Grassland climates can be described as wet or dry, hot or cold (typically in the same season), but on average are intermediate between the climates of deserts and forests. The climate of grasslands is best described as one of extremes. Average temperatures and yearly amounts of rainfall may not be much different from desert or forested areas, but dry periods during which the plants suffer from water stress occur in most years in both temperate and tropical grasslands. An excellent example of this comes from North America, where in the area around Washington, DC (dominated by eastern deciduous forest), the annual precipitation is ~102 cm whereas at Lawrence,
KS (dominated historically by tallgrass prairie), the annual precipitation is ^100 cm. But the way the rainfall is distributed is notably different. At Lawrence, KS, over 60% of the rainfall occurs in the growing season (AprilSeptember), whereas at Washington, DC, the precipitation is uniformly distributed throughout the year. The open nature of grasslands is accompanied by the presence of sustained high wind speeds. Windy conditions increase the evaporation of water from grasslands and this increases water stress in the plants and animals. Another factor that increases water stress is the high input of solar radiation in these open ecosystems. This leads to the convective uplift of moist air and results in intense summer thunderstorms. Rain falling in these intense storms may not be effectively captured by the soil and the subsequent runoff of this water into streams reduces the moisture available to grassland plants and animals. In addition to periods of water stress within the growing season, consecutive years of extreme drought are more common in grassland than in adjacent forested areas. Such droughts may kill even mature trees, but the grasses and other grassland plants have extensive root systems and belowground buds that help them survive and grow after drought periods (Figure 3).
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