As planted forests expand in area, they are beginning to dominate landscapes as a matrix and cause the fragmentation of remaining natural forests. To understand and predict the responses of biological assemblages to maturing planted landscapes, examining the effects of forest type (natural vs planted) and forest age on such assemblages is particularly important. Therefore, to document the effects of forest type and age on longhorned beetle assemblages, in 2008 we collected beetles in broad-leaved natural and cedar planted forests where beetles had also been collected in 1989. Beetle species composition differed greatly between the two forest types in 1989, whereas this difference was less pronounced in 2008. Species richness and total abundance were higher in natural forests than in planted forests in 1989. In 2008, species richness had decreased in both forest types, but the difference between the two forest types had been maintained. Total abundance was also markedly lower in 2008, and the difference between forest types was much smaller. Although larval host plants were not associated with the responses of species to year (forest age or maturation), beetle species whose larvae fed on either broad-leaved or coniferous trees (or both) exhibited slight preferences for natural forests. These results suggest that longhorned beetle assemblages become impoverished in planted landscapes as the planted matrix matures. Changes in species composition with forest maturation may be difficult to predict based on larval host plants. However, consideration of larval host plants may enable the prediction of changes in species composition caused by the replacement of natural forests by planted forests.