65% cover is the sustainable vegetation threshold on the Loess Plateau

Global temperatures will continue to increase in the future.The∼640,000-km2 Loess Plateau(LP)is a typical arid and semi-arid region in China.Similar regions cover∼41%of the Earth,and its soils are some of the most severely eroded anywhere in the world.It is very important to understand the vegetation change and its ecological threshold under climate change on the LP for the sustainable development in the Yellow River Basin.However,little is known about how vegetation on the LP will respond to climate change and what is the sustainable threshold level of vegetation cover on the LP.Here we show that the temperature on the LP has risen 0.27°C per decade over the past 50 years,a rate that is 30%higher than the average warming rate across China.During historical times,vegetation change was regulated by environmental factors and anthropogenic activities.Vegetation coverage was about 53%on the LP from the Xia Dynasty to the Spring and Autumn and Warring States period.Over the past 70 years,however,the environment has gradually improved and the vegetation cover had increased to∼65%by 2021.We forecast future changes of vegetation cover on the LP in 2030s,in 2050s and in 2070s using SDM(Species Distribution Model)under Low-emission scenarios,Medium-emission scenarios and High-emission scenarios.An average value of vegetation cover under the three emission scenarios will be 64.67%,62.70%and 61.47%,respectively.According to the historical record and SDM forecasts,the threshold level of vegetation cover on the LP is estimated to be 53-65%.Currently,vegetation cover on the LP has increased to the upper limit of the threshold value(∼65%).We conclude that the risk of ecosystem collapse on the LP will increase with further temperature increases once the vegetated area and density exceed the threshold value.It is urgent to adopt sustainable strategies such as stopping expanding vegetation area and scientifically optimizing the vegetation structure on the LP to improve the ecological sustainability of the Yellow River Basin.

Shortened duration and reduced area of frozen soil in the Northern Hemisphere

The changes in near-surface soil freeze-thaw cycles(FTCs)are crucial to understanding the related hydrological and biological processes in terrestrial ecosystems under a changing climate.However,long-term dynamics of soil FTCs at the hemisphere scale and the underlying mechanisms are not well understood.In this study,the spatiotemporal patterns and main driving factors of soil FTCs across the Northern Hemisphere(NH)during 1979–2017 were analyzed using multisource data fusion and attribution approaches.Our results showed that the duration and the annual mean area of frozen soil in the NH decreased significantly at rates of 0.13±0.04 days/year and 4.93104 km^(2)/year,respectively,over the past 40 years.These were mainly because the date of frozen soil onset was significantly delayed by 0.1±0.02 days/year,while the end of freezing and onset of thawing were substantially advanced by 0.21±0.02 and 0.15±0.03 days/year,respectively.Moreover,the interannual FTC changes were more drastic in Eurasia than in North America,especially at mid-latitudes(30
–45
N)and in Arctic regions(>75
N).More importantly,our results highlighted that near-surface air temperature(Ta)and snowpack are the main driving factors of the spatiotemporal variations in soil FTCs.Furthermore,our results suggested that the long-term dynamics of soil FTCs at the hemisphere scale should be considered in terrestrial biosphere models to reduce uncertainties in future simulations.