[H-DS08] Landslides and related phenomena

[HDS08-P12] Comparing post-fire decay rates of root reinforcement of subtropical coniferous and broadleaf tree species

*Li Han TANG¹, Ching-An Chiu², Jian-Hong Yang³, Guo-Zhang Michael Song¹ (1.Department of Soil and Water Conservation, National Chung Hsing University, Taichung 402, Taiwan, 2.Department of Forestry, National Chung Hsing University, Taichung 402, Taiwan, 3.Lienhuachih Research Center, Taiwan Forestry Research Institute, Ministry of Agriculture, Taiwan Forestry Research Institute, Nantou 555, Taiwan)

Keywords:forest fire, root strength, root decay

In recent years, extremely high temperatures and prolonged drought associated with climate change have increased the intensity and frequency of forest fires. After trees are burned, the reinforcement force of their roots on slopes decreases due to wood decomposition, ultimately increasing the probability of landslide occurrence. In temperate regions, the root reinforcement force of trees decreases to 10% of its original strength 7-10 years after death. In subtropical areas, the decomposition of dead tree roots is expected to be faster due to the humid and warm climate. Compared to broadleaf tree species, the wood of coniferous tree species has lower wood density and contains resin. Such differences in wood properties may result in the different decomposition rates of dead tree roots. This study aims to answer the following questions: (1) Does the root reinforcement force of dead trees decrease faster in subtropical than temperate regions? (2) Is there a difference in the decay rate of root reinforcement force between coniferous and broadleaf tree species?

The study site is located in a forest in central Taiwan that experienced a fire in 2021. The target coniferous and broadleaf tree species are Pinus taiwanensis Hayata and Castanopsis kawakamii Hayata, respectively. Measurements of root area ratio and root force, key factors determining root reinforcement force, will be conducted on living trees of both species. For dead trees, measurements will be taken every six months. The diameter and quantity of roots from the target trees within soil profiles (0.6m wide, 0.4m deep) perpendicular to the root direction will be recorded to calculate root area ratio. Using an instrument equipped with a load cell, displacement meter, and winch, target trees’ roots stretching out of soil profiles will be individually pulled out towards the target tree, and the pull-out force of each single root will be measured. The Root Bundle Model-Weibull will convert the collected data into root reinforcement force of the target trees. Decay models of post-fire root reinforcement force will be established based on the root reinforcement force of living and dead trees at different time points.

Relevant managers can utilize these decay models to estimate when the landslide risk will reach its peak after a fire. If there are differences in the decay of root reinforcement force between coniferous and broadleaf tree species, forestry managers can use this information to plant tree species with slower root reinforcement decay in areas prone to fires, thereby reducing the risk of post-fire landslides.