As opposed to forests, grasslands receive markedly less precipitation. This is one of the reasons why trees, which usually require a significant amount of moisture, are spaced relatively sparsely in grasslands. Due to the facts that most grasslands are situated in the center of continental landmasses, and that coastal mountain ranges can block west-to-east winds as is the case with the Great Plains in North America, air masses from the north and south are highly influential on grasslands. The Great Plains receive cold Arctic air in winter and hot tropical air in summer. A typical grassland climate includes hot summers with long periods of desiccation and cold winters with erratic snow cover. Consequently, plants in these grassland areas experience low soil moisture and, additionally, are exposed to full sun, strong dry winds, and extreme summer heat and winter cold. Grassland plants must adapt to these conditions. To support research on various types of ecosystems, during the 1970s, The National Science Foundation started what is known as the Long-Term Ecological Research (LTER) program. With the LTER program, scientists working at far-away sites can share data and collaborate to identify and understand large scale ecological patterns. At one LTER site in the Flint Hills of Northeastern Kansas, scientists made an important discovery about the influence of precipitation on plant productivity, the fuel on which ecosystems run. It was found that grasslands respond more strongly to pulses in rainfall than any other ecosystem. Dramatic bursts of plant growth and significant increases in primary productivity result from intermittent rainfall in grasslands. This pattern indicates that grasslands have a high underlying growth potential that surfaces when enough water is suddenly available. Because of these findings, LTER scientists have proposed that grassland annual primary productivity can be a useful indicator of global climate change. Similar to the canaries that miners used to carry into mines to warn of poisonous gases, grasslands may be able to serve as a warning of climate change and how it is affecting plants and humans. When maintaining grasslands, fire plays an essential role. In the summer, dry grasses and their dead remains which have accumulated over previous years on the upper layer of soil (known as mulch) make a highly combustible fuel for fires started by people or lightning. When there is an absence of rain, dry lightning is a common occurrence in grassland areas, as is the accidental escape of campfires or other human-related fires. These fires in grasslands can bum for many kilometers before being stopped by rain or wet areas. Studying charcoal buried in soil layers have suggested that under natural (pre-European settlement) conditions, at least a hectare of North American prairie may have burned once every 5 to 30 years. Plants in the grasslands are adapted to survive fire in many ways, but, just as importantly, fire is an essential component of grassland ecosystems. The invasion of both fire-sensitive plants and most trees are prevented by fire, and fire clears away dead plant material, thereby releasing necessary nutrients that facilitate new growth. Since the development of new grass was favored by Native Americans' horses and was also attractive to the buffalo they hunted, they often set grassland fires to stimulate it. These fires on grasslands was also helpful in increasing productivity of the Native Americans' wild food plants, improving visibility (which brings security), and helping control pests such as ticks. Nowadays, people use fire as an important tool in restoring and preserving grasslands. Eaters create a disturbance to which grassland plants are adapted in many ways. In grasslands, grazing is more significant than in other types of ecological communities. Up to 60 percent of energy and grassland materials flows through primary and other consumers. In other terrestrial ecosystems, on the other hand, less than 5 percent of the ecosystem's material and energy usually flows through consumer food webs. This difference coordinates with the relatively large herbivore populations that grasslands support. Benefits brought about from grazing include helping lower-growing plants by preventing shading by tall species, and providing a rich source of nutrients from grazer excrement. Grazing and fire both prevent just a few plants from dominating grasslands and help maintain the high natural plant diversity. Researchers at the University of Minnesota have found that high plant diversity has been pivotal in establishing and maintaining the high fertility in natural grassland soils.