Competition suppresses shrubs during early, but not late, stages of arid grassland–shrubland state transition
Competition suppresses shrubs during early, but not late, stages of arid grassland–shrubland state transition
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DOI：
10.1111/1365-2435.13336
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发表时间：
2019-08
影响因子：
5.2
通讯作者：
Nathan A. Pierce;S. Archer;B. Bestelmeyer
中科院分区：
文献类型：
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作者：
Nathan A. Pierce;S. Archer;B. Bestelmeyer
干旱系统中广泛报道的从草向木本植物优势的转变通常归因于干扰机制的变化与非生物反馈的结合，而竞争和促进等生物机制常常被忽视。然而，半干旱和半湿润稀树草原的研究表明，生物相互作用是处于状态转换风险的系统的重要驱动因素。我们试图弥合这一鸿沟，分别在草地-灌木状态转换的早期和晚期通过实验操纵草-灌木和灌木-灌木相互作用，并评估这些相互作用可能影响干旱土地状态转换动态的程度。目标牧豆灌木周围的草或同种邻居保持完整或杀死与叶面除草剂，和植物性能的指标进行了多年的监测灌木与和没有草或灌木邻居。在降水量高于平均水平的年份，小灌木的生产力通过清除草而提高，这一结果在较大的灌木或干旱年份并不明显。基于最近邻度量的代理证据表明灌木-灌木竞争在起作用，但我们的实验操作显示没有这种影响。来自草的竞争似乎减弱了灌木达到以自我增强的方式改变物理环境所需的大小的速度，但仅在灌木入侵的早期阶段。我们的研究结果进一步表明，在草地到灌木地状态转换的后期，灌木-灌木竞争不会减缓灌木扩张的速度，并表明最大灌木覆盖率是由密度依赖机制以外的因素调节的。我们的结论是，草对灌木的影响应包括在评估荒漠草原状态转换概率和速率，和干旱生态系统中的荒漠化模型，传统上集中在干扰和非生物反馈应扩大到包括空间和时间变化的竞争效应。本文提供了一个简单的语言摘要。
Transitions from grass to woody plant dominance, widely reported in arid systems, are typically attributed to changes in disturbance regimes in combination with abiotic feedbacks, whereas biotic mechanisms such as competition and facilitation are often overlooked. Yet, research in semi‐arid and subhumid savannas indicates that biotic interactions are important drivers in systems at risk for state transition. We sought to bridge this divide by experimentally manipulating grass‐on‐shrub and shrub‐on‐shrub interactions in early and late stages of grassland–shrubland state transition, respectively, and to assess the extent to which these interactions might influence arid land state transition dynamics. Target Prosopis glandulosa shrubs had surrounding grasses or conspecific neighbours left intact or killed with foliar herbicide, and metrics of plant performance were monitored over multiple years for shrubs with and without grass or shrub neighbours. Productivity of small shrubs was enhanced by grass removal in years with above‐average precipitation, a result not evident in larger shrubs or during dry years. Proxy evidence based on nearest neighbour metrics suggested shrub–shrub competition was at play, but our experimental manipulations revealed no such influence. Competition from grasses appears to attenuate the rate at which shrubs achieve the size necessary to modify the physical environment in self‐reinforcing ways, but only during the early stages of shrub encroachment. Our results further suggest that at late stages of grassland‐to‐shrubland state transitions, shrub–shrub competition will not slow the rate of shrub expansion, and suggest that maximum shrub cover is regulated by something other than density‐dependent mechanisms. We conclude that grass effects on shrubs should be included in assessments of desert grassland state transition probabilities and rates, and that desertification models in arid ecosystems that traditionally focus on disturbance and abiotic feedbacks should be broadened to incorporate spatial and temporal variations in competitive effects. A plain language summary is available for this article.