Grasslands
This zone is the southernmost part of the true prairie that extends as far north as Saskatchwan, Canada. Included in this zone are the tall-grasses of the east, the mixed grasses of central Oklahoma, and the short grasses of the west. Along the streams and drainages are the extensions of woody species from the post oak-blackjack oak zone. These factors explain why 70% of the aeroallergenic species occur in this zone. Native grasses are dominant here and of little allergenic significance. Bouteloua (grama grass) is the only grass mentioned in the hay fever literature. However, much of the area is being modified and the native grasses are being replaced by species that are greater causes of allergens. Carya illoensis (pecan) and Ulmus americana (American elm) are the only trees of major allergenic significance and are restridted to narrow streams and limited bottomland habitats. Quercus virginiana (live oak) occurs in this zone along the North Fork of the Red River and is an important ha fever tree. Oklahoma and Tulsa couties are situated along the boundaries of both Zone 3 and Zone 4. Consequently, individuals in these counties will be exposed to allergens fom both plant communities.