Reconstructing the invasion history of a spreading, non-native, tropical tree through a snapshot of current distribution, sizes, and growth rates

Abstract

Elucidating the invasion history of non-native species has been dependent on coarse-grain and expensive methods or long-term monitoring during which the spread may have proceeded beyond feasible control. We used the case of a relatively recent introduction and spread of the neotropical Cecropia pachystachya in Singapore to develop a method for reconstructing spatio-temporal patterns of spread through a low-cost, cross-sectional study. Size and growth rates were measured for C. pachystachya trees as well as the native Macaranga gigantea. A power-expansion exponential-decline function was a better fit than the probability density function of the log-normal distribution in describing the growth-rate to size relationship for both species. C. pachystachya trees generally grew faster (up to 5.4 ± 0.1 cm per year at 12.2 ± 0.2 cm DBH) than M. gigantea trees (up to 3.8 ± 0.2 cm per year at 11.5 ± 0.3 cm DBH). We demonstrated that the integral of the reciprocal of these growth equations provides an estimate of the age of the individuals from their size. Using the size and geographic coordinates of C. pachystachya trees from an island-wide search, we estimate that the invasion front of reproductive trees (>5 cm DBH) showed at least a 20-year lag phase from the time of initial establishment to the year 2005, before advancing exponentially at median rates between 5-466 m year-1 with maximum rates of several km year-1. The extent of occurrence expanded by nearly 10-fold from 2004-2012. Consequently, the spatial dynamics of trees can be reproduced using ontogenetic growth functions.