Evaluation of Land Surface Phenology in Northern Hemisphere Permafrost Regions

Vegetation phenology interacts strongly with climate through the exchange of carbon, water, momentum, and energy between terrestrial ecosystem and atmosphere. These vegetation dynamics in Northern Hemisphere permafrost regions have substantial uncertainties in previous studies, partly due to differences in datasets. Thus, reliable land surface phenology (LSP) retrievals are crucial for understanding the effects of climate change on ecosystems and biosphere-atmosphere-hydrosphere interactions. We assessed various LSP datasets at different spatial resolutions for 2001-2014, generated by different methods based on different satellite observations. We also assessed the accuracy of LSP by comparing with CO2 flux phenology. For start of growing season (SOS), the comprehensive evaluation indicated MODIS phenology showed better consistency and higher accuracy with flux-derived phenology observations (R = 0.54, RMSE = 36.2 days, bias<5 days). For end of growing season (EOS), we cannot conclusively determine which LSP performs best. In the Northern Hemisphere permafrost regions, SOS occurred at 100-150 days (April-May), and EOS occurred at 260-320 days (September-November). During 2001-2014, SOS occurred earlier by 0.33 +/- 0.30 days/yr. Significant trends were observed for 6.4%-27.6% of pixels, averaging -1.30 +/- 1.16 days/yr. EOS occurred earlier by 0.25 +/- 0.43 days/yr, with significant trends averaging -0.52 +/- 0.94 days/yr. Among LSPs, variability in EOS trends was significant, with even the direction of trends differing. This study may provide insights into LSP data selection and further understanding of vegetation dynamics and its mechanisms in permafrost regions.