Widespread browning of foliage and early defoliation of beech were observed in some parts of Switzerland during the severe drought in 2018. Yet, the actual frequency and extent of leaf browning and defoliation, and their actual impact on tree performance (growth) remained unclear. On the basis of the data collected in an early phase of this initiative (see below), we investigated the following research questions:
(i) What was the actual severity, extent and pattern of the short-term impact of 2018 drought on the condition of beech (considered as indicator species) in Switzerland?
(ii) Were the affected beech trees constrained in their growth during 2018?
(iii) Were tree species diversity and other ecological features of the site related to the occurrence and severity of such an effect?
Follow-up data collections are panned to additionally address the following question:
(iv) Will there be lagged (medium-term) effects in the subsequent years?
Between 27.8 and 6.9.2018, FOREMA NFI field teams re-revisited 75 plots (821 trees) out of the plots already measured in April-May as part of the regular NFI survey. In particular, we collected data on crown condition (instances of severe defoliation and browning) and diameter increment on 271 beech trees, as well as diameter increment on 212 Norway spruce and 128 Silver fir trees, plus trees from additional 24 different species.
Currently, we are designing a process to follow-up and detect lagged effects – see above question (iv).
At the time of the survey (end of August - beginning of September) 17% of the investigated beech trees showed severe defoliation and/or foliage browning. Although low in absolute terms, this is a much higher frequency as compared to long-term and 2018 data collected earlier in the season by other Swiss and European networks (e.g. Sanasilva; ICP Forests). This may reflect the worsening in beech condition occurring along the summer. Defoliated and/or brown trees were distributed over 25% of the 75 NFI sample plots included in this study.
Statistical models indicated that severe defoliation and severe leaf browning were related to drought severity, previous growth and stand density at the investigated sites, as well as to the individual tree size.
On sites containing both non-affected and severely affected beech trees, the affected trees grew significantly less than non-affected trees over the period April-August 2018. On average, severely affected beech trees grew significantly less than expected from empirical NFI growth models. The same models revealed also a more generalized growth reduction across all the species investigated.