Woody plants in water‐limited ecosystems affect their environment on multiple scales: locally, natural stands can create islands of fertility for herb layer communities compared to open habitats, but afforestation has been shown to negatively affect regional water balance and productivity. Despite these contrasting observations, no coherent multiscale framework has been developed for the environmental effects of woody plants in water‐limited ecosystems. To link local and regional effects of woody species in a spatially explicit model, we simultaneously measured site conditions (microclimate, nutrient availability and topsoil moisture) and conditions of regional relevance (deeper soil moisture), in forests with different canopy types (long, intermediate and short annual lifetime) and adjacent grasslands in sandy drylands. All types of forests ameliorated site conditions compared to adjacent grasslands, although natural stands did so more effectively than managed ones. At the same time, all forests desiccated deeper soil layers during the vegetation period, and the longer the canopy lifetime, the more severe the desiccation in summer and more delayed the recharge after the active period of the canopy. We conclude that the site‐scale environmental amelioration brought about by woody species is bound to co‐occur with the desiccation of deeper soil layers, leading to deficient ground water recharge. This means that the cost of creating islands of fertility for sensitive herb layer organisms is an inevitable negative impact on regional water balance. The canopy type or management intensity of the forests affects the magnitude but not the direction of these effects. The outlined framework of the effects of woody species should be considered for the conservation, restoration or profit‐oriented use of forests as well as in forest‐based carbon sequestration and soil erosion control projects in water‐limited ecosystems.

中文翻译：

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生育岛下面的地下沙漠？木本物种使沙质旱地的土壤下层干燥

缺水生态系统中的木本植物会在多个方面影响其环境：与开放的栖息地相比，本地的天然林分能够为草本层群落创造肥沃的岛屿，但事实表明，造林对区域的水平衡和生产力产生负面影响。尽管有这些相反的观察结果，但尚未开发出针对水受限生态系统中木本植物对环境影响的连贯多尺度框架。为了在空间明确的模型中链接木本物种的局部和区域效应，我们在不同冠层类型的森林（长，中度）中同时测量了场地条件（微气候，养分有效性和表土湿度）和区域相关性条件（更深的土壤湿度）。且寿命短）和沙质旱地附近的草地。与天然草地相比，所有类型的森林都比邻近的草地改善了立地条件，尽管天然林比人工林更有效。同时，所有森林在植被生长期都使土壤更深层干燥，冠层寿命越长，夏季的干燥作用越严重，在冠层活动期后补给时间就越延迟。我们得出的结论是，木本物种带来的场地规模的环境改善必然与更深层土壤的干燥同时发生，从而导致地下水补给不足。这意味着为敏感的药草层生物建立肥沃的岛屿的成本对区域水平衡具有不可避免的负面影响。森林的冠层类型或经营强度会影响这些影响的大小，但不会影响这些影响的方向。