Tree Cover in Mountain Regions
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The mountains dataset has been overlaid with a global data set on percent tree cover taken from MODIS 1-km resolution percent tree cover data, courtesy of University of Maryland Global Land Cover Facility. Species richness, density and forest height tend to reduce with increasing altitude; the boundary between forest vegetation and more open ground cover at higher elevation 'the treeline' is an ecological marker signifying the transition to more extreme climatic conditions. Purpose: To provide a global context for a discussion of mountain forests, it is first necessary to define the locations and types of mountain forests, and this in turn requires a definition of mountains or mountain areas. Altitude and slope and the environmental gradients they generate are key components of such a definition, but their combination is problematic. Simple altitude thresholds both exclude older and lower mountain systems and include areas of relatively high elevation that have little topographic relief and few environmental gradients. Using slope as a criterion on its own or in combination with altitude can resolve the latter problem, but not the former. The mountains dataset shows the location of mountain land estimated from a digital elevation model using criteria based on elevation alone (the upper three classes: > 2 500 metres) and at lower elevation, on a combination of elevation, slope and local elevation range. This is an update of the Mountain's of the World 2000 and was produced for the UNEP-WCMC publication Mountain Watch, 2002.
本山地数据集叠加了一套全球林木覆盖率数据集，该数据集源自马里兰大学全球土地覆盖设施（University of Maryland Global Land Cover Facility）提供的中分辨率成像光谱仪（MODIS）1千米分辨率林木覆盖率数据。物种丰富度、种群密度与森林高度通常随海拔升高而降低；高海拔区域森林植被与更为开阔的地表覆被之间的分界——林木线（treeline）——是表征向极端气候条件过渡的生态标志。研究目的：为开展山地森林相关讨论提供全球语境，首先需明确山地森林的分布位置与类型，而这又依赖于对山地或山地范围的界定。海拔、坡度及其所催生的环境梯度是此类界定的核心要素，但将二者结合却存在难题。单纯采用海拔阈值的判定方式，既会将古老低矮的山地系统排除在外，又会纳入那些海拔相对较高但地形起伏极小、环境梯度匮乏的区域。单独以坡度作为判定标准，或结合海拔与坡度进行判定，虽可解决后者问题，却无法解决前者。本山地数据集展示了基于数字高程模型（digital elevation model）估算得到的山地分布：仅以海拔为判定标准时，涵盖海拔高于2500米的最高三个等级；低海拔区域则结合海拔、坡度与局地高程变幅进行判定。本数据集为《2000年世界山地》的更新版本，系为联合国环境规划署-世界保护监测中心（UNEP-WCMC）2002年发布的《山地观察》报告所制作。
