Earthworms in tropical tree plantations and secondary forests



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We compared patterns of earthworm abundance and species composition in tree plantations and secondary forests of Puerto Rico. Tree plantations included pine (Pinus caribaea Morelet) and mahogany (Swietenia macrophylla King) established in the 1930s, 1960s, and 1970s; secondary forests were naturally regenerated in areas adjacent to these plantations. We found that (1) earthworm density and fresh weight in the secondary forests were twice those in either of the tree plantations, and did not differ between the plantations, and (2) the exotic earthworm species, Pontoscolex corethrurus M ller, dominated both plantations and the secondary forests, but native earthworm species, Pontoscolex spiralis Borges & Moreno, Estherella montana Gates, and E. gatesi Borges & Moreno, occurred only in the secondary forests. Our results suggest that naturally-regenerated secondary forests are preferable to pine and mahogany plantations for maintaining a high level of earthworm density, fresh weight, and native species.




Additional Project roles: 

Name: Miguel C Leon Role: Data Manager
Name: Grizelle Gonzalez Role: Associated Researcher
Name: Sonia Borges Role: Associated Researcher


We used three plantations of each tree species (Pinus caribaea and Swietenia macrophylla) and their adjacent secondary forests, located between 200 m and 600 m elevation in the LEF. Each of the six pairs (plantations/secondary forests) have the same age, and similar topography, soil, and climate conditions. Two pine plantations were established in 1962 and one in 1977 with a 3 x 3 m tree spacing on the abandoned croplands near Cubuy and Guzman, respectively. These lands were cleared by bulldozers before planting pine seedlings. By 1989, the canopy of the pine plantation was mainly a monoculture, but Casearia sylvestris Sw., Didymopanax morototoni (Aubl.) Decne. & Planch., and Tabebuia heterophylla (DC.) Britton were among the native woody species presented in a well developed understory. The average litter production was 7.0 Mg/ha/yr. Two plantations of Swietenia macrophylla were established in 1932 near El Verde Field Station and one in 1963 near Sabana. Tree spacing ranged from irregular to 7.6 x 2.4 m (Lugo 1992). In 1984, the overstory of the plantations were dominated by mahogany, but Ocotea leucoxylon (Sw.) Mez., Tabebuia heterophylla, and Myrcia splendens (Sw.) DC. were also found in the understory. Average litter production was 10.8 Mg/ha/yr. Secondary forests were naturally regenerated adjacent to the pine and mahogany plantations on the same abandoned lands with similar soil conditions. By 1989, the secondary forests were dominated by Tabebuia heterophylla, Casearia arboria (L.C. Rich) Urban, Syzygium jambos (L.) Alston, and Manilkara bidentata (A. DC.) Chev. Average litter production was 6.15 Mg/ha/yr (Cuevas et al. 1991, Lugo 1992). All field data were obtained between January and May, 1994. Earthworms were sampled in two plots (0.5 x 0.5 m) randomly located in each plantation or its adjacent secondary forest for a total of 24 plots. Ground litter (leaves and twigs < 2 mm) was collected from each plot and litter biomass was obtained after drying leaves and twigs at 60 C for at least 72 hrs. Leaves were analyzed for total extractable polyphenolics content using the Folin-Denis reagent (Horowitz 1970, Anderson & Ingram 1989) and were reported as percent tannic acid equivalent (% TAE). Soils beneath the litter were collected by shovel to a depth of 0.25 m and placed onto a cloth sheet. Earthworms were hand sorted and stored in plastic bags in a cooler with ice. Fresh weight of earthworms was recorded after the worms were rinsed with water and dried with paper towels on the same day when they were collected. A small subsample of the soil (0 - 0.25 m) was used for measuring pH and moisture content. Soil water content was calculated for each site by oven drying 10 g of fresh sample at 105 C for 48 hrs and reported on the oven-dry basis. Soil pH was measured using a paste of 1:1 ratio of fresh soil and deionized water. Data was analyzed using ANOVA (SAS 1987) to compare differences among the plantations and the secondary forests. Where significant differences were found, Duncan's multiple range test was employed to compare differences among pine, mahogany and secondary forest. Earthworm density and fresh weight were correlated with soil pH, moisture content, ground litter biomass, and litter polyphenolics content using a simple linear correlation analysis (SAS, 1987).



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