THE SELF-THINNING RULE - DEAD OR ALIVE

The self-thinning rule predicts that for a crowded, even-aged plant population, a log-log plot of total plant mass against plant density will give a straight line of slope -3/2. 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. Much of the 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. There was no evidence for relationships between shade tolerance or taxonomic groups and the slope, and reported relationships between the slope and various allometric growth constants, though real, were weak. Combined data for all populations are not consistent with an interspecific relationship of slope -3/2, the slope being somewhat shallower, at -0.379, possibly because only the stem mass of trees is generally measured. There is therefore no evidence at present for a -3/2 power rule of self-thinning, but final rejection of the idea that there is an ideal slope awaits experiments in which resource levels are controlled. -from Author