Time-Series of Cloud-Free Sentinel-2 NDVI Data Used in Mapping the Onset of Growth of Central Spitsbergen, Svalbard

Abstract

The Arctic is a region that is expected to experience a high increase in temperature. Changes in the timing of phenological phases, such as the onset of growth (as observed by remote sensing), is a sensitive bio-indicator of climate change. In this paper, the study area was the central part of Spitsbergen, Svalbard, located between 77.28°N and 78.44°N. The goals of this study were: (1) to prepare, analyze and present a cloud-free time-series of daily Sentinel-2 NDVI datasets for the 2016 to 2019 seasons, and (2) to demonstrate the use of the dataset in mapping the onset of growth. Due to a short and intense period with greening-up and frequent cloud cover, all the cloud-free Sentinel-2 data were used. The onset of growth was then mapped by a NDVI threshold method, which showed significant correlation (r2 = 0.47, n = 38, p < 0.0001) with ground-based phenocam observation of the onset of growth in seven vegetation types. However, large bias was found between the Sentinel-2 NDVI-based mapped onset of growth and the phenocam-based onset of growth in a moss tundra, which indicates that the data in these vegetation types must be interpreted with care. In 2018, the onset of growth was about 10 days earlier compared to 2017.

Winters are changing: snow effects on Arctic and alpine tundra ecosystems

“…There is a continued need, however, for improving the validation between remotely-sensed and ground-based observations of phenology (see, e.g. Karlsen et al 2021 for an approach). Further development of in-situ low-cost snow monitoring techniques (e.g., Lewkowicz 2008;Siren et al 2018;Tutton and Way 2021) and expanded winter field data collection should also be prioritized to provide better validation of remotely-sensed snow products (Walker et al 2021).…”

Changes in Onset of Vegetation Growth on Svalbard, 2000–2020

“…It happens rapidly, is easily observed in the field on Svalbard, and is extracted from phenocams with an inaccuracy of less than 4 days. More details regarding the observations and observation sites are found in Karlsen et al [ 16 ].…”

“…Recently, a cloud-free time series of daily Sentinel-2 data for the 2016–2019 period was processed [ 16 ]. This dataset covers the central parts of Svalbard (outlined in Figure 1 ) and was used in mapping the onset of vegetation growth validated from the described in-situ phenological data.…”

“…When Karlsen et al [ 16 ] used time series of Sentinel-2 NDVI data to map the onset of vegetation growth in central Svalbard, they first computed the 4-year (2016–2019) mean NDVI value for every pixel in the study area for the 10 July to 5 August period. This period was chosen as the period where the leaves are fully open and before senescence starts and reduced the “noise” from snow-covered ground.…”

“…Since then, Svalbard has experienced temperature increases in all months compared to 2000–2013 [ 15 ]. Recently, a cloud-free time series of Sentinel-2 NDVI data was processed and used to map the onset of vegetation growth in central Svalbard during the 2016–2019 period [ 16 ]. That study used in-situ phenological data from a network of time lapse cameras designed for upscaling to the 10 m pixel resolution Sentinel-2 data.…”