Productivity of a Tropical Forest and its Relation to a World Pattern of Energy Storage

One of the goals of the International Biological Programme is to estimate existing and potential plant production in the major climatic regions of the world (National Academy of Science 1968). Even before this goal was formally proposed for the I.B.P., ecologists realized the importance of determining the productivity of natural ecosystems, and consequently in recent years there has been an increase in productivity studies in the ecological literature. The objectives of this study were: (1) to measure the production and efficiency of a tropical forest, an ecosystem about which relatively little is known because of its remoteness from most universities and laboratories; (2) to compare it with other ecosystems in an effort to establish a world-wide pattern of productivity and efficiency. The field study was carried out at an elevation of 500 m on El Yunque Mountain in the Luqillo Experimental Forest of eastern Puerto Rico (latitude 18?N, longitude 66?W). Rainfall in the forest is relatively high (240-360 cm/year at the study site) because of the cooling and uplift of the moist air brought to the mountain by the prevailing easterly winds. A cloud cover is common over the top of the mountain. Because of the clouds, total solar radiation at the forest is relatively low compared to that of the coastal plain and western regions of the island (Briscoe 1966). The forest is of a type described as Tabanuco (Dacryodes excelsa) by Wadsworth (1951). The forest has never been clear cut, but occasional selective logging has taken place in the area during and before World War II (F. W. Wadsworth, personal communication). The average height of the top of the canopy is about 20 m, although there are a few emergents which reach 25 m. Richards (1966), speaking generally about rain forests throughout the world, states, 'The average height of the taller trees in a rain forest is rarely more than 46-55 m, though individual trees over 60 m are not uncommon.' On the island of Dominica, British West Indies, the top of the canopy of a Dacryodes-Sloanea forest extends about 30 m (Beard, cited in Richards 1966). Therefore, it is likely that the forest described here may not be a 'climax' forest in which there is no further increase in standing crop. From January to April of 1965, a portion of the forest was irradiated with a 10000 Ci source of '37Cs (H. T. Odum 1965). Die-back of the canopy began before the end of irradiation. By July 1966, the canopy had disappeared to about 15 m and was intermittent to 30 m from the source location. There was no recovery of the canopy (Plate 2) and stems of the radiation-killed trees fell during succeeding years.