Theoretical hypotheses on how the risk of spring frost damage to trees could change under climate warming. Hypothesis a, the risk of frost damage remains unchanged because both phenology and the frost-free period advance to the same extent. Hypothesis b, the risk of frost damage decreases because phenology advances less than the frost-free period. Hypothesis c, the risk of frost damage increases because phenology advances more than the frost-free period. Graphic: Vitasse, et al., 2017 / Agricultural and Forest Meteorology

28 September 2017 (Université de Neuchâtel) – [Translation by Google.] Trees growing above 800 m are increasingly exposed to spring frost, the Neuchâtel University, the Swiss Federal Institute for Forest, Snow and Landscape Research (WSL and SLF), and Agroscope Conthey reveal in a joint study a paradoxical consequence of global warming, which increases the vulnerability of trees. This work is published today in the journal Agricultural and Forest Meteorology.

In April 2017, a wave of sudden cold froze the vineyards and orchards, particularly in Valais, causing a significant loss of production. In the forest, several species have also suffered significant damage. These late frost events are likely to multiply in the future, due to an ever-increasing precociousness of the resumption of vegetation in the spring due to global warming. At higher altitudes, the vegetation shift associated with higher temperatures occurred at a higher rate than the lag of the last frost date, so that the risk of spring frost flowers is increasing and could still worsen in the future.

Led by climatologist Martine Rebetez and biologist Yann Vitasse, five researchers from the three Swiss institutions, supported by the Federal Office for the Environment, analyzed a series of uninterrupted daily data since at least 1975, from 50 stations in the network MeteoSwiss distributed throughout the country.

They also analyzed thousands of observations made by citizens regarding the dates of beech and spruce firing, as well as the flowering dates of cherry and apple. "These two dominant forest species in Switzerland, as well as these two highly cultivated fruit species, have been used to calibrate several models to predict blooming and foliage dates under different climatic conditions," explains Yann Vitasse.

The scientists then analyzed the time lag between the date of the last frost and the predicted dates of appearance of the leaves or flowers, i.e., the margin of safety of the species against frost. "Our work shows that with the intense warming observed over the last forty years, the margin of safety against frost has decreased above 800 m altitude while remaining stable at lower altitudes," continues the researcher.

It emerges that it is currently not appropriate to plant varieties of fruit species or to favor forest tree species better suited to an increasingly warm summer climate, as they often start their growing season more early in the spring and would therefore be particularly prone to frost damage.


Prof. Martine Rebetez

Dr Yann Vitasse

Réchauffement climatique : le risque de gel tardif s’aggrave en altitude

Mean date of full flowering for Malus and Prunus and of leaf-out for Fagus and Picea in Switzerland during the period 1975–2016, averaged from 8 to 11 sites, depending on species, with more than 35 years of observations. 11-year moving averages were applied and slopes of the linear regressions were reported. Graphic: Vitasse, et al., 2017 / Agricultural and Forest Meteorology

ABSTRACT: Winters and early springs are predicted to become warmer in temperate climates under continued global warming, which in turn is expected to promote earlier plant development. By contrast, there is no consensus about the changes in the occurrence and severity of late spring frosts. If the frequency and severity of late spring frosts remain unchanged in the future or change less than spring phenology of plants does, vulnerable plant organs (dehardened buds, young leaves, flowers or young fruits) may be more exposed to frost damage. Here we analyzed long-term temperature data from the period 1975–2016 in 50 locations in Switzerland and used different phenological models calibrated with long-term series of the flowering and leaf-out timing of two fruit trees (apple and cherry) and two forest trees (Norway spruce and European beech) to test whether the risk of frost damage has increased during this period. Overall, despite the substantial increase in temperature during the study period, the risk of frost damage was not reduced because spring phenology has advanced at a faster rate than the date of the last spring frost. In contrast, we found that the risk of frost exposure and subsequent potential damage has increased for all four species at the vast majority of stations located at elevations higher than 800 m while remaining unchanged at lower elevations. The different trends between lower and higher elevations are due to the date of the last spring frost moving less at higher altitudes than at lower altitudes, combined with stronger phenological shifts at higher elevations. This latter trend likely results from a stronger warming during late compared to earlier spring and from the increasing role of other limiting factors at lower elevations (chilling and photoperiod). Our results suggest that frost risk needs to be considered carefully when promoting the introduction of new varieties of fruit trees or exotic forest tree species adapted to warmer and drier climates or when considering new plantations at higher elevations.

Increase in the risk of exposure of forest and fruit trees to spring frosts at higher elevations in Switzerland over the last four decades



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