Small-scale switch in cover-perimeter relationships of patches indicates shift of dominant species during grassland degradation

Aims Grasslands are globally threatened by climate changes and unsustainable land-use,which often cause transitions among alternative stable states,and even catastrophic transition to desertification.Spatial vegetation patch configurations have been shown to signify such transitions at large spatial scale.Here,we demonstrate how small-scale patch configurations can also indicate state transitions.Methods The whole spatial series of degradation successions were chosen in alpine grasslands characterized as seven typical communities.Patch numbers,and perimeter and cover of each patch were recorded using adjacent quadrats along transects in each type of the communities.Species abundance within each patch was measured.Important Findings Across seven grazing-induced degradation stages in the world’s largest expanse of grassland,from dense ungrazed turf to bare black-soil crust,patch numbers and perimeters first increased as patch cover decreased.Numbers and perimeters then decreased rapidly beyond an intersection point at 68%of initial continuous vegetation cover.Around this point,the vegetation fluctuated back and forth between the sedge-dominated grassland breaking-up phase and the forb-dominated phase,suggesting impending shift of grassland state.This study thus demonstrates how ground-based small-scale vegetation surveys can provide a quantitative,easy-to-use signals for vegetation degradation,with promise for detecting the catastrophic transition to desertification.

草地生态系统普遍经受着全球气候变化和不可持续的土地利用方式的胁迫,这些环境胁迫往往引起草地生态系统状态在多稳态间跃变,甚至诱发灾变性跃变,直至荒漠化的发生。研究表明,植被斑块的空间构型可以指示较大空间尺度的生态系统状态跃变。本文展示了小尺度斑块构型如何指示草地生态系统的状态跃变。在高寒草甸生态系统选取7种草甸群落类型作为研究对象,这7种类型的草甸群落具有高寒草甸退化过程不同阶段的典型群落特征。在每一种群落类型内,采用样带方法进行植被斑块调查,沿样带采用邻接格子样方法记录样方内的斑块数量、斑块周长和斑块面积,并测定了每个斑块内的物种多度。研究结果表明,沿着草甸退化阶段七种类型的草甸群落(即未退化的禾草-嵩草群落、轻度退化的高山嵩草-禾草群落、中度退化的高山嵩草群落、重度退化草皮开裂的高山嵩草群落、侵蚀期的高山嵩草-杂草群落、严重侵蚀期的杂草-高山嵩草群落、零星杂草的黑土滩裸地),斑块数量和斑块周长随斑块面积的下降呈现先增加的变化趋势,而后,随斑块面积的急剧降低呈下降的趋势。斑块数量、周长与面积关系变化的拐点出现在植被总面积约占68%的状态处。在这一拐点附近,斑块上的植物组成呈现出高山嵩草或杂草占优势两种状态往复波动的不稳定状态,表明草地状态处于向荒漠化裸地跃变的边缘。本研究展示了如何基于小尺度植被斑块空间构型为草地退化、状态跃变提供数量化的早期预警信号,有望为嵩草草甸退化至黑土滩裸地的早期预警提供判定指标。