Photo by Lovemedead, Wikimedia Commons
Click on this link to listen to an audio version of this blog … Pawnee Buttes
Greely, Colorado sits on a section of the Great Plains called the “Colorado Piedmont.” The sandstone capping of the high plains has over geological time eroded away across a broad area just east of the Rocky Mountains leaving behind a landscape of rolling hills with an underlying layer of shale.
Most of the major cities of Colorado (including Denver, Colorado Springs, Fort Collins, Pueblo and Greeley) are located in or on the edges of this piedmont zone. Driving south from Cheyenne, Wyoming on Interstate 25, you can clearly see the northern edge of the Colorado Piedmont at Wellington, Colorado. There is a noticeable drop as the highway goes from the 6000 foot (or so) elevation of the high plains to the 5000 foot (or so) elevation of the piedmont.
The higher elevations of both the foothills of the Rockies to the west and the high plains to the north and east have great impacts on the weather patterns and vegetative ecosystems of the Colorado Piedmont. For example, many snowstorms and thunderstorms skip around the city of Greeley (an observation sometimes attributed to a mysterious “Greeley high!”). (See “Signs of Fall 4,” September 23, 2021). These patterns, and the severity of these storms just a hour north in southern Wyoming (the hail capital of North America!), are probably due to lower elevation of the piedmont area.
Photo by M. Hamilton
Out in the middle of the Pawnee Grasslands just to the north and east of Greeley is a place where the somewhat ragged edge of the piedmont/high plains border can be clearly seen. A series of discrete mesas made of the erosion-resistant, high plains sandstone spread out over a rolling landscape of shortgrass prairie. These mesas are called the “Pawnee Buttes,” and on the day before New Year’s Eve , we decided to go out to the buttes to go for a hike.
All of our immediate family had gathered for the holidays and, originally, five of us (Marian, Joe, Marlee, Deborah and I) along with Marian’s two wonderful dogs (Gedi and Heidi) had planned to go over to the foothills near Boulder for a hike up into the mountains. High wind warnings, though, along the mountains convinced us instead to head east into quieter landscapes. That was a fortunate decision because the Marshall Fire over near Boulder flared up late that morning. It was driven by hurricane-force winds and ended up almost instantly destroying 6000 acres of dry grass and shrublands and nearly 1000 homes! We would have been caught in the smoke and confusion of the fire if we had kept to our original plans.
Photo by M. Hamilton
We arrived at the buttes just before noon after a long drive on a series of narrow, gravel, wash-board- surfaced secondary roads. There was no wind out here in the grasslands when we arrived, so we walked in the amazing quiet of the late morning. The wind then started up as a gentle breeze from the southwest and slowly intensified over the early afternoon until it became a pretty substantial opposing force!
The low, crumbling buttes of sandstone were each ringed by gravel skirts of eroding scree. A fenced area at start of trail was well layered with cow pats. Some were fresh looking (and still had a detectable odor) while others were dry, gray, petrified masses. The buttes sit only in part on public land, and this grazing area was probably part of the privately owned range. Nutrients in the dung were tightly locked up! There had been no apparent dung beetle activity and no hoof trampling and spreading. Grasses in the grazed area were very scattered and low growing.
The grasses were taller and thicker down in the low concavities of landscape and also on the north sides of any of the gentle rises. Apparently, any relief from the intense sun delivered a significant water advantage to the growing plants.
Photo by M. Hamilton
The only green in the landscape were the scattered yuccas. We knew from previous visits to the grasslands that opuntia (“prickly pear” cacti) were abundant here, but they were not terribly obvious when we looked across the brown, grassy plain. Looking closer, though, we found the cacti flattened and desiccated, laying close to the ground. This was like the opuntia that were growing in our front yard! In the winter, the plant sends most of the water from its above ground, prickly paddles down into its roots. This helps to protect its delicate tissues from damage by ice crystals. Also the paddles, which are normally a bright green (due to chlorophyll) turn a light purple in the winter (a striking contrast with the white of the covering snow!).
Photo by D. Sillman
The changed color of the winter cactus paddles is due to a similar process seen in the color changes observed in deciduous tree leaves in the fall. With falling temperatures the cactus, like the deciduous trees, stops making chlorophyll and the subsequent decline in green pigments in its tissues reveals some of the accessory (or “secondary”) pigments in its cells. In deciduous tree leaves these pigments include the carotenoids and xanthophylls that have been assisting the functioning of chlorophyll all summer long by capturing additional wavelengths of light and also helping to disperse excessive incoming light energy. As these pigments are “revealed” the leaves then “turn” orange (from the carotenoids) or yellow (from the xanthophylls). Persisting chlorophylls in the tree leaves, however, continue to slowly carry out photosynthesis, and the sugars formed by this process are confined to the leaf tissues because of the leaf-abscission layer blockage. These sugars then stimulate the synthesis of anthocyanin pigments in the leaf. These pigments generate purple or bright red colors and are thought to possibly protect the leaf (and particularly next year’s delicate leaf buds) from insect damage.
Cactuses also have carotenoids that assist chlorophyll functioning, but they lack the ability to make anthocyanins. Instead, they synthesize a different class of cellular pigments called “betalains.” Betalains give cactus flowers, stems and many of their fruits their distinctive colors that range from red-violet to yellow-orange. These betalains are also powerful antioxidants that may function to protect the leaves from oxidative damage during freezing or desiccation stress events. In fact, nutrition scientists have been exploring these cactus betalains as potential sources of dietary antioxidants for humans!
The prickly pears, though, looked very sad all flattened down in their piles!
Photo by M. Hamilton
Outside the more distant fence the grasses were thicker and taller. This outer area must not grazed by cattle.
Shades of brown are the colors everywhere you look. The wind moves the grass stems and seed heads around in a very fluid, almost ocean-like way.
At first the only sound I hear while I am walking is the crunch of my footfalls on the gravel of the path. The great space of the plains is quiet and very still. Slowly, though, as the wind rises, its sound begins to cover-up much of the walking crunches. A hard wind possibly would easily erase all of the other sounds of this place. The last time we were out on the grasslands the wind was howling and dominated all of our senses. Out away from the shelter of our car we felt like we could not escape the roar and push of the wind. It had been a relief to get back into the car and close the door and return to a state of peace and quiet. There are stories of early settlers out here on the prairie being driven mad by the howling wind and also the immensity of the space and sky!
Photo by D. Sillman
There are a few groups of people strung out ahead on the trail and a few others who pass by on their way back to the parking lot. I cannot hear their voices until they are within a few feet of me. We nod but keep our appropriate social distances as we pass each other.
To the north (up on the rise of Wyoming plains?), there is a long line (miles and miles, I think!) of wind turbines slowly turning in the now steady breeze. I can see some of the turbines very clearly while
Photo by D. Sillman
others are tucked down below the much closer buttes only showing the tips of their turning blades. The flicking of the tops of the white turbine blades up over the sharp brown crests of buttes is very surreal. They look like whack-a-mole game pieces popping up and then retreating to their hidden safe spots.
Closer by, up on the rise off to the west, is an old fashioned windmill that probably once was used to pump water. Its sunflower-like array of blades must be locked in place because it isn’t turning even though the wind is blowing even harder. There is no relic stock tank standing nearby either to receive any trickle of pumped water.
Photo by M. Hamilton
Buffalo and blue grama grasses spread out across the rolling plain. I recognize them from the grasses in my front yard prairie back at home. Tall, golden bunches of grass stems (possibly little blue stem?) and spreads of scattered, fluffy topped grasses (maybe western wheat grass?) fill in around the very low growing buffalo and blue grama grasses. Plants with reddish brown stems (possibly thread-leaf sage?) grow thickly in some low spots. Sagebrush and rabbitbrush fill in the empty spaces and give the grass surfaces some vertical relief.
There is deep history here in the buttes. There is an unfathomably long geological history punctuated with fossils, there is a human history highlighted by artifacts, and there is an ecological history with remnants of long-past animals, abundances and then losses. There is a loneliness to this space. I love it here but don’t feel all that comfortable or, really, quite at home!