Objective  As an important tool to solve environmental problems, afforestation has been widely implemented globally, especially in rural areas. China has the largest area of plantation forests in the world, about 80.3 million hectares, equivalent to 36.3% of the total forest area in China. These plantation forests have been proven effective in preventing soil erosion, controlling sandstorms, and improving air quality. However, due to the insufficient consideration of biodiversity conservation, the tree species are generally monoculture in plantation forests. Many studies have compared pure, mixed and natural forests. Their results show that monoculture plantation forests can negatively affect biodiversity. These studies mainly focus on the effects of woody plants. Rich herbaceous plants can improve soil condition and microclimate, and furthermore, can provide habitat, shelter, breeding places and food resources for other groups of wild species. However, the relationship between understory herbs and the diversity of other groups has yet to be well understood. Beijing, the capital of China, started a plain afforestation project in 2012. By the end of 2022, two rounds of this project had been completed. A total of 146 thousand hectares of forest was planted in this process, nearly tripling the forest coverage in Beijing plain area. Due to the heavy workload and tight schedule, the plantation forests involved in this project are empirically thought to be monoculture in both tree species and structure, and thus fail to benefit biodiversity. However, experimental verification still needs to be conducted. In rural areas, arthropods such as wild bees, spiders and carabid beetles are critical functional groups because they provide pollination or pest control services necessary for agriculture. This study investigated plants and the aforesaid three arthropod groups in plantation forests in the rural plain area of Beijing. We analyzed the response of abundance and species richness of the three arthropod groups toward understory herb cover and richness. This study aimed to examine whether it is possible to enhance the biodiversity of rural monoculture plantation forests through the improvement of understory herbs. The results were expected to provide a scientific basis for the management of the existing monoculture plantation forests in China’s rural areas to strengthen their role in promoting sustainable rural and agricultural development, improving the rural ecological environment, and boosting rural revitalization.

Methods  In the rural plain afforestation project area of Beijing, we randomly selected three sampling sites of plantation forests respectively in Shunyi, Tongzhou, Daxing and Fangshan districts. In total, 12 sites were selected. The linear distances between every two sites were longer than 5 km to avoid potential spatial autocorrelation. In these sites, we recorded the cover and species of herbs, shrubs and trees, and collected insect pollinators (wild bees) and natural enemies (ground spiders and carabid beetles) using pan traps and pit traps respectively. The captured arthropod samples were identified to species level in the laboratory. In addition, we mapped the land use type within a 1,000 m radius of each site using ArcGIS 10.2 based on Google satellite image and field scale monitor. With the collected data, we firstly compared the cover and species richness of herbs and the abundance and species richness of the 3 arthropod groups between the pure and mixed forests using Two-sample t-tests. Then we analyzed the effects of the cover and species richness of herbs on the abundance and species richness of the 3 arthropod groups using linear models. “Plantation forest cover”, “arable land cover” and “plantation forest patch density” in the surrounding landscape were included in the aforesaid models as covariates to correct for potential bias caused by the differences in the surrounding landscape patterns among sampling sites.

Results  Among the 12 sampling sites of plantation forests randomly selected, 8 ones were pure forest, 3 ones were simply mixed forest with only two tree species each, and the rest one was a simply mixed forest with 4 tree species. Tree species of the sampling sites were commonly monoculture. In total, this study captured 525 wild bee individuals comprising 36 species, 159 spider species comprising 27 species, and 108 carabid beetle individuals comprising 18 species. The cover and species richness of herbs were not significantly different between pure and mixed forests. The abundance and species richness of wild bees, ground spiders and carabid beetles showed no significant difference between pure plantation forests and mixed forests neither. According to the results of linear models corrected for landscape pattern, the wild bee abundance and the carabid beetle species richness significantly increased with the increasing herb species richness. Moreover, herb cover was significantly positively linked to the abundance and species richness of carabid beetles.

Conclusion  This study found that despite the monoculture of tree species in rural plantation forests, the increasing cover and diversity of understory herbs can significantly improve wild bee abundance and carabid beetle abundance and species richness. Therefore, it may be an effective measure to improve the habitat quality and biodiversity conservation effects of plantation forests by maintaining and enhancing understory herb cover and richness. Our results provide a new idea on how to use the large amount of monoculture plantation forests in rural areas to conserve biodiversity and improve ecosystem services, and accordingly promote sustainable rural and agricultural development. However, we still need to put more effort into monitoring and analysis before putting the aforesaid measure into practice.