Changes in growing season duration and productivity of northern vegetation inferred from long-term remote sensing data
Monitoring and understanding climate-induced changes in the boreal and arctic vegetation is critical to aid in prognosticating their future.We used a 33 year (1982–2014) long record of satellite observations to robustly assess changes inmetrics of growing season (onset: SOS, end: EOS and length: LOS) and seasonal total gross primary productivity. Particular attentionwas paid to evaluating the accuracy of thesemetrics by comparing them tomultiple independent direct and indirect growing season and productivitymeasures. These comparisons reveal that the derivedmetrics capture the spatio-temporal variations and trendswith acceptable significance level (generally p<0.05).We find that LOS has lengthened by 2.60 d dec (p<0.05) due to an earlier onset of SOS (−1.61 d dec, p<0.05) and a delayed EOS (0.67 d dec, p<0.1) at the circumpolar scale over the past three decades. Relatively greater rates of changes in growing seasonwere observed in Eurasia (EA) and in boreal regions than inNorthAmerica (NA) and the arctic regions. However, this tendency of earlier SOS and delayed EOSwas prominent only during the earlier part of the data record (1982–1999). During the later part (2000–2014), this tendencywas reversed, i.e. delayed SOS and earlier EOS. As for seasonal total productivity, wefind that 42.0%of northern vegetation shows a statistically significant (p<0.1) greening trend over the last three decades. This greening translates to a 20.9%gain in productivity since 1982. In contrast, only 2.5%of northern vegetation shows browning, or a 1.2% loss of productivity. These trends in productivity were continuous through the period of record, unlike changes in growing seasonmetrics. Similarly, wefind relatively greater increasing rates of productivity in EA and in arctic regions than inNA and the boreal regions. These results highlight spatially and temporally varying vegetation dynamics and are reflective of biome-specific responses of northern vegetation during last three decades.