Microtopography and skid-trail disturbance shape drought-related seedling mortality in Japanese cedar reforestation

Although drought risk is expected to increase in response to climate change, the mechanisms underlying drought-related seedling mortality in traditionally humid regions remain poorly understood. This study examined factors associated with drought-related mortality during the early establishment stage of planted Cryptomeria japonica seedlings in a reforestation site. Using orthomosaics derived from unmanned aerial vehicle imagery and a 1-m-resolution digital elevation model, we evaluated the influence of microtopography and site conditions, including skid trails and understory vegetation, on seedling mortality using generalized additive models. Terrain variables derived from 3 to 5 m terrain representations were generally best supported for explaining seedling mortality, indicating the importance of microtopography at several-meter scales. Under representative site conditions, predicted mortality was substantially higher on skid trails than outside skid trails (65% vs. 22%) and increased with greater understory vegetation cover and terrain roughness. Mortality was also higher in topographically drier positions, represented by low topographic wetness index values, although higher wetness on skid trails did not reduce mortality, suggesting that machine-traffic disturbance may restrict root growth and reduce plant-available water. In contrast, ridge–valley position and potential solar radiation were not retained in the final model. Overall, these findings indicate that drought-related mortality in newly planted seedlings is shaped by the combined effects of fine-scale terrain variation and planting-site disturbance. Incorporating meter-scale terrain indices and avoiding or mitigating compacted skid-trail microsites may improve reforestation planning under an increasing risk of drought.