Perhaps the simplest farmer adaptations have to do with changes in on-farm management, which include decisions about what crops to grow and when and how to grow them. One of the more straightforward of these possible adaptations is the option to shift when in the year crops are planted. Current decisions about when to plant are made based on a number of factors, including available soil moisture, the expected timing of temperature extremes, and the demands of multi-cropped systems. Year-to-year shifts in planting dates are already a demonstrated farmer adaptation in the face of climate variability, particularly for farmers in rainfed environments who often must wait for the onset of the rainy season in order to plant. Farmers in parts of Africa and Asia, for instance, routinely shift planting dates by a month or more from year to year in response to variability in when monsoon rains arrive (Falcon et al. 2004; Tadross et al. 2005).
If climate change results in large shifts in the factors that determine optimal planting times, farmers could potentially gain by further changing the timing of their crop production. In a crop model simulation of US rainfed spring wheat under a warmer and wetter future climate, Tubiello et al. (2002) find that systematically shifting planting 2 weeks earlier transforms what would have been 20-25% yield losses by 2030 into modest gains. This is because cold temperatures limit early planting in current climate, subjecting the crop to heat and drought stress during critical stages of plant growth, and warmer climates appear to allow earlier planting and less stress during sensitive growth stages. Similarly, cropping systems where
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Fig. 8.2 Simulated maize yields (t/ha) in southeastern Kenya using CERES-maize. Planting is simulated independently on each day of the year, for current and hypothetical future climates. Black solid line = current climate; black dotted=+2°C; grey solid=+2°C, -20% precipitation; grey dotted=+2°C, +20% precipitation. Optimal planting for all scenarios peaks near the start of the long rains
jan mar may jul sep nov
Fig. 8.2 Simulated maize yields (t/ha) in southeastern Kenya using CERES-maize. Planting is simulated independently on each day of the year, for current and hypothetical future climates. Black solid line = current climate; black dotted=+2°C; grey solid=+2°C, -20% precipitation; grey dotted=+2°C, +20% precipitation. Optimal planting for all scenarios peaks near the start of the long rains irrigation is possible for much of the year might also benefit from shifting planting dates, particularly for crops likely to experience frequent temperature extremes in their current growing season as the climate warms.
But for rainfed systems throughout much of the tropics, where planting is typically limited by moisture rather than temperature, it is less clear that shifts in planting date will offset much of the expected damages from climate change - largely because climate change is expected to reduce growing season length throughout much of the tropics (Chapter 3). Figure 8.2 shows representative results for maize at a somewhat arid site in southeastern Kenya, with yields simulated using CERES-Maize for every possible planting date in the year under current and hypothetical future climates. The planting dates resulting in maximum yield occur near the beginning of the long rains, as expected, with a second smaller peak during the short rains (when a second crop is often planted). With planting moisture-limited, future climates suggest gains or losses in yield but no shifts in optimal planting date.
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