The effects of temperature increase and heat stress on crop yield were evaluated separately in this thesis. Higher mean temperature during the growing period could reduce growing season length in particular the duration of the grain filling period. The results of chapter 5 and 6 confirmed that a reduction in the duration of most sensitive development phases (e.g. floral initiation to end of leaf growth in maize and pearl millet) can remarkably influence the crop yield. Higher mean temperature also caused shift of phases sensitive to heat to the cooler time of the growing season. Consequently, the intensity of heat stress which is defined as short episodes of very high temperature during the thermal sensitive stages such as anthesis (Barlow et al., 2015) may decline under global warming especially for winter cereals grown in temperate climate (Chapter 3). It was also found that change in crop management can strongly influence crop responses to high temperature and heat stress. For instance, the negative impact of high mean temperature on biomass accumulation can be compensated by high soil fertility (Chapter 5) and by change of the cultivated crop species (Chapter 6). However, this does not mean that the response on management × climate interactions for a single crop species (Chapters 5 and 6) can be generalized for other crop species. Eyshi Rezaei et al. (under review) found that the direction and magnitude of changes in phenological development are crop specific And that management decisions can reverse the climate change signal on crop phenology (Eyshi Rezaei et al., under review).
This thesis also contributed to the improvements of methods to assess effects of heat stress on crop yield at different scales (Chapters 2 and 4). It was found that considering canopy temperature can improve crop models at small scale (Chapter 2) but model complexity and high data demand are still challenging issues for simulation of canopy temperature at larger scales and need therefore further investigation (Chapter 4). However, the close relationship between canopy temperature and soil water content (Siebert et al., 2014) can be a starting point for the development of simpler approaches (low parametrization requirement) for simulation of canopy temperature at large scales. Simulation of canopy temperature also links heat and drought stress in crop models. Upscaling of crop model results by different aggregation methods showed that the signal of heat
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and drought stress is not strongly modified by aggregation of input and output data from 1 km × 1 km to 100 km × 100 km (Chapter 4). Therefore, there is no need for high input data resolution and large computational effort for simulations of heat and drought stress at large scales. Nevertheless, it is necessary to test the obtained results in more heterogeneous environments to investigate the possibility of generalization of the results.
7.6. Conclusions
This thesis advanced our knowledge on important gaps in the understanding of heat and drought stress effects on cereal crops in order to improve crop models ability to assess climate change impacts. This refers specifically to interactions between heat stress and temperature effects on phenology as well as between heat stress and crop management, and the effect of spatial data aggregation on stress impacts. Important outcomes of the thesis are:
1. There is a necessity to improve the understanding of complicated interactions between different processes such as photosynthesis, transpiration, and development under heat stress conditions. In addition, crop models should not only consider canopy temperatures, instead of air temperature, but also interactions between heat and drought stress.
2. The magnitude of heat stress around anthesis may not increase due to climate change. This is largely a result of accelerated crop development rates of winter cereals under climate change.
3. It is not essential to use high resolution input data (weather and soil) for the simulation of mean crop yield at national level. Despite this, high resolution input data is required for reproducing spatial details of extreme events (heat and drought reduction factors) under heterogeneous soil conditions.
4. The relationship between climatic variables and crop production can be influenced by different fertilization management. Effective fertilization management can increase dry matter production of pearl millet under present and future climatic conditions in Niger.
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5. Crop substitution can be a suitable opportunity to reduce the vulnerability of cropping systems to various negative impacts of climate change. However, additional studies are necessary in order to evaluate the combined effect of different adaptation strategies.
7.7. Outlook
The studies described in this thesis contribute to the understanding of the effects of heat and drought on crop production across spatial scales and help to design effective adaptation strategies to cope with global warming. However, some un-resolved issues and future challenges need to be acknowledged:
1. Most importantly, it is required to better understand the underlying mechanisms explaining phenotypic responses to heat stress and then develop new crop model routines for quantifying the large variability in sensitivity of cultivars to heat stress. Cultivar screening field experiments will be an important basis to achieve such advancement. We also need to develop more simple and robust approaches to estimate canopy temperatures, particularly for large scale assessments.
2. A small effect of data aggregation on basic statistics of simulated heat and drought stress of winter wheat at large spatial scale was found. Nevertheless, it is strongly recommended to conduct similar studies for summer crops and more stress prone areas. Modeling crop responses over large areas will also need to address management variability, for instance, to cover the variability of cultivars grown in a respective spatial unit which is presently often simplified to one set of crop parameters (cultivar characteristics). Probabilistic modeling approaches of input data and crop parameters in large scale assessments may be a way to account for this uncertainty.
3. It has been shown that crop responses to heat and drought stress can change depending on the management strategy. Hence, considering combined adaptation strategies such as modification of sowing date and deficit irrigation can be recommended. In more general
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terms, interactions between combined adaptation strategies and climate change and their effects on crop yield will need to be better understood in the future.
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