Design Of An Optimal Quadrat Investigation Scheme For Plant Biomass-density Relationship Of Different Vegetation Types

Quadrat sampling is the most basic method for large-scale vegetation investigation or large plot survey in the field.During the survey process,researchers often not only ensure comprehensive information of the communities in the sampling sites,but also minimize the quadrat size,sampling time,and cost to determine the optimal quadrat size.However,owing to the complexity and diversity of natural ecosystems,the development of optimal sample survey schemes for different vegetation types in different habitats remains a challenge for academia.There is no uniform methodology or principle for the scientific design of the optimal survey schemes for different vegetation types at home and abroad,which not only ensures accurate access to true information on plant communities,such as the relationship between plant biomass and the individual number of units(biomass-density,M-N)but also greatly improves survey efficiency and saves investigation cost.The plant M-N relationship is the basic attribute of plant community structure and is also the focus of ecologists.However,when analyzing the M-N relationship of vegetation or a sample plot based on a quadrat survey,the quadrat area is too large or too small to accurately obtain the M-N relationship information of the plant community.Therefore,how to design the size and number of quadrat when surveying is particularly important.To determine the optimal quadrat size for studying the M-N relationship in plant communities and how their relationship changes with quadrat number and quadrat size,this study took two typical desert ecosystems(Jingtai and Jiayuguan)and two types of forest ecosystems(Wytham Woods,WW;Barro Colorado Island,BCI)were taken as research objects,through setting different quadrat sizes to randomly sampling from the tracking and monitoring sample plots to simulate and analyze how the quadrat size affects the plant M-N relationship,and finally get the optimal quadrat size for different vegetation types under different environmental conditions.The main steps to develop the scheme include:1)obtaining large plot data;2)simulating random sampling under different quadrat size and quadrat number gradients;3)conducting a meta-analysis on the simulation results;4)determining the optimal quadrat size through piecewise linear regression.Four large plots were randomly sampled under different quadrat numbers and quadrat sizes gradients,we got the results as follows:1.The quadrat number had no significant effect on the sampling results.The data distribution of biomass and density obtained by simulation sampling did not change significantly with the increase of sampling numbers.2.The quadrat size significantly affected the sampling results.The data distribution of biomass and density obtained by simulation sampling with fixed quadrat number changed greatly with the increase of quadrat size when the quadrat was small but did not change significantly with the increase of quadrat size when the quadrat was large.3.The plant M-N allometric exponent obtained from all the simulations presents a normal distribution.The average values of these allometric exponents are-0.845(Jingtai),-1.327(Jiayuguan),-1.300(WW),and-3.068(BCI),respectively;the allometric exponent of plant M-N at Jiayuguan and WW are close to–4/3.To explore the strength of the M-N relationship under different sampling conditions,further meta-analysis was conducted on the simulation results:4.The quadrat number can't explain the variation of M-N relationship strength,while the quadrat size can significantly explain the variation of M-N relationship strength.There was heterogeneity in the combined effect values,and the Q_m values of four large plots were obtained by meta-regression using the number of quadrats as the independent variable was not significant;using the quadrat size as the independent variable of meta-regression,the Q_m of four large plots were extremely significant.5.The change of quadrat numbers had no significant effect on the intensity of the M-N relationship,but the change of plot size had a significant effect on the intensity of the biomass density relationship.The strength of the M-N relationship did not change with the increase in the number of plots.With the increase of sample size,the strength of the M-N relationship presents a two-stage change.When the sample size is small,the strength of the M-N relationship increases rapidly with the increase of sample size;when the sample size is large,the strength of the M-N relationship increases slowly with the increase of sample size.6.Utilizing piecewise linear regression,the turning point of M-N relationship intensity change was determined as the optimal quadrat size.The results showed that the optimal quadrat sizes of Jingtai,Jiayuguan,WW,and BCI were 400 cm,560 cm,20m,and 15 m,respectively.The above study not only provides an effective method to acquire the M-N relationship of plant community more accurately,improve the sampling efficiency and effect of field investigation,and forest community sampling but also offers an important methodological structure and research idea for random survey and sampling plant population under different conditions,especially in the large plot field survey.Because the method structure of random sampling can quickly determine the optimum quadrat size,the efficiency of the plant community survey is greatly improved.At the same time,using this method,community information can be further exploited to provide new ideas and theoretical guidance for the management and evaluation of forests,grasslands,and other ecosystems,as well as the estimation of ecosystem carbon stocks.