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Tropical forest tree mortality, recruitment and turnover rates
Calculation, interpretation and comparison when census intervals vary
Mathematical proofs show that rate estimates, for example of mortality andrecruitment, will decrease with increasing census interval when obtained fromcensuses of non-homogeneous populations. This census interval effect could beconfounding or perhaps even driving conclusions from comparative studies involvingsuch rate estimates. We quantify this artefact for tropical forest trees, developcorrection methods and reassess some previously published conclusions aboutforest dynamics. Mortality rates of >50 species at each of seven sites in Africa,Latin America, Asia and Australia were used as subpopulations to simulate standlevel mortality rates in a heterogeneous population when census intervals varied: allsites showed decreasing stand mortality rates with increasing census intervallength. Stand-level mortality rates from 14 multicensus long-term forest plots fromAfrica, Latin America, Asia and Australia also showed that, on average, mortalityrates decreased with increasing census interval length. Mortality, recruitment orturnover rates with differing census interval lengths can be compared using the meanrate of decline from the 14 long-term plots to standardize estimates to a commoncensus length using the expression corr= x t^0.08, where is the rate and t is timebetween censuses in years: i.e., [corrected for time bias] = [as derived from simpleanalyses]x( time[=years between measurements] to the power of 0.08). This simplegeneral correction should reduce the biasassociated with census interval variation,where it is unavoidable. Re-analysis of published results shows that the pan-tropicalincrease in stem turnover rates over the late 20th century cannot be attributed tocombining data with differing census intervals. In addition, after correction, Old Worldtropical forests do not have significantly lower turnover rates than New World sites,as previously reported. Our pan-tropical best estimate adjusted stem turnover rate is1.81 +/- 0.16% per year (mean +/- 95% CI, n=65). As differing census intervalsaffect comparisons of mortality, recruitment and turnover rates, and can lead toerroneous conclusions, standardized field methods, the calculation of local correctionfactors at sites where adequate data are available, or the use of our generalstandardizing formula to take account of sample intervals, are to be recommended.