Tropical forest tree mortality, recruitment and turnover rates: calculation, interpretation and comparison when census intervals vary uri icon

abstract

  • 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 stand
    level mortality rates in a heterogeneous population when census intervals varied: all
    sites showed decreasing stand mortality rates with increasing census interval
    length. Stand-level mortality rates from 14 multicensus long-term forest plots from
    Africa, Latin America, Asia and Australia also showed that, on average, mortality
    rates decreased with increasing census interval length. Mortality, recruitment or
    turnover rates with differing census interval lengths can be compared using the mean
    rate of decline from the 14 long-term plots to standardize estimates to a common
    census length using the expression corr= x t^0.08, where is the rate and t is time
    between censuses in years: i.e., (corrected for time bias) = (as derived from simple
    analyses)x( time(=years between measurements) to the power of 0.08). This simple
    general correction should reduce the biasassociated with census interval variation,
    where it is unavoidable. Re-analysis of published results shows that the pan-tropical
    increase in stem turnover rates over the late 20th century cannot be attributed to
    combining data with differing census intervals. In addition, after correction, Old World
    tropical forests do not have significantly lower turnover rates than New World sites,
    as previously reported. Our pan-tropical best estimate adjusted stem turnover rate is
    1.81 +/- 0.16% per year (mean +/- 95% CI, n=65). As differing census intervals
    affect comparisons of mortality, recruitment and turnover rates, and can lead to
    erroneous conclusions, standardized field methods, the calculation of local correction
    factors at sites where adequate data are available, or the use of our general
    standardizing formula to take account of sample intervals, are to be recommended

publication date

  • 2004