Abstract: The dominant factors affecting plant growth vary across the stages of life history. Understory microhabitats play a significant role on plant growth and development during juvenile stage. In 2021, we surveyed Phellodendron amurense saplings planted in 2014 beneath a natural forest in Baihuashan Nature Reserve, and measured the biotic factors (density of con and heterospecific seedling neighbors, density of herbaceous and shrub neighbors, and adult neighborhood index) and abiotic factors (leaf area index, soil nutrients, and microtopography indices) around the saplings. Linear mixed models were used to assess the effects of biotic factors and abiotic microhabitat factors on the growth rates of the established saplings. The results showed that both biotic and abiotic factors affected the growth of P. amurense saplings, with different magnitudes in their impacts. Heterospecific adult neighbors had the most significant negative effect on the growth of saplings, explaining 34.1%-47.7% of the variance in their growth. Light intensity also had a negative effect, which explained 18.1%-25.3% of the variance. Soil nitrogen, available potassium, and organic carbon promoted the crown extension of P. amurense, accounting for 21.1% of the variance of the growth. Among the microtopographic factors, the topographic wetness index had a significant positive impact on the growth of saplings, explaining 19.5% of the variance, whereas slope had a negative but weak effect on the growth, accounting for 11.5% of the variance. Shrubs and grasses and the conspecifics did not affect the growth of P. amurense saplings. In conclusion, deciduous broad-leaved forests with flat microtopography, moist and fertile soil, rich in nitrogen and potassium, and high canopy density would facilitate the ex situ conservation of P. amurense.

Key words: microhabitat, microtopography, Phellodendron amurense, linear mixed model.