Hydrological response to long-lasting dry spell at the southern edge of Siberian permafrost

Regions experiencing prolonged dry spell exhibit intensified land-atmosphere coupling, exacerbating dry conditions within the hydrological system. Yet, understanding the propagation of these processes within the context of permafrost degradation remains limited. Our study investigates concurrent hydro-climate variations in the semi-arid Selenga River basin in the southern edge of Siberian permafrost. Driven by the natural atmospheric circulations, this region experienced two distinct dry spells during 1954-2013. It enables comparative investigations into the role of warming-induced permafrost degradations in drought dynamics under land-atmosphere coupling. Based on a comprehensive analysis of observed borehole data from 1996 to 2009 and empirical methods, we identify widespread permafrost loss in the semi-arid Selenga region. Such large-scale landscape changes may increase the infiltration of water from the surface to the subsurface hydrological system, and significantly influence the dry conditions in landsurface. First, significant decreasing trends are observed in river runoff (-0.30mm/yr, p < 0.05) and TWS (-3.16 mm/yr, p = 0.1), despite the absence of an apparent trend in annual precipitation (0.009 mm/yr, p = 0.9). Furthermore, in comparison to the first dry spell (1974-1983, 10yrs), the hydro-climatic variables show prolonged and more severe water deficits in runoff and TWS during the second dry spell (1996-2012, 17 yrs), with a reduced runoff-generation efficiency from precipitation. Such exacerbated dry conditions are coincident with amplified positive anomalies observed in air temperature, PET, as well as low-level geopotential height. These concurrent "hot-dry" phenomena indicate an enhanced land-atmospheric interaction within the hydro-climate system, which is further evidenced by the negative relationship between permafrost thawing index and runoff deficits (regression coefficient = -3.8, p < 0.001). As climate warming continues, the ongoing permafrost degradation could reinforce water scarcity, triggering an irreversible shift in water availability in water-scarce regions. Our findings could support freshwater management for regional food supply, human health, and ecosystem functions in the regions undergoing large-scale permafrost degradation.

No Signs of Long-term Greening Trend in Western Mongolian Grasslands

Trends for increased vegetation greenness based on satellite-derived data have been repeatedly published for the temperate grassland biome (including forest steppes) of eastern Inner Asia since 1982. Although this greening trend has been attenuated or partially reversed by drought in the early twenty-first century, linear increases in the Normalized Difference Vegetation Index (NDVI) or other parameters of vegetation greenness are nevertheless evident when the period since 1982 is regarded. However, the question arises whether these trends are part of a long-term trend driven by climate change, as simultaneously forests in the region show widespread drought-induced growth reductions and mortality outbreaks. Therefore, we hypothesized that the post-1982 greening trend was neither part of a long-term trend nor unprecedented. To test this hypothesis, we analyzed monthly maximum NDVI data from AVHRR time series and correlated these data with standardized tree-ring data of Larix sibirica from two regions of western Mongolia. We used linear regression to model the NDVI from tree-ring anomalies and to reconstruct the NDVI since 1940. These reconstructions show that the availability of satellite-based NDVI data coincidentally began during a dry period of low vegetation greenness in the early 1980s and was followed by a wet phase in the 1990s, producing the linear greening trend. No positive long-term trend in the reconstructed NDVI was observed from 1940 to 2010. This result rules out a recent climate change-driven greening trend for the grasslands and forest steppes of western Mongolia and calls into question its existence for all of eastern Inner Asia.

A 168-year temperature recording based on tree rings and latitude differences in temperature changes in northeast China

A ring-width series was developed from Dahurian larch (Larix gmelinii) in the northeastern forest area of Inner Mongolia, China. By analyzing the relationships between tree-ring data and climate records, an August-September mean maximum temperature (T₈₉) series during 1845 and 2012 was reconstructed based on a simple linear regression equation. This reconstructed series explained 40.9% variance of the observed temperature from 1959 to 2012. The reconstructed T₈₉ series was consistent with the historical disaster events caused by extreme climate (e.g., flood, frost disaster, and cold damage). Besides, the temperature comparisons showed that the year in which the warm months (April-September) in northeast China began to warm up has latitude differences. It started with a gradual delay from north to south, starting 1980 in the south region, after 1950 AD in the central region and after 1940 in the north region. Our study can enrich high-resolution temperature series in Northeast China and help clarify the characteristic of recent warming in northeast China.

Reconstruction of June–July Temperatures Based on a 233 Year Tree-Ring of Picea jezoensis var. microsperma

In this study, ring-width chronology of Picea jezoensis var. microsperma from the Changbai Mountain (CBM) area, Northeast China, was constructed. Growth/climate responses suggested that mean maximum temperature (Tmax) was the limiting factor affecting radial growth of PJ trees in the study region. According to the correlation analysis between the ring-width index and meteorological data, a June–July mean maximum temperature (Tmax6–7) series between 1772 and 2004 was reconstructed by using the standard chronology. For the calibration period (1959–2004), the explained variance of the reconstruction was 41.6%. During the last 233 years, there were 36 warm years and 34 cold years, accounting for 15.5% and 14.7% of the total reconstruction years, respectively. Cold periods occurred in 1899–1913, 1955–1970, and 1975–1989, while warm periods occurred in 1881–1888. The reconstructed temperature series corresponded to the historical disaster records of extreme climatic events (e.g., drought and flood disasters) in this area. Comparisons with other temperature reconstructions from surrounding areas and spatial correlation analysis between the gridded temperature data and reconstruction series indicated that the regional climatic variations were well captured by the reconstruction. In addition, multi-taper method spectral analysis indicated the existence of significant periodicities in the reconstructed series. The significant spatial correlations between the reconstructed temperature series and the El Niño–Southern Oscillation (ENSO), solar activity, and Pacific Decadal Oscillation (PDO) suggested that the temperature in the CBM area indicated both local-regional climate signals and global-scale climate changes.

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.

Mongolian pines (Pinus sylvestris var. mongolica) in the Hulun Buir steppe, China, respond to climate in adjustment to the local water supply

The growth response of Mongolian pine (Pinus sylvestris var. mongolica) to climate was studied at three sites in the Hulun Buir steppe on the eastern Mongolian Plateau, China. Correlation analysis revealed two patterns of response: (1) trees on two sites in the upstream section of the Yimin River are strongly limited by temperature and precipitation during the growing season from April to September, and (2) trees in the convergence area of the downstream section of the Yimin River and of the midstream section of the Hailar River are sensitive to precipitation during winter (December-January) and early spring (April) as well as to the early growing season temperature (April and June). These responses can be attributed to the positions where groundwater, recharged by the runoff from summer to autumn (July-September), could supply sufficient water needed for tree growth. Therefore, the patterns of growth-climate responses and of climate variation trends in this steppe region should be considered for the management and afforestation of Mongolian pines.

Rapid warming accelerates tree growth decline in semi-arid forests of Inner Asia

Forests around the world are subject to risk of high rates of tree growth decline and increased tree mortality from combinations of climate warming and drought, notably in semi-arid settings. Here, we assess how climate warming has affected tree growth in one of the world's most extensive zones of semi-arid forests, in Inner Asia, a region where lack of data limits our understanding of how climate change may impact forests. We show that pervasive tree growth declines since 1994 in Inner Asia have been confined to semi-arid forests, where growing season water stress has been rising due to warming-induced increases in atmospheric moisture demand. A causal link between increasing drought and declining growth at semi-arid sites is corroborated by correlation analyses comparing annual climate data to records of tree-ring widths. These ring-width records tend to be substantially more sensitive to drought variability at semi-arid sites than at semi-humid sites. Fire occurrence and insect/pathogen attacks have increased in tandem with the most recent (2007-2009) documented episode of tree mortality. If warming in Inner Asia continues, further increases in forest stress and tree mortality could be expected, potentially driving the eventual regional loss of current semi-arid forests.