The Response Of Reproductive Trait Of Component Species To Fertilization And Grazing In Qinghai-Tibetan Alpine Meadow

Background and AimsBuilding communities ecology with functional traits need intensively and extensively comparative study response of plant trait of community's compent species to land use and environment change.Scaling plant trait to community in response to variable environment following such theoretical framework:plant functional trait response as a fundamental trade-offs in individual level;this basic trade-offs can translate to secondary trade-offs-the trade-offs among individual ability of competition,survival and reproduction,as performance traits response;this species process response could translate to high trade-off-species abundance and distribution,as a response in community level.Therefor,the synthetic study of plant trait and species abundance in response to environment change,and linking individual trait response to community response,has been described as the "Holy Grail" of plant ecology.However,the linkage between functional trait response and species abundance distribution along environmental gradients often fail in field study.Compareing to low-level functional trait,plant performance trait as fitness components are more effectively in determine to plant performance and species fitness.As an important performance trait,plant reproductive allocation(RA)has long been a central topic in population ecology and species evolution.In this study,we choice plant RA as a proxy to compare component species in response environment change and linking individual response to vegetation process over common land use (fertilization and grazing)in Tibetan grassland.Our gist as following:(ⅰ)Seeking a general pattern of species RA in response to long-term grazing and short-term fertilization.(ⅱ)Disentangling size-dependent and size-independent plant RA in response to fertilization and grazing.(ⅲ)Studying the role of biomass allocation strategy in change of community structure due to fertilization and grazing. MethodsWe conducted 3 years experiments in perennial herbaceous communities in an alpine meadow on Tibetan Plateau.The location is The Alpine Meadow Ecosystem Field Station of Lanzhuo University,MaQu(101°53′E,35°58′N).We chose 32 component species and sampled 25-30 individual of each species at their flower and fruit phases in fertilizing(two level),natural control and grazing communities from 2004 to 2006,which was for measure biomass response in individual level. Correspondingly,10 quadrants were sampled in each treatment at middle of July and September in each year for investigates the response in communities' level.Light and soil resource in treatment was measured at same time of vegetation monitoring.From data sets,we chose collected biomass data of 32 species in fruit phase from high level fertilized,control and grazing treatment for analyse to general response pattern of biomass allocation.Total biomass was represented by totaled three parts biomass,and RA was the proportion of biomass of fruit to total biomass;stem allocation was the proportion of stem biomass to total biomass;leaf allocation was the proportion of leave biomass to total biomass.We then used these to estimate the species biomass response to fertilization and grazing with the Log Response Ratio (LRR).Same method used in mearsure to species abundance response(at September). To test for the effect of the fertilization and grazing on species abundance and biomass(allocation),we used an independent-sample t-test.The mean LRR of each functional group was calculated for relative abundance,individual biomass and biomass allocation.We performed non-parametric Kruskal-Wallis tests to test the differences among groups,and Mann-Whitney U tests for post-hoc comparisons.For disentangling size-dependent and size-independent plant reproductive allocation in response to fertilization and grazing,we chose collected data of 32 species from low level fertilized,control and grazing treatment in 2005 and 2006.We performed allometric model for best fitting relationship between reproductive biomass (RB)and vegetative biomass(VB)for each population of each species.We estimated the allometric coefficients(slope and Y-intercept)for 301 RB-VB bivariates,using typeⅡregression analyses in order to test the diference of allometic coefficients between control and fertilized(or grazing)treatement.Using data sets of the general response of species abundance and biomass,we examined the relationships between the response in species relative abundance and the response in species biomass(and biomass allocation)by calculating Kendall's tau-b correlation coefficients.Data for each growing season were analyzed separately.Key Resultsⅰ)Gerneral response pattern of RA to land useWe found fertilization and grazing significantly affected the biomass and the biomass allocation of most species.In whole,individual biomass increased and RA decrease following fertilization,and individual biomass decreased and RA increase due to grazing.The degree of RA response to fertilization and grazing were different among comonpent species and functional groups within same community,such as:the biomass response of graminoids was of stronger magnitude.ⅱ)Size-dependent and size-independent RA in response to land useOur results indicated significant changes in the RB-VB slope for 26 out of 102 cases in grazed plots,and 16 out of 88 cases in fertilized plots.Moreover,when slopes were not affected by treatment,more than 50%of the allometric models showed a significant change in the Y-intercept.Y-intercepts generally increased in fertilized treatment and decreased in grazed treatment.At the interspecific level,the Y-intercept,but not the slope,of the RB-VB relationships was significantly affected by land use.The allometry of RA for graminoids and forbs responded similarly to treatmentⅲ)Relationship between biomass allocation response and species abundance change.Following fertilization in community,a strong positive correlation exists between species relative abundance response and individual biomass response,and change in species relative abundance has a positive correlation with leaf allocation response,a negative correlation with stem allocation response,and no significant correlation with RA response. Following grazing,change in species relative abundance strong positive correlated with individual biomass response and week positive correlated with leaf allocation response;change in species relative abundance strong negative correlated with stem allocation response and RA response.Whatever plant response to fertilization or grazing,stem allocation response strong negative correlated with leaf allocation response.ConclusionsWe found that grazing/fertilization triggered a marked decrease/increases in the plant size and reproductive threshold and increase/decrease in RA for most of the component species.We considered that the difference of species life history strategy, plant form and function leads to species and functional groups different in degree of the response.We conclude that the RA response to grazing and fertilization in grasslands result from mostly change in reproductive threshold and size dependent RA,and lesser adaptive plasticity of size independent RA.The occurrence of shifts in RA was neither related to functional group nor to the magnitude of biomass changes following grazing and fertilization.The response of species biomass allocation strategy is very important in vegatation process following land use.We found species with larger response in individual biomass and leaf allocation will response more in species abundance following fertiliazation and grazing.Due to trade-offs among response of stem allocation,leaf allocation and reproductive allocation,we found increase in leaf allocation or reproductive allocation often at expense of stem allocation.We discuss the role of growth/reproduction trade-offs in structuring plant community and the response of functional diversity to land use in Tibetan grassland.