El Niño puts young pines and fresh harvest edges under pressure
Forestry crews could work two very different seasons this summer, depending on where their blocks are. Earth Sciences New Zealand’s (ESNZ) seasonal outlook for October to December 2026 says very strong El Niño conditions are present and expected to strengthen further, peaking in late spring or early summer.
ESNZ is the science agency formed when NIWA and GNS Science merged in July 2025. It expects more frequent westerlies, with stronger and gustier winds. That brings more fronts to western and southern areas and a greater chance of dry, warm föhn conditions in the east.
The regional odds show the split. Gisborne, Hawke’s Bay and Wairarapa have a 55% chance of below-normal rain, and Northland, Auckland, Waikato and Bay of Plenty a 45% chance. On the West Coast, the Southern Alps and foothills, inland Otago and Southland, above-normal rain is most likely, at 55%, with more potential for heavy rain and unusually strong winds.
Soil moisture and river flows are already below normal in parts of the North Island, ESNZ says, while the South Island is generally in good shape. Its El Niño explainer doesn’t expect a return to neutral conditions before April or May 2027.
A dry season for young trees
Matthew Waghorn of the University of Canterbury’s School of Forestry and colleagues from Scion and Landcare Research tested how water shortages at different times of year affect young radiata. They grew two-year-old clonal trees in pots in a polyhouse and held water back on set schedules. Their study ran in the New Zealand Journal of Forestry Science in 2015.
Trees given almost no water from November to April had their height cut by 24.7% and their diameter by 33.1%, compared with well-watered trees. Their total biomass was 64.7% lower.
Timing changed the result. A dry spell from February to May cut diameter growth more than one from October to January, and the authors wrote that their results suggest “late-season drought is more detrimental to absolute growth and biomass accumulation than early-season drought.”
The paper also points to earlier research showing forest managers can influence soil water through site choice, stand density and weed control. Two of those, weed control and stand density, come down to release spraying and thinning crews.
Wind, wet soils and new edges
On the wet, windy side of the country, standing trees face a different risk. Scion, the forestry research organisation now part of the Bioeconomy Science Institute, says radiata is more susceptible to wind damage than many other species, particularly in saturated soils.
Its 2012 information sheet on wind damage says damage often happens around gullies and stand openings, particularly openings created by harvesting. Any action that opens up a stand can raise the risk, and late thinning above 18m mean top height significantly increases the risk of value loss. Damage in past wind events has been concentrated in older stands, especially radiata over 30 years old.
Scion’s advice for harvest planning is to place harvest boundaries carefully and think about the order stands are cut in. Done well, that can significantly reduce the risk of damage to the stands next door.
Salvage runs on a shorter clock
If a storm does flatten trees, Scion’s research shows the salvage window depends on the season. Sapstain sets in as wood dries, and there’s less time to recover logs in spring and summer than in autumn and winter. The window is shorter again in warmer regions such as the northern North Island.
Windthrown stems deteriorate more slowly than processed ones because most of their bark stays on. Uprooted trees can also draw moisture through roots still in the ground, so they stay green longer than snapped stems. Trees that stay rooted after a winter windthrow can stay in good condition for up to a year.
Sprinklers around log piles buy more time, and Scion notes that salvage work needs specialised skills and training.
In the dry east, ESNZ also expects higher wildfire risk, which brings the industry’s fire code stop times into play. ESNZ rates its confidence in the rainfall outlook as medium, because a positive Southern Annular Mode could partly counteract the usual El Niño pattern. ESNZ’s Drought Forecasting Dashboard gives drought forecasts up to five weeks ahead, which is worth checking before planning the next few weeks of work.
