Mass-density relationships within and among self-thinning populations of Fagopyrum esculentum

Background/Question/Methods

The relationship between size and density of organisms is central to ecological science. Many studies have investigated correlated changes in mass and density as a crowded stand of plants grows and undergoes self-thinning (density-dependent mortality).  The classical "self-thinning rule" is based on simple geometrical principles and predicts a slope of –1/2 for the relationship between log total biomass (B) of survivors and log density (N). It is consistent with much field and experimental data. Metabolic Scaling Theory, based on organisms internal physiological transport processes, predicts a slope of -1/3 for log B – log N relationship, and has support from broad inter-population comparisons. We analyze data on biomass and density of self-thinning populations of Fagopyrum esculentum grown at three high initial densities and measured at 6 harvests to ask whether the B – N relationships among populations of one species mirror those within these populations. 

Results/Conclusions

Initial density did not affect on the slope of the log B – log N relationship, but there was a clear and significant effect on the intercept, with populations sown at higher densities having significantly more biomass at a given density of survivors. If the data for all densities and harvests are analyzed together, the log B – log N relationship linear with a slope of –0.377, which is consistent with the predictions of Metabolic Scaling Theory but not with the classical self-thinning rule. If the independent variable initial density is included as a factor, however, the estimated slope of the log B – log N relationships are much steeper, and consistent with the classical self-thinning rule but not Metabolic Scaling Theory. Our results confirm experimentally that inter-population scaling patterns do not reflect population processes. Broad interpopulations patterns are broadly consistent with the predictions of Metabolic Scaling Theory, while self-thinning itself is more consistent with the classical self-thinning rule.  This conflation accounts for much of the current debate about size-density relationships in plant populations and communities. Broad inter-population and inter-species patterns are an important starting point for research, but much of the variation within the broad patterns is real: it is not "noise" but rather consists of many other "signals", which do not parallel the overall trend but reflect processes occurring at different levels of biological organization.