Divergent responses of above- and belowground ecosystem functioning to shrub encroachment in the Tibetan semi-arid alpine steppes
LDD | 藏北半干旱高寒草原灌木侵入对地上和地下生态系统功能的差异性响应
Semi-arid alpine ecosystems on the Tibetan Plateau are experiencing rapid climate change and extensive anthropogenic activity, concomitant with the encroachment of shrubs. Shrub encroachment will lead to changes in both the ecosystem structure and functioning of the semi-arid alpine steppes. Yet, the encroaching impacts of shrubs on the above- and belowground ecosystem functioning of the semi-arid alpine steppes remain uncertain. To quantify shrub encroachment impacts on ecosystem functioning of the semi-arid alpine steppes, two alpine steppe sites encroached by shrub species are investigated, with one site encroached by leguminous shrub species (Caragana spinifera) and another site encroached by non-leguminous shrub species (Dasiphora fruticosa). Results shows that the individual ecosystem functions and the ecosystem multifunctionality (EMF) of alpine steppes are significantly enhanced following both leguminous and non-leguminous shrubs encroachment. We conclude that shrub encroachment tends to facilitate the belowground EMF (BEMF) but has neutral effects on the aboveground EMF (AEMF) in alpine steppes. Our findings also highlight that soil nutrients play critical roles in driving ecosystem functioning responses to shrubs encroachment. These findings further our understanding of shrub encroachment impacts on the ecosystem functioning of the semi-arid alpine steppes.
FIGURE 1
Landscape site at Damxung County on the central Tibetan Plateau and the pictures of the alpine steppes encroached (a) by a leguminous shrub (Caragana spinifera) and (b) by a non-leguminous shrub (Dasiphora fruticosa).
FIGURE 2
Differences in (a) vegetation cover (Cover), (b) plant height (Height), (c) species richness (SR), (d) aboveground biomass (AGB), (e) belowground biomass (BGB), (f) soil water content (SW), (g) soil organic carbon content (SOC), (h) soil total nitrogen content (STN), (i) soil microbial biomass carbon (MBC), (j) soil microbial biomass nitrogen (MBN), (k) soil ammonia nitrogen (NH4+–N), and (l) soil nitrate nitrogen (NO3−–N) for the alpine steppes encroached by a leguminous shrub (Caragana spinifera) and by a non-leguminous shrub (Dasiphora fruticosa). Mean ± SD is shown in error bars. Asterisks represent the significant differences between the shrub encroached plots and non-shrub encroached plots. *p < 0.05; **p < 0.01; ***p < 0.001.
FIGURE 3
Differences in the relative interaction intensity (RII) values of individual ecosystem functions for the alpine steppes encroached by (a) a leguminous shrub (Caragana spinifera) and (b) by a non-leguminous shrub (Dasiphora fruticosa). Cover, vegetation cover; Height, plant height; SR, species richness; AGB, aboveground biomass; BGB, belowground biomass; SW, soil water content; SOC, soil organic carbon content; STN, soil total nitrogen content; MBC, soil microbial biomass carbon; MBN, soil microbial biomass nitrogen;NH4+–N, soil ammonia nitrogen; NO3−–N, soil nitrate nitrogen. Bars indicated the SD of the mean value. Asterisks represent the significant differences between the shrub encroached plots and non-shrub encroached plots. *p < 0.05; **p < 0.01; ***p < 0.001.
FIGURE 4
Differences in (a) the ecosystem multifunctionality (EMF), (b) aboveground ecosystem multifunctionality (AEMF), (c) belowground ecosystem multifunctionality (BEMF) indexes between the shrub encroached plots and non-shrub encroached plots and the relative interaction intensity (RII) values of EMF, AEMF, and BEMF for the alpine steppes encroached by (e) a leguminous shrub (Caragana spinifera) and (f) by a non-leguminous shrub (Dasiphora fruticosa). Bars indicated the SD of the mean value. Asterisks represent the significant differences between the shrub encroached plots and non-shrub encroached plots. *p < 0.05; **p < 0.01; ***p < 0.001.
FIGURE 5
Correlation coefficients among the individual ecosystem functions and multiple ecosystem functions relevant to the ecosystem multifunctionality (EMF), aboveground ecosystem multifunctionality (AEMF), and belowground ecosystem multifunctionality (BEMF) in the semi-arid alpine steppes. Cover, vegetation cover; Height, plant height; SR, species richness; AGB, aboveground biomass; BGB, belowground biomass; SW, soil water content; SOC, soil organic carbon content; STN, soil total nitrogen content; MBC, soil microbial biomass carbon; MBN, soil microbial biomass nitrogen;NH4+–N, soil ammonia nitrogen; NO3−–N, soil nitrate nitrogen. Bars indicated the SD of the mean value. Asterisks represent the significant differences between the shrub encroached plots and non-shrub encroached plots. *p < 0.05; **p < 0.01; ***p < 0.001.
FIGURE 6
Correlation coefficients among the relative interaction intensity (RII) values of individual ecosystem functions and multiple ecosystem functions relevant to the ecosystem multifunctionality (EMF), aboveground ecosystem multifunctionality (AEMF), and belowground ecosystem multifunctionality (BEMF) in response to shrub encroachment. Cover, vegetation cover; Height, plant height; SR, species richness; AGB, aboveground biomass; BGB, belowground biomass; SW, soil water content; SOC, soil organic carbon content; STN, soil total nitrogen content; MBC, soil microbial biomass carbon; MBN, soil microbial biomass nitrogen;NH4+–N, soil ammonia nitrogen; NO3−–N, soil nitrate nitrogen. Bars indicated the SD of the mean value. Asterisks represent the significant differences between the shrub encroached plots and non-shrub encroached plots. *p < 0.05; **p < 0.01; ***p < 0.001.
FIGURE 7
A schematic diagram for understanding the potential ways of shrub encroachment on the individual ecosystem functions and multiple ecosystem functions in the semi-arid alpine steppes.
