Topography and planting density affect understory plant diversity in conifer plantations through development of stand canopy closure

Sustainably managed plantations not only provide high-quality timber but also contribute to biodiversity. Clarifying the direct and indirect effects of planting density and topography on the development and diversity of understory vegetation is important from the perspective of the diversity-retaining function of young plantations. This study conducted a vegetation survey in plantation stands of Japanese cedar, 4–7 years after planting. These plantations varied in initial planting density (ranging 1000–3000 trees/ha), and contained topographic variations from ridges to lower slopes. The direct and indirect effects of initial planting density and topography on species composition, diversity, and height growth of understory vegetation were investigated. Topographic variation affected stand canopy closure and the mean height growth rates of planted cedars. Topography effect was directly related to vegetation height. High initial planting density accelerated stand canopy closure through crown development of planted trees. Through stand canopy closure, initial planting density and topography indirectly affected plant diversity, although their direct effects were not significantly detected. Initial planting density and topography impacted vegetation diversity mainly through the degree of canopy closure, rather than directly. These findings are useful when planning reforestation schedules and spatial designs within the landscape while taking diversity into consideration.