Strong influences of stand age and topography on post-fire understory recovery in a Chinese boreal forest

Abstract Soil resource quantity and heterogeneity as well as overstory characteristics change along with stand age after fire disturbance, imposing strong influences on post-fire understory recovery. In this study, we aimed to assess how soil resource (nitrogen quantity and heterogeneity), topographic position (aspect), overstory characteristics (basal area), and stand age affected understory characteristics (cover, richness, evenness, and composition) during boreal forest succession after fire disturbance. We collected plant community and environmental data from 76 stands aged at 4 to 203 years post-fire in the boreal forest of Northeast China. We separately tested the effects of the considered factors on the characteristics of total understory, shrubs, and herbs. Total understory cover was higher at the south-facing slopes and flat locations than at north-facing slopes. Species richness of total understory, shrubs, and herbs was higher at the flat locations and south-facing slopes than at the north-facing slopes. The cover of total understory and shrubs showed increasing trends along with stand age, soil nitrogen quantity, and overstory basal area as well as decreasing trends along with soil nitrogen heterogeneity, while the cover of herbs showed reversed trends across all the three topographic positions. Understory richness showed decreasing trends along with soil nitrogen quantity as well as increasing trends along with soil nitrogen heterogeneity and overstory basal area. Total understory evenness showed flat trends along with soil nitrogen quantity, heterogeneity, and stand age as well as decreasing trends along with overstory basal area. Understory community composition was mainly affected by topographic position, stand age, and overstory basal area. During understory recovery, community composition shifted from shade-intolerant species to shade-tolerant species. Our results highlight the importance of topography and stand age in post-fire understory recovery in boreal forests of China, mediated through availability of light and soil nutrients.