REDUCED SOLAR IRRADIANCE EFFECTS ON NET PRIMARY PRODUCTIVITY (NPP) AND THE DELTA-C-13 AND DELTA-O-18 VALUES IN PLANTATIONS OF MUSA SP, MUSACEAE

Biomass accumulation (annual net primary productivity, NPP) in a plant ation of Musa sp., Musaceae is linearly correlated with solar irradian ce (IR) between 100 and 32% of ambient levels (NPP = 0.82 IR + 20.42, p < 0.01, for the normalized data). The irradiance effects are clearly reflected in the delta(13)C values of leaf organic matter that are al so Linearly correlated with irradiance levels (with a slope of 0.23 pa rts per thousand for any 10% decrease in IR). We conclude that changes in the organic delta(13)C values faithfully record the irradiance eff ects on the plants photosynthetic capacity since they are not accompan ied by changes in concentration and delta(13)C values of air CO2 or by changes in the delta(18)O values of the leaf cellulose. The results p rovide a unique, quantitative demonstration of (1) the importance of c hanges in irradiance such as those associated with cloudiness and aero sols for NPP and (2) the usefulness of the combined C-13, O-18 analysi s for recording and interpreting these effects.