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.

当前高山环境相对未受外来植物物种侵扰，尽管近年来外来植物的出现频率与丰度均呈上升趋势。但目前仍未明确：观测到的此类区域低入侵水平，究竟是源于高山带固有的入侵抗性，还是植物入侵的指数级增长仅为时间早晚的问题。本研究针对瑞典亚北极地区两座山脉的南北坡（对应微气候梯度）开展种子添加实验，在高于现有分布上限的海拔处，模拟增强的人为压力（包括干扰、养分添加与繁殖体压力提升），测试6种外来植物的定植情况。研究观测到，累积生长度日（cumulative growing degree days，GDD）存在显著微气候变异，变异范围达500.77℃，标准差为120.70℃；该变异由地形地貌（如坡向）与生物物理特征（如植被覆盖）共同导致，其中生物物理特征因实验干扰发生改变。沿本次研究的微气候梯度，外来植物的定植成功率与生物量生产均与GDD呈正相关关系。不过，尽管生长季内北坡的定植成功率最终追平南坡，但由于生长季更短，北坡的生物量生产受到限制。除上述微气候效应外，实验施加的所有人为因子均提升了外来植物的入侵成功率。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定植跳板，向微气候梯度的寒冷区域扩散。结合人为压力因素，本研究结果预示寒冷生态系统的植物入侵风险将持续升高——在未来气候变暖与人为压力持续共存的背景下，高山生态系统中的这类定植跳板将愈发常见。