Response of Freezing/Thawing Indexes to the Wetting Trend under Warming Climate Conditions over the Qinghai–Tibetan Plateau during 1961–2010:A Numerical Simulation

Since the 1990s,the Qinghai–Tibetan Plateau(QTP)has experienced a strikingly warming and wetter climate that alters the thermal and hydrological properties of frozen ground.A positive correlation between the warming and thermal degradation in permafrost or seasonally frozen ground(SFG)has long been recognized.Still,a predictive relationship between historical wetting under warming climate conditions and frozen ground has not yet been well demonstrated,despite the expectation that it will become even more important because precipitation over the QTP has been projected to increase continuously in the near future.This study investigates the response of the thermal regime to historical wetting in both permafrost and SFG areas and examines their relationships separately using the Community Land Surface Model version 4.5.Results show that wetting before the 1990s across the QTP mainly cooled the permafrost body in the arid and semiarid zones,with significant correlation coefficients of 0.60 and 0.48,respectively.Precipitation increased continually at the rate of 6.16 mm decade–1 in the arid zone after the 1990s but had a contrasting warming effect on permafrost through a significant shortening of the thawing duration within the active layer.However,diminished rainfall in the humid zone after the 1990s also significantly extended the thawing duration of SFG.The relationship between the ground thawing index and precipitation was significantly negatively correlated(−0.75).The dual effects of wetting on the thermal dynamics of the QTP are becoming critical because of the projected increases in future precipitation.

Variations and trends of the freezing and thaw- ing index along the Qinghai-Xizang Railway for 1966-2004

Annual freezing and thawing index of 7 meteorological stations along the Qing- hai-Xizang Railway were calculated based on daily maximum and minimum temperature records for 1966-2004. Trends of annual freezing and thawing index were analyzed using the Mann-Kendall test and a simple linear regression method. The results show that： 1） The mean annual freezing indices range from 95 to 2300℃·d and the mean annual thawing indices range from 630 to 3250℃·d. The mean annual freezing index of the 7 stations exhibited decreasing trends with decreasing rate of -16.6- -59.1 ℃·d/10a. The mean annual thawing index of these 7 stations showed increasing trends with the related decreasing rate is 19.83-45.6℃·d/10a. 2） The MK trend test indicated the significant decreasing trends （significant at 〈 0.05 significant level） in the annual freezing index for most stations except for Golmud. The significant increasing trends can be observed in the annual thawing index for 4 stations except Golmud and Tuotuohe. Golmud was the only station with no trends in both annual freezing and annual thawing index.

Analysis of the Freezing Damage to Biennial Eucalyptus dunnii and Eucalyptus smithii in Haikou Forest Farm

In winter of 2013,cold weather occurred in Kunming,and biennial E. dunnii and E. smithii in Haikou Forest Farm suffered different degrees of freezing damage. The freezing damage to them was analyzed here. The results showed that in the families of E. dunnii,families suffering severe and especially severe freezing damage accounted for 7. 7%,while 92. 3% of families suffered moderate and below freezing damage; in the families of E. smithii,56. 7% of families suffered severe and especially severe freezing damage,while families suffering moderate and below freezing damage accounted for 43. 3%. There was significant difference between two species of eucalyptuses in tree height（ P 〈0. 05）,while there was extremely significant difference in freezing damage grade and index（ P 〈0. 01）. There was significantly negative correlation between the tree height and freezing damage grade of E. dunnii and E. smithii,with the correlation coefficients of- 0. 374 and- 0. 234. That is,the freezing damage grade would decrease with the increase of tree height.

Recent changes in ground surface thermal regimes in the context of air temperature warming over the Heihe River Basin, China

Changes in ground surface thermal regimes play a vital role in surface and subsurface hydrology, ecosystem diversity and productivity, and global thermal, water and carbon budgets as well as climate change. Estimating spring, summer, autumn and winter air temperatures and mean annual air temperature（MAAT） from 1960 through 2008 over the Heihe River Basin reveals a statistically significant trend of 0.31 °C/decade, 0.28 °C/decade, 0.37 °C/decade, 0.50 °C/decade, and 0.37 °C /decade, respectively. The averaged time series of mean annual ground surface temperature（MAGST） and maximum annual ground surface temperature（MaxAGST） for 1972–2006 over the basin indicates a statistically significant trend of 0.58 °C/decade and 1.27 °C/decade, respectively. The minimum annual ground surface temperature（MinAGST） in the same period remains unchanged as a whole. Estimating surface freezing/thawing index as well as the ratio of freezing index to thawing index（RFT） in the period between 1959 and 2006 over the basin indicates a statistically significant trend of-42.5 °C-day/decade, 85.4 °C-day/decade and-0.018/decade, respectively.

The effect of low-temperature event on the survival and growth of Juglans mandshurica seedlings within forest gaps

In forest ecosystems, gap formation changes the allocation of abiotic resources and thus affects the survival and growth of understory plants. However, how tree seedling survival and growth respond to low-temperature events and the influencing mechanisms remain unclear. To clarify how low-temperature event limits the survival and growth of tree seedlings in the montane regions of eastern Liaoning Province, northeast China, we investigated temperature and light intensity within secondary forest gaps, and the survival and growth of Juglans mandshurica seedlings after a low-temperature event in the spring of 2014. Damage to seedlings due to low temperature sig- nificantly varied in different aspects. Seedlings in gaps on southeast-facing slopes were the most seriously damaged, followed by those in gaps on northeast-facing slopes. In contrast, seedlings in west-facing gaps and in control plots without slope aspect were not damaged. The freezing injury index for seedlings was negatively correlated with minimum temperature （r = - 0.608, P 〈 0.01）, but it was positively correlated with light intensity （r= 0.818, P 〈 0.01）. In addition, height and root collar diameter of damaged seedlings were significantly lower than those of the undamaged seedlings （P 〈 0.01） during the early growing season （April-July）, but no significant difference were observed during the late growing season （July-Oc- tober） （P 〉 0.05）. The extent of seedling damage was directly related to slope aspect. Low temperature and high light intensity were found to be the dominant factors affecting extent of damage to seedlings on southeast- and northeast-facing slopes.

Evaluation on Climate Suitability in Main Growth Stages of Zhaotong Apple in Recent 60 Years

Based on daily average temperature,precipitation and sunshine hours during 1961-2018 from two meteorological stations of Zhaoyang District and Ludian County at Zhaolubazi,Zhaotong City,Yunnan Province,using the freezing injury index at flowering stage(Ha),climatic suitability index during expansion period(Pa),climatic suitability index during coloring period(Dy)and climatic suitability index during fruit growing period(Si),etc.,the climatic suitability of Zhaotong apple during the main growth stages was evaluated.The results showed that the annual average temperature was 11.8-12.3℃at Zhaolu Bazi in recent 60 years,and the climate was the most suitable for Fuji apple planting both in the south and north;the average annual precipitation was 674.8-852.6 mm,and the climate was suitable in the north and sub-suitable in the south;the annual average sunshine hours was between 1845.4 and 1851.1 h,and the climate was sub-suitable in both areas.The Ha was between 0.42-0.50,serious in the north and moderate in the south;the Pa was between 0.92 and 1.02,and both the south and the north were suitable areas;the Dy was between 0.95 and 1.00,and the climate in the south was suitable,while it was sub-suitable in the north;the Si was between 1.38 and 1.59,and the climate was suitable in the south and sub-suitable in the north.There was a 4-year cycle in the interannual variation of Ha,Pa,Dy and Si.After the 1980s,the Ha decreased by 0.56-0.59,the Pa,Dy and Si respectively increased by 0.10-0.16,0.14-0.15 and 0.78-0.84 with the temperature rise,and the changes of these four indexes were all favorable for Fuji apple's fruits growth.It can be seen that under the background of the global warming,the climatic conditions in Zhaolu Bazi are becoming more and more favorable for popularizing the planting of Fuji apple.