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.

The response of trees to climate is crucial for the health assessment and protection of forests in alpine regions. Based on samples of Pinus tabuliformis and Abies fargesii, two typical evergreen coniferous species with distinct elevation differences in the vertical vegetation zones of the Qinling Mountains, we have developed two tree-ring width chronologies for the southern slope of the central Qinling Mountains in central China. The correlation analysis results showed that the radial growth of P. tabuliformis and A. fargesii responded to different climatic factors. Water stress caused by temperature in May of the current year was the main limiting factor for radial growth of P. tabuliformis, while precipitation in September of the previous year and the current year had a negative impact on A. fargesii, with lag effects of temperature and precipitation during the previous growing season. Spatial correlation and comparative analysis indicated that the P. tabuliformis chronology responded to extreme dry and wet events on a regional scale. Interannual and multidecadal periodic signals recorded by tree rings suggested that the hydrological and climatic changes on the southern slope of the central Qinling Mountains were teleconnected with the Pacific and Atlantic Oceans, including El Niño-Southern Oscillation (ENSO), Pacific Decadal Oscillation (PDO), and North Atlantic Oscillation (NAO). Our results provide new evidence for a hydroclimatical response study inferred from tree rings on the southern slope of the central Qinling Mountains.

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 (T_DS) 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 T_DS 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 T_DS 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 T_DS variations in Ejina Oasis were regulated by solar radiation and atmospheric circulations at the interannual and interdecadal time scales.

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.