Droughts force temporal change and spatial migration of vegetation phenology in the northern Hemisphere

Abstract

Ecosystems are exposed to continuous shifts in droughts. This shows up as a point on a pixel that moves continuously in a specific direction and angle (spatial migration) to preserve its original climate niche. We currently do not have a good understanding whether ecosystems exposed to droughts are more sensitive to droughts temporal trends or spatial migrations. Using dataset of vapor pressure deficit (VPD), total water storage anomaly (TWSA), and standardized soil moisture index (SSMI), we tested the spatiotemporal sensitivity of vegetation phenology to extreme droughts and compound extreme droughts, respectively. On the temporal scale, we found that the differences in the sensitivity of vegetation phenology to VPD, TWSA, and SSMI were mainly reflected in the lag time. Compared to normal years, however, the start of growing season (SOS) was advanced and the end of growing season (EOS) was delayed in most pixels even under extreme preseason droughts. Such results can be attributed to the extreme atmospheric driving forces. On the spatial scale, the spatial migration velocity of SOS and EOS was best correlated with the extreme atmospheric drought (r = 0.5, p < 0.01) and the extreme soil drought in the third layer (r = 0.43, p < 0.01), respectively. Compared to extreme droughts, however, the effects of compound extreme droughts on the spatial migration of vegetation phenology only produced weak negative feedback (|r| < 0.1, p < 0.01). This indicate that the velocity of vegetation phenology was consistent with the velocity of single extreme droughts. Our results emphasize the temporal sensitivity of droughts on vegetation phenology and the accompanying effects of extreme droughts on the spatial shifts in vegetation phenology.