Roots of large trees can increase slope stability through increasing soil strength. On the other hand, the downslope force contributed by their weight may reduce slope stability. This study aims to identify the net effects of large trees on slope stability with a big dataset. The study site is in the 25-ha Lienhuachi Forest Dynamic Plot (23°54′ 49″ N, 120° 52′ 43″E), central Taiwan. Totally, 203,640 trees (with diameter at breast height (DBH) ≥ 1 cm) in this lowland evergreen broad-leaved forest were recorded in the 2007 tree-by-tree census. Torrential rains in the 2008 typhoon season resulted in 11 landslides with total area of 1.2 ha in this plot. A census conducted right after the typhoon season showed that 4,236 trees were missing due to landslides, 773 trees survived from landslides and 198,631 trees remained undisturbed.
Results/Conclusions
Analyses showed that the mean DBHs of these three groups of trees differed significantly (P<0.05). The landslide survivors had largest DBHs (5.26±0.27 cm), followed by undisturbed trees (4.15±0.01 cm) and missing trees (3.82±0.08 cm). Trees in the landslide sites were further divided into large (DBH≥ 20 cm) and small (DBH< 20 cm) trees for distribution analyses. Survived small trees distributed within 10 m away from survived large trees, and at least 4 m away from missing large trees. Our results indicated that large trees can reduce the occurrence of landslides and protects small trees from landslides.