Integrated assessment of site quality for coastal Casuarina equisetifolia shelterbelts using ground-based modeling and remote sensing

Accurate assessment of site quality in coastal Casuarina equisetifolia (C. equisetifolia) plantations is essential for enhancing the protective function of shelterbelts and implementing site-specific afforestation strategies. However, traditional ground-based surveys are limited in spatial coverage and efficiency, hindering effective forest management. To overcome these limitations, this study developed an integrated assessment framework that couples ground-based modeling with remote sensing inversion to achieve large-scale site quality mapping. Field investigations on Pingtan Island, Fujian Province, China, were used to establish a ground-based evaluation model. Soil fertility was quantified using Principal Component Analysis (PCA), and principal components were classified into discrete fertility grades through K-means clustering. These grades, together with topographic variables, were incorporated into a site quality classification model constructed using Quantification Theory I. The point-based model was subsequently extrapolated using Landsat 9 imagery to generate a spatially continuous site quality map. Spatial autocorrelation (Moran's I) and LISA clustering were further employed to interpret spatial patterns. Results indicate that coastal sandy soils in the study area are generally nutrient-poor, with tree growth primarily constrained by total nitrogen, organic matter, available phosphorus, and total phosphorus. The five most influential site factors, ranked by importance, are soil fertility, distance from the coastline, aspect, slope gradient, and elevation. Optimal conditions for C. equisetifolia growth include fertile soil, location > 1000 m from the coastline, south-facing or semi-sunny slopes, slope gradients < 15°, and elevations between 10–100 m. Only 11.94% of the area was classified as high-quality (Grade Ⅰ), while 61.74% fell into moderate or poor grades (Grades Ⅲ and Ⅳ), indicating that most plantations are located on suboptimal sites. This study provides scientific support for improving the precision and sustainability of coastal shelterbelt planning and management, offering practical insights for afforestation strategies, forest restoration, and ecological forestry development in coastal zones.