Field measurements of photosynthetic CO2 exchange were made on saplings of a C4 tree species, euphorbia forbesii, and a C3 tree species, Claoxylon sandwicense, in a shaded mesic forest on Oahu, Hawaii. Both species had light responses typical of those generally found in shade plants. Light saturated photosynthetic rates were 7.15 and 4.09 umol m-2 s-1 and light compensation points were 6.3 and 1.7 umol m-2 s-1 in E. forbesii maintained a higher mesophyll conductance and a higher water use efficiency than C.sandwicense as is typically found in comparisons of C4 and C3 plants. Under natural light regimes, both species maintained positive CO2 uptake rates over essentially the entire day because of low respiration rates and light compensation points. However, photosynthesis during sunflecks accounted for a large fraction of the daily carbon gain. The results show that the carbon-gaining capacity of E.forbesii is comparable to that of a C3 species in a moderately cool. shaded forest environment. There appears to be no particular advantage or disadvantage associated with the C4 photosynthetic pathway of E.forbesii in this environment.
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