范围。这意味着 P. flexilis 可能扩大范围，对高山植物多样性和生态系统功能产生影响。