Leaf nitrogen content per area (Narea) is a good indicator of assimilative capacity of leaves of deciduous broad-leaved trees. This study examined the degrees of increase in Narea in response to canopy openings as leaf mass per area (LMA) and leaf nitrogen content per mass (Nmass) in saplings of eight deciduous broad-leaved tree species in Hokkaido, northern Japan. Five of the species were well-branched species with a large number of small leaves (lateral-growth type), and the other three species were less-branched species with a small number of large leaves (vertical-growth type). The degrees of increase in Narea were compared between the two crown types. In closed-canopy conditions, leaves of the vertical-growth species tended to have a lower LMA and higher Nmass than those of the lateral-growth species, which resulted in similar Narea for both. LMA increased in canopy openings in the eight species, and the degrees of increase were not largely different between the lateral- and vertical-growth species. On the contrary, Nmass was unchanged in canopy openings in the eight species. As a result, Narea of each species increased in canopy openings in proportion to the increase in LMA, and the degrees of increase in Narea were similar in the lateral- and vertical-growth species. Therefore, this study showed that the degrees of increase in Narea were not correlated with the crown architecture (i.e., the lateral- and vertical-growth types).