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).