Height Growth of Mahogany Seedlings, St. Croix, Virgin Islands
~/)/, ... :As ij .s - oU.fJJ.~[JJ~ /4 ---- 00~$~~00~[X) OO@V~ OO©o ~,r[f ~@ INSTITUTE OF TROPICAL FORESTRY RIO PIEDRAS, PUERTO RICO FOREST SERVICE - U.S DEPARTMENT OF AGRICULTURE December 1966 HEIGHT GROWTH OF MAHOGANY SEEDLINGS, ST. CROIX, VIRGIN ISLANDS By R. w. Nobles an<l c. B. Briscoe Summary Small-leaf mahogany seedlings grew more rapidly than bigleaf and their hybrids grew more rapidly than either for the first two years. However, by age 4 and conti- nuing through age 7, both hybrids and bigleaf were significantly taller than small-leaf. Bigleaf suffered the most mortality, followed by small- leaf, then the two hybrids. Resumen Los estudios de crecimiento de caoba en Santa Cruz, Islas V1rgenes, indicaron que durante los primeros dos afios las h1bridos de la caoba de hoja grande y de la caoba de hoja pequefia sobrepasaron la caoba de hoja pequefia, que a su vez sobrepaso la de hoja grande. …
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~/)/, ... :As ij .s - oU.fJJ.~[JJ~ /4 ---- 00~$~~00~[X) OO@V~ OO©o ~,r[f ~@ INSTITUTE OF TROPICAL FORESTRY RIO PIEDRAS, PUERTO RICO FOREST SERVICE - U.S DEPARTMENT OF AGRICULTURE December 1966 HEIGHT GROWTH OF MAHOGANY SEEDLINGS, ST. CROIX, VIRGIN ISLANDS By R. w. Nobles an<l c. B. Briscoe Summary Small-leaf mahogany seedlings grew more rapidly than bigleaf and their hybrids grew more rapidly than either for the first two years. However, by age 4 and conti- nuing through age 7, both hybrids and bigleaf were significantly taller than small-leaf. Bigleaf suffered the most mortality, followed by small- leaf, then the two hybrids. Resumen Los estudios de crecimiento de caoba en Santa Cruz, Islas V1rgenes, indicaron que durante los primeros dos afios las h1bridos de la caoba de hoja grande y de la caoba de hoja pequefia sobrepasaron la caoba de hoja pequefia, que a su vez sobrepaso la de hoja grande. Desde los 4 hasta las 7 afios, la altura alcanzada por la caoba de hoja grande y por el h1brido de los arboles portagranos de las dos especies fue esencialmente igual, y todos sobrepasaron en altura a la caoba de hoja pequefia. La caoba de hoja grande sufrio la mortalidad mayor segui<la por la caoba de hoja pequefia y finalmente por las dos h1bridos. * In cooperation with the University of Puerto Rico. LIBRA RY JUN 27 1967 .RO.CKY MOUNTAIN STATION • ., 1/ Since the recogn1t1on of natural hybridization of small-leaf A (West Indies) mahogany (Swietenia rnahagoni Jacq.) with bigleaf W (Honduras) mahogany (~. macrophylla King) there has been con- jecture about their relative growth rates. One would expect small-leaf to be the fastest growing on dry sites, the hybrids to be fastest on intermediate sites, and bigleaf to excel on wet sites. In 1960 a comparison was begun on dry and intermediate sites on St. Croix, Virgin Islands. Four types of 1-0 seedlings were planted: Seedling Bigleaf Medium leaf Mediumleaf Small-leaf Mother-tree Bigleaf Bigleaf Small-leaf Small-leaf Each plot contained one seedling of each type, a total of 4, at 3-meter (10-foot) spacing. There were 9 well-scattered plots at each of 11 locations. Elevations range up to 180 meters (600 feet) above sea level. Mean annual rainfall varies from 700 to 1400 mm (29 to 54 inches) per year for the 7 locations. Results from 7 locations are given in Table 1. The remaining locations were eliminated: three because of drouth mortality and one because of destruction by bulldozer. For the first two years: (a) Both types of hybrid seedlings outgrew both species at all locations but one, where small-leaf grew slightly more11. (b) Small-leaf growth exceeded bigleaf growth. (c) The hybrids were not different from each other. After 7 years, both hybrids average 15 centimeters (6 inches) taller than the bigleaf, but the difference is not significant. The bigleaf averages 1.5 meters (5 feet) taller than small-leaf, a highly significant difference. !/ c. B. Briscoe and F. B. Lamb. 1962. Leaf size in Swietenia. Car. For. 23:2:112-115. II c. B. Briscoe and R. w. Nobles. 1962. Height growth of mahogany seedlings. Trop. For. Notes #13. 2 Because of prolonged dry spel~, mortality in 7 years was high: 109 of 252 trees in all plots. In two of the wettest sites, Ham Bluff and Cane Bay, mortality jumped 44 percent and 36 percent, respectively, in the past 5 years. As expected, mortality was highest for bigleaf, 63 percent for all sites. Somewhat surprisingly, mortality for hybrids was less than for small-leaf, 37 and 32 percent compared to 44 percent. The present pattern of development was established by age 4, and has remained constant since, Table 1. 3 Table 1.--Mean height growth in meters of mahogany seedlings, 1960-67 Location New Works Thomas Sima Hill Betsy Jewel Cane Bay Ham Bluff Bodkin I Combined (1960-62) :Approximate: : Mediumleaf, : Mediumleaf, : :Mortality Rainfall :Bigleaf:Bigleaf mother:Small-leaf mother:Small-leaf:Percentage mm/Yr. 700 1100 1100 1200 1300 1300 1400 4.6 6.6 0.8 6.3 4.6 (0.5) 3.8 5.1 3.1 7.5 3.3 5.6 5.5 4.8 (1. 1) 4.4 4.7 2. 7 7.8 2.6 s.o 6.0 4.7 (1.2) 2 .1 4.2 1.2 4.1 1.7 3.1 5.0 3.0 (0. 8) 33 42 78 14 53 61 31 Mortality Percent 63 37 32 44 43 4