Evaluation of the dry weight rank method for botanical analysis of grassland by means of simulation

With the Dry Weight Rank (DWR) method of 't Mannetje & Haydock (1963) for b otanical analysis in pastures, the dry weight proportions of species are es timated from their first, second and third ranks in dry weight in single qu adrats. The yield correction of Haydock & Shaw (1975) is used additionally to solve the problem of the respective under- and overestimations of the dr y weight proportions of high and low yielding species when these grow in pa tches. In this paper the DWR method is evaluated by means of computer simul ation. Main element of the simulation model is a computer sampling program with wh ich a fictitious vegetation can be sampled with a circular quadrat. The out put shows that the DWR method works well using relatively small sampling qu adrats with, on average, only a few plants per quadrat, irrespective of the horizontal vegetation structure. In vegetations where species grow patchwi se, satisfactory results are also obtained using large quadrats with much m ore plants (i.e. tens) per quadrat. The reason is that in these cases also minor species can compete successfully for first, second and third ranks. H owever, it appeared that only a certain degree of patchiness is necessary, and with the usually applied quadrat sizes up to 25 dm(2), probably in most vegetations this condition is fulfilled. Care should be taken in applying the DWR method for estimating species composition in recently sown grasslan ds where species usually occur more or less at random. In those cases, in p rinciple a very small sampling quadrat (smaller than 1 dm(2)) could be used . However, this has practical limitations since the quadrat size should not be too small for realistic yield estimations, needed for the Haydock & Sha w yield correction. The simulations revealed that one condition (i.e., that the sampling quadra t should be at least as large that it usually contains three or more specie s) is not necessary because of the almost always perfect functioning of the correction for missing ranks ('t Mannetje & Haydock, 1963). Generally spea king, a sampling quadrat should be chosen not larger than is strictly neces sary from the viewpoint of horizontal vegetation structure and from the vie wpoint of realistic yield estimations. Multipliers calculated from simulation data could satisfactorily mimic the original multipliers of DWR given by 't Mannetje & Haydock (1963). It is po stulated that the DWR method is well suited for studying vegetation changes in old, floristically diverse grasslands with dominant species often in mo derate dry weight proportions and species usually growing in patches.