Factors affecting seedling establishment in a warm-temperate secondary forest affected by oak wilt disease

Diseases are major natural disturbances to forest ecosystems that cause changes to overstory tree structure and influence seedling dynamics by altering environmental conditions via functional traits. Thus, identifying factors affecting seedling dynamics would improve broader understanding of the seedling regeneration process after disease disturbance. We investigated 13,010 current year seedlings from 59 woody species, 44 genera, and 21 families in a Japanese warm-temperate secondary forest, following Japanese oak wilt disease (JOW) in 2009-2013 and in 2018-2020. We also quantified temporal changes in canopy openness and soil moisture. Structural equation model (SEM) was used to explore the effects of environmental conditions and seedling functional traits on current year seedling density in autumn for 13 dominant species. Canopy openness gradually increased, whereas soil water content decreased throughout the study site. After disruption by JOW, recruitment of current year seedlings increased through improvements in light conditions. The functional traits of current year seedlings were likely affected by soil nutrients in the late JOW period (2011-2013), whereas litterfall production influenced functional traits of current year seedlings in the post-JOW period (2018-2020). Factors affecting seedling density varied as environmental conditions changed across the JOW periods. Seedling establishment was enhanced by increased canopy openness in the late JOW period and by low litterfall, high soil nutrient content, and high soil moisture in the post-JOW period. Our study provides evidence that temporal changes in environmental conditions and seedling densities occurring after disruption by JOW differ from the changes in forests impacted by other natural disturbances.