Abstract: Understory herbs are the basic components of forest ecosystem, which play a vital role in forest regeneration, as well as in the maintenance and improvement of ecosystem structure and function. Understanding the community characteristics and influencing factors of herbaceous layer in the understorey is helpful to construct an ideal forest-grass combination model. In this study, the undisturbed Larix gmelinii var. principis rupprechtii plantations, the thinned L. gmelinii var. principis rupprechtii plantations, and the natural mixed forest of L. gmelinii var. principis rupprechtii  and Betula platyphylla in Jingyuan County of the south mountainous area of Ningxia were examined. We explored the relationship between community characteristics (i.e., species diversity and community biomass) of understory herbs, soil properties (i.e., soil bulk density, water content, total carbon content, total nitrogen content and total phosphorus content) and stand structure (i.e., stand density and canopy density). The results showed that: (1) Compared with the monoculture plantations, the natural mixed forest had more herbaceous species (34 species), higher species diversity and aboveground biomass; (2) Soil properties under the natural mixed forest were better than those under the plantations, but soil properties under the thinned plantation were significantly improved than those under plantation without human disturbance; (3) Redundancy analysis showed that species diversity of herbaceous layer was positively correlated with soil water content, soil total carbon content, total nitrogen content, and total phosphorus content, but negatively correlated with soil bulk density, stand density and canopy density. Stand density and soil water content of 0-20 cm soil layer significantly affected species diversity of understory herbs (P<0.05); (4) The regression analysis showed that soil properties of 0-20 cm layer, species richness and dominance, stand density and canopy density significantly affected aboveground biomass of herbaceous species (P<0.05). The random forest model and variance decomposition further showed that the explanation rate of soil factors on aboveground biomass was 30.99%, among which soil bulk density, total carbon content, total nitrogen content and soil water content of 0-20 cm layer were the most important ones. The explanation rate of stand structure was 12.74%, of which stand density was the most important explanatory factor. The explanation rate of species diversity was only 3.42%. The natural mixed forests as the most ideal model of forest-grass combination were conductive to the healthy development of understory herbs and the improvement of understory ecological environment. Species diversity and aboveground biomass of herbaceous layer were affected by different environmental factors. Our results provide references for the near-natural management of plantations (such as thinning and replanting) and the high-quality development of understory economy.

Key words: understory herb, biomass, species diversity, stand structure, soil physicochemical property.