Field Observations, Vulnerability Factors, and Management Implications
John Ballinger1, Dean Satchell2 , Simon Webb3
- Senior Land Management Advisor, Northland Regional Council
- Te Uru Rākau -New Zealand Forest Service
- Northland Regional Council
Cyclone Gabrielle (12–16 February 2023) was one of the most damaging wind events to impact Northland in recent decades, combining extreme gusts exceeding 130 km/h with prolonged soil saturation following the wettest start to a year on record for Whangārei. While damage was widespread across the region, anecdotal reports suggested significant windthrow in poplar (Populus spp.) plantings. Given the species’ importance for erosion control, shelter, and timber, the Northland Regional Council, in collaboration with Te Uru Rākau, undertook a rapid post-event assessment to investigate the extent, patterns, and drivers of poplar windthrow.
Seven sites across Northland were surveyed, representing a range of soil types, topographies, planting regimes, and poplar clones—predominantly Kawa (P. deltoides × yunnanensis), the region’s most widely planted clone since the 1980s. Observations revealed highly variable outcomes: some stands experienced near-total loss (up to 82% toppled), while others remained largely unaffected.
Key vulnerability factors included:
- Soil conditions: Poorly drained or saturated soils, shallow rooting due to high water tables, boulders, or pan layers reduced root anchorage strength.
- Exposure and wind direction: Sites exposed to the most damaging south-westerly gusts suffered greater losses, with turbulence effects sometimes causing mid-stand collapse rather than edge failure.
- Clone differences: Kawa showed higher susceptibility than other clones such as Veronese, potentially due to smaller root plates relative to above-ground biomass.
- Age and form: Older, tall, unthinned stands with high height-to-diameter ratios were more prone to windthrow.
- Proximity to waterways: Trees planted too close to drains or eroding streambanks were undermined, with root systems compromised on the water-facing side.
Notably, some mature Kawa stands on well-drained soils and in sheltered positions showed no windthrow, underscoring the role of site-specific conditions. In contrast, other species—such as adjacent radiata pine shelterbelts—were sometimes unaffected, highlighting interspecies differences in wind resilience.
The investigation identified several management implications to improve cyclone resilience without compromising the primary functions of poplar plantings:
- Site selection: Avoid planting Kawa in poorly drained soils or immediately adjacent to waterbodies; set back 3–5 m from bank edges.
- Clone diversification: Incorporate clones with demonstrated windthrow resistance into planting programmes, alongside other desirable traits such as rust resistance, possum resistance, and timber quality.
- Stand design: Prioritise the primary purpose (e.g., erosion control, shelter) over timber production in high-risk sites; where timber is a goal, manage stocking and thinning to reduce height-to-diameter ratios.
- Root system considerations: Recognise that soil constraints (e.g., boulders, pans) can limit root depth and spread, increasing vulnerability.
Cyclone Gabrielle’s impacts on poplar in Northland illustrate the complex interplay between extreme weather, site conditions, and genetic factors. While Kawa remains a proven performer for erosion control in the region, its relative vulnerability to cyclonic winds warrants a more diversified and site-specific approach to clone selection and planting design. Ongoing field trials by the Northland Regional Council will inform future recommendations, ensuring that resilience to windthrow is integrated into breeding, selection, and land management strategies.
These findings are summarised in the following report:
www.nrc.govt.nz/resource-library-summary/publications/land/poplar-windthrow-following-cyclone-gabrielle/