Satellite monitoring of boreal forest phenology and its climatic responses in Eurasia

The fragile ecosystems in boreal Eurasia are sensitive to global climate change. Land surface phenology provides an important tool for us to better understand the current status of boreal forest and its climatic responses in this remote zone. This study utilizes the new-generation AVHRR GIMMS NDVI3g products in 1982-2011 to extract four phenological metrics in the study region, including start of season (SOS), end of season (EOS), season length (LOS), and middle of season (MOS). Linear and Mann-Kendall trend analyses are performed to examine their spatiotemporal patterns and relationships with climatic variables assisted with the Climate Research Unit re-analysis climatic data sets. While advanced spring greening is observed in agreement with past studies, our results reveal that the SOS advancemostly occurs in mixed forests in southern Eurasia. More importantly, this study extracts the opposite trends for the end of season - advanced EOS in coniferous forests above 60 degrees N and delayed EOS in mixed forests below. Overall, temperature in May-October has consecutively increased in the past 30 years. Precipitation has also increased but with a fragmented pattern. The advanced SOS across Eurasia is highly correlated with a warmer spring (April and May) in Eurasia. The EOS has a strong, negative relationship with fall precipitation (September). Further investigation is suggested to examine the opposite EOS trends and their environmental/ecological consequences in different forest zones of boreal Eurasia.