The impact of significant earthquakes on Christchurch, New Zealand's urban forest

Abstract

The resilience of Christchurch, New Zealand's urban forest has been tested during a year of major earthquakes and aftershocks. Tree loss has resulted from mass soil movement, soil liquefaction, rockfalls, and land slips. At the time of writing, only 384 trees have been documented as removed, however, thousands more are scheduled for removal. Additionally, the changes to the soil environment resulting from liquefaction will require existing trees to adapt quickly to their new soil environment. Their fate will not be known for years. Though the total number of trees removed is unlikely to reduce city-wide canopy cover appreciably, it is important to recognize that spatial patterns of tree loss were highly localized and thus local canopy cover has been drastically reduced in some areas. Short-term management of the urban forest in the aftermath of the earthquake focused on removal of trees deemed unstable or unsafe. In the medium-term, attempts have been made to monitor the health and stability of remaining trees using an asset management system, though some difficulties related to consistency and ubiquity of its use have been identified. Though short and medium-term management have been largely effective, long-term management of Christchurch's urban forests will largely be dictated by government policy, which will be informed by public consultation and land ownership. Many of the benefits provided by urban forests are well understood. However, trees and greenspaces provided additional benefits during earthquakes and in the aftermath. Planted trees in the hills surrounding the city prevented rockfall from damaging infrastructure and human life downslope, while greenspaces were used as a base for search and rescue staff during rescue and recovery operations, and as temporary living spaces for residents who lost their homes.