Monitoring Thermal Responses and Growth Characteristics of Pinus densiflora Seedlings Using Multispectral Imaging

Thermal stress induces physiological changes in forest tree seedlings, and multispectral imaging offers a non-destructive approach for monitoring associated changes in spectral reflectance. This study evaluated the responses of Pinus densiflora seedlings to sustained high-temperature stress and short-term extreme thermal exposure using multispectral vegetation indices, photosynthetic pigments, and soil environmental measurements. Seedlings were subjected to four thermal treatments: 38 °C at 60% relative humidity (RH), 38 °C at 40% RH, 45 °C at 60% RH, and short-term exposure to 90 °C. Spectral responses were assessed using NDVI, GNDVI, NDRE, and CIRE. Temporal changes in vegetation indices and soil variables were evaluated using linear mixed-effects models, while pigment contents were analyzed using linear models. NDVI and GNDVI generally declined under sustained heat exposure, whereas NDRE remained comparatively stable and CIRE exhibited treatment-dependent responses. Photosynthetic pigment contents also varied among treatments, with total chlorophyll increasing under 38 °C at 60% RH but decreasing under 38 °C at 40% RH, while carotenoid contents increased under both conditions. Soil temperature and moisture changed significantly over time under all treatments (all p<0.001). Soil moisture reached a recorded value of 0% under both sustained 38 °C treatments, whereas relative reductions of 88.7% and 37.0% were observed under the 45 °C and short-term 90 °C treatments, respectively. Time-adjusted association analyses identified only one statistically significant relationship after false discovery rate correction, between GNDVI and soil temperature under 38 °C at 40% RH (FDR-adjusted p = 0.0499). Nevertheless, the observed temporal changes in vegetation indices indicate that multispectral imaging can capture spectral responses of seedlings to thermal stress, although the associations between vegetation indices and physiological or environmental variables were limited.