Abstract: Poplar (Nanlin-95) plantations with different stand densities and spacing were investigated to study the effects of stand structure on understory characteristics in Chenwei Forest Farm, Sihong County, Jiangsu Province. The results showed that the Shannon index of the understory was higher under the low density stands than under the high density ones, while the distribution of understory was much homogeneous under high density stands, with a higher Pielou evenness index. The diversity index of the understory was lower in May than in November. The biomass of the understory was higher in the low density stands than in the high density stands. Low stand densities increased the concentration and accumulation of N, P, K, and Mg in understory, while no significant effects of stand structure were detected on the concentration and accumulation of Ca in understory. The concentration and accumulation of N, P, and Ca were higher in May than in November while the concentration and accumulation of K and Mg were lower in May than in November.

Key words: melatonin, alleviating effect, autotoxicity, p-hydroxybenzoic acid, cucumber