Changes in climate variability and climate can affect microclimatic conditions is many ways (Sivakumar et al. 2005). Modifications to the microclimate of crop stands were used in ancient cultures such as the Incas in the Peruvian highlands (Vogl 1990). ttese ancient camellones and qochas are a combination of water-filled canals and plots designed to improve the microclimate (especially to decrease nocturnal cooling) and water availability of crops and are still in occasional use. Another examples is kanchas (stone fences around small fields) and terraces on slopes, which can increase both air temperature and water regime and reduce the wind speed of crop stands in these semi-arid and cold environment. Similar systems can also be found in other parts of the world, developed by ancient farmers on their own experience.
In current semi-arid low-input systems there are known examples not only for improving water resources but also for optimizing the temperature and radiation regimes of crop stands (Stigter 1988, 1994). A classic example is oasis agroecosys-tems with complex crop mixing and patterns to permit efficient use of radiation in a small area, to increase air humidity for the shaded crops and to avoid extreme diurnal temperature variations.
Agroforestry systems including shelterbelts are another farm management option to improve microclimatic conditions and not just to reduce wind and evapotranspiration. As crops respond especially to climatic extremes, any measure to reduce these extremes in most cases has had an accumulating positive effect on the yield level. Easterling et al. (1997), for example, show in a simulation study for the Great Plains that shelterbelts may provide nighttime cooling that could partially compensate the tendency of warming to shorten the growing season, ttis effect is even more significant under extreme climates or severe warming trends. On the other hand, heat stress on crops can be reduced by shading, which has been documented as a significant yield factor (Southworth et al. 2000, 2002).
Many examples of agroforestry systems in different climates and regions are known, all adapted to the specific characteristics of the relevant agroecosystem (climate, soils, crop production, farm input level, socioeconomic conditions). Such systems are already well established in many agricultural regions, especially in subtropical and tropical climates with extreme temperatures and/or weather variability. As Salinger et al. (2005) reported, soil surface heat extremes may surpass critical limits in many regions, especially with changing climate variability and extended drought periods. Under extreme climatic conditions, in semi-arid and arid tropics, for example, physiological critical temperature thresholds for crops were attained more frequently, tte establishment of agroforestry systems as a long-term measure is probably the most effective option and solution to this problem.
Tree shading, on the other hand, can also prevent frost damage to crops and reduce nocturnal radiation cooling on the crop surfaces, ttis method is used not only in temperate regions but also in tropical highlands, for example in tea plantations in Sri Lanka. Other frost production methods, such as covering plants with sheets or foil, are also used in small plots for low-input systems. For orchards or large fields methods such as frost irrigation, foil covering, or applying aerosols are costly and are therefore found mainly in medium- and high-input farming and for cash crops. Long-term measures that are very important for avoiding damage to crops from radiation frost include planning of plantations in relation to topography in order to avoid impacts from cold air lakes, ttese measures are often ignored, especially when frost occurs seldom, but the effect on perennial crops can be more devastating than hail damage, as the whole crop can be damaged.
Hail is another danger, which can occur in almost all climatic regions. Although it is normally limited to a small region, its frequency can cause devastating damage. Protection against hail is not possible for annual crops on large fields (except by cloud injection, the effectiveness of which is uncertain). Only on perennial cash crops such as orchards is the investment on a hail net, for example, profitable. Hail insurance is probably the most effective protection against financial losses as a result of hail, but it is mainly used in developed countries and high-income farm ing systems. For low-input farmers in developing countries institutional support might be the only solution for hail protection.
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