The rich sides of mountain summits — a pan-European view on aspect preferences of alpine plants

AIM: In the alpine life zone, plant diversity is strongly determined by local topography and microclimate. We assessed the extent to which aspect and is related to temperature affect plant species diversity, and the colonisation and disappearance of species on alpine summits on a pan-European scale. Location: Mountain summits in Europe’s Alpine Life Zone. Methods: Vascular plant species and their percentage cover were recorded in permanent plots in each cardinal direction on 123 summits in 32 regions across Europe. For a subset from 17 regions, resurvey data and 6-year soil temperature series were available. Differences in temperature sum and Shannon index as well as species richness, colonisation and disappearance of species among cardinal directions were analysed using linear mixed-effects and generalised mixed-effects models, respectively. Results: Temperature sums were higher in East and South-facing aspects than in the north-facing ones, while the West-facing ones were intermediate; differences were smallest in northern Europe. The patterns of temperature sums among aspects were consistent among years. In temperate regions, thermal differences were reflected by plant diversity, whereas this relationship was weaker or absent on Mediterranean and boreal mountains. Colonisation of species was positively related to temperature on Mediterranean and temperate mountains, whereas disappearance of species was not related to temperature. Main conclusions: Thermal differences caused by solar radiation determine plant species diversity on temperate mountains. Advantages for plants on eastern slopes may result from the combined effects of a longer diurnal period of radiation due to convection cloud effects in the afternoon and the sheltered position against the prevailing westerly winds. In northern Europe, long summer days and low sun angles can even out differences among aspects. On Mediterranean summits, summer drought may limit species numbers on the warmer slopes. Warmer aspects support a higher number of colonisation events. Hence, aspect can be a principal determinant of the pace of climate-induced migration processes.研究目的：在高山生物带（alpine life zone）中，植物多样性主要受局地地形与微气候调控。本研究旨在探究坡向及其与温度的关联对植物物种多样性的影响程度，以及泛欧洲尺度下高山峰顶的物种定植与消失动态。 研究区域：欧洲高山生物带内的山地峰顶。 研究方法：在欧洲32个区域的123座峰顶的四个正方位（cardinal direction）上设置永久样地，记录维管植物（vascular plant）物种及其盖度百分比。其中17个区域的样地配有重复调查数据与6年的土壤温度序列。分别采用线性混合效应模型（linear mixed-effects model）与广义线性混合效应模型（generalised mixed-effects model），分析不同正方位间的积温（temperature sum）、香农指数（Shannon index）、物种丰富度以及物种定植与消失情况的差异。 研究结果：东向与南向坡向的积温高于北向坡向，西向坡向居中；欧洲北部的方位间积温差异最小。不同年份间的方位积温格局保持一致。在温带区域，热环境差异与植物多样性呈显著对应关系，但在地中海与寒带山地（boreal mountains）中，这一关系较弱或不存在。在地中海与温带山地中，物种定植与温度呈正相关，而物种消失与温度无显著关联。 主要结论：太阳辐射引发的热环境差异决定了温带山地的植物物种多样性。东坡的植物生长优势可能源于两个因素的共同作用：一是午后对流云效应带来的更长日照时长，二是可抵御盛行西风的遮蔽位置。在欧洲北部，夏季昼长与低太阳高度角抹平了不同坡向间的热环境差异。在地中海峰顶上，夏季干旱可能限制了暖坡的物种数量。暖向坡支持更多的物种定植事件。因此，坡向可成为气候诱导的物种迁移进程速率的主要决定因素。