The Danish Meteorological Institute (DMI) has forecast more overnight frost this week, well into May 2020, and a new study shows that we should expect more of this kind of weather in the future. In other words, late spring frosts. And it will be tough on trees.

As global warming increases, trees are starting to bud and grow foliage earlier in the year, but when the young leaves begin to show their greenery is exactly the same time as when they are most vulnerable to freezing temperatures.

"And the new study shows that late spring frosts are becoming increasingly frequent in line with general warming," explains Jens-Christian Svenning, a professor at the Department of Biology at Aarhus University and head of the Center for Biodiversity Dynamics in a Changing World (BIOCHANGE), who has contributed to the study.

The trend is most serious for trees in the temperate regions of Europe and Asia, where late spring frosts threaten ecosystems, agriculture and the economy, warns a large international team of researchers, including researchers from Aarhus University. The results have just been published in the scientific journal Proceedings of the National Academy of Sciences (PNAS).

New calculations show that the risk of damage from spring frosts in these areas increased significantly in the period 1959-2017.

One-third of European forests at risk

The risk has risen most dramatically in large parts of Europe – including Denmark and East Asia, where there have hitherto only rarely been late spring frosts. In these areas, many local trees are ‘opportunistic’, and they start to blossom and grow leaves even after only a brief period of warmer weather. This means that climate change is leading to an imbalance between warm early springs, night frosts in late spring, and tree foliation strategies.

The result is that 35 per cent of European temperate forests and 26 per cent of temperate forests in Asia have become increasingly vulnerable to spring frosts.

"We don't know what it will mean for forests as ecosystems. But in natural forests, this could affect the conditions for competition between different trees and bushes and, in the long term, change the composition of forests," explains Jens-Christian Svenning.

Frost affects the economy and leads to more warming

Global warming is typically associated with excessively hot weather and heatwaves in the summer. However, climate change will exacerbate all forms of extreme weather, including frost incidents after the spring warmth has started.

In March 2017, late spring frost killed almost half of the iconic cherry blossoms in Washington DC, which had blossomed early after an unusually warm February. In the same year, an unprecedented warm period caused plants to bud in Western Europe before a late spring frost in April. This led to serious damage to natural and cultivated plants, and to an estimated total financial loss of EUR 3.3 billion – of which only 18% were insured, according to the reinsurance company, MunichRe.

"Denmark was also struck in 2017, with considerable damage to the fruit harvest and lower seed production in trees for the forestry sector. Spring frost also caused huge losses for the fruit harvest again in 2019," says Jens-Christian Svenning.

And this is a vicious circle. When leaves are damaged by frost, photosynthesis is impaired and plants do not absorb as much CO₂. This in turn means higher carbon emissions into the atmosphere, and then further warming.

American trees are more cautious

"In this new big-data study, we analysed hourly temperatures all over the world over a 60-year period and mapped almost 1,500 species on the basis of their vulnerability to frost. By matching annual frosts with plant properties globally, it was possible for us to predict the vulnerability of forests to late frosts during climate change," explains Assistant Professor Alejandro Ordonez, from the Department of Biology and Biochange at AU, who has also contributed to the study.

The mapping shows that late spring frosts are generally more frequent in North America than in Europe and Asia. This is mainly because there are no mountain ranges running east-west in North America, so warm air from the Gulf of Mexico and cold air from the Arctic can move unhindered across the continent and create large fluctuations in temperature in a short period of time.

The study also finds that many trees in areas like North America, where frosts in the spring are common, have developed certain protective measures. These 'cautious' trees don't blossom and bud unless they have experienced long-term warmth. This caution is valuable insurance against frost damage, which can affect their growth, reproduction and survival.

Further information:

The study “Late-spring frost risk between 1959 and 2017 decreased in North America but increased in Europe and Asia” was authored by researchers at ETH Zurich, Munich University, Aarhus University, WSL Swiss Federal Institute of Forest, Snow and Landscape Research, and the Global Forest Biodiversity Initiative (GFBI).

The article is available here: www.pnas.org/cgi/doi/10.1073/pnas.1920816117

Contact:

Professor Jens-Christian Svenning
Department of Biology
Center for Biodiversity Dynamics in a Changing World
Aarhus University
Mail:svenning@bios.au.dk
Mobile: +45 2899 2304