The European Heat Wave 2018: The Dendroecological Response of Oak and Spruce in Western Germany

Abstract

The European heat wave of 2018 was characterized by extraordinarily dry and hot spring and summer conditions in many central and northern European countries. The average temperatures from June to August 2018 were the second highest since 1881. Accordingly, many plants, especially trees, were pushed to their physiological limits. However, while the drought and heat response of field crops and younger trees have been well investigated in laboratory experiments, little is known regarding the drought and heat response of mature forest trees. In this study, we compared the response of a coniferous and a deciduous tree species, located in western and central–western Germany, to the extreme environmental conditions during the European heat wave of 2018. Combining classic dendroecological techniques (tree–ring analysis) with measurements of the intra–annual stem expansion (dendrometers) and tree water uptake (sap flow sensors), we found contrasting responses of spruce and oak trees. While spruce trees developed a narrow tree ring in 2018 combined with decreasing correlations of daily sap flow and dendrometer parameters to the climatic parameters, oak trees developed a ring with above–average tree–ring width combined with increasing correlations between the daily climatic parameters and the parameters derived from sap flow and the dendrometer sensors. In conclusion, spruce trees reacted to the 2018 heat wave with the early completion of their growth activities, whereas oaks appeared to intensify their activities based on the water content in their tree stems.

Towards Continuous Stem Water Content and Sap Flux Density Monitoring: IoT-Based Solution for Detecting Changes in Stem Water Dynamics

“…The average annual air temperature and precipitation for 2020 in the selected site were reported at 9.7 • C and 374 mm, respectively. As reported in the previous study about the European heatwave's impact on Western Germany [63], correspondingly, in this site, from 15 July to 15 August, we showed an increasing trend in the VPD up to a maximum of 3.8 KPa, due to high temperatures and few rainfall events (Figure 6). The maximum recorded rain event was 21.2 mm, which occurred on 17 June.…”

“…The accuracy of the sensor, however, seems not yet to be adequate for diurnal trend Observed hourly data, captured by the TreeTalker (Figure 12a,b) for one growing season, accurately reveal the impact of elevated temperature and an acute heatwave from 15 July to 15 August 2020 across Germany. Subsequently, a large impact on the trees' water storage level in beech trees was evident (Figure 12a,b) and can be explained by [63,71].…”

“…Forests 2022, 13, x FOR PEER REVIEW 14 of 18 15 July to 15 August 2020 across Germany. Subsequently, a large impact on the trees' water storage level in beech trees was evident (Figure 12a,b) and can be explained by [63,71].…”

Environmental Drivers of Gross Primary Productivity and Light Use Efficiency of a Temperate Spruce Forest

“…As an indicator of stomatal responses, sap flow density ( J S ) was derived from three spruce trees about 20 m to the southeast of the tower (Figure 1). Each tree was instrumented with a Granier sensor comprising four needles (Ecomatik SF‐L, Ecomatik, Dachau, Germany), mounted at 1.5 m height, from which the average J S of the trees was derived from the temperature difference between two probes (Bogena et al., 2015; Neuwirth et al., 2021). The respective equation follows empirical relations (Granier, 1987): $$${J}_{\mathrm{S}}=119\ast {\left(\frac{{\increment}{T}_{\mathrm{max}}-{\increment}T}{{\increment}T}\right)}^{1.231},$$$ where J S is the sap flow density (g m −2 s −1 ), Δ T is the actual temperature gradient between the two probes and Δ T max the maximum temperature gradient measured between the probes in a given time period.…”

Effects of deforestation on dissolved organic carbon and nitrate in catchment stream water revealed by wavelet analysis

“…This process involves replacing monospecific, "artificial" conifer plantations with broadleaved mixed stands often via clear-cutting and it can lead to abrupt changes in forest ecosystems altering C fluxes and stocks (Bala et al, 2007;Saner et al, 2012;Nogueira et al, 2017) and causing abrupt shifts in the N cycle (Hamilton et al, 2016;Guiry et al, 2020). In addition to the current drive, forest loss has been exacerbated by the droughts and heat waves of recent years (e.g., Neuwirth et al, 2021).…”