MORTALITY AND RECRUITMENT RATE EVALUATIONS IN HETEROGENEOUS TROPICAL FORESTS

1 The methods commonly used to estimate stem turnover rates (i.e. mort ality and recruitment) in species rich tropical forests suffer from a previously unrecognized artefact. The estimated rate is not independen t of the census period. 2 An average rate estimate will decrease with time if the sample population cannot be characterized as homogeneous. This artefact may have considerable significance for comparisons betwe en permanent plot studies that have used different census periods. 3 W e present a theoretical consideration of this census effect. The artef act will be severe when a fraction of the population has a very much h igher mortality rate than the average. 4 Using a simple formulation we provide a mathematical proof that rate estimates will decline with in creasing census periods for all but perfectly uniform populations. 5 T he phenomenon of apparent rate decrease may be used to provide ecologi cally significant information about the diversity and dynamics of the population as it is related to the variance of life expectancies withi n the sample. 6 Such an artefact complicates evaluation of change over nonstandard time intervals and requires careful and detailed attentio n. Similar problems will effect any estimation procedure which cannot account for all the rate variation within a study population. Recognit ion of the problem is a necessary first step.