Forest gaps alter the soil bacterial community of weeping cypress plantations by modulating the understory plant diversity

Abstract

Weeping cypress is an endemic tree species that is widely planted in China, and the simple stand structure and fragile ecosystem of its plantation are common issues. Exploring the effect of different gap sizes on the soil bacterial community structure of weeping cypress plantations can provide a theoretical basis for the near-natural management of forest plantations. We, therefore, constructed three kinds of forest gaps with different sizes in weeping cypress plantations, namely, small (50-100 m²), medium (100-200 m²), and large gaps (400-667 m²), for identifying the key factors that affect soil bacterial communities following the construction of forest gaps. The results suggested that the herb layer was more sensitive than the shrub layer, while the Simpson, Shannon, and richness indices of the herb layer in plots with gaps were significantly higher than those of designated sampling plots without any gaps (CK). The presence of large gaps significantly increased the understory plant diversity and the Shannon and Simpson indices of the soil bacterial alpha diversity. There were obvious changes in the community composition of soil bacteria following the construction of forest gaps. The dominant bacterial phyla, orders, and functions were similar across the plots with different gap sizes. Of the indicator bacterial species, the abundance of the nitrogen-fixing bacteria, Lysobacter_ yangpyeongensis, and Ensifer_meliloti, was significantly different across plots with different gap sizes and accounted for a large proportion of the bacterial population of plots with medium and large gaps. The understory plant diversity was mostly related to the soil bacterial community than to other soil factors. The results of structural equation modeling indicated that the understory plant diversity was the most important environmental factor in driving the composition and diversity of bacterial communities. The construction of forest gaps significantly improved the understory plant diversity, physicochemical properties of the soil, and bacterial diversity in weeping cypress plantations, and the results of the comprehensive evaluation were in the order: large gaps > small gaps > medium gaps > CK. Our results suggested that large gaps are beneficial for the diversity of above-ground plant communities and underground soil bacterial communities.

Keywords: forest gap sizes; near-natural forest management; soil bacteria; soil properties; understory vegetation.

Figure 1

Redundancy analysis (RDA) of understory plant diversity with soil properties. “S” stands for small gaps; “M” stands for medium gaps; and “L” stands for large gaps.

Figure 2

Principal coordinate analysis (PCoA) of the soil bacterial community based on Bray–Curtis distance among different groups. The ellipses represent the 95% confidence interval. “S” stands for small gaps; “M” stands for medium gaps; and “L” stands for large gaps.

Figure 3

Relative abundance of bacteria taxa at the phyla level (A) and the orders level (B). Only the bacteria in the top ten relative abundance are shown. A single asterisk indicates p < 0.05. “S” stands for small gaps; “M” stands for medium gaps; and “L” stands for large gaps.

Figure 4

Soil bacteria community indicator species in the three different forest gaps. Only species with abundance value > 0, total proportion > 0.1%, and p < 0.05 are shown. “S” stands for small gaps; “M” stands for medium gaps; and “L” stands for large gaps.

Figure 5

Variance partitioning analysis (VPA) of dominant phyla of soil bacteria and environmental factors.

Figure 6

A heat map depicts the relative abundances of indicator species and Spearman's correlations between abundances of indicator species and environmental factors. Double asterisks indicate a p-value < 0.01; single asterisk indicates a p-value < 0.05. RH, DH, JH, and HH denote the richness index, the Simpson index, the Pielou index, and the Shannon index in the herb layer, respectively. RS, DS, JS, and HS denote the richness index, the Simpson index, the Pielou index, and the Shannon index in the shrub layer, respectively. This notation signifies the same meanings in the text. “S” stands for small gaps; “M” stands for medium gaps; and “L” stands for large gaps.

Figure 7

Structural equation model (SEM) of bacterial diversity and composition in relation to environmental factors. Solid arrows indicate significant effects (P < 0.05). Arrow width corresponds directly to the standardized path coefficient. The blue arrow represents the positive impact, and the red arrow represents the negative impact. R² values associated with response variables indicate the proportion of explained variation by relationships with other variables. Values associated with arrows represent standardized path coefficients. Herb diversity: four plant diversity indices of the herb layer; SOM: soil organic matter; shrub diversity: four plant diversity indices of the shrub layer; bacterial composition: the sequencing data for bacteria at the phyla level; Simpson: Simpson index of bacteria community. ***P < 0.001; **P < 0.01; *P < 0.05.

Figure 8

Standardized total effects (direct plus indirect effects) derived from the structural equation modeling. Herb diversity: four plant diversity indices of the herb layer; SOM: soil organic matter; shrub diversity: four plant diversity indices of the shrub layer; bacterial composition: the sequencing data for bacteria at the phyla level; Simpson: Simpson index of bacteria community.