Assessing Moisture Content Variability in Tropical Soils and Trees at the Luquillo Experimental Forest (LEF) Using Multi-Frequency Ground-Penetrating Radar Surveys

Soil and tree moisture content strongly influence eco-hydrological processes, dictating carbon and nutrient pathways. As such, effective methods for imaging its spatial distribution are essential for a better understanding of water availability at the ecosystem level. For many decades, ground-penetrating radar (GPR) has become an increasingly common and effective tool for estimating and modeling subsurface moisture content due to its non-invasive nature and its high spatial resolution. However, its application in trees has been limited. In this study, we use a unique array of GPR measurements to evaluate the variability of moisture content within both soils and trees in the tropical Luquillo Experimental Forest (LEF), Puerto Rico. GPR measurements with frequencies ranging from 1,200-160 MHz were taken across seven study sites in the rainforest during four campaigns: January 2023, June 2023, March 2024, and January 2025. Data includes: 1) GPR transects (15-30 m) along a 2–3-meter soil column to trace changes in moisture content; 2) GPR measurements along tropical tree trunks to assess moisture variability; and 3) laboratory-based site-specific soil calibrations relating soil moisture measurements to GPR permittivity. Time-domain reflectometry moisture probes were used to validate GPR-derived moisture values from field transects. Preliminary results indicate lower soil moisture in June within the top meter that are consistent with moisture probe and rainfall measurements. Deeper layers remain highly saturated across all sampling periods. In contrast, GPR-derived tree moisture shows little consistency across campaigns and does not relate to rainfall. Rather, the values align closer with site elevation and diameter, with larger trees at higher elevations containing more moisture. This research suggests that GPR is a powerful tool for capturing fine-scale hydrological dynamics in tropical rainforest ecosystems and highlights its potential for assessing ecosystem vulnerability, particularly in an environment prone to extreme weather events.