Effects Of Stocking Rate On The Biomass And Its Allocation Of Vegetation In The Stipa Breviflora Desert Steppe

The desert steppe is the most arid and the least productive grassland type in the Eurasian steppe,and its characteristic is very unique in ecology.Sheep grazing is the most common,economical and almost the only way to use these kind of grasslands.Stocking rate is an important factor to determine the effects of livestock on biomass productions,community structures and functions in grassland ecosystems.Biomass is the main manifestation of ecosystems to acquire energy,and is also the foundation of ecosystem structures and functions.However,the effects of stocking rate on biomass and its allocation of plant community and dominant species in the desert steppe remain understudied.A 4-yr(2013-2016)field experiment was carried out in a randomized complete block designed grazing site with four stocking rates[0,0.15,0.30,and 0.45 animal unit months · ha-1 in the control(CK),light grazing(LG),moderate grazing(MG)and heavy grazing(HG),respectively]in the Stipa breviflora desert steppe of Inner Mongolia,China that had begun in 2003.The objectives of the present study were to evaluate the effects of stocking rate on aboveground biomass(AGB)and belowground biomass(BGB)and their allocations at the plant community and dominant species(S.breviflora,Cleisogene songorica,Artemisiafrigida,Convolvulus ammannii)levels.This study would provide references for sustainable grazing utilization in the desert steppe and be of importance to the desert steppe conservation.The results showed that:(1)Total biomass of the plant community in the S.breviflora desert steppe decreased with an increase in stocking rate(P<0.001).The year and stocking rate and year interaction had little effect on total biomass(P>0.05).Both AGB and BGB of the plant community decreased with an increase of stocking rate(P<0.001),they were also impacted by the year(P<0.001).The stocking rate and year interaction had significant effects on AGB(P<0.001)but not BGB(P>0.05),AGB decreased more with increased stocking rate in 2013 and 2016 with greater precipitation.(2)Total biomass of S.breviflora,C.songorica,A.frigida and C.ammannii decreased with an increase in stocking rate(P<0.001),and they were all affected by year(P<0.001).The stocking rate and year interaction had significant effects on total biomass of C.songorica and A.frigida(P<0.001),which decreased more with increased stocking rate in 2013 and 2016 with greater precipitation.Both AGB and BGB of four dominant plant spicies decreased with an increase in stocking rate(P<0.001),they were also affected by the year(P<0.001).The stocking rate and year interaction had significant effects on the AGB of C.songorica and A.frigida and BGB of C.songorica(P<0.001),they decreased more with increased stocking rate in 2013 and 2016 with greater precipitation.(3)The percentage for aboveground biomass of S.breviflora,A.frigida and C.ammannii in the community aboveground biomass were impacted(P<0.001)by either stocking rate,year or their interaction.The percentage for S.breviflora increased while for A.frigida decreased with an increase of stocking rate(P<0.001).The percentage for C.ammannii decreased with an increase in stocking rate in most years(P<0.001).However,the percentage for C.songorica was little influenced by stocking rate,year and their interaction(P>0.05)(4)The BGB:AGB ratio of the plant community increased with an increase of stocking rate(P<0.01),and it also affected by the year(P<0.001)and the stocking rate and year interaction(P<0.05).The ratio increased more with increased stocking rate in 2014 and 2015 with less precipitation.The trade-off of community AGB and BGB showed that more trade-offs for BGB in all treatments,and which were greater in LG,MG and HG than in CK with slight variances among years.(5)The BGB:AGB ratio of S.breviflora,C.songorica and C.ammannii all increased with an increase of stocking rate(P<0.05),while the ratio of A.frigida was little affected(P>0.05).The ratio of the four species were also impacted by the year(P<0.001).The stocking rate and year interaction had significant effects on the ratio of C.songorica(P<0.001)but not other species(P>0.05),indicating that the response of C.songorica ratio on stocking rate is determined by precipitation.The trade-off of AGB and BGB for four dominant species showed that more trade-offs for BGB in all treatments in the most cases Compared to CK,grazing increased the trade-offs for BGB of S.breviflora,C.songorica and C ammannii,while decreased the trade-offs for BGB of A.frigida,and there were variances among years.The relationship between AGB and BGB of S.breviflora and A.frigida were isometric,while it was allometric for C.songorica and C.ammannii in the most cases and the BGB grew faster,although there were considerable variances among years.(6)The percentage of reproductive mass in AGB of S.breviflora was higher(P<0.001)in LG and MG than in CK and HG.The percentage of A.frigida decreased with an increase of stocking rate(P<0.001).The trade-off of vegetative and reproductive mass for S.breviflora and A.frigida showed that more trade-offs for vegetative mass in all treatments.The relationships between vegetative and reproductive mass of S.breviflora and A.frigida were allometric in all stocking rate treatments,and greater for reproductive than vegetative mass.Plant height,leaf length,leaf thickness,leaf biomass,leaf area and specific leaf area of four dominant species decreased(P<0.001)with an increase in stocking rate.