Growth and photosynthetic responses of Canarium pimela and Nephelium topengii seedlings to a light gradient

We analyzed the growth and photosynthetic responses of Canarium pimela K. D. Koenig (Chinese black olive) and Nephelium topengii (Merr.) H. S. Lo. (Hainan shaozi) to a light gradient to recommend better procedures for optimizing seedling establishment and growth of both species in restoration and agroforestry practices. One-month-old seedlings were exposed to four irradiance levels (46, 13, 2 and 0.2 % full sunlight) inside shade cloth covered shadehouses for 1 year. With decreased sunlight both species displayed trends of decreased relative growth rate (RGR) and leaf area (LA), and increased specific leaf area and leaf area ratio (LAR). The mean values of light-saturated net photosynthetic rate (Pmax) in 46 and 0.2 % full sunlight were 10.11 and 3.44 μmol CO₂ m^?2 s^?1 for C. pimela and 6.26 and 3.47 μmol CO₂ m^?2 s^?1 for N. topengii, respectively. C. pimela had higher RGR in 46 and 13 % full sunlight than in 2 and 0.2 % full sunlight. Differences in growth rates can be explained by the different values of LA, LAR and leaf mass ratio, as well as by the different values of photosynthetic saturation irradiance and net photosynthetic rate (Pmax) between the two species. Both morphological and physiological responses to shading indicate N. topengii could be rated as “very shade-tolerant,” while C. pimela could be rated as “intermediately shade-tolerant”.