| Drylands are critical terrestrial environments,considered to be Earth's most abundant biome and known for their extensive global.They are regarded as vulnerable areas to climatic variations and human activities at global scale because of low precipitation,high evapotranspiration and low vegetation cover.Land degradation has become one of the most serious social-economic-environmental issues in more than one hundred countries of the world.Considering the difficulty of reversion and the low restoration success of many dry degraded areas,many research efforts have focused on developing useful indicators to detect it at the early stages.The early signals of land degradation has often been studied in the framework of critical transitions.According to this theory,drylands exhibit two stable states(non-degraded vs.desertified)and the transition between them,triggered by disturbances such as increased aridity and grazing pressure,is abrupt and sudden,rather than gradual and continuous.Multiple theoretical studies have suggested the existence of alternative stable states and of abrupt,non-linear,shifts between them in drylands.However,empirical verification of alternative stable states in the field has been scarce so far.In the past decades,remote sensing has become a valuable tool to obtain parameters characterizing dryland ecosystems and gain more and more attentions because it can provide detailed information quickly about ecosystem state across wide regions and over increasingly long series of years at a low cost.Thus,it is a potential valuable approach to supply a solution for observation of ecosystem state,which is the key access to validate the existence of alternative stable states in real drylands ecosystems.Berdugo et al.(2017)compared the performance of three indicators,i.e.,power law range,vegetation cover and ecological multifunctionality index(MUL),to reveal the presence of alternative stable states in dryland based on field data,and he found only ecological multifunctionality index can show two stable states.Therefore,our study firstly employed the ecological multifunctionality index as the reference,to select the remotely sensed indicators which has potential to reveal the alternative stable states.Then,we carried out the case studies on alternative stable states and early signs of degradation in the Sahel(tropical drylands)and Mongolian Plateau(temperate drylands)by indicator derive from remote sensing.The main conclusions are as follows.(1)The linear relationships of EVI(Enhanced Vegetation Index)and RUE(RainUse Efficiency,defined as aboveground net primary production(ANPP)divided by rainfall)with MUL are both significant positive.However,RUE explained more variation(about 44%)in MUL than summed EVI(about 32%)did.The results obtained by partial correlation analysis by controlling the rainfall showed that correlation coefficient between summed EVI and MUL,increased about 20%,while correlation coefficient between RUE and MUL increased very slightly(about 3%).Similar results were also found by using the Normalized Difference Vegetation Index(NDVI).These facts indicated that both remotely sensed vegetation indices and RUE could be indicators of ecological multifunctioning.However,RUE was better due to its robustness to rainfall.Also,we must take care that the core assumptions related to the RUE should be fulfilled before using it as an indicator,i.e.,(1)a significant positive linear correlation between precipitation and the ANPP proxy applied,(2)a near-zero correlation between RUE and precipitation.The relationships between RUE and nutrient cycles showed that in comparison to phosphorus cycle(PCY),the carbon and nitrogen cycles(CCY and NCY)had an apparent higher weight in determining the relationship between RUE and MUL.(2)There was a negative relationship between land surface albedo and multifunctionality.Black-sky albedo had a stronger correlation with multifunctionality than white-sky albedo.Visible black-sky albedo showed the strongest correlation with multifunctionality(MUL,-0.314),as well as with functions related to carbon(-0.216)and nitrogen cycling(-0.410),while near-infrared(-0.339)and shortwave black-sky albedo(-0.325)showed stronger correlations with functions related to phosphorus cyclingthan visible black-sky albedo(-0.233)did.VIs showed significant positive correlations with MUL,CCY,and NCY,and the magnitudes were higher than those observed between albedo metrics and the multifunctionality indices.However,VIs were not correlated with PCY,which had significant correlations with both shortwave and near-infrared albedo.Among the 7 narrowband albedos from MODIS,the red band albedo has the best ability to indicate MUL,CCY and NCY,while the narrowband albedo near 2.13 ?m has the best correlation with PCY.Though the magnitudes of the correlations observed were not high,which may result from the wide variability in soil and vegetation types in our dataset,our findings indicate that remotely sensed albedo correlates to multifunctionality,which has been linked to alternative states in global drylands.(3)In the case study of the Sahel(tropical drylands),three remotely sensed albedo(shortwave,visible and near-infrared black-sky)metrics the albedo metrics evaluated showed two stable states(low and high)in this area,and exhibited an abrupt and discontinuous increase with increases in aridity.The two albedo states cooccurred along aridity values ranging from 0.67 to 0.82.Low and high albedo stable states were associated with high and low vegetation cover,respectively,which showed a rapid but continuous decrease along these aridity values.By identifying regions on the edge between high and low albedo states,we also identified vulnerable areas within the Sahel where critical transitions may occur in the future as a consequence of forecasted increases in aridity due to climate change.Our findings indicate that remotely sensed albedo indices have potential to assess the vulnerability of sites to experience discontinuous transitions,which might be linked to the onset of dryland desertification.(4)In the case study of Mongolian Plateau(temperate drylands),probability density distribution of shortwave white-sky albedo showed double peaks,which indicated there were possible two albedo modes in the study area.Potential analysis model showed there were two white-sky albedo states(low and high)co-occurred persistently in aridity range from 0.35 to 0.45.The low and high albedo stated were represented by coniferous forest and grass,respectively.When aridity researched 0.38,potential energy of the high albedo state became less than that of low albedo state,and it implied the low albedo state tended to tip into the high albedo states.The low albedo states exhibited an abrupt and discontinuous increase with when aridity exceeded 0.45.This implied at this aridity,coniferous forest may started to degrade to grass.(5)Land surface albedo derived from remote sensing can reveal two stable states in both Sahel and Mongolian Plateau.But the aridity range during which two albedo states co-occurred and the threshold at which critical transition occurred were different in the two areas.In additions,in both Sahel and Mongolian Plateau,potential energy of the high albedo state became less than that of low albedo state in the aridity range during which two albedo states co-occurred.It implied the low albedo state tended to tip into the high albedo states,accompanied by trees degraded to grass.Therefore,the turnover of the potential energy have the potential to be an earlywarning signal of critical transitions(i.e.,degradation)in drylands.Totally,this study provids direct proofs for the existence of alternative stable states in real dryland ecosystems.The results are helpful for uncovering the mechanism of appearance of alternative stable states and developing general earlywarning signals of critical transitions by comparative studies in drylands under contrasting climate.This study can provide decision-making basis and scientific support for the government and management departments when combating dryland degradation. |
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