swissEO Product Line: New Forest Indicators for Drought Monitoring – swissEO NDVIz and swissEO NDVIdiff
The effects of climate change, particularly drought periods and disturbances such as pest infestations or storms, pose major challenges for Swiss forests. Detailed, nationwide monitoring is essential to detect changes in vitality at an early stage and to support management decisions. Building on the existing swissEO products for drought monitoring, swisstopo is now launching two new geodata products specifically designed for forest areas.
Interdisciplinary collaboration for Swiss forests
The Federal Office of Topography swisstopo is expanding the operational monitoring of vegetation conditions. In close cooperation with the Federal Office for the Environment (FOEN), whose platform waldmonitoring.ch is a key user of these data, these specific forest products were defined. They are based on open satellite data with a spatial resolution of 10 m and utilize the foundations prepared in swissEO S2-SR (Surface Reflectance). This data infrastructure enables precise and timely monthly analyses during summer (June/July, July/August, August/September) of forest conditions across Switzerland.
swissEO NDVIz: Forest vitality in long-term comparison
The product “swissEO NDVIz” provides detailed information on forest vitality anomalies since 2017. It sheds light on changes in vegetation vitality, as NDVI (Normalized Difference Vegetation Index) allows the chlorophyll content — the “greenness” of plants — to be inferred.
For practical use, current NDVI values are statistically classified. Using the Z-score, the current values are compared with the median of the climate reference period (1991–2020).
- A positive Z-score at a given location (pixel) indicates more vigorous vegetation than the long-term average for that time of year (e.g. due to growth or increased productivity).
- A negative Z-score indicates reduced vitality. This may point to natural disturbances (drought, insect damage, storm damage) or to forest management activities (e.g. timber harvesting).
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swissEO NDVIdiff: Annual forest changes at a glance
The “swissEO NDVIdiff” product focuses on annual changes in forests since 2018 by directly comparing NDVI values from two consecutive years.
For practical use, the NDVI values of the previous year’s period are subtracted from those of the current period to obtain a measure of annual change.
- Positive differences are observed in cases of early leaf-out, general tree growth, or recovery after a stressful previous year (e.g. drought).
- Negative differences indicate disturbances such as early leaf discoloration, windthrow, forest fires, disease or insect infestation, as well as forest management measures (timber harvesting)
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Outlook
The new swissEO products NDVIz and NDVIdiff represent an important step toward detailed, long-term forest monitoring in Switzerland. They complement existing indicators (such as the VHI) and provide essential foundations for platforms like waldmonitoring.ch. Future developments will focus on further reducing data gaps (clouds, shadows, missing satellite coverage) and refining the temporal resolution of the monitoring, in order to address the challenges of climate change in forest management more effectively.
Further information
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