LIGHT REQUIREMENTS OF SOME BROADLEAF TREE SEEDLINGS IN NATURAL CONDITIONS

Light conditions prevailing in natural regenerating stands of four broadleaf tree species in south-western Quebec were compared for two successive growing seasons. Mean irradiance (W m-2 s-1) within the 390-1100 nm waveband and mean red:far-red ratio were determined from measurements of solar radiation conducted under sunny skies at ground level within eight stands of each species studied. American basswood (Tilia americana L.) and black cherry (Prunus serotina Ehrh.) seedlings were generally found in more shaded understories than white ash (Fraxinus americana L.) and bur oak (Quercus macrocarpa Michx.) seedlings. Differences in light conditions observed among stands of the four species were more quantitative than qualitative and could result mainly from unequal density of the canopies. Leaf absorptance (%) within the 390-1100 nm waveband was also measured on 80 seedlings of each species to estimate the leaf absolute absorption (W m-2 s-1). In both years mean leaf absolute absorption in photosynthetically active radiations of basswood and black cherry seedlings was lower than that of bur oak and white ash seedlings. These results lead to the conclusion that, under natural conditions, regeneration of basswood and black cherry seedlings occurs in lower levels of light than for white ash and bur oak. Qualitative differences observed in the mean absolute absorption spectrum of the four species seem to indicate that seedlings of these species not only absorb a different quantity of light but also a different quality of light.