DECAY-RATE AND SUBSTRATE QUALITY OF FINE ROOTS AND FOLIAGE OF 2 TROPICAL TREE SPECIES IN THE LUQUILLO-EXPERIMENTAL-FOREST, PUERTO-RICO

Decomposition rates, initial chemical composition, and the relationship between initial chemistry and mass loss of fine roots and foliage were determined for two woody tropical species, Prestoea montana and Dacryodes excelsa, over a gradient of sites in two watersheds in the Luquillo Experimental Forest, Puerto Rico. At all locations, fine roots decayed significantly more slowly than foliage during the initial 6 months. Substrate quality of the initial tissue showed marked differences between roots and foliage when using cell wall chemistry, secondary chemistry and total elemental analysis as indices. Quantity of acid detergent fiber (ADF) (non-digestible cell wall fiber) and lignin content were higher for roots than leaves: D. excelsa roots had 55.3% ADF and 28.7% lignin while leaves had 36.2% ADF and 11.8% lignin; P. montana roots had 68.0% ADF and 26.8% lignin while leaves had 48.5% ADF and 16.1% lignin. Aluminum concentrations were higher in fine roots (843 mg kg-1 in D. excelsa, 1500 mg kg 1 in P. montana) than leaves (244 mg kg-1 in D. excelsa, 422 mg kg-1 in P. montana), while calcium concentrations were higher in foliage (5.5 mg g-1 in D. excelsa, 7.8 mg g-1 in P. montana) than roots (3.4 mg g-1 in D. excelsa, 3.1 mg g-1 in P. montana). Nitrogen did not show any trend with tissue or species type. A linear model between mass remaining after 6 months and initial tissue chemistry could be developed only for calcium (r2 = 0.64).