Responses of tropical forest tree seedlings to irradiance and the derivation of a light response index

1. Seedlings of 16 West African timber tree species were grown in six neutral shadehouses with irradiances 2%, 6%, 10%, 28%, 44% and 66% of unshaded values. 2. Tested species included well-known pioneers, which showed negative growth in 2% irradiance but which responded strongly to increased irradiance, and very shade-tolerant non-pioneers, which had positive growth in 2% irradiance but responded only slightly to increased irradiance. 3. For all species, maximum growth occurred at an irradiance varying between 10% and 44%. The inhibition of growth at higher irradiance was greater in the more shade-tolerant species. 4. Data from growth analyses were used in a principal components analysis to quantify the species' positions on a light response gradient. The growth variables most strongly correlated with this gradient were relative growth rate (RGR) and relative diameter growth in 2% irradiance, and also apparent quantum efficiency and leaf mortality rate in both 2% and 66% irradiance. These results draw attention to the importance of performance in deep forest shade in defining differences among tropical forest tree species. 5. The second (independent) axis of the principal components analysis separated three species that are known from other experiments to be drought tolerant. Other species' traits of functional significance (adult deciduousness, seed size, seed dispersal) were only weakly, if at all, associated with the gradient of light response. 6. Possible methods for efficient placement of other species on the gradient of light response are discussed. Formulation of standardized conditions simulating deep shade would allow diameter growth to be used as a simple non-destructive measure of a species' position on the gradient of light response, which could then be applied to any tropical forest flora.