Summer heatwaves are currently receiving a lot of attention in Europe because they now cause more deaths than floods or storms .
But winters are also warming. While they are generally less deadly, they influence and disrupt human and natural systems in many subtle ways.
Aotearoa New Zealand has experienced a particularly warm start to this winter , with record high June temperatures in the capital and warm conditions across the country.
Many will welcome the unseasonably warm weather, but milder winters have a range of impacts, especially for plants and insects.
Extra winter growth, but loss of carbon
In forests, warmer temperatures can extend the growing season of trees.
Usually, many trees are dormant during winter as conditions are too cold for growth. But our ongoing measurements of kauri tree growth in Auckland indicate trees have continued to grow throughout recent winters.
One might assume a longer growing season would increase carbon uptake and storage in trees. However, overall carbon changes are actually negative because warmer temperatures also increase respiration , which returns more carbon to the atmosphere.
In Aotearoa, few plant species lose their leaves in winter. But according to traditional Māori knowledge (mātauranga Māori), flowering time has changed and fruit biomass has declined with warming in forests of the central North Island since the 1950s .
This has had a negative impact on the numbers, breeding rates and health of kererū (native wood pigeons) and has reduced nutrient cycling in the soil.
Risk of new invasions
Insects are also very sensitive to winter temperatures.
Like trees, many insects have a dormant period during colder months. Some insects from warmer climates have established as pests in Aotearoa, but they usually struggle to survive cold conditions. As winters warm, the numbers of species able to get through the cold season is increasing.
For instance, in temperate climates such as in New Zealand, wasp colonies have a strong seasonal cycle. Wasp numbers increase during spring after the queen emerges from overwintering and lays eggs. The workers expand the colony during summer but when temperatures drop in autumn, most of them die off.
However, in warmer conditions, sightings of winter-active workers have increased in Aotearoa. This means a warming climate will likely lead to higher wasp numbers and increased ecological and economic impacts.
There are a range of other invertebrate pests that may become more problematic in natural systems, plantation forests and agricultural and horticultural settings as winters warm. This includes rising numbers of parasites of sheep and cattle , more insect pests in plantation forests , increased risk of overwintering of the Queensland fruit fly and bigger range sizes for mosquitoes and ant and cattle ticks.
Shrinking alpine refuge
New invasive plant species from subtropical regions may also be able to establish or expand their ranges and shift into the alpine zone .
Similarly, the upward expansion of invasive mammals will reduce the availability of refuge areas for native birds, including the endangered rock wren.
Known as ” thermal squeeze “, the movement of rats and stoats to higher elevations reduces the availability of safe spaces for large alpine birds such as the kea, exacerbating the risk of extinction.
The alpine zone is especially vulnerable to winter warming because plants and animals living there are highly adapted to the specific environmental conditions and are often poorly prepared for invasive predators or competitors.
Horticultural winners and losers
In the horticulture industry, cold winter nights are important as a trigger for spring flowering . Economically important fruits such as apples, avocados and kiwifruit may not flower well and have poor-quality fruit under future climates.
Potatoes and onions are also sensitive to warming conditions because heat stress reduces the quality of tubers and produces smaller bulbs, causing lower yields of both crops.
Plant breeding and gene technologies offer opportunities to develop fruits and vegetables that are better prepared for a warmer world.
And there is some good news in other areas, including that flea infestations are predicted to decline in regions where warming is associated with drying. There may also be opportunities for the establishment of new crops, such as bananas .
As the climate continues to warm, there is more to learn about the impacts and options for adaptation in Aotearoa. Research needs to focus on finding solutions for native species and primary industries because healthy ecosystems are essential for a healthy economy and thriving communities.
