Except for desert species, which are equipped with a deep tap root, a widely radiating root system, small transient leaves and twigs capable of photosynthesis (among other adaptations), deciduous trees generally need at least 20 inches of precipitation per year and grow in soil that is capable of absorbing and retaining that moisture.
Heading back to Missouri, yesterday, we first crossed the High Plains of eastern Colorado and western Kansas, where trees (mostly cottonwoods) are confined to the primary steam channels. Lying within the rain shadow of the Rocky Mountains and located far from the Gulf of Mexico, the streams of the High Plains are often dry, running only after thunderstorm downpours or during the rapid melt of a heavy snowpack. East of Wakeeney, Kansas, trees become more numerous, covering the main stream valleys and stretching up along the tributaries. Further east, near Ellsworth, Kansas, forests begin to cloak the shaded, north-facing hillsides, where soil moisture is protected from solar evaporation; by the time one reaches the longitude of Junction City, now well within the Gulf of Mexico moisture plume, woodlands grow on south-facing slopes as well.
The thin, rocky soil of the Flint Hills, between Junction City and Topeka, discourages tree growth; prairie grasslands cover the ridges and riparian woodlands are confined to the stream valleys but the topography of these Hills, in comparison to the flat terrain of the High Plains, attests to the greater annual precipitation in this region. Finally, in eastern Kansas and throughout Missouri, forests dominate the landscape, broken primarily by human environments (urban/suburban developments and cleared agricultural fields); thanks to Gulf of Mexico moisture, pumped northward ahead of advancing storm fronts, the southern Midwest receives about 35 inches of precipitation each year (compared to 10-12 inches across the High Plains).
