Demographic differentiation among pioneer tree species during old-field succession of a Neotropical rainforest

Early pioneer species share life histories enabling them to colonize disturbed sites, but how much they differ demographically and how such differentiation determines pioneer species turnover during succession are still open questions. Here, we approached these issues by comparing the demography of dominant pioneer tree species during the old-field succession of tropical rainforest in Southeast Mexico.
We assessed changes in population density, population structure, vital rates, and intrinsic population growth rate (r) of the pioneer species Trema micrantha, Cecropia peltata, and Trichospermum mexicanum during the first 35 years of succession. For this, we combined chronosequence and long-term (from 2000 to 2018) data from 14 old-fields with 0.5-35 years fallow age.
Trema colonized and disappeared first during succession (< 15 years), followed by Cecropia (< 28) and Trichospermum (> 31). All species exhibited hump-shaped successional trajectories of population density and biomass with Trema reaching a peak first, followed by Cecropia and later Trichospermum. Species exhibited a fast reduction in r with fallow age, with Trema reaching negative growth rates (r < 0) in the third, Cecropia in the fourth, and Trichospermum in the seventh year of succession. Recruitment, growth, and mortality rates of seedlings and juveniles defined the period of population increase and the age of succession at which each species reached maximum density and biomass. The mortality rate in mature stages determined how long each species persisted during succession. An important variation in species replacement occurred among study sites. In some sites one species was abundant and the others were almost absent, while it was the opposite in other sites. We inferred that priority inhibitory effects operated among species during the field colonization.
Synthesis: Although Trema, Cecropia, and Trichospermum are considered typical pioneer trees, these species differed importantly in their demographic attributes during succession. The speed at which r declined with age of succession indicated the moment at which each species reached its maximum density and species replacement sequence during succession. However, inter-specific priority inhibitory effects during field colonization may also be involved in the chance of colonization and replacement between species with similar regeneration strategies.