Prairie. Candace Savage

toward the west, the vegetation diminished in step, gradually reducing in height like a living bar graph. Somewhere around 100 degrees west longitude (give or take a few degrees), the tall-grass species dwindled away, leaving the wind to ripple through knee-high stands of mixed-grass prairie. To the west and southwest, the midheight grasses in turn gave way to a carpet of ground-hugging grasses, as a dry land clothed itself in drought-resistant plants.

      The boundaries between tall-, mixed-, and short-grass prairies are not as tidy as they look on the map. In the patchwork quilt of the grasslands, each of the major blocks of vegetation is composed of many smaller blocks. And just as the overall picture is determined by large-scale climatic patterns, so each of these distinctive patches is a response to local variations in microclimate. Conditions are different on a south-facing slope than on the north and on lowlands than on hilltops, and these subtle differences are reflected in the vegetation. Moving uphill, from humid bottomlands to the drier crest, reproduces the moisture gradient of moving from east to west. And so, a sea of tall-grass prairie is broken by islands of mixed (or even short) grasses that grow on uplands and arid slopes. Meanwhile, out on the short-grass prairie, blue grama and its diminutive associates follow the opposite trend, ceding ground to midheight or tall grasses in moist valley bottoms.