Blitzschlagbrände und Sommertrockenheit: Gibt es einen Zusammenhang?

281–287

Artikel

200207866

Schweizerische Zeitschrift für Forstwesen 2020, 171(5): Trockenheit 2018: Was haben wir gelernt? ; Jrg. 171 ; 5 (2020) , 281–287
waldwissen.net 2024 : Information für die Forstpraxis (2024) 

Blitzschläge sind weltweit die wichtigste natürliche Ursache für Waldbrände. In der Schweiz machen die Blitzschlagbrände im Schnitt rund einen Drittel (30.5%) der im Hochsommer (Juni, Juli und August) vorkommenden Brandereignisse aus. In diesem Beitrag analysieren wir das Auftreten und die herrschenden meteorologischen Bedingungen, welche die 275 in der Waldbranddatenbank «Swissfire» für die Periode 2000–2018 registrierten Blitzschlagbrände charakterisieren. In der Schweiz sind Blitzschlagbrände fast ausschliesslich in gebirgigen Lagen zu verzeichnen, wobei ihre Häufigkeit von den Südalpen über die Zentralalpen bis zu den Nordalpen abnimmt. Am meisten betroffen von Blitzschlagbränden sind Nadelbäume, vor allem die Fichte und die Lärche. Anhaltende Sommertrockenheit ist bei Weitem der wichtigste Auslöser für Blitzschlagbrände. Unsere Resultate zeigen, dass sich die Gefahr von Blitzschlagbränden im Schweizer Alpenraum deutlich erhöht und auch der Jura und das Mittelland von Blitzschlagbränden betroffen sein können, wenn sich, wie im Zusammenhang mit dem Klimawandel prognostiziert, wärmere Sommer mit lange andauernden Trockenperioden häufen. Noch im Detail abzuklären sind die genauen meteorologischen Bedingungen, die Blitzschlagbrände begünstigen.
The Intercantonal Forest Monitoring Programme has been studying the health of European beech, Norway spruce and oaks in currently 185 designated observation plots in Switzerland since 1984. This programme is a useful method to document early signs of changes in these trees. The data that is currently being collected is helping to identify the consequences of the dry summer of 2018 and to differentiate the effects of that summer between tree species and regions. Drought indicators such as the minimum site water balance suggest that 2018 was not drier than 2003. However, 2018 was preceded by a series of dry years. The damage that became evident in 2019 exceeded previous findings from across the monitoring network. The proportion of beech trees with a crown transparency of >60% was six times higher than the average measurement from the whole period of the programme. In some plots up to 41% of the beech trees were transparent by more than 60%. The mortality of beech increased by a factor of four or five in 2018 and 2019, respectively. The increase was higher in plots with an insufficient foliar phosphorus concentration. Mortality in Norway spruce increased as a consequence of infestation by European spruce bark beetle and reached 4.9% in the autumn of 2019, which was much higher than the impact of the dry summer of 2003. The common, sessile and pubescent oaks did not show any losst vitality in 2018–19, however pubescent oak showed a higher crown transparency than sessile oak. Staining of active vessels and measurements of the water content of shoots showed that drought in the previous year reduced vessel activity and reduced the water content of beech shoots, but in oaks this was not the case. These findings are a strong indication that the damage to trees observed in 2019 was not only the consequence of the extreme drought in 2018 but also of a series of dry years.
Lightning strikes are the most important natural cause of forest fires worldwide. On average, lightning fires in Switzerland account for around one third (30.5%) of the events occurring in the high summer months (June, July and August). In this paper, we analyse the distribution and the prevailing meteorological conditions at time of occurrence of the 275 lightning fires registered in the Swissfire database for the period 2000–2018. Lightning fires occur in Switzerland almost exclusively in mountainous areas, especially in the Alps, although with a clear south-north decreasing gradient. The most affected species are conifers, especially spruce and larch. Persistent summer dryness is by far the main triggering factor for lightning fires, which even show an exponential increase in fire frequency and the related burnt area in particular with increasing drought severity. Our results clearly show that the predicted intensification of warm and prolonged summer dry periods in the context of climate change increases the probability of lightning fires in the Swiss Alps and extends the phenomenon to the Jura and Plateau. However, the exact meteorological conditions that favour lightning fires still need to be clarified in detail.
Keywords: lightning-caused fires, summer drought, climate change, Switzerland, Alps