In recent decades much progress has been made in understanding how frequency and density dependence in key stage-based demographic transitions contribute to the maintenance of diversity in tropical forests. However the time frames over which these stage-based dynamics play out are usually not apparent because the age of plants through their ontogeny is typically unknown. While size-age relationships for some species have been constructed, these often omit the very smallest plants and the ages of short-statured, probably suppressed understory individuals are largely unknown for any tropical forest. We used individual-based, long-term records of height growth across multiple tree species at the Davies Creek Forest Dynamics Plot in north Queensland, Australia, to estimate the age of individuals at 1 cm dbh, a common lower size limit on forest dynamics plots worldwide. New seedling recruits of all tree species were tagged, mapped and measured for height along permanent transects in 1971, 1974, 1978, 1981 and 1986, and survivors were remeasured in 1978, 1986, 1992, 1998, 2006 and 2013. Because plants on the study plot are measured for height until they reach 2.5 cm dbh, we established through multispecies survey that on average, a 1 cm dbh stem is 2.14 m tall.
Results/Conclusions
The median height of seedlings 42.7, 39.2, 35.2, 32.1 and 27.7 years old in 2013 were 0.55 m, 0.49 m, 0.52 m, 0.40 and 0.40 m, respectively. Only 1.6% of seedlings surviving to 2013 from all these cohorts reached 2.14 m tall during several decades of observation. We extrapolated the observed linear relationships between age and height determined over several decades for each of the seedling cohorts, to estimate median age at 2.14 m tall. The extrapolated median ages of seedlings this tall from the 1971, 1974, 1978, 1981 and 1986 cohorts were 187.9, 188.5, 190.8, 196.8 and 234.0 years, respectively. The growth rate of seedlings that were already >0.46 m tall at the start of the study and followed for several decades was not significantly different to the growth rates of the initially smaller recruit cohorts, strongly suggesting that in the shaded understory growth rate is size-independent and therefore our extrapolations are realistic. These data indicate that relatively small plants at Davies Creek are highly suppressed and extremely old, and this work gives a new appreciation of the likely time frames over which the processes that determine patterns of abundance and diversity through these influential early stages play out.