Abstract

Socotra Island, in the western Indian Ocean, harbors high biodiversity and endemism and makes up the largest part of the Socotra archipelago UNESCO World Heritage site. Its climatic, pedological, and geomorphological characteristics, together with the long geological isolation and inaccessibility, led to the flourishing of unique tree diversity, with great cultural and ecological value. Lately, trees on Socotra are facing new threats linked to the abandonment of traditional management practices, climate change impacts, and growing human pressure. This study assesses the relative influence of environmental factors and human activity as drivers of tree cover and density, which can be used to support land management policies for the conservation of this key local resource. Tree cover and density were assessed on a total of 3600 plots of 0.5 ha each, laid on a regular grid covering the entire area of the island through Collect Earth, a free open source software tool that allows an augmented visual interpretation of high-resolution satellite images. A total of 770,000 trees, with an average of 2.14 trees/ha, were estimated on the island. The collected data was validated by ground surveys in 26 plots. The relative influence of selected predictors (seven environmental, two anthropic) on tree cover and density was analyzed through logistic and survival regression. Tree cover and density were found to be impacted positively by steeper slopes, higher rainfall, and marked precipitation seasonality and negatively by relatively warmer temperatures and a higher sand content in the soil. Distance of each plot from the nearest road and settlement was calculated and neither of these anthropic predictors emerged as significant, indicating that the limited number of trees present are not currently as affected by anthropic pressure, but, instead, their presence is mostly limited by the harsh climatic and pedological factors. However, as human population and development increase, periods of drought rise, and extreme and random climatic events intensify, these additional stresses on the environment could negatively impact the availability of natural resources in the terrestrial environment. Lastly, potential recommendations to the existing conservation policies and Socotra Conservation Zoning Plan are discussed based on the results.

Graphic abstract

中文翻译：

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也门索科特拉岛上环境因素和人类活动是树木覆盖和密度的驱动因素

摘要

位于印度洋西部的索科特拉岛（Socotra Island）具有丰富的生物多样性和特有性，是索科特拉群岛联合国教科文组织世界遗产的最大组成部分。它的气候，生态和地貌特征，加上长期的地质隔离和交通不便，导致了独特的树木多样性的繁荣，具有很大的文化和生态价值。最近，索科特拉岛上的树木正面临着与放弃传统管理实践，气候变化影响以及不断增加的人类压力有关的新威胁。这项研究评估了环境因素和人类活动作为树木覆盖率和密度的驱动因素的相对影响，这些影响可用于支持土地管理政策以保护这一重要的本地资源。在总共3600个面积为0.5公顷的土地上评估了树木的覆盖率和密度，通过“收集地球”（一个免费的开放源代码软件工具）将其放置在覆盖整个岛上的规则网格上，该工具可对高分辨率卫星图像进行增强的视觉解释。岛上估计共有770,000棵树，平均每公顷2.14棵树。收集的数据通过对26个样地的地面调查进行了验证。通过逻辑和生存回归分析了选定预测因子（七个环境变量，两个人类变量）对树木覆盖率和密度的相对影响。树木的覆盖率和密度受陡坡，较高的降雨和明显的降水季节的影响，而相对较高的温度和土壤中较高的沙含量则对其产生负面影响。计算了每个地块距最近道路和居民点的距离，并且这些人类预测因子均未显示为重要变量，这表明当前存在的有限数量的树木不受人类压力的影响，但相反，它们的存在主要受到人类活动压力的限制。恶劣的气候和教育因素。但是，随着人口和发展的增加，干旱时期的增加以及极端和随机气候事件的加剧，这些对环境的额外压力可能会对陆地环境中自然资源的可利用性产生负面影响。最后，根据结果讨论了对现有保护政策和《索科特拉保护区划》的潜在建议。这表明目前存在的树木数量不受人类压力的影响，但是它们的存在主要受到恶劣的气候和人为因素的限制。但是，随着人口和发展的增加，干旱时期的增加以及极端和随机气候事件的加剧，这些对环境的额外压力可能会对陆地环境中自然资源的可利用性产生负面影响。最后，根据结果讨论了对现有保护政策和《索科特拉保护区划》的潜在建议。这表明目前存在的树木数量不受人类压力的影响，但是它们的存在主要受到恶劣的气候和人为因素的限制。但是，随着人口和发展的增加，干旱时期的增加以及极端和随机气候事件的加剧，这些对环境的额外压力可能会对陆地环境中自然资源的可利用性产生负面影响。最后，根据结果讨论了对现有保护政策和索科特拉保护区划计划的潜在建议。这些对环境的额外压力可能会对陆地环境中自然资源的可用性产生负面影响。最后，根据结果讨论了对现有保护政策和索科特拉保护区划计划的潜在建议。这些对环境的额外压力可能会对陆地环境中自然资源的可用性产生负面影响。最后，根据结果讨论了对现有保护政策和索科特拉保护区划计划的潜在建议。

图形摘要