Oecologia (1991) 88:494-503 Oecologia 9 Springer-Verlag 1991 Model simulations of spatial distributions and daily totals of photosynthesis in Eucalyptus grandis canopies R. Leuning 1'*, Y.P. Wang 2, and R.N. Cromer 1 1 CSIRO Division of Forestry, P.O. Box 4008, Queen Victoria Terrace, Canberra, A.C.T., 2600, Australia z CSIRO Division of Atmospheric Research, CSIRO Private Bag No. 1, Mordialloc, Victoria 3195, Australia Received March 20, 1991 / Accepted in revised form July 27, 1991 Summary. A simulation model for radiation absorption and photosynthesis was used to test the hypothesis that observed nonuniform distributions of nitrogen con- centrations in young Eucalyptus 9randis trees result in greater amounts of daily assimilation than in hypotheti- cal trees with uniform N distributions. Simulations were performed for trees aged 6, 9, 12 and 16 months which had been grown in plantations under a factorial com- bination of two levels of fertilization and irrigation. Ob- served leaf N distribution patterns yielded daily assimila- tion rates which were only marginally greater (< 5%) than for hypothetical trees with uniform distributions. Patterns of assimilation distribution in individual tree crowns closely resembled those for absorbed radiation, rather than for N. These conclusions were unaffected by three choices of alternative leaf area density distribu- tions. The simulation model was also used to calculate hourly and daily rates of canopy assimilation to inves- tigate the relative importance of radiation absorption and total canopy nitrogen on assimilation. Simulated hourly rates of carbon assimilation were often light- saturated, whereas daily carbon gain was directly proportional to radiation absorbed by the tree crown and to total mass of N in the leaves. Leaf nitrogen concentra- tions determined photosynthetic capacity, whereas total leaf area determined the amount of radiation absorbed and thus the degree to which capacity was realized. Ob- served total leaf area and total crown N were closely correlated. The model predicted that nitrogen use effi- ciences (NUE, tool CO2 mo1-1 N) were 60% higher for unfertilized than for fertilized trees at low levels of ab- sorbed photosynthetically active radiation (PAR). Nitro- gen use efficiency was dependent on fertilizer treatment and on the amount of absorbed PAR; NUE declined with increasing absorbed PAR, but decreased more rap- idly for unfertilized than for fertilized trees. Annual pri- mar)' productivity was linearly related to both radiation absorbed and to mass of N in the canopy. *Present address and address for offprint requests: CSIRO Centre for Environmental Mechanics, PO Box 821, Canberra City, A.C.T., 2601, Australia Key words: Eucalyptus 9randis - Photosynthesis model- ling - Canopy light climate - Nitrogen use efficiency - Light use efficiency Individual sun and shade leaves allocate N differently between compounds involved in light harvesting, elec- tron transport and the Calvin cycle (Boardman 1977). Whole plants also distribute their available N differenti- ally between leaves within their canopies, leading Field (1983) and Field and Mooney (1986) to suggest that plants allocate N to maximise the amount of daily carbon assimilation. To achieve this, leaves which receive the greatest irradiance should have the highest N concentra- tions, while shaded leaves should have relatively lower concentrations. Field (1983) showed that N distributions within the plant species examined were close to optimum for canopy assimilation, although there were only small differences in calculated daily assimilation between crowns with optimal or uniform N distributions. Cal- culations by Hirose and Werger (1987) suggested that, relative to uniform N distributions, significant enhance- ments in daily canopy assimilation may be obtained when N concentrations decrease exponentially with depth, because irradiance also decreases exponentially with depth in closed canopies. A simulation model described by Wang (1988) and Wang and Jarvis (1990) was used to test the hypothesis that observed N distributions in young Eucalyptus 9ram dis trees (Leuning et al. 1991) result in greater amounts of daily assimilation than in hypothetical trees with uni- form N distributions. The model (MAESTRO) cal- culates hourly averages of assimilation and radiation distributions in an array of trees with given spacing, tree size and crown dimensions. Transfer of beam and diffuse radiation through the canopy is calculated separately for three wavebands, visible (PAR), near-infrared (NIR) and thermal, according to methods presented by Norman and Welles (1983). Radiation fluxes at any location with- in the crown are predicted, and Wang and Jarvis (1990) obtained good agreement between model output and