Twenty Mahogany Provenances Under Different Conditions in Puerto Rico and the U.S. Virgin Islands

Abstract

The Neotropical mahoganies (Swietenia, family Meliaceace) show great ecological and geographic amplitude. A provenance study, begun by the USDA Forest Service in the 1960s, included 20 genetic sources of the three species of mahogany from Mexico, Central America, and the Caribbean. Trees were established at 14 sites ranging from dry to wet forests in Puerto Rico and the U.S. Virgin Islands. We assessed relative amounts of environmental and genetic variation in growth and size traits, survival; insect susceptibility; the segregation of genetic variation among species and among and within populations; and how provenance and species characteristics changed across the different environments. For size and growth traits by 14 years of age, this study showed the following results: Mahogany species had about 10 times more genetic variation among species than among provenances within species. Cluster analysis did not group provenances strictly by species, however. In big-leaf mahogany, the mean estimated environmental variance was 79%, the mean estimated genetic variance was 11%, and the mean estimated geneticby-environment interaction was 8% of the total phenotypic variance. In Pacific coast mahogany, these estimates were 41%, 15%, and 29%, respectively. Because of differing sampling designs, the same comparisons could not be made for small leaf mahogany. Big-leaf mahogany showed greater similarity in estimated environmental variances within and among (means of 33% vs. 46% of the total phenotypic variance) plantation sites in Puerto Rico and the Virgin Islands than did Pacific coast mahogany (within-site at 33% vs. among-site at 7%). For both big-leaf and Pacific coast mahogany, the estimated genetic variations among and within provenances were similar, between 6% and 9%. Significant correlations between environmental gradients at sites of origin and measures of growth and size suggested the action of natural selection in differentiating populations. Across all study sites, big-leaf mahogany always showed the greatest mean growth by year 14 (height = 10.4 m, dbh = 12.8 cm, total volume per tree = 114,700cm3). Small-leaf mahogany (height = 5.9m, dbh = 7.8 cm, total volume per tree = 45,500cm3) and Pacific coast mahogany (height = 6.4 m, dbh = 8.0 cm, total volume per tree = 49,000 cm3) were not different in most of the measured variables. All species showed maximum diameter and height growth rate in years 2 to 4, diminishing thereafter. The basal-area growth rates of big-leaf mahogany continued to increase with time; for all species, total volume growth rates continued to increase with time. All three species and several populations showed distinct responses to different life zones and soil types. Big-leaf mahogany died earliest in the subtropical dry life zone, grew least by age 14 in the lower montane wet zone, and grew best in the subtropical moist and subtropical wet zones. Pacific coast mahogany grew best and survived longest in the subtropical dry and moist zones and grew least and died soonest in the lower montane wet zone. Small-leaf mahogany lived longest in the dry zone, but other differences were not significant because of the small sample size. In both big-leaf and Pacific coast mahogany, 95% of the variation in shoot borer attack was caused by the estimated large and small-scale environmental effects. Provenances became more genetically distinct as distance increased; the source populations came from 160 to 1600 km apart.