Twenty Years After Decollectivization: Mobile Livestock Husbandry and Its Ecological Impact in the Mongolian Forest-Steppe

The socioeconomics and the ecological impact of nomadic pastoralism were analyzed using interviews with 87 herder families and secondary information in the western Khangai, Mongolia. The pastoralists had an income above the national average for rural areas in Mongolia. Most herders continued traditional seasonal migration patterns, which involved ca. 10 moves per year over a total distance of ca. 100 km between summer and winter grazing grounds. As elsewhere in Mongolia, the number of goats owned by herders has greatly increased and cashmere has become the main source of cash income. Total livestock numbers rose considerably after decollectivization of the livestock sector in 1992, but in recent years have periodically been reduced by harsh winters, often combined with drought. Due to the high economic risk of these periodic livestock losses, many herders invest in better education for their children to enable them to migrate to the cities.     

Higher climate warming sensitivity of Siberian larch in small than large forest islands in the fragmented Mongolian forest steppe

Forest fragmentation has been found to affect biodiversity and ecosystem functioning in multiple ways. We asked whether forest size and isolation in fragmented woodlands influences the climate warming sensitivity of tree growth in the southern boreal forest of the Mongolian Larix sibirica forest steppe, a naturally fragmented woodland embedded in grassland, which is highly affected by warming, drought, and increasing anthropogenic forest destruction in recent time. We examined the influence of stand size and stand isolation on the growth performance of larch in forests of four different size classes located in a woodland‐dominated forest‐steppe area and small forest patches in a grassland‐dominated area. We found increasing climate sensitivity and decreasing first‐order autocorrelation of annual stemwood increment with decreasing stand size. Stemwood increment increased with previous year's June and August precipitation in the three smallest forest size classes, but not in the largest forests. In the grassland‐dominated area, the tree growth dependence on summer rainfall was highest. Missing ring frequency has strongly increased since the 1970s in small, but not in large forests. In the grassland‐dominated area, the increase was much greater than in the forest‐dominated landscape. Forest regeneration decreased with decreasing stand size and was scarce or absent in the smallest forests. Our results suggest that the larch trees in small and isolated forest patches are far more susceptible to climate warming than in large continuous forests pointing to a grim future for the forests in this strongly warming region of the boreal forest that is also under high land use pressure.     

Carbon pool densities and a first estimate of the total carbon pool in the Mongolian forest‐steppe

The boreal forest biome represents one of the most important terrestrial carbon stores, which gave reason to intensive research on carbon stock densities. However, such an analysis does not yet exist for the southernmost Eurosiberian boreal forests in Inner Asia. Most of these forests are located in the Mongolian forest‐steppe, which is largely dominated by Larix sibirica. We quantified the carbon stock density and total carbon pool of Mongolia's boreal forests and adjacent grasslands and draw conclusions on possible future change. Mean aboveground carbon stock density in the interior of L. sibirica forests was 66 Mg C ha^?1, which is in the upper range of values reported from boreal forests and probably due to the comparably long growing season. The density of soil organic carbon (SOC, 108 Mg C ha^?1) and total belowground carbon density (149 Mg C ha^?1) are at the lower end of the range known from boreal forests, which might be the result of higher soil temperatures and a thinner permafrost layer than in the central and northern boreal forest belt. Land use effects are especially relevant at forest edges, where mean carbon stock density was 188 Mg C ha^?1, compared with 215 Mg C ha^?1 in the forest interior. Carbon stock density in grasslands was 144 Mg C ha^?1. Analysis of satellite imagery of the highly fragmented forest area in the forest‐steppe zone showed that Mongolia's total boreal forest area is currently 73 818 km², and 22% of this area refers to forest edges (defined as the first 30 m from the edge). The total forest carbon pool of Mongolia was estimated at ~ 1.5?1.7 Pg C, a value which is likely to decrease in future with increasing deforestation and fire frequency, and global warming.     

Ecological stoichiometry explains larger-scale facilitation processes by shrubs on species coexistence among understory plants

Plant facilitation (positive plant–plant interactions) strongly influences biodiversity, structure, and dynamics in plant communities, and the topic has received considerable attention among ecologists. Most studies of facilitation processes by shrubs have been conducted at small spatial scales between shrubs and their neighboring species. Yet, we know little about whether facilitation processes by shrubs at a small scale (i.e., a patch scale) also work at a larger scale (i.e., a site scale) in terms of the maintenance of biodiversity. Here, we report that the facilitative effects of shrubs on plant diversity at a larger scale can be explained by changing ecological stoichiometry. The soil fertility showed unimodal shape along shrub cover gradient, suggesting that the facilitative effects of a shrub do not necessarily increase as the shrub develops. The unimodal shape of dependence of plant species richness on shrub cover probably was generated by the unimodal dependence of soil fertility on shrub cover. Soil nutrient enrichment by shrubs shifted low N:P ratios of plant communities with low levels of shrub cover to more balanced N:P ratios at intermediate levels of shrub cover. At the peak N:P ratio along the gradient in shrub cover, the maximum species richness and functional richness were observed, which was consistent with the unimodal relationship predicted by the resource balance hypothesis. Thus, our findings showed that facilitation processes by shrubs at a patch scale also work at a larger scale in terms of the maintenance of biodiversity. Because observed larger-scale facilitation processes are enhanced at some intermediate levels of shrub cover, this study offers practical insight into the need for management practices that allow some intermediate levels of grazing by livestock for optimizing the role of larger-scale facilitation processes in the maintenance of biodiversity and ecosystem functioning in arid and semi-arid rangelands.     

Cross-spatial-scale patterns in the facilitative effect of shrubs and potential for restoration of desert steppe

Facilitation (positive plant–plant interactions) is a potential means to accelerate vegetation restoration in arid areas. Shrubs can accelerate vegetation recovery by means of soil amelioration, but this effect has not been evaluated at large spatial scales or across scales. Here, we examined the facilitative function of shrub change across spatial scales at a desert steppe in Mongolia. Using a high-resolution satellite image, we established five 2500 m² plots in each of three shrub density classes (low, moderate, high) in a desert steppe in Mongolia. To evaluate the facilitative functions of shrubs at multiple spatial scales, we recorded the total number of plant species at three nested spatial scales in each plot: 25, 400, and 2500 m². The facilitative effect of shrubs on plant species richness was more pronounced at larger scales. Denser shrub communities increased plant species diversity at a larger scale. However, the increased taxonomic diversity was not clearly related to increased functional diversity in this system. This scale dependency in species diversity can be explained by the degree to which spatial heterogeneity of habitats within the plots increased as plot size increased. These results support the hypothesis of scale-dependent changes in the balance between facilitation and competition. Therefore, transplanting shrub saplings at high-density and a larger scale could potentially improve the success of vegetation restoration in arid regions.     

Effect of plant species on communities of arbuscular mycorrhizal fungi in the Mongolian steppe

Communities of arbuscular mycorrhizal (AM) fungi were investigated in Stipa krylovii, Leymus chinensis (Poaceae), Allium bidentatum (Liliaceae), and Astragalus brevifolius (Fabaceae) in the Mongolian steppe to examine the effect of plant species on the communities in this study. The AM fungal communities were examined by molecular analysis based on the partial sequences of a small subunit of the ribosomal RNA gene. The sequences obtained were divided into 23 phylotypes by the sequence similarity >98%. Many of the AM fungal phylotypes included AM fungi previously detected in high-altitude regions in the Tibet and Loes plateaus, which suggested that these AM fungi may have wide distribution with stressful conditions of aridity and coldness. Among the 23 phylotypes, 12 phylotypes were found in all four plants, and 87.4% of the all obtained sequences were affiliated into these 12 types. For the distribution of the AM fungal phylotypes, overlapping of the phylotypes among the four plant species were significantly higher than that simulated by random chance. These results suggested that AM fungal communities were less diversified among the examined plant species.     

Variation in nitrogen isotopic composition in the Selenga river watershed, Mongolia

The stable nitrogen (N) isotope ratio (??¹⁵N) has been used to examine the anthropogenic N input (i.e., septic water, wastewater, and manure) to aquatic ecosystems, because anthropogenic N generally has a ??¹⁵N signature distinct from that found in nature. Aquatic organisms and the derived organic matter such as sediments are reported to become increasingly enriched in ¹⁵N as the human population density increases in watersheds. However, little is known about the relationship in steppe ecosystems, where the livestock population is greater than that of humans. Here, we conducted a preliminary study in the Selenga river mainstream watershed in Mongolia, which covers an area of approximately 300,000?km². A multiple regression analysis revealed that the ??¹⁵N of the riverine sediment was significantly affected by the human population density and more significantly by livestock population density. The population density, including both humans and livestock, significantly influenced ??¹⁵N of the macrophytic Potamogeton spp. The results showed that ??¹⁵N of riverine organic matter can be an indicator of the human and livestock population density, which is likely associated with the status of N cycles in livestock-dominated watersheds.     

Effects of Caragana microphylla patch and its canopy size on “islands of fertility” in a Mongolian grassland ecosystem

In arid and semiarid regions, variations in “islands of fertility” accompanied by discontinuous vegetation is frequently observed. However, the effects of vegetation patches on soil, including the influence of canopy size, are not fully understood, particularly under conditions of severe grazing. We examined the effects of patches of mound-forming shrub, Caragana microphylla, and the plant’s canopy size on these islands of fertility in a heavily grazed Mongolian grassland. In 11 patches with various canopy sizes (32.5–180 cm in diameter), we compared the chemical properties of soils among three microsites: Mound, Below, and Around, which were inside, below, and outside of C. microphylla mounds, respectively. Total carbon (C) and most essential elements for the plants were more concentrated in Mound, but total nitrogen (N) and nonlimiting elements, such as exchangeable sodium (Na), did not significantly differ among microsites. Larger canopies more strongly affected the enrichment of total C and most essential elements, including total N, in Mound. These results suggest that C. microphylla patches substantially enrich total C and most essential elements and that the extent of enrichment was intensified with canopy size. However, under severe grazing, total N may be relatively more affected by the redistribution of resources through grazing, particularly when the canopy size is small.