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Spatial vegetation patterns and imminent desertification in Mediterranean arid ecosystems

机译：地中海干旱生态系统中的空间植被格局和即将到来的荒漠化

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干旱生态系统占地球土地面积的约40％，是20多亿人生活的家园，但 它们在气候变化和人类行动面前仍然很脆弱。Kefi等人利用数值模型 及来自西班牙、摩洛哥和希腊的地中海生态系统的数据发现，植被斑 块大小分布遵从一个“幂次法则”。随着动物吃草对植被造成的压力 的增大，斑块大小分布将偏离该“幂次法则”，接近向荒漠条件过渡 的状态。所以，斑块大小分布可能是荒漠化的一个有用的预警。本期 封面所示为西班牙Belchite的El Planeron自然保护区的一个干旱景观 (在封面照片的上部)，下部所示为这一景观退化的情况。在另一篇 论文中，Scanlon等人用卫星影像表明，卡拉哈里盆地的树丛大小分 布也遵从一个无标度(scale—free)“幂次法则”。这种现象可以用与现 有树木附近的有利环境相关的正反馈来解释。在一篇News & Views文 章中，Ricard Sole对这两篇论文都进行了讨论。%Humans and climate affect ecosystems and their services, which may involve continuous and discontinuous transitions from one stable state to another. Discontinuous transitions are abrupt, irreversible and among the most catastrophic changes of ecosystems identified1. For terrestrial ecosystems, it has been hypothesized that vegetation patchiness could be used as a signature of imminent transitions. Here, we analyse how vegetation patchiness changes in arid ecosystems with different grazing pressures, using both field data and a modelling approach. In the modelling approach, we extrapolated our analysis to even higher grazing pressures to investigate the vegetation patchiness when desertification is imminent. In three arid Mediterranean ecosystems in Spain, Greece and Morocco, we found that the patch-size distribution of the vegetation follows a power law. Using a stochastic cellular automaton model, we show that local positive interactions among plants can explain such power-law distributions. Furthermore, with increasing grazing pressure, the field data revealed consistent deviations from power laws. Increased grazing pressure leads to similar deviations in the model. When grazing was further increased in the model, we found that these deviations always and only occurred close to transition to desert, independent of the type of transition, and regardless of the vegetation cover. Therefore, we propose that patch-size distributions may be a warning signal for the onset of desertification.
机译：干旱生态系统占地球土地面积的约40％，是20多亿人生活的家园，但 它们在气候变化和人类行动面前仍然很脆弱。Kefi等人利用数值模型 及来自西班牙、摩洛哥和希腊的地中海生态系统的数据发现，植被斑 块大小分布遵从一个“幂次法则”。随着动物吃草对植被造成的压力 的增大，斑块大小分布将偏离该“幂次法则”，接近向荒漠条件过渡 的状态。所以，斑块大小分布可能是荒漠化的一个有用的预警。本期 封面所示为西班牙Belchite的El Planeron自然保护区的一个干旱景观 (在封面照片的上部)，下部所示为这一景观退化的情况。在另一篇 论文中，Scanlon等人用卫星影像表明，卡拉哈里盆地的树丛大小分 布也遵从一个无标度(scale—free)“幂次法则”。这种现象可以用与现 有树木附近的有利环境相关的正反馈来解释。在一篇News & Views文 章中，Ricard Sole对这两篇论文都进行了讨论。%Humans and climate affect ecosystems and their services, which may involve continuous and discontinuous transitions from one stable state to another. Discontinuous transitions are abrupt, irreversible and among the most catastrophic changes of ecosystems identified1. For terrestrial ecosystems, it has been hypothesized that vegetation patchiness could be used as a signature of imminent transitions. Here, we analyse how vegetation patchiness changes in arid ecosystems with different grazing pressures, using both field data and a modelling approach. In the modelling approach, we extrapolated our analysis to even higher grazing pressures to investigate the vegetation patchiness when desertification is imminent. In three arid Mediterranean ecosystems in Spain, Greece and Morocco, we found that the patch-size distribution of the vegetation follows a power law. Using a stochastic cellular automaton model, we show that local positive interactions among plants can explain such power-law distributions. Furthermore, with increasing grazing pressure, the field data revealed consistent deviations from power laws. Increased grazing pressure leads to similar deviations in the model. When grazing was further increased in the model, we found that these deviations always and only occurred close to transition to desert, independent of the type of transition, and regardless of the vegetation cover. Therefore, we propose that patch-size distributions may be a warning signal for the onset of desertification.

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