Regeneration of forest floor-grown seedlings of Sakhalin fir can be promoted through shading by shelter trees

Abstract

ABSTRACT Natural regeneration of forest-floor seedlings of Sakhalin fir (Abies sachalinensis) is expected to contribute to low-cost afforestation. However, removal of canopy trees often causes photoinhibitory damage in forest-floor seedlings of Sakhalin fir as a typical shade-tolerant species. In the present study, we investigated factors related to solar radiation influencing survival rate and growth of Sakhalin fir seedlings after a canopy tree cutting in 62-year-old forest plantations. Survival rates, derived from aerial photographs taken by an unmanned aerial vehicle, could be predicted by the maximum consecutive direct solar radiation and shade period calculated from the sun track simulated by hemispheric photographs. Needle shedding rate of old shoots, which were developed under shade, increased with increasing daily integrated direct solar radiation in the first summer after the winter cutting of canopy trees. Conversely, growth rates of current-year-old shoots in the second summer after the winter cutting of canopy trees decreased with increasing daily integrated direct solar radiation. Needle shedding might suppress the growth rate of current-year-old shoots in the second summer, possibly due to the decreased nitrogen supply from the old needles. Thus, shading achieved by topography (e.g. north-faced slope) and retained shelter trees would be of relevance for the natural regeneration of forest-floor seedlings of Sakhalin fir, a typical shade-tolerant species.