Data from: Microclimate variability in alpine ecosystems as stepping stones for non-native plant establishment above their current elevational limit

Alpine environments are currently relatively free from non-native plant species, although their presence and abundance have recently been on the rise. It is however still unclear whether the observed low invasion levels in these areas are due to an inherent resistance of the alpine zone to invasions or whether an exponential increase in invasion is just a matter of time. Using a seed-addition experiment on north- and south-facing slopes (cf. microclimatic gradient) on two mountains in subarctic Sweden, we tested the establishment of six non-native species at an elevation above their current distribution limits and under experimentally enhanced anthropogenic pressures (disturbance, added nutrients and increased propagule pressure). We found a large microclimatic variability in cumulative growing degree days (GDD) (range = 500.77 °C, SD = 120.70 °C) due to both physiographic (e.g. aspect) and biophysical (e.g. vegetation cover) features, the latter being altered by the experimental disturbance. Non-native species establishment and biomass production were positively correlated with GDD along the studied microclimatic gradient. However, even though establishment on the north-facing slopes caught up with that on the south-facing slopes throughout the growing season, biomass production was limited on the north-facing slopes due to a shorter growing season. On top of this microclimatic effect, all experimentally imposed anthropogenic factors enhanced non-native species success. The observed microclimatic effect indicates a potential for non-native species to use warm microsites as stepping stones for their establishment towards the cold end of the gradient. Combined with anthropogenic pressures this result suggests an increasing risk for plant invasion in cold ecosystems, as such stepping stones in alpine ecosystems are likely to be more common in a future that will combine a warming climate with persistent anthropogenic pressures.

当前高山生态系统整体上鲜有外来植物物种入侵，但其出现频率与种群丰度近年来呈上升趋势。但目前仍无法确定，此类区域观测到的低入侵水平，究竟是源于高山带对生物入侵的固有抗性，还是说入侵程度的指数级增长仅为时间问题。本研究依托瑞典亚北极地区两座山地的南北向坡面（以此模拟微气候梯度）开展种子添加实验，在高于各物种当前分布上限的海拔区域，结合实验性增强的人为压力（包括生境干扰、养分添加与繁殖体压力提升），测试了6种外来植物的定植情况。研究发现，受地貌特征（如坡向）与生物物理特征（如植被覆盖度，后者会因实验性干扰发生改变）的共同作用，样地内的累积生长度日（cumulative growing degree days，GDD）存在显著微气候异质性，其波动范围为500.77 °C，标准差为120.70 °C。沿本次研究的微气候梯度，外来植物的定植率与生物量积累均与GDD呈显著正相关。不过，尽管在整个生长季内，北向坡面的植物定植率最终追平了南向坡面，但由于生长季更短，北向坡面的植物生物量生产仍受到显著限制。除上述微气候效应外，所有实验施加的人为干扰因子均提升了外来植物的入侵成功率。本次观测到的微气候效应表明，外来植物可借助温暖微生境作为"垫脚石"，向该梯度末端的低温区域完成定植。结合人为活动压力来看，该结果预示着寒冷生态系统的植物入侵风险将持续升高——在未来气候变暖叠加持续存在的人为干扰背景下，高山生态系统中的这类温暖微生境或将愈发普遍。