Circumpolar vegetation dynamics product for global change study
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Land surface phenology (LSP) and vegetation growth of the circumpolar north are changing in response to more pronounced warming in the region. We here introduce the first phenology index (PI) based vegetation dynamics product, comprising start (SOS), end (EOS), length of growing season (LOS), and growing season integrated annual normalized difference vegetation index (NDVI), specifically designed for the entire circumpolar north (>45°N) using SPOT VGT data starting from 1999. PI combines the merits of NDVI and normalized difference infrared index (NDII) by taking the difference of squared greenness (from NDVI) and wetness (from NDII) to remove the soil and snow cover dynamics from key vegetation LSP cycles. The results show that the circumpolar vegetation dynamics and their spatial distributions are realistically detected. Further validation based on North American and European deciduous broadleaf, evergreen needleleaf and mixed forests, and wetland flux tower sites shows good agreements between the LSP dates from the circumpolar vegetation dynamics and ground phenology estimates from CO2 flux measurements. The validation also proves that the circumpolar vegetation dynamics product is an improvement over the operational global MODIS Combined Land Cover Dynamics (MCD12Q2) product for the circumpolar region. The results are further compared with the interannual variability of sea ice extent and leading teleconnection patterns in the region. The circumpolar averaged results show that, the growing season integrated annual NDVI is significantly increasing (0.68%year−1, p=0.006) and well correlated with the growing season sea ice extent trend (p=0.007). The circumpolar vegetation dynamics is more related to Polar/Eurasia pattern (i.e., indicator of circumpolar vortex) than to Scandinavian Pattern (SCA) and North Atlantic Oscillation (NAO). In view of the considerable scientific and policy importance of the circumpolar region, particularly the arctic ecosystems, the presented circumpolar vegetation dynamics product will greatly contribute to study changes in plant growth, phenology, photosynthetic capacity, and associated feedbacks under climate change.
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