Abiotic stress in cultivated plants
Temperature largely determines the suitability of a crop species for cultivation and significantly influences growth and development processes. All cultivated plants have a temperature range within which they grow optimally. It differs from species to species and also depends on the particular stage of development of a plant. For example, the optimum for plant growth of wheat is between 10 and 25°C. If the temperature deviates upwards from the optimum temperature range, the plant suffers from heat stress, which manifests itself through certain symptoms on the plants (e. g. wilting symptoms, poor flower initiation, lack of fertilization, fruit deformation, poor coloration of fruits). In combination with high solar radiation, temperatures can be reached at the plant surface that lead to burns on leaves (e.g. of wheat, vegetables) and fruits. Damage occurs to proteins and thus to metabolic processes. At very high temperatures, this usually leads to growth stagnation and to losses in quality and yield. The heat year 2003 shows for various regions in Germany (e. g. Rheingraben) that a significant drop in crop yields correlates with the number of heat days.
The individual crops tolerate heat differently: rapeseed, sugar beet and potatoes are relatively sensitive to heat. Cereals, especially barley, are somewhat more tolerant, but also show yield depression with increasing heat. Crops of subtropical origin, such as corn, millet, and soybeans, on the other hand, are fairly heat tolerant. In grasslands, there are also grass species that are particularly sensitive to heat, e. g common bluegrass (Poa trivialis), while other grass species are more heat tolerant, e.g., Kentucky bluegrass (Poa pratensis) and tall fescue (Festuca arundinacea). Most crops are particularly sensitive to heat stress at certain sensitive stages of development. In wheat, flower unfolding and pollen differentiation in May/June are such phases when temperature rises above 30 °C can lead to sterile pollen and thus a decline in grain number. In rapeseed, very high temperatures during the growth phase decrease the content of nutritionally desirable unsaturated fatty acids, while increasing the content of saturated fatty acids. Cereals react to heat stress in the ripening phase with so-called emergency ripening. The grains remain smaller and the yield is lower. The decisive factor here is therefore when the heat phases occur in the near and distant future and whether they occur more frequently during the sensitive development phases of the crop plants. Crop heat sensitivity or tolerance is not an isolated phenomenon, but must always be seen in the context of water supply, whether through precipitation or soil water supplies. Heat has a worse effect the less evaporative cooling (transpiration) can provide relief, i.e., the drier it is.
In terms of the potential extent of damage, areas with a high proportion of fruit cultivation, viticulture and horticulture are particularly vulnerable, since yield and quality reductions in these special crops with high added value have a particularly strong impact. In contrast, regions with a high proportion of grassland are more adaptable, as grasslands can change their species composition.
Cold stress means stressing plants at low temperatures. At lower temperatures, chemical processes run more slowly, which for plants generally means less energy from operating metabolism, lower nutrient and water uptake from the soil, and slower growth. As a result of cold stress, leaves may turn yellow and wilt. Below 0°C, frost damage occurs on many crops due to ice formation in the cells, which destroys cell structures. For some crops, such as winter wheat, a climate change-induced decrease in frost days is problematic because a cold stimulus is required during a certain growth phase, the plant's transition from the vegetative to the generative phase ("shooting"). If this is lacking, crops suffer. Late spring frosts are among the most dreaded extreme weather events, especially in fruit cultivation, viticulture and horticulture. They cause damage to leaves, shoot tips and flowering plants and inhibit fruit development. Frost cracks and frost scars on the fruit lead to qualitative impairments, some of which can no longer be marketed as a result. As a rule, the risk of late frost damage is higher in early-sprouting varieties, very early-sown or planted stands, and at lower altitudes. In the case of apple and wine crops, the more advanced plant development is in spring, the more devastating the damage can be. Special attention should be paid to the increasingly earlier start of vegetation due to climate change, which increases the probability that plants are already in very late frost-sensitive development stages.
If the soil water content falls below a critical value, a supply of water and nutrients to the plant roots and their transfer to other parts of the plant is no longer sufficiently guaranteed. The plant goes into drought stress. During drought stress, many plants close their leaf openings (stomata) to evaporate less water. However, this also results in less evaporative cooling and the tissue temperature of the plants increases. For agriculture and fruit cultivation, wine- and horticulture, the temporal distribution of precipitation plays an important role in addition to the annual precipitation total. Long periods without precipitation lead to changes and damage in all stages of plant growth. The effects are particularly severe when water stress occurs during sensitive plant growth stages, such as leaf formation. The lack of soil water impedes the availability and uptake of nutrients and leads to a reduction in photosynthesis. This can cause growth and maturity inhibition, yellowing, early autumn coloration, and leaf fall.
Agricultural crops have different levels of tolerance to drought stress. While shallow-rooted plants such as potatoes are very sensitive to low soil moisture, apple, grapevine and asparagus have very deep-reaching roots and can thus access a larger volume of soil water. They are comparatively less susceptible to drying out of the upper soil horizons. However, even these deep-rooted crops suffer from inadequate coverage of their water requirements as a result of prolonged drought, leading to damage. According to data from the German Meteorological Service, the number of days with low soil moisture has already increased significantly in Germany since 1961 and, depending on the region, has led to significant crop losses with corresponding economic damage. Particularly in southwestern Germany and parts of the eastern states, decreasing precipitation in the summer half-year and more consecutive dry days can be observed as a result of climate change. In these regions, which are already comparatively warm or dry, climate change is becoming increasingly problematic for agriculture.
Continuous and heavy rain
Excessive precipitation can also affect plant growth and have negative consequences for agriculture and horticulture, fruit cultivation and viticulture. In this context, extended periods of wetness (continuous rain) with waterlogged soil represent an extreme weather situation in two respects. First, plant roots suffer from a lack of oxygen, which damages the root system. As a result, there is stunted growth, delayed ripening, yellowing, and increased vulnerability to pests. In the worst case, the plants and trees die. Secondly, the soil is so wet that it is impossible to drive agricultural machinery over it to sow, tend and harvest the crops without causing damage. As climate change progresses, it must be assumed that heavy precipitation in Germany will increase with a high to very high probability in the coming decades. In addition to yield losses due to excessive wetness, heavy rainfall is often accompanied by increased surface runoff (water erosion) and thus causes damage to agricultural land and infrastructure.
In extreme cases, hail can destroy the entire harvest of a cultivated area in arable farming and in the special crops of horticulture, fruit growing and viticulture, thus causing high economic damage. In particular, hail causes mechanical damage to above-ground plant parts, leaves and crop. By perforating or even knocking off the leaves, the assimilation area and thus the photosynthetic output are reduced. Damage to the crop can cause secondary infections and a delay in ripening, in addition to visually affecting quality. According to insurance companies, hail is already occurring more frequently than in previous decades. In general, an increase in the risk of hailstorms from northern to southern Germany is assumed in the course of climate change.
Storms can also cause mechanical damage to agricultural crops and specialty crops, either directly by the wind or by wind-transported particles (wind erosion). Strong wind gusts are capable of toppling entire trees, causing damage to support equipment in viticulture, or breaking entire shoots in vegetable crops. Often, significant delays in development and maturity occur as a result of this damage. Pressure spots, scuff marks (e.g. from support devices) or abrasions can occur on shoots or crops. This results in qualitative losses and creates entry points for pathogens. In general, the risk of major storm damage to permanent crops increases as development and leaf surface progress. For example, vulnerability to storms is particularly high when apple and wine have high fruit hanging. Excessively high wind speeds also hinder the application of crop protection products. Wind erosion can lead to drifting and thus to a transfer of pesticide active substances to other areas.
Indicators from the DAS monitoring: Yield fluctuations | Hailstorm damage in agriculture
Heat stress for and performance of livestock
In times of climate change, increasing heat stress, water shortages and feed shortages are probably the biggest challenges for livestock farms. Heat stress reduces the well-being of livestock and can affect their health and performance.
Dairy cows, which are very sensitive to temperature increases, respond to heat stress by reduced feed intake, increased water intake and standing. Consequences may include a reduction in milk quantity and milk quality (fat and protein content), impairment of the immune system, metabolic disorders, lower fertility, and increased embryonic lethality. If heat stress occurs in the last three months of gestation, calves may have lower birth weight and metabolic problems after calving. In addition, the milk yield of these animals may be limited when they are old enough to produce milk. High-performance breeds among dairy cows, in particular, are considered increasingly sensitive to heat. They have difficulties compensating for heat exposure because milk production is accompanied by high heat production of their own. Other factors influencing heat stress in individual animals include coat color and health status. Animals of the same breed from cooler regions suffer from heat stress more quickly than animals from warmer regions.
Pigs are also particularly sensitive to heat stress because they do not have functioning sweat glands and can only remove heat by panting. Therefore, heat can quickly become critical for the animals. Critical temperatures are reached in pig farming with outside temperatures as low as 25°C. To prevent a rise in body temperature due to digestive heat, pigs cool themselves by increasing respiration, which leads to water loss and reduced feed intake. Another strategy of the animals is to avoid physical contact with conspecifics when space permits. As a result of heat stress, there is decreased welfare to circulatory failure and increased mortality. During hot periods, the fertility performance of boars and the farrowing rate of sows also decline, and litter sizes decrease. Sows react to high temperatures with decreasing milk yield and become more susceptible to disease. These and other parameters have a direct impact on economic efficiency.
Poultry is also very sensitive to heat. For example, heat stress in young fattening poultry begins at around 30°C. Laying hens tend to prefer 18 to 24°C. When it gets too hot for chickens, they spread their wings sideways from their bodies. This allows cooler air to get under the feathers to the skin and warm air to escape. Chickens also release heat through their tongues and mucous membranes. When breathing with an open beak, moisture is released and the resulting evaporative cooling is used for cooling. Hens react to the onset of overheating with a decline in laying performance, accompanied by smaller eggs, thinner shells and lower egg quality.
In addition to heat stress, climate change can also affect the performance of livestock through more indirect pathways. For example, climate impacts on crop production can reduce forage quality and availability.
Other climate impacts
Lengthening of the growing season: The continuously rising average air temperature extends the growing season. In particular, fewer frost days in spring can lead to an earlier start to growth. The overall higher temperature totals can thus increase yields in arable farming, fruit cultivation, viticulture and horticulture. However, an earlier start to vegetation increases the risk of late frosts and thus the risk of yield losses. This affects fruit cultivation in particular. Decoupling of flowering and pollinators is also possible for some crops. Changing plant phenology also has implications for agricultural operations and crop management. Corn and sugarbeets in Germany are already sown and harvested ten days earlier on average than in the period from 1961 to 2000.
Shift of cultivation areas: On average, higher temperatures and milder winters lead to a shift of cultivation areas towards the north and higher regions. Crops that require a certain cold stimulus in winter can no longer be cultivated regionally due to higher temperatures. However, new warmth-loving crops can be added, whose cultivation was previously not possible or not economical in Germany. In particular, cultivation areas in Germany that are comparatively cool and humid today (e.g. northern Germany, low mountain ranges) can benefit from these developments and, given a moderate rise in temperature and an adequate water supply, may certainly expect increased yield potential for many types of crops.
Stress from pests and diseases: As a result of rising temperatures, we can expect a spread of plant diseases as well as pests that were previously found only in warmer areas. Other pests, on the other hand, which depend on longer periods of moisture, could decline. As a result of climate change, shifts in the species spectrum of plant pests can therefore be expected in the coming years. Damage caused by fungal diseases - with the exception of diseases triggered by warmth-loving fungal species such as mildew - is likely to decrease in many areas.
Indicators from the DAS monitoring: Agrophenological phase shifts | Infestation with harmful organisms - case study