The Vascular Plants of Iowa: An Annotated Checklist and Natural History

Iowa and Its Flora

IowaandItsFlora The Physical Setting Location Iowa lies near the center of the continental United States, between approximately 40° 35" and 43° 30" north latitude and between 90° and 96° 30" west longitude, and encompasses an area of 56,290 square miles. It is located between two of the world's principal rivers, being bounded on the east by the Mississippi River and on the west by the Missouri River from the Missouri-Iowa border north to Sioux City. Running north from Sioux City, the Big Sioux River forms the remainder of the state's western boundary. Climate The climate of Iowa is characterized as extreme midcontinental (Reed 1941) or as humid continental, warm summer, with a small portion of northern and northeastern Iowa in the cool summer subtype (Espenshade 1964). These climatic types are characterized by warm or occasionally hot summers, with rainfall in the form of showers and thunderstorms. Iowa is subjected to seasonal extremes and frequent rapid local weather changes due to the convergence of cold dry arctic air, moist maritime air from the Gulf of Mexico, and Pacific air masses that have lost their moisture while crossing the Rocky Mountains. Summers are warm and humid due to the influence of relatively unstable maritime air masses. Hot winds and periods of high temperatures occasionally occur from May to September. The average annual temperature ranges from 40°F along the northern border to 52°F in the southeast corner (Reed 1941). The mean July maximum temperatures range from 86°F to 90°F, with minima of 60°F to 66°F. The mean January maxima range from 24°F to 34°F, with minima of 4°F to 14°F. Winters are dominated by cold dry air masses from the Arctic, and there may be extended periods of subfreezing temperatures. Winter precipitation is of frontal origin and falls mostly in the form of snow. The lowest temperatures occur in the northern portion of the state, where minima of -20°F to -25°F are not uncommon . In southern Iowa, comparable lows of -10°F to -15°F occur. In the southeast, the January mean is 24°F; in the northwest, it is 13°F. A considerable difference in the length of growing season can be found across Iowa (map 2). The extreme northwestern and northeastern portions average 135 days, which increases to 175 days in southern Lee County (Reed 1941). The statewide average is 158 days, from May 2 to October 7. A gentle precipitation gradient exists generally from southeast to northwest (Reed 1941), with an average of over 34 inches in the extreme southeast and east to less than 26 inches in the northwest (map 3). This gradient is of importance in Map 2. Length of the growing season (in days). 0 Less Ihan 26 ~ 30 - 32 ~ ago 26 - 28 32 - 34 @ 28 - 30 0 More Ihan 34 Map 3. Annual rainfall (in inches). limiting the distribution of numerous plant species. The low rainfall of northwestern Iowa restricts woodlands to stream corridors and to some sparse or shrubby upland areas. The increased available moisture in eastern Iowa permits the growth of upland deciduous forests, an extension of the eastern deciduous forest. These interdigitate with the tallgrass prairie farther west. About two-thirds of the state's precipitation falls from April to September, with a peak in late spring and early summer. An approximate twenty-year drought cycle occurs in Iowa. It may be of importance in limiting the occurrence of some prairie species and certain northern wetland species and, perhaps, is most critical in restricting woody species. The average annual daytime relative humidity of Iowa is 72 percent, ranging from 69 percent in the southwest to 78 percent in the northeast (Reed 1941). The preceding factors, working in concert, have subtle but important effects on plant distributions . The longer growing season and high rainfall of southeastern Iowa allow certain southern species to enter that part of the state. On the other hand, the arid conditions of extreme western Iowa allow a number of Great Plains species to enter that part of the state. Geology Most of Iowa is underlain by sedimentary rocks such as shales, sandstones, limestones, and dolomites. These are generally deeply buried below glacial deposits and have little influence on plant distribution and abundance. Exceptions are in the northeast, where the bedrock is very near the surface and where many outcrops occur. In the...