The central Mongolian forest steppe forms a transition between different ecozones and is as such particularly sensitive to environmental changes. It is commonly affected by disturbances such as logging and forest fires. Intensified drought events aggravate stress on the trees that are anyway at their drier limit in the forest steppe. Climate change increases evapotranspiration and reduces the distribution of discontinuous permafrost, which leads to drier soil conditions. The motivation for this study came about through our previous observation that forest stands show great differences with respect to their recovery after disturbance by fire or logging. Sometimes, no regrowth of trees takes place at all. As water availability is the main limiting factor of forest growth in this region, we hypothesised that differences in soil hydrology control the forest recovery pattern.To test this hypothesis, we analysed soil properties under forests, predominantly consisting of Siberian larch (Larix sibirica Ledeb.), in the forest steppe of the northern Khangai Mountains, central Mongolia. We distinguished the following four vegetation categories: (1) near-natural forest (FOR), (2) steppe close to the forest (STE), (3) disturbed forest with regrowth of trees (DWIR), and (4) disturbed forest showing no regrowth of trees (DNOR). A total of 54 soil profiles were described in the field and sampled for soil chemical, physical, and hydrological analysis. We found a significant difference in soil texture between soils under DWIR and DNOR. Sand generally dominated the soil texture, but soils under DWIR had more silt and clay compared to soils under DNOR. Soil pF curves showed that soils under DWIR had higher plant-available field capacity in their uppermost parts than soils under DNOR. In addition, hydraulic conductivity tended to be higher in the uppermost horizons of soils under DWIR compared to their counterparts under DNOR. Chemical properties of the soils under DWIR and DNOR showed no significant differences.We conclude that the differences in post-disturbance tree regrowth are mainly caused by different soil hydrology. High plant-available field capacity is the key factor for forest recovery under semi-arid conditions. High hydraulic conductivity in the uppermost soil horizons can further support tree regrowth because it reduces the evaporation loss and the competition of larch saplings with grasses and herbs for water. Another important factor is human impact, particularly grazing livestock on cleared forest sites, which often keeps seedlings from growing and, thus, inhibits forest recovery. None of the disturbed sites (DWIR and DNOR) had permafrost. We, thus, conclude that permafrost is no major factor for the post-disturbance tree regrowth pattern, although it generally supports tree growth in the forest steppe by preventing meltwater from seasonal ice from seeping below the root zone, thus increasing the water supply in summer.

中文翻译：

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蒙古中部森林草原森林干扰后水文土壤特性控制树木再生

蒙古中部森林草原形成了不同生态区之间的过渡，因此对环境变化特别敏感。它通常受到诸如伐木和森林火灾等干扰的影响。加剧的干旱事件加剧了森林草原中处于干燥极限的树木的压力。气候变化增加了蒸发量并减少了不连续永久冻土的分布，从而导致土壤条件更加干燥。这项研究的动机来自我们之前的观察，即林分在火灾或伐木干扰后的恢复方面表现出很大差异。有时，树木根本不会再生。由于水资源供应是该地区森林生长的主要限制因素，西伯利亚落叶松。)，在蒙古中部康盖山脉北部的森林草原上。我们区分了以下四种植被类别：(1) 近天然林 (FOR)，(2) 靠近森林的草原 (STE)，(3) 树木再生的受干扰森林 (DWIR)，以及 (4) 受干扰森林显示没有树木再生（DNOR）。在田间共描述了 54 个土壤剖面，并取样进行了土壤化学、物理和水文分析。我们发现 DWIR 和 DNOR 下土壤之间的土壤质地存在显着差异。沙子通常在土壤质地中占主导地位，但与 DNOR 下的土壤相比，DWIR 下的土壤含有更多的淤泥和粘土。土壤p ˚F曲线显示，DWIR 下的土壤在其最上部具有比 DNOR 下的土壤更高的植物可用田间容量。此外，与 DNOR 下的对应物相比，DWIR 下土壤最上层的导水率往往更高。DWIR和DNOR下土壤的化学性质没有显着差异。我们得出结论，扰动后树木再生的差异主要是由不同的土壤水文引起的。高植物可用田间容量是半干旱条件下森林恢复的关键因素。最上层土壤的高导水率可以进一步支持树木的再生，因为它减少了蒸发损失和落叶松树苗与草和草本植物对水的竞争。另一个重要因素是人为影响，尤其是在清理过的林地放牧牲畜，这通常会阻止幼苗生长，从而抑制森林恢复。受干扰的地点（DWIR 和 DNOR）都没有永久冻土。因此，我们得出结论，永久冻土不是扰乱后树木再生模式的主要因素，尽管它通常通过防止季节性冰的融水渗入根区下方来支持森林草原中的树木生长，从而增加夏季的供水.