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Global late-spring frost risks


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Global warming is associated with excessively hot weather and summer heatwaves. However, counterintuitively, climate change might also exacerbate cold snaps that have a negative impact on plant growth.

In April 2017, cold air from the Arctic penetrated western Europe following an unprecedented warm period that had induced vegetation to spring out. This frost event caused severe damage to natural and cultivated plants, which altogether led to an estimated 3.3 billion euros of economic loss. Furthermore, the reduced photosynthesis resulting from leaf damage contributed to an increase of atmospheric CO2.

Altogether, the burden these late-spring cold snaps will represent for vegetation and carbon cycling still needs to be explored.

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Late-spring frost is defined as a cold snap happening after the spring season and warmth has started. These events do not necessarily take place in cold regions, in fact, temperate regions are more likely to experience these swings of temperature. Depending on the species, trees and shrubs are more or less sensitive to late-spring frost. This sensitivity depends mainly on one characteristic: how soon after the start of spring warming leaves emerge. “Cautious” trees do not leaf-out unless they have experienced enough cumulated warmth. “Opportunistic” trees on the other hand leaf-out soon after short periods of warming.

Our study analyzes the change in late-spring frost events over nearly 60 years, their geographic distribution, and the resistance strategies plants have developed to avoid damage. Pairing annual frost occurrences and plant characteristics at the global scale allows us to forecast the vulnerability of forests to late frost events under climate change.

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Our study analysed the distribution of late-spring frost in space and time using hourly climate data gathered between 1959 and 2017 by the Climatic Research Unit (CRU) and the National Center for Atmospheric Research (NCAR).

Then, we analyzed plant-traits of 1,500 temperate and boreal woody species cultivated in botanical gardens, and identified where they naturally occur within over 530,000 forest and shrubland locations around the world. Using these data, we mapped the distribution of vegetation according to their leaf-out strategies (“cautious” vs. “opportunistic”) and leaf frost susceptibility. We were then able to test the correlation between the distribution of late-spring frost and plant strategies.

Finally, we projected the future distribution of late-spring frost using the trend of extreme events over the past 60 years. Crossing these data with plant-traits allows us to forecast forest vulnerability under climate change.

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When mapping all late-spring frost events in temperate and boreal regions between 1959 and 2017, the results show that these cold snaps are generally more frequent in North America than in Eurasia. This is mainly due to the absence of east-west mountain ranges in North America allowing warm spells from the Gulf of Mexico and cold spells from arctic regions to move across the continent unimpeded, resulting in high short-term temperature variability.

Our results also show that, at the continental scale, the incidence of late-spring frost events decreases with higher latitude, where temperatures are more consistently cold. However, their frequency increases with distance to the sea or higher elevation, most likely due to more variable daily temperatures.

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The model we developed tests, amongst other variables, the correlation between late-spring frost events and vegetation strategies. The results highlight that, in regions where late-spring frosts were unlikely to happen, trees are mainly “opportunistic.” However, in regions where these events are more frequent, trees have evolved to be “cautious.” These results suggest that these characteristics evolved in trees as a safety strategy that minimizes their risk of frost damage.

We further show that in cold, high latitude regions, trees have evolved freeze-resistant leaves. In contrast, in lower latitudes, where late-spring frost risk is high, leaves are more susceptible to frost, implying negative consequences if climate change will lead to more frequent frost occurrences.

Altogether, the geographic distribution of “cautious” vs “opportunistic” species reflects the historical occurrence of late-spring frost events: more “cautious” species in North America and more “opportunistic” species in Europe and Asia.

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The most drastic increase of late-spring frost incidence under ongoing climate change is occurring where the risk used to be low: in the coastal and eastern parts of Europe, and East Asia. In those areas, many local trees are “opportunistic”, which suggests that climate change will lead to a mismatch between late-spring frost incidences and trees’ strategies to cope with frosts. Therefore, we can expect that, respectively, 35% and 26% of the European and Asian temperate forests will experience increasing spring frost damage in the future.

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Our study shows that the geographic distribution of late-spring frosts has strongly impacted the growth strategies of temperate trees. However, climate change is quickly altering geographic late-frost patterns, implying a future mismatch between late-frost occurrences and local tree growth characteristics. Trees impacted by more frequent late-frosts will show reduced growth and competitive ability. The cascading effects this might have on the carbon cycle and biodiversity remain to be elucidated. By detecting vulnerable forests, our study allows us to be better prepared for future extreme climate conditions and ultimately guide decision-making in land-management, forestry and agriculture.
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