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. High-latitude treeless ecosystems represent spatially highly heterogeneouslandscapes with small net carbon fluxes and a short growing season. Reliableobservations and process understanding are critical for projections of thecarbon balance of the climate-sensitive tundra. Space-borne remote sensing is theonly tool to obtain spatially continuous and temporally resolved informationon vegetation greenness and activity in remote circumpolar areas. However,confounding effects from persistent clouds, low sun elevation angles,numerous lakes, widespread surface inundation, and the sparseness of thevegetation render it highly challenging. Here, we conduct an extensiveanalysis of the timing of peak vegetation productivity as shown by satelliteobservations of complementary indicators of plant greenness andphotosynthesis. We choose to focus on productivity during the peak of thegrowing season, as it importantly affects the total annual carbon uptake. Thesuite of indicators are as follows: (1) MODIS-based vegetation indices (VIs) as proxiesfor the fraction of incident photosynthetically active radiation (PAR) that is absorbed (fPAR), (2) VIs combined with estimatesof PAR as a proxy of the total absorbed radiation (APAR), (3) sun-induced chlorophyll fluorescence (SIF) serving as a proxyfor photosynthesis, (4) vegetation optical depth (VOD), indicative of totalwater content and (5) empirically upscaled modelled gross primaryproductivity (GPP). Averaged over the pan-Arctic we find a clear order of theannual peak as APAR  ≦ GPP SIF VIs / VOD . SIF as an indicator ofphotosynthesis is maximised around the time of highest annual temperatures.The modelled GPP peaks at a similar time to APAR. The time lag of the annual peakbetween APAR and instantaneous SIF fluxes indicates that the SIF data docontain information on light-use efficiency of tundra vegetation, but furtherdetailed studies are necessary to verify this. Delayed peak greennesscompared to peak photosynthesis is consistently found across years and land-cover classes. A particularly late peak of the normalised differencevegetation index (NDVI) in regions with very small seasonality in greennessand a high amount of lakes probably originates from artefacts. Given the veryshort growing season in circumpolar areas, the average time difference inmaximum annual photosynthetic activity and greenness or growth of 3 to 25 days(depending on the data sets chosen) is important and needs to be consideredwhen using satellite observations as drivers in vegetation models.

Assessing the dynamics of vegetation productivity in circumpolar regions with different satellite indicators of greenness and photosynthesis