Late spring frost impacts on radial growth of European beech near its upper elevational limit

Global warming has considerably advanced the start of the growing season of temperate trees. However, the rate of this phenological change does not necessarily track the changes in the date of the last spring frost, also induced by climate change, which may result in a higher risk of false spring. When a late spring frost (LSF) occurs during tree leaf emergence, it can lead to complete tree defoliation. Although the impacts of LSFs are rarely fatal for a tree, they may play a decisive role in combination with extreme droughts in determining species distribution limits in the near future.

Here we aimed at assessing the impact of LSFs on tree growth of a frost-sensitive species, European beech (Fagus sylvatica L.), and retrospectively quantify the LSF regime in two sites of the Swiss Jura mountains. We collected increment cores of beech and a more freezing tolerant species, Norway spruce (Picea abies (L.) Karst) from a site where LSF damage was observed in May 2020 located at 1,365 m asl and in a second site where no frost damage was observed in 2020 at 1,065 m asl. Climate-growth relationships were established at both sites and for two different periods (1953–1986 and 1987–2020) to identify species-specific climatic drivers and potential temporal shifts. To further distinguish years with LSF impacts on beech radial growth, climatic signals not related to LSF recorded in the spruce series were removed from the beech chronologies.

Our preliminary analyses indicated that tree growth was dominated by different climatic factors in the two study sites: tree growth was limited by cold temperatures during both study periods in the higher elevation site whereas drought signals were apparent in tree growth during the second study period in the lower elevation site. Interestingly, beech growth was initially negatively and then positively related to spring minimum temperature at the higher elevation site. At the lower elevation site, warm temperatures in spring promoted tree growth of both species only during the second period (1987–2020). By subtracting the climatic signals of spruce on beech chronologies, we identified five and two years potentially affected by LSF during the last 30 years at the upper and lower sites, respectively. We are currently calibrating phenological models to climatically identify the years with potential frost damage and verify if these years are consistent with the ones identified with the previous dendrochronological analyses.

We further hypothesized that a damaging spring frost followed by a severe drought during summer may have a much larger impact than drought alone. Further investigations should be conducted on this aspect as the frequency and severity of extreme droughts are expected to increase while spring onset will continue to advance under a warmer climate, potentially increasing the risk of frost damage.