Compared with pure forest, mixed forest alters microbial diversity and increases the complexity of interdomain networks in arid areas

IMPORTANCE

The results provide a comparative study of the response of soil microbial ecology to the afforestation of different tree species and deepen the understanding of the factors controlling soil microbial community structure.

Recent decreasing precipitation and snowmelt reduce the floods around the Chinese Tianshan Mountains

Understanding and managing mountain floods has become increasingly urgent, with global climate change and human activities exacerbating flood risk. However, flood research in Tianshan Mountains, a typical flood-prone mountainous region in China, is still insufficient. Here, we customized a set of flood research methods based on rainstorms and extreme snowmelt events, including a new flood counting method that comprehensively considered the frequency and magnitude of floods and the methods of flood classification and change attribution. We found that floods around the Chinese Tianshan Mountains (CTM) increased from 2014 to 2016 but decreased rapidly from 2016 to 2021, with storm floods, snowmelt floods, and mixed floods accounting for 38.3 %, 26.5 %, and 34.6 % of total flood events, respectively. The variation of floods was most significantly correlated with the average and extreme precipitation, followed by the temperature-driven average snowmelt change. Furthermore, atmospheric circulation anomalies and water vapor input from the western boundary of CTM caused decreasing precipitation and storm floods. Meanwhile, the warming hiatus also greatly impacted declining flood frequency. Notably, flood frequency is projected to rebound soon because of the rising precipitation and temperature, infrastructure aging, and reservoir abandonment, implying the present flood decline unsustainable. Our research develops a strategy to investigate short-term flood anomalies under climate oscillations around the CTM, providing insights into flood research and prevention in global mountainous regions.

Assessment of lake area in response to climate change at varying elevations: A case study of Mt. Tianshan, Central Asia

Changes in lake area (water surface area) are often considered accurate and sensitive representations of climate change. However, the role that elevation plays in this dynamic is somewhat unclear; studies remain inconclusive as to whether lake responses are consistent across elevation gradients. Here, we used Landsat and keyhole satellite images to quantify lake area changes from the 1960s to 2020 at different elevations in Central Asia's Tianshan Mountains and relate them to both climatic and anthropogenic factors. The results revealed that all low-elevation lakes showed a decreasing trend, and the total area of all monitored low-elevation lakes was reduced by 18.50 %. The total area of the mid-elevation lakes decreased by 0.16 %, while the total area of the high-elevation glacial lakes increased by 4.35 %. Lakes are recharged by a variety of influxes including glacial meltwater and precipitation. Notably, human activities (urban and agricultural water consumption) were the dominant factors in the shrinkage of low-elevation lakes. Climatic factors were the main driving factors of mid-elevation lake changes, and these lakes appeared to be more sensitive to temperature changes than lakes at other elevations. In addition, significant warming dominated area changes in high-elevation proglacial and unconnected glacial lakes. Overall, those results emphasized that when using lakes to reconstruct paleoclimates or predict lake evolution, it is necessary to consider how elevation gradients and recharge types may affect lake sensitivity to variations in climatic and anthropogenic activity.

[Water sources of Picea schrenkiana and Berberis heteropoda in the Tianshan Mountains in summer]

Water use patterns of trees and shrubs in the Picea schrenkiana coniferous forest remain unclear, due to a lack of quantitative analysis on water use dynamics. In this study, the xylem water hydrogen and oxygen stable isotope compositions of P. schrenkiana and the companion shrub species Berberis heteropoda were measured to detect their water sources. The IsoSource model was used to analyze the relative contribution of each potential water source for both species during summer. The results showed that during July, P. schrenkiana and B. heterocarpa mainly extracted water from the 0-60 cm soil layer due to the relatively sufficient soil water content, with the relative contributions being 73.8% and 63.2% for the two species, respectively. In August, with the decreases in soil water content, water source of P. schrenkiana remained stable, and the relative contribution of soil water above 60 cm was 69.5%. In contrast, B. heterocarpa reverted to water source from deeper soil layer, with the relative contribution of shallow soil (0-20 cm) water decreasing to 14.3% and that of middle (20-60 cm) to deep (60-100 cm) soil water increased to 67.7%. In September, with the increases of water content in the shallow soil layer, both species extracted water from shallow soil layers, with the relative contribution reaching to 95.0%. In summary, P. schren-kiana exhibited typical shallow root characteristics, while B. heterocarpa extracted water from the 0-100 cm soil profile and could flexibly change its water source corresponding to changes in soil water content to cope with changing environmental water condition.

Characteristics of tree-ring density at different stem heights and their climatic responses

Taking windfall woods of Picea schrenkiana in the southern mountainous area of the Ili Prefecture as the research object, tree-ring density chronologies were developed from the discs for maximum density (MXD), minimum density (MID), mean earlywood density (EWD), and mean latewood density (LWD) at five different stem heights (1.3, 5, 10, 15 and 20 m) to examine the climatic responses of tree-ring density by correlation analysis with local meteorological data. The results showed that there was a good coherence among the four types of tree-ring density chronologies for the same stem height, which was relatively significant for the data from 10, 15 and 20 m. The LWD had good coherence among different stem heights, while the climatic responses of tree-ring density at different stem heights varied. The MXD and LWD at 15 m were sensitive to mean tempera-ture from July to September in the previous year and from May to September in the current year. It might underestimate the response of P. schrenkiana to temperature if we sample tree-ring at 1.3 m.

Extreme climate historical variation based on tree-ring width record in the Tianshan Mountains of northwestern China

The increasing frequency and intensity of extreme climate events have caused serious impacts on the service functions of terrestrial ecosystems and the production and life of human society in recent years. The warm nights (TN90p) variable of the 26 extreme climate indicators was the main factor controlling the tree radial growth of Schrenk spruce (Picea schrenkiana) in the Tianshan Mountains region based on the responses of tree-ring width in the 5 sample sites. Therefore, TN90p in the growth season from May to September (TN90p_5-9) during 1735-2016 was reconstructed on the basis of the time stability of the growth-climate relationships. The interpretation rate of variance of the reconstructed equation was 45.4% (R²_adj = 44.4%, F = 45.7). The reconstruction showed four relatively high TN90p_5-9 historic intervals (1747-1798, 1856-1872, 1906-1951, and 2002-2016) and four low intervals (1735-1747, 1798-1856, 1872-1900, and 1951-2002). The occurrence frequency of extreme high values was higher than that of extreme low values during the reconstruction period of 1735-2016. The extreme values of reconstruction were consistent with historical droughts and large-scale volcanic eruptions, indicating that the reconstruction series had high accuracy. Multi-window spectral periodic analysis and spatial correlation analysis revealed that TN90p_5-9 variation in the study area was affected by large-scale sea-air stress factors. In particular, the TN90p_5-9 obtained by using R/S analysis (rescaled range analysis) will continue to show an upward trend in the relative period of time in the future. This trend will lead to a further decrease in the radial growth of trees and even trigger forest death events.

Climatic change in southern Kazakhstan since 1850 C.E. inferred from tree rings

Although global warming is an indisputable fact, there is still uncertainty about how climate change will occur at regional levels. Kazakhstan is the largest landlocked country in the world. To best manage this country's limited water resources, socio-economic development and environmental protection, a solid understanding of regional climate change impacts is needed. In this study, tree-ring width and δ¹³C chronologies were established based on 99 tree-ring samples of Schrenk spruce (Picea schrenkiana Fisch. et Mey.) collected in Almaty, Kazakhstan. Climate response analysis between the tree-ring chronologies and climate data indicates that summer mean temperature is the strongest climate signal recorded by tree-ring δ¹³C. We reconstructed temperature change in southern Kazakhstan since 1850 C.E. using the tree-ring δ¹³C_corr chronology. The results show that the temperatures in southern Kazakhstan have risen at a rate of about 0.27 °C per decade over the past 166 years. However, the rate has increased by as much as 0.44 °C per decade over the past 30 years. Analyses of temperature and precipitation data show that the climate has alternated between warm-dry and cold-humid periods over the past 166 years. The extreme droughts of 1879, 1917 and 1945 were caused by the combination of continuously high temperatures and reduced precipitation.

Spatial distribution and controlling factors of surface water stable isotope values (δ18O and δ2H) across Kazakhstan, Central Asia

Climate change is expected to alter hydrological and biogeochemical processes in Central Asia (CA), and surface water stable isotope values (δ¹⁸O and δ²H) can be used to examine these changes. Spatially extensive stable isotope data, however, are sparse, which constrains the understanding of hydrological processes in transboundary rivers across Kazakhstan. Therefore, we conducted a survey of surface water stable isotopes across the region. River and lake water isotope values exhibit spatial variability that was closely associated with isotope values of precipitation, physiographic factors, landscape characteristics, and local hydrological processes, e.g., evaporation and mixing of waters from different sources. River water was characterized by lower δ¹⁸O and δ²H values and higher d-excess relative to lake water, suggesting evaporative enrichment of lake water. Analysis of δ¹⁸O versus δ²H for rivers and lakes yielded distinct regressions, (river [RWL], δ²H = 6.08δ¹⁸O - 16.7, r² = 0.837, p < 0.001) and (lake [LWL], δ²H = 6.23δ¹⁸O - 22.1, r² = 0.924, p < 0.001). The slope and intercept of the RWL and LWL were slightly lower than the local meteoric water line [LMWL] (δ²H = 6.96 δ¹⁸O - 1.0, r² = 0.942, p < 0.001). River water δ¹⁸O showed a significant negative correlation with elevation and longitude, but not with latitude. The spatial distributions of δ¹⁸O and d-excess values showed a remarkable gradient from west to east across Kazakhstan that was associated with moisture moving from the Mediterranean region to Kazakhstan. We also found generally higher δ¹⁸O values and lower d-excess values in low-elevation areas because of high evaporation rate in Kazakhstan. These baseline data will be useful for documenting the effects of climate change on the hydrological cycle in Central Asia.

Phylogeographic patterns of the desert poplar in Northwest China shaped by both geology and climatic oscillations

Background

The effects of historical geology and climatic events on the evolution of plants around the Qinghai-Tibetan Plateau region have been at the center of debate for years. To identify the influence of the uplift of the Tianshan Mountains and/or climatic oscillations on the evolution of plants in arid northwest China, we investigated the phylogeography of the Euphrates poplar (Populus euphratica) using chloroplast DNA (cpDNA) sequences and nuclear microsatellites, and estimated its historical distribution using Ecological Niche Modeling (ENM).

Results

We found that the Euphrates poplar differed from another desert poplar, P. pruinosa, in both nuclear and chloroplast DNA. The low clonal diversity in both populations reflected the low regeneration rate by seed/seedlings in many locations. Both cpDNA and nuclear markers demonstrated a clear divergence between the Euphrates poplar populations from northern and southern Xinjiang regions. The divergence time was estimated to be early Pleistocene based on cpDNA, and late Pleistocene using an Approximate Bayesian Computation analysis based on microsatellites. Estimated gene flow was low between these two regions, and the limited gene flow occurred mainly via dispersal from eastern regions. ENM analysis supported a wider distribution of the Euphrates poplar at 3 Ma, but a more constricted distribution during both the glacial period and the interglacial period.

Conclusions

These results indicate that the deformation of the Tianshan Mountains has impeded gene flow of the Euphrates poplar populations from northern and southern Xinjiang, and the distribution constriction due to climatic oscillations further accelerated the divergence of populations from these regions. To protect the desert poplars, more effort is needed to encourage seed germination and seedling establishment, and to conserve endemic gene resources in the northern Xinjiang region.

Electronic supplementary material

The online version of this article (10.1186/s12862-018-1194-1) contains supplementary material, which is available to authorized users.