Challenges and Limitations in Comparing Satellite Microwave Vegetation Optical Depth (VOD) Against in-Situ Tree Hydraulics in the Canadian Boreal Forest

Microwave remote sensing can be used to estimate vegetation optical depth (VOD), a measure of the attenuation of microwave radiation by vegetation. VOD is tightly linked to vegetation properties such as water content and above-ground biomass. Satellite-based VOD appears to be sensitive to tree hydraulics at larger scales, highlighting its potential to improve understanding of tree water status across the boreal biome. However, a detailed comparative analysis of boreal tree hydraulic functioning from microwave VOD is still lacking. This study explored empirical relationships between seven microwave VOD data products at three frequencies (L-, C- and X-bands) against in-situ tree hydraulic measurements. Additionally, we examined the correlations between VOD and above-ground biomass. Continuous measurements of relative dielectric constant, stem radius variations, and sap flux density across boreal forest stands were used to quantify tree hydraulic functioning across the Canadian boreal forest. Inter-comparisons between the different microwave VOD data products across forest stands and sensors revealed high spatiotemporal differences between all VOD data products for AM and PM orbit times. We observed higher VOD values in the Taiga Plains ecozone and lower values in the Boreal Plains and Boreal Shields ecozones, which may be attributed to differences in vegetation density across these regions. As expected, tight linear correlations were found between mean annual VOD values and above-ground biomass, with correlation coefficients exceeding 0.7 across all products and boreal forest stands. Analysis comparing microwave VOD products against in-situ tree hydraulic data demonstrated considerable differences across forest stands and years. Unexpected negative correlations (ranging from −0.02 and −0.8) were obtained between passive VOD and tree dielectric measurements. This relationship could be driven by soil moisture dynamics, which generally exhibited negative behaviors with VOD. Conversely, correlations were positive with the active VOD product, reaching approximately 0.9 in some cases. Our findings indicate strong interannual and spatial variability in the relationships between VOD and in situ tree hydraulic data and suggest that the VOD retrievals are limited in the boreal region. Further work should explore how to improve daily VOD retrievals in boreal forests.