EFFECT OF LIGHT INTENSITY ON GROWTH, LEAF PRODUCTION, LEAF LIFESPAN AND LEAF NUTRIENT BUDGETS OF ACACIA MANGIUM, CINNAMOMUM INERS, DYERA COSTULATA, EUSIDEROXYLON ZWAGERI AND SHOREA ROXBURGHII

Saplings of Acacia mangium, Cinnamomum iners, Dyera costulata, Eusideroxylon zwageri and Shorea roxburghii were grown side by side at 4, 7, 25, 50 and 100% relative light intensities (RLIs) to facilitate comparison under identical soil and climate conditions. The comparisons cover height increment, diameter increment, rate of production of new leaves, leaf lifespan and leaf nutrient content. The study was carried out in just under two years and the best plants grew from 30 cm to over 300 cm tall. Acacia mangium and S. roxburghii grew fastest at 100% RLI. The other three species grew fastest at 25% RLI. In height, A. mangium at 100% RLI grew four times faster than the second best species, S. roxburghii, and 21 times faster than the slowest species, E. zwageri. Leaf lifespan, found to be maximum at 4% RLI, was shortened by a constant amount by each doubling of light intensity. For example, each doubling of light intensity shortened the leaf lifespan of S. roxburghii by 70 days, and that of A. mangium by 40 days. Photo-senescence of leaves is proposed as a mechanism driving phenological events in the non-seasonal humid tropics. Old leaves at the point of shedding had lower NPK contents than newly expanded leaves. The difference is a measure of internal nutrient recycling of NPK. Mg was sometimes internally recycled, sometimes not, while Ca was usually accumulated. In the two fastest-growing species, A. mangium and S. roxburghii, the NPK content in new leaves tended to peak at about 25% RLI while the growth rate reached its maximum at 100%. Leaf nutrient content did not mirror the rate of growth. Acacia mangium had the highest rate of nutrient uptake per growing shoot—10 to 16 times that of S. roxburghii. At 100% RLI it grew fastest, had the shortest leaf lifespan and the highest leaf turnover rate.