Exploration of spatial biases in natural hardwood regeneration in conifer plantations in southwestern Japan

Japan has adopted biodiversity-oriented forest management, necessitating the diversification of extensive conifer plantations and the identification of geographic conditions that favor natural hardwood regeneration. The increasing availability of high-resolution airborne laser scanning (ALS) data provides new opportunities to analyze spatial patterns in forests. In this study, we applied exploratory approaches to quantify the prevalence of natural hardwood regeneration within mature conifer plantations in Kochi Prefecture, southwestern Japan. Hardwood regeneration was defined as areas recorded as conifer plantations in forest registry data (2005–2009) but dominated by hardwoods based on ALS data collected in 2018. Across postwar afforestation sites (1949–1978 planting), hardwood regeneration consistently occupied 20–25% of the total area, regardless of the planting year. Using a logistic generalized additive model, we found that hardwood regeneration was favored on slopes steeper than 40° and in ridges and valleys. In the low-elevation zone (< 600 m a.s.l.), where evergreen Castanopsis and Quercus species were the dominant vegetation, the likelihood of finding hardwood regeneration increased with decreasing elevation and greater southern slope exposure. This trend was particularly evident within specific geologic zones. Spatial analyses to identify site characteristics that favor natural hardwood regeneration could be used to support biodiversity-oriented forest management. Furthermore, high-resolution ALS data that will soon be publicly available hold significant promise for uncovering geographic patterns and generating novel insights on forest ecosystem dynamics.