Analyse der geoökologischen Steuerungsfaktoren für die Verbreitung von Waldstandorten und diskontinuierlichem Permafrost unter den Einflüssen von Waldbränden, Waldnutzung und Klimaentwicklung in den Waldsteppen der zentralen Mongolei
Zusammenfassung der Projektergebnisse
In this project, we aimed to identify geoecological and anthropogenic factors controlling spatiotemporal patterns of forest distribution, disturbance and post-disturbance forest regrowth in the forest-steppe of central Mongolia. The related key question of our research was, which roles relief, permafrost, climate, soils, and anthropogenic activities play in this system. Since forest patches in the central-Mongolian forest-steppe are generally limited to north-facing slopes, our research focused on these. Based on our central research question, we derived six hypotheses (H1-H6) to be tested through a multi-method approach, combining remote sensing, fieldwork, laboratory analyses, and statistics. H1: Patterns of permafrost and permafrost-table depth follow certain rules, and can thus be assessed and also modelled, based on high-resolution digital data. This hypothesis was confirmed, whereby especially the Normalized Difference Vegetation Index (NDVI) turned out to be helpful for this purpose, as it reflects the vitality of forest vegetation, and of the steppe vegetation downslope, below the respective forest patch. We found that enhanced NDVI of the downslope steppe vegetation indicates the presence of permafrost in the above forest, as permafrost leads to increased soil moisture in the forest as well as downslope. H2: Permafrost, together with soil properties, has a major effect on the spatiotemporal distribution of soil moisture in forests and thus on tree vitality and forest structure. This hypothesis was generally verified, although it turned out that permafrost is not the decisive factor for post-disturbance forest regrowth. Water availability is the key factor for forest distribution, susceptibility to forest fire and post-disturbance forest regrowth. Water availability in turn is strongly controlled by soil texture and permafrost distribution. Even small differences in soil texture that result in different plant-available field capacity of soils can be decisive for post-disturbance tree regrowth. As the seasonal ice above the permafrost progressively melts down during summer, the released meltwater provides additional soil moisture. The underlying permafrost table prevents the meltwater from deep infiltration, thus keeping it within the root zone and in this way supporting the vegetation. H3: Logging and fire substantially alter soil functions, the state of permafrost and the active layer. Indeed, we observed that selective logging and non-lethal forest fire lead to lowering of the permafrost table, whereas clear-cutting and severe forest fires lead to permafrost degradation. Permafrost was absent at all studied disturbed forest sites, both with and without tree regrowth. Thus, the presence of permafrost cannot be a precondition for post-disturbance tree regrowth. H4: Altered soil properties, resulting from forest fire, directly affect the direction and temporal course of vegetation succession after forest destruction by fire. We found that post-disturbance tree regrowth depends mainly on soil texture, which is not affected by forest fire. Loamy soils can store enough water for forest recovery, whereas sandy soils mostly do not store enough water for tree regrowth under the present climatic conditions. Forest regeneration is also favoured by high hydraulic conductivity in the uppermost soil horizons, as rapid percolation through the upper soil horizons reduces evaporation loss. In addition, trees benefit from rapid water percolation into the subsoil, as it reduces competition for water with grasses and herbs. H5: The past - somewhat more humid and/or cooler - climate allowed for post-disturbance tree regrowth and permafrost preservation, whereas under the present climatic conditions, permafrost can only partially regenerate, which leads to impeded tree regrowth after disturbance. Investigation of sedimentological archives in our study area showed that forest expansion took place during the early to mid-Holocene “climatic optimum”, which most likely was more humid than today. Thus, soil moisture was sufficient to support forest growth, even without permafrost. Indicators in vertical sediment successions (e.g., solifluction layers and cryoturbation features) point to permafrost formation later on. During late Holocene, frequent fires, permafrost thawing, and enhanced erosion occurred. Under the present conditions, permafrost disappears after forest fire and does not generally regenerate after canopy closure of the recovering forest. This happens only in places in the mountains. Permafrost distribution in the forest-steppe is in transition from climate- and ecosystem-triggered (active formation) to ecosystem-preserved (passive) under the present climate warming. H6: Both, at present and in the past, there have been periods in which climatic conditions led to an enhanced risk of forest fire. Until now, we did not detect a clear periodicity in charcoal abundance in sediment sequences that would allow for identifying distinct dry periods with enhanced fire frequency in the past or present. This may be due to an insufficient dataset size. Alternatively, it seems also plausible that the frequency of forest fires is more strongly affected by human activity than by periods of dry conditions. Thus, the present state of research does not allow for a definite verification of hypothesis H6.
Projektbezogene Publikationen (Auswahl)
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(2019): Spatial pattern of Late Glacial and Holocene climatic and environmental development in Western Mongolia - A critical review and synthesis. In: Quaternary Science Reviews 210, S. 26–50
Klinge M, Sauer D
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(2020): Geoecological analysis of forest distribution and post-disturbance tree regrowth in the forest-steppe of central Mongolia. PhD Thesis, University of Göttingen
Schneider F
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(2020): Hydrological soil properties control tree regrowth after forest disturbance in the forest-steppe of central Mongolia. SOIL Discuss
Schneider F, Klinge M, Brodthuhn J, Peplau T, Sauer D
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(2020): Modelled potential forest area in the forest-steppe of central Mongolia is about three times of actual forest area, Biogeosciences Discuss
Klinge M, Dulamsuren C, Schneider F, Erasmi S, Hauck M, Bayarsaikhan U, Sauer D
