Land-use decisions and extreme weather
Our land-use and management decisions have consequences in extreme weather events. Natural ecosystems provide vital infrastructure that helps to reduce the impacts of natural hazards. For example, wetlands and forested areas in river catchments act as buffers during extreme weather events. When we make changes to natural infrastructure, we risk losing the ecosystem services they provide.
Our land 2024 is an environmental report produced by the Ministry for the Environment and Stats NZ. The report includes a case study regarding damage caused by Cyclone Gabrielle – in part due to the consequences of our land-use and management decisions.
Impact on our land – infographic
Since the 1880s, we’ve used an extractive approach that emphasises the improvement potential of land for both productivity and economic purposes. Land-use changes can make us more vulnerable to extreme weather events.
Download a PDF of this infographic.
Source: Ministry for the Environment, Stats NZ and data providers and licensed by the Ministry for the Environment and Stats NZ for reuse under the Creative Commons Attribution 4.0 International licence.
Cyclone Gabrielle
Severe tropical cyclone Gabrielle hit the northern and eastern part of the North Island, with a national state of emergency declared on 14 February 2023. It devastated the area with slips and flooding and took the lives of 11 people.
Over 300,000 landslides carried large volumes of soil from pasture and forest down the hills behind Te Tairāwhiti, Hawke’s Bay, Wairoa and the Wairarapa. Each landslide moved about 1,000 tonnes of soil on average – the equivalent weigh t of 548 single-cab utes – and deposited it on floodplains and waterways below.
Forests played a role as natural infrastructure to reduce the risk of erosion. Most of these landslides occurred where intense rainfall fell on steep land without protective forest cover. Trees can protect erosion-prone landscapes during intensive storm events by providing a canopy that intercepts rainfall, reducing water in the soil and binding the soil together with their roots.
Extreme erosion at Te Tairāwhiti
This is an example of erosion caused by Cyclone Gabrielle. Exotic forests and pasture on steep terrain were impacted by the heavy rains.
Prior damage from cyclones
Cyclone Gabrielle is not the first storm to cause devastating landslides in Te Tairāwhiti. The region is known for its extremely erodible land. It has a long history of extreme weather carrying sediment and woody debris down its slopes. The impacts of Cyclone Bola (1988) showed a clear difference in landslide vulnerability between pastoral land and forested land. This began a decades-long movement to plant forests on the steep hills in the region, mainly converting pasture to production forest.
While the main driver of the planting post Cyclone Bola was to stabilise steep, highly erodible hillsides, recently harvested forests in Te Tairāwhiti left landscapes vulnerable once again to extreme weather events like Cyclone Gabrielle.
Forestry blocks in Hawke’s Bay
Commercial forests are routinely harvested when the trees mature. Harvesting (as seen on the left of the forest block) exposes the soil to rainfall and erosion. Replanting (as seen on the right) provides partial protection.
Native vegetation can lessen impacts
The type of forest has an impact on landslide probability. After Cyclone Gabrielle, in Hawke’s Bay and the Wairarapa hill country, it was estimated that land covered by indigenous forest was 90% less likely to slide than hill country pastoral land, while land under exotic forest was 60–80% less likely to slide than pastoral land.
Coastal hill country in Te Tairāwhiti under indigenous forest was estimated to be 50% less likely to slide than hill country pastoral land. However, exotic forestry, harvested land and pasture had similar estimated extents of land sliding in this region.
Generally, indigenous forest had less probability of landslides compared to other landcover types relative to pasture.
Native vegetation in Hawke’s Bay
Steep, hilly land covered by native forest is less prone to slips during extreme weather events.
There are several possible reasons why exotic forestry and some native vegetation were less effective than indigenous forest at reducing landslide probability in Te Tairāwhiti and Hawke’s Bay.
These reasons could include soil and rock type and rainfall intensity alongside the age, densit y and maturity of vegetation cover at the time of the storm. Other factors that can increase vulnerability to landslides include thin soils caused by a long erosion history and forestry management practices such as non-thinning or multiple rotations of forestry.
While the root systems of exotic tree species can often outperform indigenous ones in reinforcing soil, these erosion control benefits are lost when forests are harvested. Benefits only return when trees in the next rotation have grown enough to close the forest canopy.
The type of tree, the place it is planted in a catchment and the way it is managed all have consequences downstream. For example, whether a forest is standing or harvested can determine the extent of erosion, sedimentation and flood impact in an extreme event. These considerations are crucial in planning decisions for forests as effective natural infrastructure .
Related content
Environment Aotearoa 2022 – introduction uses the whetū in Te Kāhui o Matariki to explore the pressures we put on te taiao.
Deforestation explores the impacts of felling our native forests.
Read about extreme weather and cyclones, typhoons and hurricanes.
Useful link
Stats NZ and the Ministry for the Environment report on different aspects of Aotearoa New Zealand’s environment every 6 months. Access their reports here.
Visit Te Uru Rākau – New Zealand Forest Service’s website Tirohanga Ngahere Canopy. Use the data and research section to find fact sheets on topics including plantation forestry on erodible land.
Use NIWA's serious game Township Flood Challenge. The game engages people in strategic thinking about actions as a result of climate change and extreme weather. The game is supported by educational resources that support learning aboutchange, choices, wellbeing, values and decision making.
Acknowledgement
This resource has been produced with the support of the Ministry for the Environment and Stats NZ. © Crown copyright.
Our land 2024
The Ministry for the Environment and Stats NZ produce New Zealand’s Environmental Reporting Series. Our land 2024 describes how the ways we use the land have wide-ranging effects on our diverse ecosystems and the biodiversity they support, with cascading impacts on our economy, our resilience to disasters and our cultural, mental and physical health.
