Recovery of a Tropical Forest after Gamma Irradiation: A 23-Year Chronicle

Abstract

An area of subtropical wet forest of the “tabonuco” type in northeastern Puerto Rico was irradiated for 3 months with a 10,000-curie cesium source, and regeneration was followed for 23 years. Plant densities, species names, growth habits, and diameters (basal or dbh) were recorded for all plants in 1- by 1-m quadrats within a 26- by 26-m square centered on the radiation source. Radiation killed most of the plants (including seeds) within 40 m of the source, forming a light gap in the forest. During recovery, overall plant density generally increased, peaking 8 and 23 years later. Regeneration has been exclusively by native species, some of which appear to have been resistant to radiation damage. Ten years after irradiation, vines, root sprouts, and graminoids began to decrease in density, herbs and saplings remained relatively constant in density, and ferns increased in density. Biomass of saplings fell to nearly zero immediately after irradiation but rose to about a third of its original value 23 years later. Most of this biomass was produced by secondary species that colonized the study site after irradiation. The number of species in all life forms in the study plot increased for the first 10 years after irradiation but then fell as a result of losses of species of herbs and ferns. Dominant species of vines remained the same throughout the study period, but the importance of herb, fern, and sapling species changed over time. Most of the saplings of primary species disappeared from the study site after irradiation and were poorly represented or had not reappeared 23 years later. The effects of radiation thus included not only damage to the vegetation but also a strongly slowed regeneration. Comparisons of the patterns observed here with regeneration in “natural” gaps highlight (by their absence) the contribution of biotic factors to site recovery.