Abstract: Taking 16-, 23-, and 51 years old Manchurian walnut (Juglans mandshurica) plantations at Maoershan of Heilongjiang Province, Northeast China as test objects, and with the consideration of allelochemicals, this paper studied the differences in the understory vegetation biodiversity among the plantations. With the increasing age of the plantations, the understory shrub richness index (I_Ma), diversity index (I_sw), and Pielou evenness index (J) all presented an increasing trend, while the understory herb I_Ma and I_sw were decreasing, and the herb species number decreased from 14 to 10. The most important plant species in the differentaged plantations were Rubus kanayamensis, Erigeron annuus, Taraxacum officinale, and Potentilla centigrana in 16 years old plantation, Ulmus japonica, Syringare ticulata, and Diarrhena manshurica in 23 years old plantation, and Syringare ticulata, Hippochaete hiemale, and Brachybotrys paridiformis in 51 years old plantation. The plant biodiversity of the plantations was less affected by juglone, but the understory shrub biodiversity was more affected by soil available P and K. The understory shrubs and herbs adapted differently to various soil pH ranges. Other soil factors such as bulk density, moisture content, organic matter, and total N had opposite effects on the biodiversity of understory shrubs and herbs.

Key words: agroforestry system, carbon storage, carbon sequestration, structure optimization, climate change.