The Self-Thinning Rule: Dead or Alive?

The self-thinning rule predicts that for a crowded, even-aged plant popu- lation, a log-log plot of total plant mass against plant density will give a straight line of slope - x. It has been described as one of the more general principles of plant population biology, but the evidence supporting it has recently come under close critical scrutiny. Recent reevaluations have concluded that the slope is much more variable than previous authors have claimed, that straight lines are the exception rather than the rule, and that the slope varies with aspects of the biology of the plant. Using a range of statistical tech- niques, I examined the published evidence to test the strength of these conclusions. Much of the recently reported variability in slope has resulted from the inclusion of inappropriate data sets: not all populations for which biomass and density data are available are undergoing self-thinning. I also found that there was no evidence for relationships between shade tolerance or taxonomic groups and the slope, and that the reported rela- tionships between the slope and various allometric growth constants, though real, were weak. The combined data for all populations are not consistent with an interspecific rela- tionship of slope - x, the slope being somewhat shallower, at -0.379, possibly because only the stem mass of trees is generally measured. I conclude that there is no evidence at present for a - % power rule of self-thinning, but that final rejection of the idea that there is an ideal slope (which may or may not be - x), awaits experiments in which resource levels are carefully controlled.

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