Hydrological soil properties control tree regrowth after forest disturbance in the forest-steppe of central Mongolia

Abstract

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.

Holocene vegetation reconstruction in the forest–steppe of Mongolia based on leaf waxes and macro-charcoals in soils

“…The semi-arid conditions promote frequent forest fires in this region, which occur simultaneously with droughts and are often caused by human fire setting (Nyamjav et al, 2007;Hessl et al, 2016). Due to these conditions, tree regrowth on burned sites strongly depends on soil hydrological properties and occurs irregularly (Schneider et al, 2021). Forest distribution in the study area since 9.5 ka has been shown by Unkelbach et al (2021), and fire intensity increased after 4.5 ka (Klinge et al, 2022).…”

Fires, Vegetation, and Human—The History of Critical Transitions During the Last 1000 Years in Northeastern Mongolia

“…The Khentii Mountains range is covered by a boreal forest and forest-steppe mosaic with discontinuous permafrost (Kopp et al, 2017) and has a diverse vegetation distribution. The main factors for influencing this variation are the exposition of the slopes and the associated occurrence of permafrost (Etzelmüller et al, 2006;Ishikawa et al, 2018;Schneider et al, 2021). Patches of permafrost can be found on the northern slopes and valley bottoms, and they play an important hydrological/ecological role in regulating local humidity and climatic conditions.…”

“…It is reasonable to presume that the long-term rise in temperature and changes in precipitation resulted in significant shifts in the structure of the vegetation and an increase in the thickness of the active layer over permafrost (Hiyama et al, 2021). Severe fires have undoubtedly played a great role in these changes (Klinge et al, 2021a;Schneider et al, 2021). Northern Mongolia (the Khentii Mountains region) is on the southern margin of the boreal forest zone (Dashtseren et al, 2014), which is the second-largest contiguous forest zone (approximately 30% of the world's forested area) in the world and stores around 20% of the world's terrestrial carbon sink (Kuuluvainen and Gauthier, 2018).…”

The reconstruction of Holocene northwestern Mongolian fire history based on high-resolution multi-site macro-charcoal analyses

“…The climate in both areas is highly continental due to their situation within Central Asia and their alpine conditions. Mean annual temperatures are −0.9 °C (Ulgii, Mongolian Altai) and −5.9 °C (Tosontsengel, northern Khangai), and the mean annual precipitation ranges from 150 to 500 mm, reaching its maximum in the short summer period (Schneider et al, 2021;AM Online Projects, 2022).…”