Lifespan of tropical trees from seed to 1-cm diameter

Background

The full lifespan of long-lived trees includes a seedling phase, during which a seed germinates and grows to a size large enough to be measured in forest inventories. Seedling populations are usually studied separately from adult trees, and the seedling lifespan, from seed to sapling, is poorly known. In the 50-ha Barro Colorado forest plot, we started intensive censuses of seeds and seedlings in 1994 in order to merge seedling and adult demography and document complete lifespans.

Methods

In 17 species abundant in seedling censuses, we subdivided populations into six size classes from seed to 1 ​cm dbh, including seeds plus five seedling stages. The smallest seedling class was subdivided by age. Censuses in two consecutive years provided transition matrices describing the probability that a seedling in one stage moved to another one year later. For each species, we averaged the transition matrix across 25 censuses and used it to project the seedling lifespan, from seed until 1 ​cm dbh or death.

Results

The predicted mean survival rate of seeds to 1 ​cm dbh varied 1000-fold across species, from 2.9 × 10⁻⁶ to 4.4 × 10⁻³; the median was 2.0 × 10⁻⁴. The seedling lifespan, or the average time it takes a seed to grow to 1 ​cm dbh, varied across species from 5.1 to 53.1 years, with a median of 20.3 years. In the median species, the 10% fastest-growing seeds would reach 1 ​cm dbh in 9.0 years, and the slowest 10% in 34.6 years.

Conclusions

Combining seedling results with our previous study of lifespan after 1 ​cm dbh, we estimate that the focal species have full lifespans varying from 41 years in a gap-demanding pioneer to 320 years in one shade-tolerant species. Lifetime demography can contribute precise survival rates and lifespans to forestry models.