Effects of heat and drought stress and their co-occurrence on winter wheat yields in Germany under climate change

In our changing climate, heatwaves and droughts and their spatio-temporal co-occurrences are likely to intensify. This will inevitably challenge future agricultural production and calls for adaptation strategies to protect future yields. To find suitable climate adaptation strategies for Germany’s major staple crop - winter wheat - it is important to know how heat stress, drought stress or their compound effects drive wheat yield failures. The principal aim of this study is, therefore, to quantify the impacts of heat, drought, and their compound effects on winter wheat yields in Germany, in a spatially and temporally discrete manner.

To address our aim, we develop a statistical crop-climate model for the time period 1991-2019 at the county level. We first create agroclimatic proxies for heat stress, drought stress and their compound effects and use these to construct a separate time series model with the addition of time-dependent interaction terms. Our approach constructs separate regression models for each county, based on common elements that allow for comparing and jointly interpreting individual models.

Preliminary results show that more than 50% of Germany’s wheat yield variability can be explained by climate effects. Compound effects of heat and drought stresses are responsible for approx. 42% of the variability in Germany’s winter wheat yields. Drought stress alone explains approx. 7%, with higher impacts in the east of the country, and heat stress alone explains approx. 3% of the year-to-year yield variability, with higher impacts in the north-west of Germany. The results confirm the importance of compound effects and underline their dominating impacts on winter wheat yields, when compared to individual heat and water stress impacts – a finding which should guide future adaptation strategies. Furthermore, our study shows that heat stress is becoming increasingly important for wheat yield failure in Germany – alone and in conjunction with moisture stress.

In conclusion, we suggest that climate change adaptation strategies for winter wheat in Germany should focus on combined measures against drought and heat extremes. While the increase of multi-stress resilience should be the main goal for entire Germany, north-western areas should prioritize strategies to increase heat resilience and eastern areas should prioritize strategies to increase drought resilience.