Jamaican Montane Forests: Nutrient Capital and Cost of Growth

Abstract

(1) Concentrations of N, P, K, Ca and Mg were measured in the biomass of two 10 x 10 m plots at about 1550 m altitude in the Jamaican mountains: one in Mor Ridge forest and one in well-developed Mull Ridge forest. (2) Arithmetic mean concentrations in leaf and composite stem samples in Mor Ridge forest were, respectively (in mg g-D), N, 11.1, 2.0; P, 0 64, 0.15; K, 4.3, 1.6; Ca, 8.0, 1.9; Mg, 3.3, 0.80. The corresponding concentrations for well-developed Mull Ridge forest were N, 17 8, 3.1; P, 0.76, 0 13; K, 11.7, 3.1; Ca, 10.0, 3.0; Mg, 4.6, 0.61. Concentrations calculated by dividing the total weight of an element by the biomass dry weight were lower than arithmetic means in every case. (3) Concentrations per unit area (kg ha-') and (in parentheses) per unit dry weight (mg g-1) of above-ground standing crop were lower in the lower stature, slower growing Mor Ridge forest: N, 426 (2.0); P, 29.7 (0.14); K, 272 (1.3); Ca, 353 (1.7); Mg, 155 (0.74) than in the well-developed Mull Ridge forest: N, 857 (2-5); P, 41.2 (0-12); K, 829 (2.5); Ca, 940 (2.8); Mg, 193 (0-57). (4) First estimates of the amounts of nutrients used in trunk growth in the Mor Ridge forest each year are 1.9% of extractable soil N (NO3 + NH4), 0.4% of exchangeable soil K, 2.3% of exchangeable soil Ca and 0.08% of exchangeable soil Mg. For well-developed Mull Ridge forest the corresponding values are N, 2.0%; K, 1.2%; Ca, 1. 1%; Mg, 0.3%. (5) The impoverishment of the Mor Ridge forest compared to the well-developed Mull Ridge forest is probably due to the limitation of growth by the supply of N and K, and possibly by P and Ca. (6) The impoverishment of the Jamaican Upper Montane Rain forest compared to better-developed Montane Rain forests at higher altitudes on larger mountains is probably not a result of poor supply of N, K, Ca or Mg, though the role of P is still not clear. (7) Tropical montane rain forests in general may be short of N, P, K and Ca compared to the most nutrient-rich tropical lowland rain forests.