Slow recovery from soil disturbance increases susceptibility of high elevation forests to landslides

Natural hazards such as shallow landslides are common phenomena that disturb soil and damage forests. Quantifying the recovery of forest vegetation after a hazard is important for determining the window of susceptibility to new disturbance events, especially at high elevations, where extreme weather events are frequent and the growing season is short. Plant roots can reduce the size of this window on unstable hillslopes, by adding mechanical reinforcement (c(r)) to soil and increasing its matric suction, termed here as hydrological reinforcement (c(h)). These data are used in landslide models to calculate the Factor of Safety (FoS) of a hillslope. We calculated temporal variations in c(r) and c(h) in naturally regenerated mixed, montane forests in the French Alps. In these closed-canopy forests, open-canopy gaps were present, with understory vegetation comprising herbs, forbs and shrubs. At three altitudes (1400, 1700 and 2000 m), we dug small trenches as proxies for shallow landslide events and calculated cr before soil disturbance in both open gaps and closed forests. Then, using monthly tree root initiation and mortality data measured in rhizotrons, we calculated monthly cr for four years after the disturbance. To compare results with c(r), c(h) was estimated using matric suction data that were measured in trenches at 1400 m for >1 year. Temporal FoS was then calculated using an infinite slope stability model. Results showed that finer, short-lived roots contributed little to soil reinforcement compared to thicker, longlived roots. After disturbance, mean cr (over the entire soil profile) never fully recovered to the initial value at any site, although >90% recovery was observed in open gaps at 1400 m. Mean cr was slow to recover in closed forests, especially at 2000 m, where only 19% recovery occurred after 41 months. The ch in closed forests was considerable during the summer months, but marked increases in soil water moisture resulted in lower FoS, especially during December to April, when soil was near saturation. As cr changed little throughout the year, it was a more reliable contributor to slope stability. Our results show therefore, that particular attention should be paid to high elevation forests after a disturbance. Also, during the process of recovery, the highly variable soil water dynamics in closed forest can result in seasonal hotspots of vulnerability. Therefore, when tree transpiration is low, our results highlight a need for careful monitoring on steep or unstable slopes, especially in disturbed closed-canopy forests.

Automated summary

Natural hazards like shallow landslides are common in disturbing soil and damaging forests. Plant roots play a crucial role in reducing the window of susceptibility for new disturbance events on unstable hillslopes. Research shows that thicker, long-lived roots contribute more to soil reinforcement than finer, short-lived roots, especially during the recovery process. In high elevation closed forests, seasonal soil water dynamics may result in vulnerable hotspots, highlighting the need for careful monitoring on steep or unstable slopes, especially after disturbances.