Biome: Midlatitude Grassland

The midlatitude grasslands are not easily defined. Most grasslands are areas of low relief and low rainfall, the combination of which protects them by reducing erosion disturbances that allows rapid invasion by trees and shrubs. Much of the former grasslands in higher-rainfall regions were removed by cultivation. The midlatitude grasslands lie mostly between 25° and 50° of latitude in both hemispheres. The grass species of the midlatitude grasslands come from rich sources both in the tropics and high latitudes. The species did not arrive together and many grasslands have been in place for thousands to millions of years. New grass species invade along disturbance corridors, such as valley walls or bottoms. The dominant grass species use a variety of seed dispersal mechanisms. Some are adapted to transport by wind, while others move in the digestive tracts of animals or adhere to the hides of animals. This entry will discuss the nature of the midlatitude grasslands, the grasses of these regions, human interactions with and maintenance of grasses, and animals that use the grasslands.

At the global scale, scant and uneven data limit our descriptions to higher-taxonomic levels, such as tribes. Available data do not permit examination of taxa by abundance. We can compensate for this deficiency to some degree by looking at the diversity or dominance of some taxa in the total grass flora of different regions. The most widely available data are species lists for various areas of the world. Hartley and colleagues have done this for the seven taxonomic groups that contain more than 75% of the global taxonomic diversity of grasses. These most important tribes are divided into two distinct photosynthetic types that evolved as adaptations to distinctly different environments. Agrosteae, Aveneae, Festucae, and the largest genera, Poa, comprise C3 species. This photosynthetic pathway, common in other plants, is adapted to high-atmospheric carbon dioxide and cool summer temperatures. For these grasses, production declines above 20 °C (68 °F). They have migrated to the midlatitudes from higher latitudes in both hemispheres. They also dominate in areas of wet winters and dry summers. This seasonal adaptation allows these grasses to complete their growth cycle before the summer heat limits production. Domesticated species from this group include rice (Oryza spp.), wheat (Triticum spp.), oats (Avena sativa), barley (Hordeum spp.), and rye (Secale cereale). These and other domesticated grasses have larger seeds that are presented high on the plant and, therefore, easily harvested and handled than plants with seeds that are small designed for transport by the wind.

A later evolution, the C4 (warm season) photosynthesis grasses, adapted to higher temperatures and lower carbon dioxide, such as occurred during glacial maxima, by enriching the CO2 taken in the stomata and feeding it to a deeper C3 tissue layer. Production declines for these grasses above 35 °C (95 °F). Eragrosteae (now regrouped with Chlorideae), Paniceae, and Andropogoneae are dominated by species that use C4 photosynthesis, although a few of the Paniceae are C3 species. The highest species diversity of these grasses is found within 30° latitude of the equator. Figure 1 illustrates the dependence of the midlatitudes on the adjacent tropical and high-latitude source areas for the species that migrated into midlatitude grasslands. Domesticated species from this group include corn (maize; Zea mays) and sorghum (Sorghum bicolor).

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