| Precipitation is an important source of water for terrestrial ecosystems,and precipitation changes affect species distribution and diversity patterns in communities by altering physiological and ecological processes such as soil physicochemical properties,plant photosynthesis rate,transpiration intensity,and water use efficiency.With the intensification of global climate change,the important role of plant diversity and plant functional traits in maintaining ecosystem functions has been of increasing concern.Therefore,exploring the relationships between plant functional traits,community structure and abiotic factors can help to study the response of ecosystem functions and processes to precipitation changes from the perspective of the individual plant,community and ecosystem,and provide an in-depth understanding of the regulation mechanisms of precipitation changes on grassland productivity in desert steppe,which is of great significance for scientific management of grassland ecosystem.In this study,taking the grass community of Stipa glareosa and the shrub community of Reaumuria songarica in the desert steppe of Inner Mongolia as research subjects.We carried out a situ control experiments with precipitation increase(+20%,+40%,+60%)and precipitation reduction(-60%,-40%,-20%)treatments and control(natural precipitation)to study the influences of precipitation changes on soil properties,plant community structure and the regulation on aboveground biomass.We came to the following main conclusions:(1)The soil pH of grass community was between 8.01 and 9.08,and that of shrub community was between 8.75 and 9.4,all of which were alkaline soils.The influence of precipitation changes on soil pH value of 0-10 cm and 10-20 cm layers of the two community types were not significant.According to the Second National Soil Census Nutrient Grading Standard,the soil organic carbon content(grass community:6.8 g/kg,shrub community:9.5 g/kg)was very low;the soil total nitrogen contents(0.36 g/kg,0.52 g/kg)of grass community and shrub community were extremely low and very low respectively,and soil total phosphorus(grass community:0.32 g/kg,shrub community:0.27 g/kg)was very low.The increase of precipitation in grass community increased the contents of soil organic carbon and total nitrogen in 0-10 cm soil layer,but had no significant effect on soil total phosphorus and potassium.The higher intensity of precipitation changes in shrub communities significantly affected soil total phosphorus and total potassium content,which showed that:+40%precipitation treatment significantly increased total potassium content in 0-10 cm soil layer,+60%precipitation treatment significantly increased total phosphorus content in 10-20 cm soil layer.(2)The increase of precipitation increased plant diversity of grass and shrub communities,as follows:with precipitation increasing,the species richness,species diversity of grass community and species richness of shrub community increased significantly(p<0.001),however,the species evenness decreased significantly(p<0.001).Changes in precipitation affected the spatial distribution of plants,in precipitation increasing treatments,the coverage of Allium mongolicum,Allium polyrhizum,and Peganum harmala increased;rare species such as Eragrostis Pilosa and Bassia dasyphylla appeared in the grass community;the coverage of Artemisia frigida,Convolvulus ammannii,Cleistogenes squarrosa increased,and rare species such as Astragalus scaberrimus and Corispermum mongolicum appeared in the shrub community.(3)There were differences in the effects of precipitation on leaf morphology and chemical properties of grass and shrub communities.The relationship between leaf morphological traits and precipitation was as follows:plant height(R~2=0.17,p<0.001),leaf area(R~2=0.1,p<0.05)and leaf dry matter content(R~2=0.13,p<0.01)were significantly positively correlated with precipitation,while specific leaf area(R~2=0.13,p<0.001)was significantly negative.There was no significant relationship between leaf morphological traits and precipitation in shrub community.The interspecific variation of grass community was as follows:leaf thickness>leaf area>height>leaf dry matter content>specific leaf area,the variation degrees of leaf area,height,leaf dry matter content were more than 85%,and the intraspecific variation of specific leaf area was 50.1%;The interspecific variation of shrub community was:leaf area>height>leaf thickness>specific leaf area>leaf dry matter content.The interspecific variation of leaf area was 97.6%,and the intraspecific variation of leaf dry matter content was the largest,which was 67.8%.The relationship between chemical properties of plant leaves and precipitation was as follows:leaf nitrogen content(R~2=0.19)and potassium content(R~2=0.15)of grass community decreased significantly with precipitation(p<0.01),while leaf carbon nitrogen ratio(R~2=0.29,p<0.001),carbon phosphorus ratio(R~2=0.1,p<0.01)and carbon potassium ratio(R~2=0.24,p<0.001)increased significantly with precipitation.Leaf carbon content(R~2=0.1),carbon nitrogen ratio(R~2=0.23),carbon phosphorus ratio(R~2=0.13)and nitrogen phosphorus ratio(R~2=0.1)of shrub community increased significantly with precipitation(p<0.01).(4)The results of principal component analysis of 13 plant functional traits showed that functional traits of grass community were sensitive to precipitation and showed strong plasticity;precipitation reduction treatments in grass communities were mainly distributed at one end of the trait axes of higher leaf nitrogen content,leaf phosphorus content,leaf potassium content,specific leaf area and lower plant height and leaf dry matter content.The precipitation increase treatments were mainly distributed at the end of the trait axes of higher leaf carbon nitrogen ratio,carbon phosphorus ratio,carbon potassium ratio,nitrogen phosphorus ratio,and higher height and leaf dry matter content.The distribution of different precipitation treatments in the trait space of shrub communities were concentrated,which showed higher functional stability.The significant relationship between soil organic carbon,soil total nitrogen content,leaf nutrient content and stoichiometric ratio and species diversity showed that the limitation of nitrogen and phosphorus content significantly reduced the species richness,diversity and evenness of grass and shrub communities.(5)Grass and shrub communities showed different patterns of aboveground biomass in response to precipitation changes.Community structure and plant functional traits were important intermediate regulatory factors.The optimal models of plant height,leaf dry matter content and aboveground biomass in herb community were exponential models,and the optimal model of leaf thickness,specific leaf area,leaf nitrogen content,phosphorus content,potassium content and aboveground biomass were linear models with negative slope.The optimal models of plant height,leaf thickness,leaf carbon content and aboveground biomass in shrub community were linear models with positive slope,and the optimal models of specific leaf area,leaf dry matter content and aboveground biomass were logarithmic models.In conclusion,the plant functional traits of dominant species at the community level had a significant impact on community aboveground biomass,thus the results supported the"Mass Ratio Hypothesis".Through the structural equation modeling of the causal relationships among community structure,plant functional traits and aboveground biomass,it was found that the pathways of aboveground biomass regulation by precipitation were different for grass and shrub communities;for grass communities,changes in precipitation and year indirectly affected aboveground biomass through height(R=0.522),leaf area(R=-0.279),and leaf dry matter content(R=0.351);for shrub communities,aboveground biomass was indirectly affected by species richness(R=0.242),height(R=0.284),and leaf area(R=-0.333).In summary,changes in soil organic carbon and total nitrogen content caused by precipitation changes regulated species diversity(species richness and evenness)and further changed species diversity and functional traits to affect above-ground biomass of communities.Plant height,leaf area,and leaf dry matter mass can be used as key factors indicating ecosystem function in response to precipitation changes.The results of the study reveal the process and regulatory pathways of precipitation changes on above-ground biomass in desert grasslands,which are important for maintaining plant diversity and ecosystem stability in desert grasslands in China and coping with global climate change. |
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