Effects of Caragana microphylla encroachment on community structure and ecosystem function of a typical steppe

doi:10.17521/cjpe.2019.0283

Abstract

Abstract:

Aims Shrub encroachment is a critical ecological problem in arid and semi-arid ecosystems worldwide. The effects of shrub encroachment on ecosystem structure and function of grasslands are complicated and need to be explored in future studies. Our objective is to examine the effects and pathways of shrub encroachment on ecosystem structure and function in a typical steppe of the Inner Mongolia grassland.
Methods Three grassland sites with different degrees of shrub encroachment (i.e. light, moderate, heavy) were selected in the Xilingol Nei Mongol, of which Caragana microphylla was the dominant shrub. Species richness and composition, aboveground net primary productivity (ANPP), soil property, and plant functional traits of dominant species were determined in this study. In addition, species diversity, functional attribute diversity, community-weighted mean traits, and vegetation leaf and soil carbon and nitrogen pools were further calculated.
Important findings 1) The species richness, functional attribute diversity and community-weighted mean traits differed significantly among three grassland sites, and species diversity and functional diversity were relatively higher in the moderate shrub-encroachment site, indicating moderate shrub-encroachment favors biodiversity maintenance. 2) The aboveground net primary productivity of heavy shrub-encroachment grassland was significantly higher than those of light and moderate shrub-encroachment grasslands, which was mainly due to a shift in functional group composition, that is, the proportion of annuals and biennials to perennial grasses and forbs increased greatly with intensifying shrub encroachment. The vegetation leaf and soil carbon and nitrogen pools differed little among three sites. 3) Shrub encroachment did not directly affect ecosystem function, including ANPP, vegetation and soil nutrient pools, but it indirectly affected them through pathways of the shift in functional group composition and changes in soil property and functional diversity. Particularly, the shift in functional group composition and intensified soil drought and basification was separately important biotic and abiotic factors for variations in ecosystem function.

Key words: shrub encroachment, species diversity, functional diversity, plant functional trait, functional group composition, ecosystem function

DING Wei,WANG Yu-Bing,XIANG Guan-Hai,CHI Yong-Gang,LU Shun-Bao,ZHENG Shu-Xia. Effects of Caragana microphylla encroachment on community structure and ecosystem function of a typical steppe[J]. Chin J Plant Ecol, 2020, 44(1): 33-43.

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URL: https://www.plant-ecology.com/EN/10.17521/cjpe.2019.0283

https://www.plant-ecology.com/EN/Y2020/V44/I1/33

Figures/Tables 7

Fig. 1 Three grassland sites with light (A), moderate (B) and heavy (C) shrub encroachment by Caragana microphylla in Nei Mongol typical steppe.

Table 1 Shrub characteristics of Caragana microphylla at light, moderate and heavy encroachment sites in Nei Mongol typical steppe

灌丛特征 Shrub characteristic	不同程度灌丛化 Degree of shrub encroachment
灌丛特征 Shrub characteristic	轻度 Light	中度 Moderate	重度 Heavy
数量 Number of bunches (No.·25 m^-2)	2 ± 0.5^b (45)	6 ± 0.6^a (94)	5 ± 0.9^a (80)
高度 Height (cm)	24.13 ± 1.45^b (45)	25.71 ± 0.91^b (94)	38.90 ± 2.23^a (80)
冠幅 Crown (cm²)	8 505.6 ± 1 453.0^ab (45)	6 907.3 ± 1 048.8^b (94)	13 083.9 ± 2 289.0^a (80)
冠幅面积比 Crown area ratio (%)	7.66 ± 1.81^b (20)	12.99 ± 2.47^ab (20)	20.93 ± 4.37^a (20)
分蘖株数 Number of individuals (No.·25 m^-2)	25 ± 6^b (504)	32 ± 5^ab (719)	47 ± 8^a (942)

Fig. 2 Species richness and functional attribute diversity (FAD) at light, moderate and heavy encroachment sites in Nei Mongol typical steppe (mean + SE). FAD was calculated by plant height, plant biomass, stem:leaf biomass ratio and specific leaf area. Different lowercase letters indicate significant differences among sites (p < 0.05).

Fig. 3 Community-weighted mean traits (CWM) at light, moderate and heavy encroachment sites in Nei Mongol typical steppe (mean + SE). PBCWM, community-weighted plant biomass; PHCWM, community-weighted plant height; SLACWM, community-weighted specific leaf area; SLRCWM, community-weighted stem:leaf ratio. Different lowercase letters indicate significant differences among sites (p < 0.05).

Fig. 4 Aboveground net primary productivity (ANPP) and relative biomass (RAB) of different functional groups at light, moderate and heavy encroachment sites in Nei Mongol typical steppe (mean + SE). AB, annuals and biennials; PF, perennial forbs; PG, perennial graminoids. Different lowercase letters indicate significant differences among sites (p < 0.05).

Fig. 5 Effects of shrub encroachment on C and N pools of vegetational leaf and soil in Nei Mongol typical steppe (mean + SE). No significant differences among sites (p > 0.05).

Fig. 6 Structural equation model (SEM) analyses of direct and indirect effects of shrub encroachment on grassland ecosystem function, including aboveground net primary productivity (ANPP), vegetation and soil nutrient pools (C and N). Values associated with solid arrows are standardized path coefficients, indicating positive or negative effects. r2 values associated with response variables indicate the proportion of variation explained by relationships with other variables.

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