Plant Phenology in Northern Scandinavia

Vegetation phenology, the timing of seasonal biological events, is a key indicator of ecosystem responses to environmental changes (Jin et al., 2014). Environmental changes are prevalent and have a higher impact on the northern hemisphere. By using remotely sensed satellite data, it is possible to capture phenological variation; this variation is most commonly captured by use of vegetation indices (VI). The Plant Phenology Index (PPI) is a physically based vegetation index. It exhibits a strong linear relationship with green Leaf Area Index (LAI) and gross primary productivity (GPP), making it a more biophysically meaningful measure of vegetation activity (Xiao et al., 2004; Jin & Eklundh, 2014). PPI is also less sensitive to snow cover, presence of clouds, and soil brightness, which often distort traditional indices (Jin & Eklundh, 2014).

Study Area

The study area (longitude: 0.00–29.26; latitude: 60.00–70.00) encompasses the Scandinavian Peninsula with Norway and central/northern Sweden, including the Scandinavian mountain range. The area is primarily under a cold climate regime with a subarctic climate, whereas the mountainous regions are classified as tundra (ET), and the coasts have a subpolar oceanic climate.

Data

Plant Phenology Index (PPI) was made from MODIS reflectance data (2001 to 2021).

The time-series analysis was done in TIMESAT (MATLAB).

The image processing in IDRISI TerrSet and maps created using ArcGIS Pro.

Methods

The time series were smoothed by applying the global double logistic function.

The change over time was analyzed using five variables:
- - Start of the season (SOS)
  - End of the season (EOS)
  - Length of the season (LOS)
  - Large Integral (LI)
  - Small Integral (SI)

Trend analysis methods applied to each variable:
- - Median-trend (Theil-sen);
  - Non-linear monotonic trend (Mann-Kendall).

Results

Over the study period, 2001–2021, a general increase in LOS for the central and northern parts of the Scandinavian peninsula is apparent. This has been brought about by an earlier SOS and a later EOS, resulting in an increase in productivity, which is reflected in the SI and LI. This increase is not uniform nor present in all areas. High-altitude and high-latitude regions show a decreasing trend in the phenological parameters, likely due to the fact that the vegetation in these areas is not as capable of adapting to the recent changes in climate compared to vegetation in the more southern and coastal regions. This indicates that the northern ecosystems are more sensitive to the ongoing changes in climate, which can result in more significant changes in the future if the climate continues to change rapidly.

Conclusion

The use of phenology parameters, such as the SOS, EOS and LOS, as well as the productivity indicators SI and LI can aid in the understanding of how vegetation reacts to changes in its environment. Over the study period, 2001 – 2021, a general increase in LOS for the central and northern parts of the Scandinavian peninsula is apparent. This has been brought about by an earlier SOS and a later EOS, resulting in an increase in productivity, as reflected in the SI and LI. This increase is not uniform nor present in all areas. High altitude and latitude regions show a decreasing trend in the phenological parameters, likely due to the fact that the vegetation in these areas are not as capable to adapt to the recent changes in climate compared to vegetation in the more southern and coastal regions. This indicates that the northern ecosystems are more sensitive to the ongoing changes in climate which can result in more significant changes in the future if the climate continues to change rapidly.