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ABSTRACT On the basis of the daily minimum temperature, records of 109 meteorological stations over Northeastern China ( NEC) from 1961 to 2013 and the output data of eight models from CMIP5, this study investigates the recent and future spatiotemporal changes of the frost-free season. The results show that the date of last spring frost ( LSFD) has a significant decreasing trend. Stations exhibiting significant negative trends are mostly distributed in the eastern Jilin Province and the Midwest of Heilongjiang Province. The date of first fall frost ( FFFD) and length of frost-free season ( LFFS) both show significant increasing trends. The spatial patterns of stations showing significant positive trends in the FFFD and LFFS are similar, which are mainly located in the middle part of Eastern Inner Mongolia and the south and middle parts of Changbai Mountains. The LFFSs have significant increasing trends with the magnitudes of 0.49 days/decade for RCP2.6, 1.62 days/decade for RCP4.5, and 3.85 days/decade for RCP8.5 during 2006-2100. The southern Liaoning Province and the northwestern NEC will experience the longer LFFS under each scenario between 2071 and 2100. The frost-free season indices have significant correlations with the Northern Hemisphere Polar Vortex Area index. The atmospheric circulation patterns of an anticyclonic circulation anomaly prevailed over the Lake Baikal region and a cyclonic circulation anomaly located in the northwest Pacific Ocean in springs and an anticyclonic circulation anomaly prevailed over the south part of the Russian Far East in falls block more cold air flowing into NEC, which result in the earlier LSFD and later FFFD. The adverse patterns in springs and in falls could bring out the later LSFD and earlier LSFD, respectively. Our results could benefit the prevention and reduction of frost disaster and provide a reference for the policy-making of future crop production. [ABSTRACT FROM AUTHOR]

Accelerated tree growth in the alpine treeline ecotone (ATE) has been linked to the recent rapid climate warming. However, the role of tree age and elevation in the growth response of trees to climate change remains unclear. Here, we developed basal area increment chronologies of Betula ermanii from the closed forest belt (CFB) at lower elevation to the ATE of Changbai Mountain with three tree age classes (young: 0–60 years, middle-aged: 60–120 years, and old: > 120 years). In ATE, all age classes of B. ermanii have been experiencing dramatic growth acceleration since 1981. Young trees, with higher temperature sensitivity, exhibited the most prominent warming-induced growth enhancement relative to middle and old-aged trees. In CFB, young and middle-aged trees exhibited similar sensitivity to the temperature increase during the warm period relative to the cold period, with strikingly accelerated growth enhancement trends. In contrast, old trees did not show any significant growth trend. Moreover, young trees in ATE showed significantly (p < 0.01) higher growth rates than those in CFB, whereas middle-aged trees in ATE showed significantly (p < 0.05) lower growth rates than those in CFB. Our results reveal that the response of tree growth to climate change varies with tree age and elevation. Therefore, these factors should be considered to avoid overestimation or underestimation of tree growth trends and bias in estimating potential forest production. [ABSTRACT FROM AUTHOR]

Interday temperature fluctuations (ITFs) have a series of direct and visible impacts on human productivity, daily life, and some rapidly occurring processes in the Earth's critical zones. The Loess Plateau of China (LPC) is characterized by Chinese continental monsoon climate and fragile ecological environment, and thus, is sensitive to ITFs. To evaluate the spatiotemporal distribution of the ITF frequency and level, and the related large‐scale circulation background, we collected daily temperature data from 53 meteorological stations in the LPC for 1961–2015, and calculated the number of days per year in which the average ITF was greater than 1°C (ITFD) and the corresponding variance of the ITF (ITFL). The annual average ITFs were mostly warming; the first half of the year experienced mainly warming, while the second half underwent cooling. The level and frequency of the interday maximum temperature fluctuation contributed greatly to increases in the interday mean temperature fluctuation level and frequency. The spatial distributions of ITFD and ITFL were related to their latitude and longitude. Stations with positive ITFD indicator trends were mostly concentrated in the southwestern LPC, while stations with negative ITFL indicator trends mainly occurred in the northwestern LPC. At an 850 hPa geopotential height, the stronger ITFs in spring more strongly correlated with the Mongolian high and Aleutian low. While at a 500 hPa geopotential height, the ITF were more strongly correlated with the Mongolian high. These results suggest that ITFs are more complex than previously reported and may need to be considered in future climate change assessments. [ABSTRACT FROM AUTHOR]

Vegetation in the alpine tundra area of the Changbai Mountains, one of two alpine tundra areas in China, has undergone great changes in recent decades. The aggressive herb species Deyeuxia angustifolia (Komarov) Y. L. Chang, a narrow-leaf small reed, was currently encroaching upon the alpine landscape and threatening tundra biota. The alpine tundra of the Changbai Mountains has been experiencing a warmer climate and receiving a high load of atmospheric nitrogen deposition. In this study, we aimed to assess the respective roles of climate warming and atmospheric nitrogen deposition in promoting the upward encroachment of D. angustifolia . We conducted experiments for three years to examine the response of D. angustifolia and a native alpine shrub, Rhododendron chrysanthum , to the conditions in which temperature and nitrogen were increased. Treatments consisting of temperature increase, nitrogen addition, temperature increase combined with nitrogen addition, and controls were conducted on the D. angustifolia communities with three encroachment levels (low, medium, and high levels). Results showed that 1) D. angustifolia grew in response to added nutrients but did not grow well when temperature increased. R. chrysanthum showed negligible responses to the simulated environmental changes. 2) Compared to R. chrysanthum , D. angustifolia could effectively occupy the above-ground space by increasing tillers and growing rapidly by efficiently using nitrogen. The difference in nitrogen uptake abilities between the two species contributed to expansion of D. angustifolia . 3) D. angustifolia encroachment could deeply change the biodiversity of tundra vegetation and may eventually result in the replacement of native biota, especially with nitrogen addition. Our research indicated that nutrient perturbation may be more important than temperature perturbation in promoting D. angustifolia encroachment upon the nutrient- and species-poor alpine tundra ecosystem in the Changbai Mountains. [ABSTRACT FROM AUTHOR]