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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 and
recruitment, will decrease with increasing census interval when obtained from
censuses of non-homogeneous populations. This census interval effect could be
confounding or perhaps even driving conclusions from comparative studies involving
such rate estimates. We quantify this artefact for tropical forest trees, develop
correction methods and reassess some previously published conclusions about
forest 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.

Topic: carbon dioxide,environmental factors,environmental degradation,monitoring,models,tropics,rain forests,trees,forest dynamics

Publication Year: 2004

ISSN: 0022-0477

Source: Journal of Ecology 92: 929-944

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