Spatiotemporal Evolution Of Global Vegetation Phenology And Its Response To Multi-Scale Drought

Vegetation phenology directly affects the dynamic of carbon budget in terrestrial ecosystems and provides feedbacks to the climate system.Vegetation phenology has been the focus of research in recent years due to the closely measurable link between biological events and climate.Drought is one of the more complicated anomalous climate phenomena that affect terrestrial ecosystems,which will increase in intensity and frequency in the context of global warming and have increasingly severe impacts on terrestrial ecosystems.The effect of climate change on vegetation with temporal cumulative and lagged effects,the limited understanding of these effects has generated uncertainty in exploring the response of vegetation growth and ecosystem carbon sinks to climate change,thus increasing the difficulty of quantifying the temporal variability of vegetation response to drought.Consequently,the accurate assessment of drought-induced decline in vegetation vigor is important for developing mitigation strategies in drought-prone areas and enhancing the understanding of abnormal spatial variability in vegetation cover under environmental stress.Based on the land surface phenology dataset and Standardized Precipitation Evapotranspiration Index（SPEI）,this study examined the spatial and temporal dynamics of global vegetation phenology and drought during 1982—2015 and quantified the cumulative and lagged effects of drought on global terrestrial ecosystems according to the maximum correlation coefficients(R_max-cml and R_max-lag)between phenology and SPEI at different time scales.The intensity of drought effects on surface phenology in different vegetation types and moisture gradients was also analyzed,and the effects of climatic factors and vegetation growth conditions on phenology-drought relationships were further investigated with the ERA5-Land climate dataset.The main findings are as follows.（1）The onset of SOS was mainly concentrated in 1—150 d（about 68.83%of the vegetated area）,the onset of EOS was mainly distributed in 250—350 d（72.78%of the vegetated area）,and the duration of growing season length（LOS）was mainly 150—250 d（80.63%of the vegetated area）.SOS was extremely significantly advanced(-0.10 d·a^-1,p<0.001),EOS was extremely significantly delayed(0.21 d·a^-1,p<0.001),and LOS was extremely significantly prolonged(0.50 d·a^-1,p<0.001)at the global scale;about 33.54%of the terrestrial area showed a significant aridity trend,and about 25.97%showed a significant wetting trend.（2）Approximately 25.36%of the SOS,26.43%of the EOS and 26.57%of the LOS of the vegetated areas globally were significantly affected by the cumulative effect of drought.Among them,SOS is mainly negatively correlated with SPEI on long-term time scales,while EOS and LOS are positively correlated with SPEI on short-term time scales.For the different vegetation areas,R_max-cml was greatest for the shrubland and the shortest cumulative time scales for SOS and LOS responses were found for the Forest.At different moisture gradients,the SOS of grassland was most significantly affected by the cumulative effect,R_max-cml was greater in the arid semi-arid zone than in the humid semi-humid zone,while vegetation in the humid semi-humid zone responded to cumulative effect on a longer time scale than in the arid semi-arid zone.（3）The area affected with drought lagged effect on vegetation phenology was larger than the cumulative effect,which showed that the vegetated areas significantly affected by lagged effect were 46.12%,47.93%and 50.45%for SOS,EOS and LOS,respectively,and accompanied with longer lagged time scales.SOS was mainly negatively correlated with lagged SPEI,while EOS and LOS were positively correlated with lagged SPEI.R_max-lagshowed a trend of increasing and then decreasing from Forest to Cropland,with the largest in Shrubland,followed by Grassland,Forest and Cropland.Grassland SOS had the shortest lagged month（6.35 months）and Forest EOS had the longest lagged month（6.61 months）.R_max-lag showed a significant decreasing trend from the arid to the humid zone,with the longest time scales of 6.61 months and 6.77 months for SOS and LOS affected by the lagged effect in the humid area,respectively.（4）The maximum correlation coefficients of phenology-SPEI(R_max-cml and R_max-lag)were mainly dominated by moisture conditions（precipitation and evaporation）,while the cumulative and lagged time scales of phenology response to drought were mainly driven by thermal conditions（temperature and radiation）.Moreover,plant greening and senescence rates also affect the relationship,suggesting that hydrothermal conditions modulate the timing of phenology by regulating plant greening/senescence rates,thus indirectly or directly affecting the phenology-drought relationship.