During the BorealScat radar tower experiment (2016 to 2021) in southern Sweden, a single forest stand was observed using a multi-polarization tomographic radar for several years. A variety of measurement sequences at multiple frequency bands were used to study the effects of soil moisture, weather and seasonal changes on radar signals at time scales ranging from less than a second to several years. The experiment was partly an ESA campaign for studying weather-induced radar measurement disturbances in support of present and future synthetic aperture (SAR) radar missions with the goal of estimating above-ground biomass, tree height and forest structure. In addition to characterising the temporal variation of boreal forest radar reflectivity, effects of tree water dynamics, drought-induced stress and insect damage were observed during the experiment.
The experiment consisted of a 50-m high tower with 30 antennas mounted at the top. The tower overlooked a mature stand of Norway spruce. The radar system allowed tomographic imaging whereby images could be produced representing the vertical cross section of the forest. This allowed backscatter contributions from different parts of the forest (e.g., ground and canopy) to be separated. Radar measurements were conducted at P-band (436 MHz), L-band (1270 MHz) and C-band (5400 MHz), offering different degrees of forest penetration. These frequency bands are supported by the main current and future SAR missions for forest radar observations such as ESA’s BIOMASS, ROSE-L and Sentinel-1 missions, NASA/ISRO’s NISAR mission and JAXA’s ALOS-2/4 missions. BorealScat observations were also used for studying short-term decorrelation in preparation for the ESA’s candidate Earth Explorer mission Hydroterra. Tomographic images were acquired at 5-minute intervals while Doppler observations were carried out over 10-s periods to study sub-second variations.
A weather station was installed for correlating radar time series measurements to weather variables. Freeze-thaw cycles caused the largest fluctuations in radar time series observations as moisture in the forest turned to ice in sub-zero temperatures and thawed again in warmer conditions. Soil moisture variations caused large fluctuations in ground-level backscatter, while canopy backscatter was relatively stable during non-frozen conditions, supporting P-band forest parameter estimation methods based on tomography and interferometric ground notching. During the hot summer of 2018, diurnal cycles were observed in radar time series, which were suspected to be connected to diurnal tree water content fluctuations. Water stored in tree tissues contribute to transpired water during hot days to sustain high transpiration rates. This effect was accentuated by strong winds that accelerate the rate of transpiration under certain conditions, showing clear drops in backscatter during the day. In sustained hot and dry conditions, this diurnal cycle would decrease in magnitude, indicating water stress. Digital point dendrometers were installed on tree stems to observe fluctuations in tree water content. Sap flow sensors, measuring the rate of water transport in stems, were also installed to complement the dendrometers. Forest damage due to bark beetles were detected in 2019, causing progressive deaths of trees in the forest stand. These effects could be observed in radar backscatter time series as a gradual drop in reflectivity. Early onset of damage was most clearly observed in L-band canopy attenuation observations, acquired by placing a large reflector within the forest. A similar effect was observed at C-band as trees died from bark beetle attacks.
BorealScat observations were also used to assess temporal coherence without the effects of spatial baselines and volume decorrelation that are always present in airborne and spaceborne SAR data. Temporal coherence cycles at diurnal timescales were observed during warm periods, offering the possibility of repeat-pass L- and C-band interferometry. Correlation tomography, a tomographic imaging method for overcoming temporal decorrelation, was investigated at L- and C-band. Vertical backscatter distributions from correlation tomography were not significantly affected by dynamic weather conditions. These results strongly motivate the implementation of a single-pass bistatic interferometer mission at L- or C-band for forest applications.
The BorealScat experiment provided forest radar observations that reveal a close connection between vegetation water transport mechanisms and tree vitality. The dataset has been made publicly available. The long overpass times of satellites do currently not facilitate the observation of some of these effects from space. To continue this study of the relationship between forest radar observations, tree water dynamics, forest-atmosphere water exchange and tree vitality, a new long-term experiment has been initiated in northern Sweden: BorealScat-2. A better understanding of how tree evapotranspiration, tree water content, stress and tree vitality can be sensed using radar will pave the way for new data exploitation opportunities and new earth observation missions.