Abstract
Abstract. High-latitude treeless ecosystems represent spatially highly heterogeneous landscapes with small net carbon fluxes and a short growing season. Reliable observations and process understanding are critical for projections of the carbon balance of the climate-sensitive tundra. Space-borne remote sensing is the only tool to obtain spatially continuous and temporally resolved information on 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 the vegetation render it highly challenging. Here, we conduct an extensive analysis of the timing of peak vegetation productivity as shown by satellite observations of complementary indicators of plant greenness and photosynthesis. We choose to focus on productivity during the peak of the growing season, as it importantly affects the total annual carbon uptake. The suite of indicators are as follows: (1) MODIS-based vegetation indices (VIs) as proxies for the fraction of incident photosynthetically active radiation (PAR) that is absorbed (fPAR), (2) VIs combined with estimates of PAR as a proxy of the total absorbed radiation (APAR), (3) sun-induced chlorophyll fluorescence (SIF) serving as a proxy for photosynthesis, (4) vegetation optical depth (VOD), indicative of total water content and (5) empirically upscaled modelled gross primary productivity (GPP). Averaged over the pan-Arctic we find a clear order of the annual peak as APAR ≦ GPP<SIF<VIs/VOD. SIF as an indicator of photosynthesis 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 peak between APAR and instantaneous SIF fluxes indicates that the SIF data do contain information on light-use efficiency of tundra vegetation, but further detailed studies are necessary to verify this. Delayed peak greenness compared to peak photosynthesis is consistently found across years and land-cover classes. A particularly late peak of the normalised difference vegetation index (NDVI) in regions with very small seasonality in greenness and a high amount of lakes probably originates from artefacts. Given the very short growing season in circumpolar areas, the average time difference in maximum annual photosynthetic activity and greenness or growth of 3 to 25 days (depending on the data sets chosen) is important and needs to be considered when using satellite observations as drivers in vegetation models.
Highlights
Landscapes in circumpolar regions are characterised by sparse vegetation, bare soil, rocks, large surface areas inundated by open water and a long snow-covered period
While a proxy of the total absorbed radiation (APAR) is highest around DOY191 (10 July), the METGPP indicates the maximum photosynthetic activity at a similar time and the RSGPP 4 days later
Removing the effect of incoming radiation from the sun-induced chlorophyll fluorescence (SIF) measurement by dividing by cos(sun zenith angle) shifts the annual maximum for SIF.cosSZA GFZ by 6 days compared to SIF GFZ and by 11 days for SIF.cosSZA NASA compared to SIF NASA, and there is comparatively large scatter
Summary
Landscapes in circumpolar regions are characterised by sparse vegetation, bare soil, rocks, large surface areas inundated by open water and a long snow-covered period. NEE, GPP and respiration are governed by variable conditions regarding the wetness and temperature, microtopography, geomorphology and type, and acidity of the soils (Kwon et al, 2006; Walker et al, 1998; Olivas et al, 2011; Emmerton et al, 2016; Pirk et al, 2017) It is not clear whether, where and when the land surface in Arctic tundra acts as a sink or source of CO2 (Cahoon et al, 2012; McGuire et al, 2012) and what the direction and magnitude of changes in altered climatic conditions will be (Oechel et al, 1993; Billings, 1987; Sitch et al, 2007). This has given rise to extensive and long-term project studies of the Arctic like the Arctic-Boreal Vulnerability Experiment (ABoVE, https: //above.nasa.gov/about.html, last access: 22 August 2018) or the Carbon in Arctic Reservoirs Vulnerability Experiment (CARVE, https://carve.jpl.nasa.gov/Missionoverview/, last access: 22 August 2018), both of which are not limited to CO2
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