A tree-ring width chronology was developed from the Chinese pine (Pinus tabuliformis) in northern North China. To acquire a long-term perspective on the history of droughts in this region, the Standardized Precipitation Evapotranspiration Index (SPEI) from August of the previous year to February of the current year was reconstructed for the period of 1903–2012 AD. The reconstruction explained 46.6% of the instrumental records over the calibration period of 1952–2012. Five dry periods (1916–1927, 1962–1973, 1978–1991, 1994–1999 and 2002–2005) and three wet periods (1908–1915, 1928–1961 and 1974–1977) were found in the reconstructed period, and most of the dry years (periods) in the reconstruction were supported by historical records. Comparisons between the reconstruction and other nearby dryness/wetness indices and precipitation reconstructions demonstrated a good repeatability and high reliability in our reconstruction. Spatial correlation implied that the reconstruction could represent regional hydroclimatic characteristics on a larger regional scale. Significant periodicities and correlations were observed between the reconstructed data and the quasi-biennial oscillation (QBO), El Niño–Southern Oscillation (ENSO) and Pacific Decadal Oscillation (PDO), which suggested that the hydroclimatic variation in northern North China may be closely connected to remote oceans. The significant and high correlation between the reconstructed series and sea surface temperatures (SSTs) in the eastern equatorial and Southeast Pacific Ocean indicated that ENSO may be the main factor influencing the regional climate.
Fitting mathematical models to describe the influence of topographic factors and coexisting plants on wild jujube distribution was performed to provide a scientific basis for wild jujube forestation. Investigation quadrats, with straight-line distances between adjacent quadrats of longer than 100 m, were set up in areas of wilderness or low human disturbance, which were rich in wild plant species. Data concerning altitude, slope aspect, slope position and slope degree of each investigation quadrat, as well as the type and number of coexisting wild plants, were collected. Based on this, correlations with the average number, occurrence probability and density of wild jujube and these variables were analyzed, and data models were established. Results of analyses show that topographic factors such as altitude, aspect, gradient, slope and position, play an important role in the distribution of wild jujube; and that Vitex negundo var. heterophylla (Franch.) Rehd. coexistence is related to wild jujube distribution. Both average number and occurrence probability of wild jujube conform to a GaussAmp model with altitude. The highest average number was recorded at 581.24 ± 13.78 m above sea level, and the highest occurrence probability at 462.53 ± 36.67 m above sea level. Average number and occurrence probability of wild jujube were fit to a linear model with slope aspect—with mathematical slope 0.49 ± 0.16—indicating that wild jujube is a light-loving and drought-tolerant plant. Average number and density of wild jujubes were fit to GaussAmp models with slope position. The highest average number and the highest density of wild jujube appears on the upper part of the middle slope. Wild jujube occurrence probability was correlated to slope degree in a quadratic equation model. With an increase in slope degree, the distribution number of wild jujube increased sharply. The survey data of slope position and slope degree further reinforced the observed drought-resistance qualities of wild jujube. Average number and density of wild jujubes were correlated to the number of Vitex negundo var. heterophylla by quadratic equation models. No other plants investigated conformed to a statistically significant relationship with wild jujube distribution. Our results suggest altitude, slope aspect, slope position and slope degree play an important role in wild jujube distribution, and that Vitex negundo var. heterophylla is an important coexistent plant species for wild jujube.
The inter-annual stable carbon isotope ratio (δ13C) of three tree-ring cores of P. euphratica (Populus euphratica Oliv.) was determined from Ejina Oasis in Northwest China. A robust and representative δ13C chronology is generated from the three δ13C series using an arithmetic mean method. After eliminating the influence of the δ13C from elevated atmospheric carbon dioxide (CO2) concentration, we obtained a carbon isotopic discrimination (Δ13C) chronology. According to the significant correlation between the tree-ring Δ13C and instrumental data, we reconstructed the mean maximum temperature anomalies from previous December to current September (TDS) for the period 1901–2011. The reconstruction explained 43.6% of the variance over the calibration period. Three high-temperature periods (1929–1965, 1972–1974, and 1992–2006) and three low-temperature periods (1906–1926, 1966–1968, and 1975–1991) were found in the reconstructed series. Comparisons between the reconstructed TDS and the observed mean temperature from previous December to current September in Anxi meteorological station and the temperature index in north-central China demonstrated the reconstructed TDS has the advantage of reliability and stability. The significant spatial correlation declared that the reconstruction has a broad spatial representation and can represent the temperature variation characteristics in a wide geographical area. In addition, we found that the area of Ejina Oasis is smaller (larger) when the mean maximum temperature is higher (lower), which may be due to a conjunction effect of natural and anthropogenic activities. Significant periodicities and correlations suggested that the TDS variations in Ejina Oasis were regulated by solar radiation and atmospheric circulations at the interannual and interdecadal time scales.
The Vaganov‒Shashkin process-based model was used to explore the variation characteristic of the radial growth rate of Pinus tabulaeformis in the agro-pastoral transition zone in northern China. The tree-ring width chronologies of the four sampling sites were significantly positively correlated with the simulated series (p < 0.01), and the simulated onset and end dates of tree radial growth indicated that April to October was the main growing season. Temperature affects the radial growth rate of tree stems at the start and end of the growing season, while soil moisture availability affects the radial growth rate in the main growing season. Despite the differences in amplitude, the integral growth rate showed a bimodal pattern, which to some extent responded to the hydrothermal configuration of the East Asian summer monsoon climate. Compared with the peak changes in the summer monsoon fringe area in Northwest China, the highest peak of the integral growth rate in this study area appeared around August in the late growing season, reflecting the adaptability of trees to the local climatic environment. The average values of the integral growth rate and rate due to soil moisture, inferred from extreme wide-ring and narrow-ring years, were significantly different (p < 0.01), while the average growth rates due to temperature were not significant (p > 0.05). The analysis results indicate that moisture availability is a key limiting factor for the radial growth of Pinus tabulaeformis. Our study provides valuable knowledge about the growth processes of the main tree species related to the hydroclimatic variables in northern China and offers a new perspective on mitigating the adverse effects of a warmer climate on the forest in the semi-arid region in the future.
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