Contrasting Demographic Structure of Short- and Long-lived Pioneer Tree Species on Amazonian Forest Edges

Although tropical forests have been rapidly converted into human-modified landscapes, tree species response to forest edges remains poorly examined. In this study, we addressed four pioneer tree species to document demographic shifts experienced by this key ecological group and make inferences about pioneer response to forest edges. All individuals with dbh ≥ 1 cm of two short-lived (Bellucia grossularioides and Cecropia sciadophylla) and two long-lived species (Goupia glabra and Laetia procera) were sampled in 20 1-ha forest edge plots and 20 1-ha forest interior plots in Oiapoque and Manaus, Northeast and Central Amazon, respectively. As expected, pioneer stem density with dbh ≥ 1 cm increased by around 10—17-fold along forest edges regardless of species, lifespan, and study site. Edge populations of long-lived pioneers presented 84—94 percent of their individuals in sapling/subadult size classes, whereas edge populations of short-lived pioneers showed 56—97 percent of their individuals in adult size classes. These demographic biases were associated with negative and positive net adult recruitment of long- and short-lived pioneers, respectively. Our population-level analyses support three general statements: (1) native pioneer tree species proliferate along forest edges (i.e., increased density), at least in terms of non-reproductive individuals; (2) pioneer response to edge establishment is not homogeneous as species differ in terms of demographic structure and net adult recruitment; and (3) some pioneer species, particularly long-lived ones, may experience population decline due to adult sensitivity to edge-affected habitats.