Study On The Response Of Vegetation To Extreme Events Based On Passive Microwave Remote Sensing Dat

The global climate change leads to frequent extreme events,e.g.,extreme drought,wet and fire are the three common events with great impacts on vegetation dynamics.Investigating the influences of the three extreme events on vegetation dynamics will improve the understanding of the interaction between vegetation and climate change.The passive microwave-based vegetation optical depth(VOD)is minimally affected by clouds and atmospheric aerosols due to the longer microwave wavelength and its stronger penetration capacity.VOD is an indicator of the vegetation water content in total aboveground biomass,including both photosynthetic(leaf)and non-photosynthetic(woods)components.Compared with the typical optical vegetation indices which are more sensitive to canopy greenness,it can provide more comprehensive information on vegetation characteristics.This paper uses VOD,optical vegetation indices and multisource hydrometeorological database to study the response of vegetation to extreme events,taking the extreme drought and wet events in southern Amazon,Australia and the 2019 fire in southern Amazon as examples.In the past 20 years,the high frequency of extreme events in southern Amazon and Australia had played a pivotal role in the interannual variability of the global carbon cycle.The main conclusions are as follows:(1)VOD was used to investigate the response of canopy to drought(2005 and 2010)and wet(2009)events in southern Amazon.A two-stage canopy growth anomaly in the recordbreaking wet year 2009,i.e.,reduced-preceding-enhanced canopy growth.A two-stage dynamic pattern was also observed in 2005 and 2010 droughts,but in the opposite phase of 2009.The opposite phase of canopy growth between wet and droughts was caused by different hydrometeorological processes.In the wet-to-dry season(April–July)with sufficient water,the above average radiation enhanced canopy growth in drought years,while the below average radiation retrained canopy growth in wet 2009.In the late dry season(August–September),water and heat stress in drought years adversely affected canopy growth,while the increased radiation with sufficient water enhanced canopy growth in wet 2009.Interestingly,if we examined the regional canopy growth during the typical dry season(July–September),both drought and wet years showed similarly negative anomalies,but resulting from opposite eco-hydrological processes.These results help to clarify the argument regarding Amazon rainforest canopy greening or browing in the dry season(July–September)of 2005,2010 and 2009.(2)Both VOD and optical vegetation indices were used to investigate the response of vegetation to drought(2017–2019)and wet(2010–2012)events in Australia.Vegetation growth was maily driven by water in this tyical semi-arid region.In the early stage of extreme drought and wet events,the response of optical vegetation indices was stronger than that of VOD.With the enhancement of drought and wetness,VOD can monitor more intense changes in vegetaion.The response of Australia vegetation to drought and wet events is asymmetric,that is,the response to wet is stronger than that to drought.The higher magnitude of vegetation growth was detected in wet event relative to the decrease of vegetation in drought event.(3)VOD and optical vegetation indices were used to characterize canopy dynamics in the southern Amazon during the 2019 fire season(July–October).VOD can reflect the spatiotemporal distribution of fire-driven canopy changes,while optical vegetation indices can only reflect the temporal pattern of canopy changes of grid cells with fire activities.The magnitude of canopy damage increased with the rising fire occurrences from July to September.In October,with fires decreasing or stopping,the canopy showed signs of recovery in areas with fewer fires.The lower magnitude of canopy damage and recovery detected by the VOD relative to optical indices.The discrepancy between VOD and optical vegetation indices in response to extreme events showed that the response of non-photosynthetic components of vegetation to extreme events is slower than that of leaves.Because of the different physical meaning,the combination of VOD and optical vegetation indices can provide a more accurate information on the vegetation changes caused by extreme events.This helps to better understand the impact of extreme events on ecosystems.Better understanding the influences of the three extreme events on vegetation dynamics will improve our prediction capacity of vegetation responses to future extreme events which are predicted to be more frequent.