Extreme summer heat and drought lead to early fruit abortion in European beech

Abstract

Years with high fruit production, known as mast years, are the usual reproduction strategy of European beech. Harsh weather conditions such as frost during flowering can lead to pollination failure in spring. It has been assumed that mast is controlled by flowering, and that after successful pollination, high amounts of fruits and seeds would be produced. However, the extremely hot and dry European summer of 2018 showed that despite successful pollination, beechnuts did not develop or were only abundant in a few forest stands. An in-depth analysis of three forest sites of european beech from the Swiss Long-Term Forest Ecosystem Research Programme over the last 15-19 years revealed for the first time that extreme summer heat and drought can act as an "environmental veto", leading to early fruit abortion. Within the forest stands in years with fruit abortion, summer mean temperatures were 1.5 °C higher and precipitation sums were 45% lower than the long-term average. Extreme summer heat and drought, together with frost during flowering, are therefore disrupting events of the assumed biennial fruiting cycle in european beech.

Contrasting Resource Dynamics in Mast Years for European Beech and Oak—A Continental Scale Analysis

“…This is in accordance with earlier observations in European beech (Müller-Haubold et al, 2015;Braun et al, 2020b). A negative effect of fruit production on leaf production was also recently found in a subset of the ICP Forests Level II plots (Swiss beech stands; Nussbaumer et al, 2020). However, in the warmer climate regions, beech leaf production was not significantly reduced during mast years.…”

“…Furthermore, summer droughts and prolonged heatwaves can lead to years with mast failure in beech, i.e., years with successful pollination in spring but failing fruit development during summer. This has already been observed at beech stands in Switzerland during the very hot and dry summer 2018 (Nussbaumer et al, 2020). For oak, on the other hand, high summer temperatures have not yet had a negative impact on fruit production (Shibata et al, 2020).…”

“…Standard errors and confidence intervals of the SEA for all analyses at a 60% threshold can be found in Supplementary Tables 6, 10. Beech is considered a species with a basic 2 years mast cycle (Matthews, 1955;Braun et al, 2020a;Nussbaumer et al, 2020). In our beech data, such a pattern was partly present but not dominant (Supplementary Figure 3).…”

“…In a new study on the impact of the 2018 summer drought on F. sylvatica in Switzerland, Nussbaumer et al (2020) found that prolonged summer heat waves and droughts during fruit development lead to fruit abortion. There is thus evidence that some tree species may struggle to reproduce under the projected climate scenarios for the next 50-100 years.…”

Unravelling resilience mechanisms in forests: role of non-structural carbohydrates in responding to extreme weather events

“…European beech (Fagus sylvatica L.), one of the most diffused native tree species in Europe, is known to be drought sensitive (Bolte et al, 2016). Hence, drought events can negatively affect physiological performance (Rezaie et al, 2018), carbon allocation (D'Andrea et al, 2020a), reproductive capacity (Nussbaumer et al, 2020), as well as the growth and competitive strength of the species (Peuke et al, 2002) which may all impact its future distribution (Noce et al, 2017).…”