Effects Of Shrub Encroachment On Plant Diversity And Biomass Of Grassland On The Loess Plateau

Shrub encroachment is widespread in the grasslands of northern China.With the expansion of shrubs,the plant community structure,plant diversity and productivity have been changed,which directly influence grassland conservation and its sustainable utilization.This study elucidated the differences of plant community structure and above-ground biomass between primary grassland,grass patches,shrub patches formed after shrub encroachment in a long-term fenced steppe on the Loess Plateau.We observed species composition,species diversity,litter mass and above-ground biomass in these patches.Meanwhile,we used the community assembly and the functioning of ecosystems（CAFE）approach to analyze the impact of community assembly changes on above-ground biomass.The main result as follows:（1）The difference in species composition between primary grassland and shrub-encroached grassland is mainly at a small scale（<4m²）.The non-metric multidimensional scaling analysis（NMDS）showed that the plant community composition of primary grassland,grass patches and shrub patches are significantly different（R=0.28,P=0.009）.Primary grassland and grass patches are mainly composed of perennial forbs and perennial bunch grasses.The dominant species is Stipa bungeana.Compared with primary grassland,there are 11 species and 10 species disappeared in grass patches and shrub patches,respectively.In shrub patches,the dominant species was replaced by Caragana brachypoda.（2）The species richness of grass-and shrub patches were significantly lower than that of primary grassland（P<0.01）.The dominance index was decreased but the evenness of shrub patches was increased,the Shannon Wiener diversity was not change.Shrub encroachment significantly increased the above-ground biomass by 374.4 g·m^-2and 486.7g·m^-2compared with grass patches and primary grassland,respectively.The dominant functional group of primary grassland and grass patches is the perennial bunch grasses（77.5%-83.9%）.However,shrub patches eroded the perennial bunch grasses abundance to12.5%,while semi-shrub and shrub group abundance reached 81.3%.It indicated that the replacement of dominant functional groups improved shrub-encroached grassland’s above-ground biomass.The above-ground biomass of grass patch was positively correlated with dominance index,and negatively correlated with diversity index and evenness.The above-ground biomass of shrub patches was positively correlated with species richness,but negatively correlated with evenness.The newly emerging species,Caragana brachypoda,was the dominant specie in the shrub patches.And the species ranking of the community was also variated,which jointly affected the community biomass production.（3）Most of the missing species are rare species.From primary grassland to grass patch,the missing species reduced the above-ground biomass by 23.9 g·m^-2.The biomass of newly recruited species only accounted for 8.8%.The above-ground biomass of coexisting species was increased by 103.5 g·m^-2.From primary grassland to shrub patches,the missing species eroded the above-ground biomass by 26.4 g·m^-2,and the biomass of newly recruited species accounted for 80.1%,the biomass of coexisting species decreased by 70.3%.These results suggested that the replacement of dominant species promoted the above-ground biomass,but Caragana brachypoda inhibited the coexisting species.The dominant species（Stipa bungeana and Caragana brachypoda）were coexisted in grass-and shrub patches.The missing species lost the above-ground biomass to 28.7 g·m^-2.The newly recruited species accounts for 33.3%of the community biomass.The coexisting species had increased by154.4 g·m^-2.The variation of above-ground biomass depends on the change of newly recruited species and coexisting species.In conclusion,Shrub encroachment in grassland eroded the plant species richness,the while increased above-ground biomass.The rare species and species with weak light competitiveness are easily disappeared.However,it has less negative effect on the above-ground biomass.The newly recruit species and dominant species promoted the above-ground biomass,but it has an inhibitory effect on the growth of coexisting species.These result suggested that the species reordering and species turnover jiontly mediated the grassland plant diversity and biomass production.