Wednesday, July 31, 2013

Grassland, Forest and an Igneous Intrusion

Devils Tower -- a puzzling igneous intrusion in northeast Wyoming.
Recently I spent a day wandering around Devils Tower looking for just the right shot.  It had to feature mixed-grass prairie and ponderosa pine forest, with Devils Tower in the background.
Replacement needed, in full color (from Knight 1994).
I was sent on this mission by Dennis Knight, Professor Emeritus of Botany at the University of Wyoming, who is revising his 1994 book, Mountains and Plains -- the Ecology of Wyoming Landscapes.  How fitting! ... it was an encounter with Dennis at Devils Tower that led to my enrollment as a Botany grad student at the University.  He was in the audience at an evening campfire program, and afterwards explained he had enjoyed my discussion of vegetation and bedrock.  The rest is history, as they say, and now 35 years later we’re back on the same topic.

At Devils Tower National Monument, ponderosa pine forest is best developed on sandstone ridges, and on buried talus around the base of the Tower.  With deeper roots, pines can tap into accumulated water among buried rocks or in fractures.
Grassland - pine forest mosaics are common in the northwest Black Hills.
Shallow-rooted grasses do well on fine soils that hold water close to the surface, for example soils derived from Permo-Triassic Spearfish red beds and shaley members of the Jurassic Sundance Formation.

What a nice orderly arrangement -- grasslands on fine soils, pines on buried talus and sandstone ridges.  Our pattern-seeking minds love such things!
From ArcGIS online; click on photo for details.
The Black Hills were named for the ponderosa pine forests that make them appear dark against the surrounding plains (below, from Google Earth).  Elevations are lower than in other mountain ranges in the region, and the relatively fast-growing ponderosas are the basis for a timber industry that costs tax-payers little or nothing (usually we subsidize timber harvest on public lands in our region).
But the Black Hills are not entirely forested.  Mixed-grass prairies are common.  These are indeed mixes -- of tall-grass prairie and short-grass prairie species.
From Regional Trends of Biological Resources  (USGS); mixed-grass prairie zone in purple.
The photo above currently is the front-runner for Dennis’s book.  The grassland on Joyner Ridge is an excellent example of mixed-grass prairie, with the following grasses common:
tall-grass species:  big bluestem, green needlegrass, needle-and-thread
short-grass species:  blue grama, buffalo grass
I especially like this shot because of the yellow coneflowers (Ratibida columnifera) in the foreground -- they’re classic prairie wildflowers.  But it will be Yale University Press that decides if the photo is appropriate.
Hiking trails at Devils Tower (NPS); click on photo to enlarge.
You can tour the vegetation of Devils Tower National Monument via hiking trails.  The Tower Trail goes through pine forest on buried and partially-buried talus.  There’s also an interesting rock to check out along the way.
Needle-and-thread grass on Joyner Ridge.
The Joyner Ridge Trail passes through mixed-grass prairie with great views of the Tower, and draws with oak, ash, chokecherry, wild plum and other hardwoods.  A connector trail hooks up with the Red Beds Trail through badlands of the Spearfish Formation.  From there, you can head down to Dog Town, where prairie dogs serenade you as you wander through the amazing diversity of plants they manage, both native and non-native.
Dog Town -- a highly-managed landscape.
Two of the managers.
I used to live within view of Devils Tower, and have spent a lot of time there in the years since.  Even so, I think it will always look improbable and mysterious to me.


  1. Great post Hollis. I loved the photos as well. I have got to visit and hike some of those trails.

    1. thanks, Matt -- I hope you make it there sometime!