Taiwan has a long history of reforestation due to land degradation. However, there is a lack of understanding of how tree species grow on reclaimed lands. This study looked at tree sizes, vulnerability, and health of economically important Zelkova serrata and Quercus glauca trees planted on reclaimed agricultural lands. Thirteen former agricultural sites with trees of six to seven years old were sampled along the elevation from 107 to 2514 m above sea level. Results showed that increasing inter-tree competition reduced tree sizes and health and increased vulnerability to damage, primarily wind, for both tree species. For example, a 1 m² ha⁻¹ increase in inter-tree competition was associated with a 5.68 cm decrease in tree diameter, a 3.21 m decrease in tree height, a 59.31% decrease in tree health for Z. serrata. Responses of Z. serrata to inter-tree competition were generally stronger than those of Q. glauca. Elevation generally reduced tree sizes of both species and reduced health of only Z. serrata trees. Stand density has minimal effects on the tree attributes of both species. Our study suggests that Z. serrata responds strongly to inter-tree competition leading to stratification of stand structures, which agrees with past studies showing Z. serrata developing different growth strategies. Q. glauca could resist inter-tree competition so that suppressed trees could compete with its neighbors. This supports past observations that Q. glauca could persist under suppression. Our findings of the elevation caution planting both tree species outside their native habitat ranges, which was not shown before.