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Food & Climate
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Crops
Extreme meteorological events, whether related to the El Niño phenomenon, other large-scale forcing factors, or simply the chaotic nature of the climate system, can have strongly detrimental effects on crop yields. The effects of extreme weather events on crops may be either direct or indirect, or both.
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Higher temperatures increase the demand of water on crops directly by increasing the evaporation rate of the soil and the transpiration rate of the plants (process known by the name of evapotranspiration) as well as the atmospheric holding capacity for water vapour.
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1. Energy exchange between vegetation and atmosphere.
From Piker et al
This diagram shows how vegetation is not a passive element on Earth, it interacts with the atmosphere and the surrounding air by absorbing energy, as we saw in the basic level and also by emitting part of this energy in form of latent heat or evapotranspiration.
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An indirect effect is created when higher temperatures hasten the breakdown of organic matter in soils, which in turn leads to lower soil organic matter levels, culminating in less soil-moisture retention and additional crop moisture stress (less available water). Both direct and indirect effects threaten yield. Plants in the early stages of plant development are especially vulnerable to extreme weather events
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When the optimal range of temperature values for a crop in a particular region is exceeded, crops tend to respond negatively, resulting in a drop in yield. The optimal temperature varies for different crops.
Most agronomic crops are sensitive to episodes of high temperature. Air temperatures between 45 and 55oC that occur for at least 30 minutes directly damage crop leaves in most environments; even lower temperatures (35 to 40oC) can be damaging if they persist longer. Temperature greater than 36oC causes corn pollen to lose viability, while temperature about 20oC depresses tuber initiation and bulking in potato.
Vulnerability of crops to damage by high temperatures varies with developmental stage. High temperatures during reproductive development are particularly injurious - for example, to corn at tasseling, to soybean at flowering, and to wheat at grain filling. Soybean is one crop that seems to have an ability to recover from heat stress, perhaps because of it is indeterminate (i.e., grows continuously).
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Precipitation
Precipitation, being the primary source of soil moisture, is probably the most important factor determining the productivity of crops. While global climate models predict an overall increase in mean global precipitation, their results also show the potential for changed hydrological regimes (either drier or wetter) in most places. A change in climate can cause changes in total seasonal precipitation. The water regime of crops is also vulnerable to a rise in the daily rate and potential seasonal pattern of evapotranspiration, brought on by warmer temperature, dryer air, or windier conditions.
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2.Corn affected by excessive humidity
Harold Kaufman, TAEX, 1996
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Drought conditions may also be brought on by lower amounts of precipitation falling as snow and earlier snowmelt. In arid regions, these effects may reduce subsequent river discharge and irrigation water supplies during the growing season. Episodes of high relative humidity, frost, and hail can also affect yield and quality of fruits and vegetables (especially corn and other grains).
Crop yields are most likely to suffer if dry periods occur during critical developmental stages such as reproduction. In most grain crops, flowering, pollination, and grain-filling are especially sensitive to water stress. For example, the effects of drought can be escaped by early planting of cultivars with rapid rates of development; fallowing and weed control can help to conserve moisture in the soil.
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3. Measurement of the soil moisture by comparing the colours of the soil and a test paper.
Photo by USDA NRCS
Please click on the photo, to see details of the scale! (100 K)
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Temperature stress and water stress often occur simultaneously, the one contributing to the other. They are often accompanied by high solar irradiance and high winds. When crops are subjected to drought they reduce transpiration and, consequently, plant temperature rises.
Excessively wet years, on the other hand, may cause yield declines due to waterlogging (there is too much water and the plant DROWNS or dies because its roots ROT in water) and increased appearance of pests. Intense bursts of rainfall may damage younger plants, as well as promote soil erosion. The extent of crop damage depends on the duration of precipitation and flooding, crop developmental stage, and air and soil temperatures. |
Table1. High temperature and soil moisture effects on the major field crops. |
Corn |
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- Temperature higher than 36ºC causes pollen to lose viability.
- Extremely sensitive to soil moisture deficits.
- Very intolerant to flooding at nearly any stage; the effect of flooding depends on temperature. When the crop is less than 6 inches high, 24 h of flooding reduces yield by 18% at any temperature.
- Continuous presence of excessive water in the soil causes long-term problems related to rot development and increase damage by diseases (e.g., crazy top and common smut). |
Soybean |
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- Soil temperature higher than 35ºC at planting causes seedling death. Very sensitive to temperatures above 35ºC during the first three weeks after bloom. Great ability to recover from temperature stress at other times.
- Sensitive to soil moisture deficits and drought at planting and other stages.
- Relatively tolerant to excess soil humidity, but saturated soils increase the risk of seedling diseases especially at temperatures above 32ºC. |
Wheat |
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- Flowering, pollination, and grain-filling sensitive to water stress.
- Excess soil moisture causes lodging, and increases risk of fungal infestations. |
Cotton |
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- Temperature above 40ºC for more that 6 hours causes bolls to abort.
- Relatively tolerant to temperatures under 40ºC.
- Sensitive to soil moisture deficits and drought at planting and flowering. Requires at least 20 inches of rain per growing season.
- Excess rainfall at maturity damages quality of crop. |
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Author: Marta Moneo and Ana Iglesias- Universidad Politécnica de Madrid - España
1. Scientific reviewer: Alex de Sherbinin - CIESIN, Columbia University - USA
2. Scientific reviewer: Lily Parshall - Goddard Institute for space studies, Columbia University - USA
Educational reviewer: Emilio Sternfeld - Colegio Virgen de Mirasierra - España
Last update: 12/05/2004 |
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