Investigating Soil and Plant Xylem Dielectric Relationships in Boreal Forest

The boreal forest is the second largest terrestrial biome and owing to its vast extent, is a critical component of the global climate system, functioning as a major carbon reservoir and regulating land–atmosphere water and energy exchanges. However, boreal ecosystems are highly sensitive to climate-driven changes in water availability, exacerbating drought stress, wildfire risk, and widespread, drought-induced tree mortality, demonstrating the need for improved characterization of soil and plant water dynamics. More specifically, soil moisture is a fundamental control on boreal forest productivity and disturbance dynamics, governing water availability for transpiration, photosynthesis, and internal plant water storage. While microwave remote sensing instruments provide valuable large-scale soil moisture observations, their interpretation in forested environments remains challenging due to the combined influence of soil and vegetation water on the microwave signal and a lack of species-specific validation data. In particular, the contribution of internal plant water storage to microwave observations is poorly constrained in boreal ecosystems.

In this study we examined coupled soil–plant water dynamics in a mixed boreal forest in central Saskatchewan as part of the SMAPVEX22-Boreal field campaign. Hourly measurements of real dielectric constant (RDC) were collected from near-surface organic soil (5 cm), mineral soil, and tree xylem across 27 forested sites during the 2022 growing season. Measurements focused on three dominant boreal species representing contrasting functional types: jack pine (Pinus banksiana), black spruce (Picea mariana), and trembling aspen (Populus tremuloides). To independently characterize internal plant water storage, destructive vegetation sampling was conducted to quantify gravimetric water content in primary branches, secondary branches (including foliage), and whole branches. Soil water potential was estimated using texture-based parameterizations to better represent plant-available water.

Time series analyses revealed a strong and consistent relationship between soil RDC and tree xylem RDC, indicating tightly coupled soil–plant water dynamics throughout the growing season. Soil moisture exhibited greater short-term variability than tree RDC, while xylem RDC showed a gradual seasonal drydown and became less responsive to individual precipitation events as summer progressed. Pronounced species-specific differences were observed: trembling aspen exhibited significantly higher and more variable xylem RDC than the conifer species, whereas black spruce sites were characterized by persistently wetter soils associated with thicker organic layers. Lag-correlation analysis showed virtually no delay between soil moisture and tree RDC at an hourly timescale for jack pine and black spruce, and a short (~1 hour) lag for aspen, with the strongest correlations (~ 0.80) occurring in the mineral soil layer, suggesting the influence of relatively shallow rooting depth in water access strategies.

These results reflect contrasting species-specific hydraulic strategies, with jack pine adapted to drier conditions and black spruce and aspen maintaining greater internal water storage. The strong, near-synchronous coupling between soil and plant water at hourly timescales suggests limited temporal separation between soil wetting and vegetation uptake in boreal forests, constraining their use as a signal-separation mechanism in microwave remote sensing. Thus, species-level hydraulic differences should be explicitly considered in soil moisture retrieval and validation frameworks.