Aotearoa New Zealand is experiencing an unseasonably warm winter, marked by record-high June temperatures in the capital and widespread mild conditions. According to Professor Cate Macinnis-Ng of the University of Auckland, while these warmer winters are generally less lethal than summer heatwaves, they are significantly disrupting natural ecosystems, agricultural yields, and the survival rates of native species.
How warmer winters impact carbon storage and native forests
Warmer temperatures are extending the growing season for trees, but the ecological result is a net loss of carbon. While kauri tree measurements in Auckland show that trees continue to grow throughout winter, the increased heat also triggers higher rates of respiration. According to Macinnis-Ng, this process releases more carbon back into the atmosphere, offsetting the gains from a longer growing season. Furthermore, traditional Māori knowledge (mātauranga Māori) indicates that since the 1950s, warming has shifted flowering times and reduced fruit biomass in the central North Island, negatively affecting the breeding and health of kererū, the native wood pigeon.
Why insect populations and pests are increasing
Warmer winters allow more pests to survive the colder months, increasing the risk of invasive species establishing themselves across the country. Macinnis-Ng notes that while many insects have historically relied on a dormant period to survive, milder temperatures are leading to more sightings of winter-active workers, particularly among wasp colonies. This trend is likely to result in higher wasp numbers and greater economic and ecological impacts. Other invertebrates, including cattle and sheep parasites, mosquitoes, ant and cattle ticks, and the Queensland fruit fly, face an increased risk of overwintering as the climate warms.
What is the risk to alpine environments?
The alpine zone is facing a “thermal squeeze” as invasive plants and predators move into higher altitudes. According to Macinnis-Ng, the upward expansion of rats and stoats reduces available refuge for native birds like the endangered rock wren and the kea, which increases their risk of extinction. Because alpine species are highly adapted to cold, they are often poorly prepared to compete with or defend themselves against these encroaching predators and subtropical plant species.
How the horticulture industry may change
Future agricultural productivity faces significant uncertainty as warming disrupts traditional seasonal triggers. Cold winter nights are essential for the flowering of apples, avocados, and kiwifruit, and a lack of these temperatures may lead to poor-quality fruit or reduced flowering. Additionally, heat stress in potatoes and onions is expected to produce smaller bulbs and lower yields. While plant breeding and gene technologies could offer potential solutions, Macinnis-Ng suggests that the industry may see a shift in crop viability, with some regions potentially becoming suitable for new crops like bananas, even as flea infestations are predicted to decline in drier, warmer areas.
