Implications of the sampling procedure used to estimate mean plant biomass for the analysis of plant self-thinning

Abstract

In even-aged, crowded plant populations that are actively growing, intraspecific competition determines at some time that plant density decreases as total biomass increases, if interspecific competition and density-independent stresses do not occur. In a bilogarithmic scale, the temporal variation of the relationship between total stand biomass (expressed per unit area) and plant density during this stage is described by a straight line with negative slope, called the line'' (Weller 1987). For the study of self-thinning, two different questions may be of interest, although they are closely related to one another: (i) is self-thinning associated with an increase in total biomass for a given stand?, and (ii) do surviving plants grow in biomass during self thinning? To answer the first question, the temporal relationship between total stand biomass and plant density during selfthinning may be plotted. In a bilogarithmic scale, the straight lines obtained by regression for different stands of the same or different species growing under different conditions are used to compare their growth dynamics (Weller 1987). To answer the second question, one might simply note from the above analysis that surviving plants do grow in biomass as density decreases, because mean plant biomass is equal to total biomass (which increases) divided by number of plants (which decreases) for a given stand. A different way to answer this question might be to plot the temporal relationship between mean plant biomass and plant density in a bilogarithmic scale (Yoda et al. 1963 and other papers cited in Weller 1987) and test for the significance of the correlation between both variables. Then, regression lines might be hypothetically used to compare growth rates of plants during self-thinning among different stands, but the utility of this method is affected by some considerations that will be outlined below. Recently, it was stated (Petraitis 1995a) that the choice of either mean or total plant biomass for correlation studies against plant density depends solely on the research question and, thus, is not affected by other factors. I believe that such a conclusion is potentially misleading towards a correct utili-