Monday, September 28, 2015

Botanists in Paradise—a letter to the Earth

Last week I received a long hand-written letter.  After reading the first few paragraphs, I found myself in a state of shock!  It had been written by Aven Nelson, first botany professor at the University of Wyoming; his wife, Celia Alice; and Leslie Goodding, one of the early botany undergrads.  But this is not the first Letter to the Earth from Professor Nelson.  Several years ago, he left one on my truck—about the Laramie columbine.

I’ve transcribed the letter verbatim.  Bracketed comments are mine.

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September 23, 2015
34985 North Pearly Gates East
Elysian Fields, Paradise

Dr. Hollis Marriott
Associate, Rocky Mountain Herbarium
Department of Botany, University of Wyoming
Laramie, Wyoming

Dear Dr. Marriott:  [not sure why they think I have a PhD]

We were most happy to hear of the upcoming open house for the Rocky Mountain Herbarium.  Professor Nelson received the flyer by email, with the help of a recent arrival.  We have such fond memories of the momentous project that laid the foundation of the herbarium.  We are writing to describe that trip and its lasting impact: the Rocky Mountain Herbarium, Professor Nelson’s greatest legacy.  Perhaps you will find occasion to share these stories and photos with attendees at the open house.

In this account, we occasionally refer to an individual writer for clarity.

Leslie Goodding:  In the fall of 1898, extraordinary news spread across campus: Professor Nelson would be going to Yellowstone Park the next summer!  This would be a botanical expedition of vast importance—three or four months collecting plants.  He had announced that he would take along two student assistants.  Of course many students were anxious to accompany Professor Nelson on that expedition, and were willing to work for nothing just to see the Park.  I was only 19, and just finishing high school where I took a botany class from Professor Nelson.  Apparently I made a good impression, for in spite of my inexperience, he hired me as a student assistant—at $10 per month and all expenses paid.  I couldn’t believe my good fortune!
Geyser-gazing in 1899.
Yellowstone—a wonderland of geysers, hot springs, waterfalls and wildlife—was America’s first National Park, designated in 1872.  We Wyoming citizens were proud to have the Park in our state but … most of us could only dream of visiting.  The cost of a commercial stage tour was prohibitive.  Thus it was quite extraordinary that a university professor—annual salary $1800—would be traveling in Yellowstone for 14 weeks with his family and two student assistants!

Professor Nelson's first graduate student, Elias Nelson (no relation), was the other assistant.  On June 13, 1899, he and Mr. Goodding rode a Union Pacific freight train to Monida, Montana—western gateway to the Park—with equipment and supplies.  The local citizens were puzzled when the boys unloaded a wagon, three horses, harnesses and saddle; canvas tent and bedding; cook stove, table with detachable legs, benches, chairs, and crates of provisions; and thousands of sheets of heavy felt paper.

Mrs. Nelson:  Professor Nelson, our daughters, and I arrived two days later.  We were all eager to begin the trip!  I began my diary entries, and encouraged Neva [older daughter] to do the same.  The girls made a strong impression on young Mr. Goodding.  He came from a poor family in the tiny town of Granite east of Laramie, and his English was hardly perfect.

Leslie Goodding:  I remember that day!  When they arrived, I realized I had acquired two English teachers, a young lady thirteen years of age and a tiny tot of eight.  Professor Nelson and his wife tolerated no sloppy English in their daughters.  Naturally it hurt like the blazes to have my speech corrected by two little girls but I swallowed my pill.

We left Monida on June 19, in our light lumber wagon.  In bad weather, we stretched a canvas bonnet over the wooden hoops, but more often we traveled coverless, enjoying the scenery.  The driver and a second man sat on state-of-the-art spring seats.  The third rode the small black saddle horse named Grace.  Mrs. Nelson and the girls sat on the passenger seat behind the driver.
Mr. Goodding at the reins.  Nelson ladies sit behind.
Professor Nelson:  We were six in all, and not a shirker in the lot.  We carried a brand new canvas tent, 12 x 14 feet in size, with a stout ridgepole and a reinforced hole for the stove chimney.  For twelve consecutive weeks, no one slept under a roof other than the tent, and the two boys usually under the vaulted star-studded skies.
We all wore felt campaign hats.
The sheet iron woodstove could be used inside the tent for cooking as well as warmth.  But when the weather was fine, we cooked and ate outside.  Can you imagine?  It was absolutely wonderful to dine in flower-filled meadows with snowy peaks in the distance!
The box-like object by Neva Nelson's feet is a plant press.
We carried enough food for the entire trip, but fortunately did not have to subsist entirely on rations.  The streams and lakes teemed with fish so large that they broke the only line we had with us.  Most evenings the men fished, but they caught nothing … until Elias developed the technique of throwing his plant-digging chisel though a big fish as it moved upstream.  On July 2, he caught 23 fish!
Most days we broke camp early.  We traveled Park roads, stopping at promising sites where there might be plant species we hadn’t yet found.  The men went out to collect, each with a vasculum across his shoulder [for carrying collected plants; today we mostly use plastic bags].  Smaller plants were taken in their entirety, including roots.  For larger species, we selected representative parts—a section of the stem with leaves, a good number of flowers, and fruit if available.
Professor Nelson's vasculum and several of our Yellowstone field books.
We collected many many duplicates—30,000 in all.  [This number is unbelievable! ... but true.]  These were sent to herbaria around the world, in exchange for specimens to add to the University of Wyoming herbarium.  Some sets were sold to institutions and private collectors (including one in India!), to raise money for summer field work and a student assistant during the school year.  At that time, the University provided no financial support for the herbarium.

Most days we travelled and collected until late afternoon, and then looked for a camping site with water, firewood, a flat spot for the tent, and grass for the horses.  Plant pressing commenced as soon as the tent was pitched and materials unloaded, often continuing into evening and sometimes the next morning.

We know you are a field botanist, Dr. Marriott, but it’s unlikely that you are familiar with field methods of our time, so we shall explain.  To preserve plants, we pressed and dried them in the field, as you modern botanists do on extended trips, but of course we had no electricity, refrigeration, nor inside facilities.  After removing a collected plant from the vasculum, we cleaned it of any dirt, and carefully arranged it between sheets of thin paper.  It was added to a growing stack of specimens, alternating with “blotters”or "driers"—12 x 17 inch sheets of heavy felt paper used to absorb moisture.  Stacks of pressed plants were tightly bound between wooden covers.  [Today we still press plants, but generally we use corrugated cardboard and driers of some kind; see this post.]

The next day we took the presses apart.  Damp blotters were replaced with dry ones, and the presses reassembled.  We continued this way until the plants were dry … in addition to pressing daily collections.
Professor Nelson checks drying plants (near the end of the expedition, hence the whiskers).
We dried damp blotters by spreading them carefully on the ground in the sun.  But this got us into trouble on our first day in the Park!  A soldier appeared and was appalled to see so many papers scattered about.  He demanded they be picked up at once.  Then he found our rifles.  After sealing them, he sent us to Mammoth to meet with Captain Brown, an extra 46 miles—two days of travel.  And Professor Nelson had already obtained permission from the Army in January!  [In 1899, the US Army was in charge of the Park; there was no National Park Service until 1916.]

Though we carried several thousand reusable blotters, this wasn’t enough when it rained for days at a time.  Then we set up the tent, gathered wood, and kept a fire going all day to dry plant presses and blotters carefully arranged around the stove.  You can see for yourself that we kept working during rainy weather—if you look closely at our Yellowstone specimens, you will sometimes find bits of felt blotter paper stuck to the plants.

Hydrothermal features are the Park's greatest attractions!  We visited many. [Nelson ladies by trees lower left.]
At the Spone with retired superintendent George Henderson, such a wonderful guide!
Mrs. Nelson:  One major mishap befell us.  Perhaps we were fortunate there was only one given the wildness of the Park in those days, but I so wish it hadn’t happened!  On July 26, Elias and Leslie were collecting near the popular “Artist Paint Pots”—
“They consist of numerous openings in the highly colored clay, and are intensely curious, their brilliant coloring and fantastic shapes being the admiration of all.  But visitors should avoid leaving the regular paths, as the treacherous character of this formation renders it quite unsafe.” (1894 Yellowstone Park Guide, A.B. Guptill)
Indeed, when Elias stepped off the path a bare two feet, his left leg sank into hot mud.  He jumped to higher ground, and pulled off his shoe and sock along with a large patch of skin from his ankle.  A huge blister ran up his leg.  Leslie raced back to camp, saddled Grace, and returned to Elias, who rode to camp at a gallop.  With the help of several nearby tourists, I sprinkled the wound with soda, bandaged it, and covered the bandage with flour.  Elias was in great pain, but never uttered one groan.

I redressed the burn morning and evening.  At Upper Geyser Basin we met a Dr. Irish, who examined it and found it serious.  Elias must go to the hospital at Fountain, or return home.  We drove to Madison and put Elias on the stage back to Monida, all broken up over leaving.
Park roads were well-constructed, but many turned to mud when it rained.
This is the curious Golden Gate.  [Click on image to view sign and approaching wagon.]
We had to spend several dreary days at Yellowstone Lake, unable to travel due muddy roads.
By mid August, it was obvious that the season was ending.  We were finding fewer plants to collect.  The weather was deteriorating, and the roads turned to mud.  Once we had to completely unload the wagon to get it unstuck.  On August 19, it snowed!  But we continued to travel during the spells of good weather, collecting occasionally, taking photographs. Finally, in early September, we drove back to Monida to catch the train to Laramie.

Leslie Goodding:  I could see the Nelsons were ready to go home.  They collected very little, and Mrs. Nelson was busy with laundry in preparation for the train trip.  I suppose I was ready too … after all, I had worn the soles off my only pair of boots!  But to be honest, I was still just as excited as the day I found out I was going to Yellowstone Park ... the spark was still in my eyes:
It was just as well the expedition was almost over—the soles were gone from my boots!

Dr. Marriott, we hope that you find field botany as exciting and satisfying as we did.  We greatly appreciate the contributions you and many others continue to make to the Rocky Mountain Herbarium.  We sincerely hope that there’s a bright and bountiful future ahead for that great institution.

Dr. Aven Nelson
Mrs. Celia Alice Nelson
Mr. Leslie N. Goodding

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Now home to one million specimens, the Rocky Mountain Herbarium is the largest collection of Rocky Mountain plants and a world-renown institution.  We are celebrating with an open house on Thursday, October 1, 4-6 pm.  Rumor has it that Mrs. Nelson and Mr. Goodding will return to Earth to share their stories of Yellowstone.  We hope you can come!


Photos from the American Heritage Center, with the exception of the vasculum.

Aven Nelson Papers, 1870-1983.  American Heritage Center, University of Wyoming, Laramie.  Guide.

Goodding, LN.  1944.  The 1899 botanical expedition into Yellowstone Park.  University of Wyoming Publication 11:9-12.

Goodding, LN.  1958.  Autobiography of the Desert Mouse.  San Pedro Valley News; Thursday, July 10.

Nelson, A. No date [1930s]. The Rocky Mountain Herbarium, in Aven Nelson papers, American Heritage Center, University of Wyoming, Laramie. Guide.

Williams, RL.  1984.  Aven Nelson of Wyoming.  Colorado Associated University Press.

Friday, September 25, 2015

Fall Colors: Pink & Green

Salsola kali, one of many synonyms (experts are still debating).

They’re among the most notorious characters of the western US … have been since the 1870s when they entered illegally.  Locals will tell you:  they take over pastures, fuel wildfires, clog fences, and either poison livestock or starve them to death by driving out palatable plants.  These scoundrels are Russian thistles—the iconic tumbleweeds of the American West.

I'll keep rolling along
Deep in my heart there's a song
Here on the range I belong
Drifting along with the tumbling tumbleweeds

(Tumbling Tumbleweeds by Bob Nolan)

The first Russian thistles arrived in the US in a batch of flax seed carried by Russian immigrants to South Dakota.  They spread quickly, and now are abundant in drier parts of the American West.  They're also common along the Atlantic and Gulf coasts, and anywhere with regular disturbance—roadsides, ag fields, overgrazed pastures and waste areas.  Throughout its North American range, Russia thistle is designated noxious:  “a weed designated by an agricultural authority as one that is injurious to agricultural or horticultural crops, natural habitats or ecosystems, or humans or livestock” (source).

But this is too harsh.  Immature Russian thistles are palatable to livestock, and show promise as hay crops in semi-arid regions.  During the Dust Bowl—the great droughts of the 1930s—ranchers fed tumbleweeds to their cattle, thereby saving the industry.  Deer and elk will eat small amounts of Russian thistle, until it becomes too prickly.  Pronghorn antelope feast on it, and it’s an important food for prairie dogs.  Small mammals and birds eat the protein-rich seeds.  Dead tumbleweeds serve as nursery plants for seedlings of other species, providing shade and protection.  Ironically, these same species may well out-compete Russian thistle, eradicating it from the site.
Some Russian thistles near my house are now bright red and prickly.
Others are still green, with some linear leaves of youth.

The widespread success of Russian thistle is intriguing, for it’s a very poor competitor.  A Russian thistle seed contains no endosperm—no nutritious tissue to sustain the embryonic plant until it can photosynthesize on its own.  Seeds that don’t germinate within a year die, so there’s no Russian thistle seed bank.  A seed won’t germinate in compacted soil, or if it’s buried more than 5 cm (2.5 in).  If a seedling is shaded, it will die.  Russian thistle can’t grow in soils with mycorrhizal fungi—normally beneficial partners of plants.  These fungi invade and kill them root by root.

But chances are that at least one of the 250,000 seeds scattered by a single tumbleweed (!) will land on a favorable spot—unshaded loose soil that's poor in mycorrhizal fungi, and with few competitors.  In other words, Russian thistles require recently disturbed habitat, and we have plenty of that.

The seeds are programmed to refrain from germination until spring, but then … they go crazy.  A seed needs only a little moisture to germinate, and can tolerate  “virtually any seedbed temperature.”  Germination is fast—in as little as 15 minutes!  Each seed contains a fully-formed coiled embryo, complete with a care package of chlorophyll (remember, there’s no endosperm).  The little plant unwinds, driving a tiny root into the ground, and starts growing.
Russian thistle seedling.  From UC Davis.
Young Russian thistles have soft linear leaves, and seedlings look a bit like pine seedlings.  Initially, plants grow and branch into a roughly conical shape.  Later the branches grow longer and curve up, creating rounded forms ready to roll.  The largest are five feet in diameter.  Leaves on mature branches are short and stiff, with a sharp spine at the tip.  This is why livestock and wildlife don’t eat mature Russian thistles unless they’re starving, and why we wear sturdy gloves to remove them.
Flowers and spine-tipped leaves (bracts).
Russian thistle flower is about 5 mm across; from Britton and Brown (1913).
The inconspicuous petal-less flowers in the leaf axils are beautiful up close, with their translucent veined sepals and Victorian colors.
Striped stems are characteristic.  In younger plants, they're red and green.

At the end of the season, when Russian thistle plants are fully dry, they break off at the base.  Now they become tumbleweeds, scattering seeds as they roll before the wind, and finally coming to rest in fences, in trees and under cars, as well as in some rather surprising places.
Tumbleweeds in Kansas.  Source.
Photo by PD Tillman.

NOTE:  Other plants besides Russian thistle become tumbleweeds—like tumble mustard and kochia weed in our neighborhood.

Sources (in addition to links in post)

University of California at Davis Pest Management Program

University of Utah.  The Great Basin and invasive weeds.

USDA Forest Service Fire Effects Database.

Wednesday, September 16, 2015

Drama at the Canyon Mouth

Clarks Fork Canyon and southeast Beartooth front; photo courtesy Tim Schoessler.

In northwest Wyoming, roughly 30 miles northwest of Powell, the Clarks Fork of the Yellowstone River leaves the Beartooth Mountains with great dramaby way of a canyon mouth so geologically spectacular that it’s included in field trips for everyone from high school teachers to professionals.  Unfortunately, the rocks are a bit hard to explain.  But the scenery’s great, even on cloudy days.
Looking downstream at south side of canyon mouth and basin beyond.
North side of canyon mouth.
Steeply-tilted Paleozoic sedimentary rocks rise abruptly from the lowlands, and are in contact with Precambrian rocks to the west.  Between them is a 2-billion-year gap in the rock record, a Great Unconformity.
Arrow points to Great Unconformity between brownish Cambrian Flathead sandstone (500 million years old) and Archean metamorphic rocks (2.7 billion years) on north side of canyon.  See the magpie? (lower right)

Most mountain ranges in Wyoming were uplifted roughly 70-40 million years ago during the Laramide orogeny, the event that created the Rocky Mountains.  They have a distinctive style:  broad blocks of crust shoved up along reverse faults over younger rocks.
Laramide-style uplift; from Regional Geomorphology, University of Wyoming, 1984.
This diagram shows a typical Laramide range.  On the east, Precambrian (ancient) igneous and metamorphic rocks (brown) have been shoved up and over younger sedimentary rocks along high-angle reverse faults; Precambrian rocks make up the east flank and crest.  On the opposite side, the rocks were folded rather than faulted, and there are outcrops of tilted sedimentary strata (in this case eroded to form hogbacks and strike valleys).

At the canyon mouth, the Clarks Fork cuts through steeply-tilted sedimentary strata.  Here the Beartooth front is steep and abrupt, and decorated with contorted sculpted rock.
Can you imagine rocks being folded like this?
In less than a quarter mile, the tilted strata start to flatten out.
Immediately west, the strata are nearly horizontal—note limestone bed on skyline.  Further west sedimentary strata are gone, having been eroded off the crest of the range.

When I first saw the dramatic exposures of eroded sedimentary rocks on the Beartooth front, I presumed I was looking at a steeply-tilted, fault-free flank.  Wrong.  Here the Beartooth uplift was shoved east over younger rocks, via high-angle reverse faults (like the east side of the "typical" Laramide uplift above).  So what’s with the sedimentary strata?
Cross-section through Beartooth front in vicinity of Clarks Fork Canyon (unlabeled diagram from Lageson & Spearing 1991, no source given).
The story is complicated and controversial.  Popular geology guides wisely keep explanations short.  In his great guide to geology east of Yellowstone Park, Bob Carson quickly passes by deformed sedimentary strata, and focuses on Precambrian rocks, the Great Unconformity and glacial features.  In Roadside Geology of Wyoming, Lageson and Spearing include a cross-section showing the Beartooth reverse fault cutting through Precambrian and sedimentary rocks (basis for diagram above).  Sedimentary strata are draped over faulted Precambrian rock.  But discussion is brief:  “Beds of Paleozoic limestone were folded over the edge of the Beartooth uplift, then eroded”
Precambrian metamorphic rocks dominate the Clarks Fork Canyon and Beartooth Plateau; view southwest from inside canyon mouth, at end of paved road.
I defer to experts for further discussion (see Sources below).  Here’s some of what they’re thinking:
In the “spectacular fold at the mouth of the canyon” sedimentary strata are draped over a faulted basement (Precambrian) block.  This structure is slightly older than the Beartooth uplift and therefore was deformed by those faults (various authors; summarized in Heasler et al. 1996; PDF).
Laramide models used to incorporate vertical uplift.  But with seismic and drill-hole data, it has fallen out of favor, replaced by uplift via high-angle reverse faults, accommodating crustal shortening.  However in situations where faults are very steep (e.g. Beartooth fault at Clarks Fork Canyon), the vertical uplift model may be valid, and sedimentary strata may well be draped over faulted basement rock (Brown 1995). 
O’Connell (1996) argued for multiple stages of faulting, and invoked zones of preexisting weakness in basement rocks.  He argued against drape-folding. 
The southeast Beartooth front is in a complex transition zone involving the western Bighorn Basin, with its own faults and Laramide history.  Thus the front has been affected by multiple episodes of deformation (Neely 2006; PDF).
North side of canyon mouth (labeled based on Heasler et al. 1996).
Looking downstream at south side of canyon mouth; arrow points to rotated Bighorn dolomite (based Heasler et al. 1996, Wise 1983).
“We gather at the base of the crag to study the rocks.  The instructor assigns them to type and age, and proclaims the cause of their deformation—all the while waving his arms in the air while we frantically write everything down in our field books.  Later, we read that others think differently.”  (anonymous geology student ca. 1985)

Sources (in addition to links in post)

Brown, WG.  1995.  Structural style of Laramide basement-cored uplifts and associated folds in Snoke, AW, Steidtmann, JR, and Roberts, SM, eds.  Geology of Wyoming.  Wyoming State Geological Survey Memoir 5:312-371.

Carson, RJ.  2010.  East of Yellowstone; geology of Clark’s Fork Valley and the nearby Beartooth and Absaroka Mountains.  Sandpoint, ID: Keokee Books.

Heasler, HP, Jaworowski, C, Jones, RW, De Bruin, RH, and Ver Ploeg, AJ.  1996.  A self-guided geologic tour of the Chief Joseph Scenic Highway and surrounding area, northwestern Wyoming.  Laramie, WY:  Wyoming State Geological Survey Public Information Circular No. 35.  PDF

Lageson, DR and Spearing, DR.  1991.  Roadside geology of Wyoming, rev. 2nd ed.  Missoula: Mountain Press Publishing.

Neely, TG.  2006.  Three-dimensional strain at foreland arch transitions: structural modeling of the southern Beartooth arch transition zone, northwest Wyoming.  MS thesis, Colorado State University, Fort Collins.  PDF

O’Connell, PJ.  1996.  Kinematics of the eastern flank of the Beartooth Mountains, Montana and Wyoming, in Wyoming Geological Society 47th Annual field Conference Guidebook.

Wise, DU.  1983.  Overprinting of Laramide structural gains in the Clarks Fork Canyon area and eastern Beartooth Mountains of Wyoming, in Wyoming Geological Society 34th Annual field Conference Guidebook.

Tuesday, September 8, 2015

Plants on Rock

Not much happening with my willow (peeking out from behind rock)—just kicking back and hanging out with juniper pals in the ‘hood.

It’s September 7 as I write, therefore I must be preparing a report on my willow for this month’s gathering of tree-followers, kindly hosted by Lucy Corrander.  But there's not much change since August 7, so I’m reporting on its neighbors instead.

The small canyon where the willow grows has sheer limestone walls 5 to 10 meters high.  They're steep but not featureless—thin crevices and small ledges are common.  And plants, being the resolute creatures they are, grow on these improbable sites in forms are striking, beautiful, even spectacular (at their scale).
Limestone rock garden.
Some plants don’t need much—just a tiny cavity, a bit of dirt or debris, a little water on rare occasions.  Growth is slow, but that’s ok when there’s little competition.
Fleabane above (Erigeron sp.); bluebells below (Mertensia sp.).  These photos were taken back in late May.

The green blob on the rock face below is thick-leaf clover, Trifolium dasyphyllum:
Thick-leaf clover was blooming in late May.

I love the tough little trees with their bonsai forms.  They’ve grown to fill their crevices, and probably are breaking down the rock a bit, enlarging the cracks.
Limber pine (Pinus flexilis), with an anonymous friend.  "Trunk" is about 2 cm in diameter.
Rocky Mountain juniper (Juniperus scopulorum).
This limber pine grew out of a crack and then straight up toward the sun:

What is this next limber pine going to do? … grow out around the overhang?  It appears to be headed that way:

My willow grows out of a crack in the rock too.  It has done quite well, with multiple stems and a canopy that extends above the rock wall (the willow is roughly 3 meters tall).
American pussy willow (Salix discolor) in February.

But my favorite plants of the canyon walls are the ferns.  Yes, ferns!  Wait … ferns are supposed grow in humid places—shady forests, lush stream banks, mossy slopes.  But here they are, growing just fine on bare dry rock that gets many hours of sunshine each summer day.

Supposedly ferns are not as good as flowering plants at controlling water loss from leaves, and have less efficient water transport systems.  And of course a moist environment is critical for fern gametophytes [Click link to see a cool interactive flow chart about the fern life cycle.]  This is why flowering plants took over the Earth about 120 million years ago; they're much better adapted than ferns, especially for dry habitats.  But Mother Nature rarely fulfills our over-simplifications.  There actually are quite a few xerophytic ferns (xerophytic = dry plant).  And they’ve kept their adaptations mostly secret.  Botanists are still trying to figure out how these "poorly-adapted" plants do so well.  See Heitz’s 2010 discussion of the “surprising drought tolerance of ferns” (page 140 via Google Books).
Slender lip fern (Cheilanthes feei) in late May, looking rather dormant.
Now the lip ferns look healthy and happy in their spartan accommodations.

In a post for the American Fern Society, Hope Diamond and Lucinda Swatzell point out that microsites where xerophytic ferns grow are not without humidity:
“To us, their habitat is arid.  But look closely ... Rock crevices, particularly sedimentary rock, silt catchments, or humus mats on stone outcrops retain moisture ... And remember, these ferns are adept at extracting moisture out of thin air”

Another mystery surrounds the slender lip fern: it grows only on calcareous rocks (made of calcium carbonate), such as limestone and dolomite.  Thus it’s not just a xerophyte but also a calciphile—a lover of calcareous habitats.  Calciphilic plants are as poorly understood as xerophytic ferns, even though botanists have studied them for at least 180 years—since 1836 when Fritz Unger wrote about the distinctive flora of calcareous areas in the Alps.  This became die Kalkfrage (the limestone question); a full answer remains elusive.

Western cliff brake, Pellaea occidentalis, is another calciphilic xerophytic fern growing in Willow Canyon.
On May 29, cliff brake ferns were just starting to show new growth (green spot in center).
On September 6, they were dressed in rich summer greens.
Cliff brake ferns lined up along crevices where there's a bit of moisture now and then.
Rock pockets with ferns, finger for scale.

Back to tree-following.  At the start of the post I said there was little change compared to a month ago.  Yet something’s different.  There’s this eerie premonition, a foreboding, a feeling that summer is winding down.  The willow leaves might be more tattered, though I can't say for sure.  Or maybe the canyon is quieter—fewer birds, fewer insects.  Or the sun’s a bit lower.  In any case, I suspect next month there will be changes to report.

Consider following a tree!  Instructions here (it’s easy).