Background Rill erosion is the main form of soil degradation, threatening ecological integrity. Studies show that adjusting planting density can affect rill formation on slopes and in forest land. Cinnamomum camphora is an important tree in the southern red soil region, but the impact of its planting density on rill morphology is unclear. This study aims to clarify the rill morphological characteristics under different densities and provide guidance for cultivation and erosion prevention.

Methods The sample plot was selected in the ecological science and technology park of Nanchang Institute of Technology, which has a suitable climate. Three slope plots were set up, namely 5° (S1), 10° (S2) and 15° (S3), and each slope had three planting densities (D1: 1.5 m×1.5 m, D2: 1.0 m×1.5 m, D3: 1.0 m×1.0 m), totaling nine treatments. The data of the sample plot was obtained through UAV aerial photography and analyzed by Agisoft PhotoScan Professional and ArcGIS 10.8 software. Relevant parameters were extracted to characterize the rill features, and SPSS and Origin were used for data analysis and graphing.

Results The main results show that: 1) A higher planting density leads to a decrease in the length, width and depth of rills. When comparing D3 with D1, they are reduced by 0.34 times, 0.19 times and 0.31 times respectively. A steeper slope will cause them to increase. When comparing S3 with S1, they are increased by 1.24 times, 0.77 times and 1.91 times respectively. 2) The rill density, complexity, dissection degree, volume and width-depth ratio vary within the ranges of 0.19-1.48 m/m², 1.01-1.42, 0.01-0.23, 0.06-1.70 m³, and 1.63-3.25 respectively. A higher planting density will increase the width-depth ratio (with an average increase of 7.5%), but decrease other indicators (with an average decrease of 24.7%, 3.4%, 33.6% and 37.1% respectively). A steeper slope will reverse this situation, reducing the width-depth ratio (with an average decrease of 22.7%) and increasing other indicators (with an average increase of 126.6%, 14.7%, 219.1% and 513.65% respectively). 3) The cross-sections of rills are mostly U-shaped. A higher planting density will reduce the cross-sectional area and change the V-shape in the middle of the slope to a U-shape. A steeper slope will increase the cross-sectional area, make the V-shape in the middle of the slope more prominent, and make the upper and lower parts of the slope more likely to be V-shaped but still remain U-shaped. 4) The amount of slope erosion is highly positively correlated with the rill density, complexity and dissection degree (with correlation coefficients of 0.975, 0.991 and 0.988 respectively), and highly negatively correlated with the rill width-depth ratio (-0.937).

Conclusions The analysis indicates that C. camphor play an important role in inhibiting the formation and development of slope rills. Although the impact of slope is significant, the dwarf forests can still impede it on all slopes. The D3 planting density has the best effect. The research results are conducive to the planting management of dwarf forests and the prevention and control of soil erosion in the red soil hilly area, providing a scientific basis.