Examining the "Window of Vulnerability" following steepland plantation forest removal in New Zealand

Abstract

Harvesting of steepland forests results in a short period where the landscape is particularly susceptible to rainfall-triggered shallow landslides between crop rotations. This is known as the ‘window of vulnerability’ (WoV) and has been considered to occur up to 6-8 years following clearfelling. The WoV represents a period in the forest's rotation where the interplay between declining root strength from the previous crop coincides with changes in soil hydrology creating conditions where soil strength is at its lowest, and the slope is vulnerable to failure.This project aimed to focus on when maximum susceptibility to rainfall triggered landslides occurs within the WoV. We examined three areas in New Zealand where significant rain events (AEP's  < 1%) had resulted in many landslides on forest land harvested in the years immediately preceding those events.Using forest company imagery, LiDAR and satellite information we manually discriminated rainfall-triggered landslides for each study area. In all three areas, landslides were 'tagged' to vegetation cover, time since harvesting and whether associated with forest infrastructure such as roads and landings or not.Maximum landslide number and density occured on land clear-felled 2-4 years prior to the event and was slightly different for each study area. Landslides also occurred in older forest age classes and on areas with different vegetation covers, i.e., mature indigenous forests, pasture, scrub, etc. There were fewer landslides associated with forest infrastructure such as roads and landings than those deemed to be ‘natural’ slope failures.  Better information on the period of susceptibility to rainfall-triggered landslides following forest removal may help forest managers and regulators better understand the nature of this hazard and what can and can’t be done to mitigate the effects of rain events that result in landslides and in some cases often ‘disastrous’ off-forest impacts.