Field measurements were conducted between 21 and 23 September 2024 within the Tatra National Park, in accordance with permits issued by the Tatra National Park (DBN.503/11/24) and the Ministry of Climate and Environment (DOP-WPN.61.132.2022.RS). A total of 40 study plots were established, characterized by homogeneous Norway spruce (Picea abies) stands, allowing for improved validation of satellite data due to the absence of spectral reflectance mixing with other objects. Measurements were taken on spruce needles located on the most sun-exposed side of the canopy to best represent the conditions of the treetop sections, which receive maximum sunlight, similarly to the treetop areas observed by satellites. Measurement points were recorded using LocusMap software installed on a smartphone equipped with a GNSS receiver receiving signals on L1 and L5 frequencies, providing high spatial accuracy (PDOP < 2.5 m). Spatial data with location of field plots is stored in GeoPackage (.gpkg) format which is an open OGC standard for storing vector and raster geospatial data in a single SQLite file, fully supported by open software like QGIS and other GIS tools. The timing of field campaigns was coordinated with Sentinel-2 satellite overpasses, during which four cloud-free satellite images were acquired (20 and 23 September), enabling precise temporal and spatial validation of field data using Sentinel-2 imagery.
During the field campaigns on the selected Tatra National Park plots, comprehensive physiological and spectral measurements of Norway spruce needles were performed. Detailed assessments included canopy temperature, spectral reflectance, chlorophyll fluorescence, and chlorophyll content. Air temperature was additionally recorded, providing a full spectrum of environmental conditions for the measured trees. Each parameter was measured at least 10 times per plot, ensuring reliability and statistical representativeness. All methods were fully non-invasive, an essential consideration for preserving valuable natural resources in the Tatra National Park. To achieve robust validation and reduce bias, measurements were conducted on various parts of the tree, including both young shoots and older needles, allowing for a more complete representation of the physiological state of the trees within the study plots.
Spectral measurements
Spectral reflectance was measured using an ASD FieldSpec 4 spectrometer, capturing signals from 350 to 2500 nm across 2151 spectral channels. This advanced spectrometer enables detailed plant spectral analysis, critical for monitoring tree health and responses to environmental variability. Prior to each measurement, the device was calibrated using a standard white reference panel (spectralon), ensuring high accuracy and repeatability. The spectrometer was used in conjunction with an ASD Plant Probe equipped with a Leaf Clip clamp, providing stable and controlled illumination conditions. Each recorded measurement consisted of 25 independent scans, averaged to produce a single measurement, minimizing errors from individual readings and ensuring consistent spectral data.
Chlorophyll fluorescence measurements
Chlorophyll fluorescence was measured using an OptiScience OS1p fluorometer, with the main parameter being Fv/Fm (maximum photochemical efficiency of photosystem II – PSII). Measurements were conducted both after dark adaptation and under ambient light conditions to obtain comprehensive information on plant physiological status:
- Dark-adapted chlorophyll fluorescence: Needles were adapted to darkness for 30 minutes using specialized dark clips provided by project funding, allowing assessment of maximum PSII photochemical efficiency and ensuring accurate, repeatable readings.
- Non-dark-adapted chlorophyll fluorescence: Additional measurements on random plots under ambient light were performed to assess needle fluorescence under natural light conditions.
Chlorophyll content measurements
Chlorophyll content in Norway spruce needles was measured using a Chlorophyll Content Meter CCM-300. The device provides absolute chlorophyll content [mg/m²], reflecting photosynthetic efficiency and overall plant health. The meter was calibrated prior to each measurement session using a calibration filter to ensure accuracy. Measurements employed specialized clamps to stabilize needles, ensuring precision and repeatability. Two parameters were recorded:
- CFR (Chlorophyll Fluorescence Ratio): Ratio of fluorescence emission at 735 nm to 700 nm (735/700), an important chlorophyll content indicator.
- Chl (Chlorophyll Content): Absolute chlorophyll content in mg/m², calculated using Gitelson’s equation.
Evapotranspiration (thermal stress) measurements
Thermal stress was assessed through both air temperature and vegetation radiative temperature, key indicators of microclimatic conditions at the study plots. Spruce canopy radiative temperature (Ts) was measured using an iRtec MiniRay-100 pyrometer, providing non-contact and precise surface temperature readings. Air temperature (Ta) was recorded at each plot, enabling calculation of the thermal stress index (Ts–Ta), representing the difference between canopy radiative temperature and ambient air temperature. These measurements facilitate accurate monitoring of tree thermal responses to environmental conditions and are crucial for analyses of plant adaptation to changing environmental factors.
Research funded by the National Science Centre (NCN), under the project Preludium 22, grant no. 2023/49/N/ST10/00517, entitled: ‘Spruce Forest Damage Assessment Using Machine Learning on Sentinel-2 Time Series in the Tatra Mountains’.
(2025)
