Abstract. The central Mongolian forest-steppe is a sensitive ecotone, commonly affected by disturbances such as logging and forest fires. In addition, 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. 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 hypothesized that differences in hydrological soil properties 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 in central Mongolia. We distinguished 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). 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 than soils under DNOR. In addition, hydraulic conductivity was higher in the uppermost horizons of soils under DWIR compared to soils 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 soil hydrological properties. 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 evaporation loss and competition of larch saplings with grasses and herbs for water. Another important factor is human impact, particularly grazing on cleared forest sites, which often keeps seedlings from growing and thus inhibits forest recovery. Permafrost was absent at all studied disturbed sites (DWIR, DNOR). We thus conclude that it is not a major factor for the post-disturbance tree-regrowth pattern, although it does contribute to water availability in summer.

Abstract. 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.

<p>The distribution of forest patches in the foreststeppe of central Mongolia reflects the interplay of several environmental factors that together control the vegetation pattern of the landscape. Since the mean annual precipitation of this semiarid area rarely exceeds 300 mm, the existence of forest strongly depends on the hydrological properties of the system. Only north-facing slopes provide suitable conditions for the growth of larch trees (Larix sibirica Ledeb.) due to their reduced evapotranspiration. Plains and south-facing slopes are covered by open steppe.However, after disturbance of the forest patches by fire, the regrowth of larch trees does not proceed equally in all areas. During fieldworkat the northern edge of the Khangai Mountains, we identified areas that seemed to havesimilar site conditions but neverthelessshowed different regrowth of larch trees after fire, ranging from intensive regrowth to no regrowth at all.Thisobservation stimulated us to carry out a comprehensive study of soils, vegetation and landscape development in field campaigns in 2017 and 2018, followed by laboratory analyses of soil samples.Through this work, we aimed at identifying the role of soil hydrology for forest succession in this sensitive ecotone.</p><p>We described and sampled 57 soil profiles, including sites (i) under forest, (ii) under steppe, (iii) on sites with succession after forest fire, (iv) on sites without succession after forest fire. In the field, we carried out measurements of water conductivity (by use of a compact constant head permeameter). In the laboratory, we analyzed particle size distribution and carried outkfand pF measurements.</p><p>These analyses showed that the dominant grain size of the soils was sand, whereby soils with forest regrowth had slightly loamier texture than those without regrowth. We concluded that already slightly loamier texture may be important for water storage during dry periods and thus for forest regrowth.Soils with forest regrowth had higher hydraulic conductivity in the first 25 cm and lower conductivity below.Soils without forest regrowth showeda reverse depth pattern of hydraulic conductivity. We concluded that quick drainage through the upper horizons supports forest regrowth, as it reduces competition for water with grass roots in the upper part of the soil.Soils with forest regrowth hadgreater plant-available water capacity than those without regrowth. We conclude that under the given climatic conditions, storage of plant-available water is a key factor for regrowth / no regrowth of forest after disturbance.</p>

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