Sunday, December 18, 2011

Plants and Rocks: Vegetation mosaic on Miocene marine strata ... sweet!

Western Transverse Ranges of California; from ArcGIS online, Google Maps.
“Man has mental limitations, and nature is infinitely complex.  To deal with this situation man invents classifications.  Nature does not classify trees, flowers, and rocks -- we do, so that we can deal with them in a reasonable fashion.” -- Robert P. Sharp
We vegetation ecologists are pattern-seekers and classifiers, that's our job.  Based on his remarks it would be easy to think Sharp was one of us, but he was not.  He was a geologist recognized for contributions in basin-and-range structure, glaciation, erosion surfaces, desert sand dunes, isotopes in snow and ice, and surface forms and processes on Mars.  He served as long-time head of geological sciences at Caltech, and received the National Medal of Honor in 1989.

To his credit, Sharp had a great interest in explaining geology to lay people.  His Southern California Field Guide (K/H Geology Field Guide Series, 1975) introduced me to the geology of the Transverse Ranges of southern California -- the ranges north of Santa Barbara that trend east-west in contradiction to the predominantly northwest-trending structural grain of California.

It was in the Transverse Ranges that I did my first vegetation study -- the beginnings of a career in field botany and vegetation ecology.  For a class project I backpacked across the Ranges from south to north, to describe and classify vegetation, and glean some understanding of the ecology.  I was young and naive, with great faith in vegetation classification and patterns in general.  While I had to have read Sharp’s introductory remarks, I’m sure they did not sink in.  Now, 35 years later, they seem profoundly insightful.

There is pattern in vegetation -- we sense this intuitively and acknowledge it when we talk about hardwood forests in New England, grasslands of the Great Plains or treeless alpine vegetation on mountain summits.  But at a finer scale, discrete associations of plant species and correlation with specific habitats often become fuzzy or break down completely.  The wraith we thought we saw may disappear when we look closely.
For my study I intended to traverse the three ranges north of Santa Barbara -- Santa Ynez Mountains, San Rafael Mountains and Sierra Madre -- and finish in the Cuyama Valley.  I was a dedicated naturalist and backpacking bum (John Muir was my hero and role model) with little money, crummy gear, and of course no camera.  Instead I kept a journal, and sketched some of my experiences.
The trip started via the Santa Barbara city bus system to a trailhead at the base of the Santa Ynez Mountains, which rise steeply from the narrow coastal plain.  It was in these mountains that some of the earliest studies were done suggesting that the Transverse Ranges did not always have their odd east-west orientation, that they are part of a crustal block that has rotated at least 90° clockwise since early Miocene time and continues to do so (Hornafius 1985).  Nicholson et al. (1994) found a possible mechanism in a partially-subducted microplate under the western Transverse Ranges (WTR) crustal block.  As the Pacific and North American plates converged in early Miocene time, the intervening Farallon plate fragmented into microplates which continued to be subducted under the continental margin.  But as subduction slowed and was transformed into right-lateral strike-slip (horizontal) motion, the microplate under the WTR block became attached to the Pacific plate and started moving northwest, causing the overlying block to rotate.
From Onderdonk 2007 (see literature cited at end of post).
It was a sunny day in early April, and warm walking up the south flank of the Santa Ynez Mountains.  As the trail left the drainage bottom for drier habitat above, I noted a transition from Riparian Woodland dominated by sycamores to Foothill Woodland dominated by California live oaks to the dense shrubland of Hard Chaparral.   I continued through chaparral to a campsite at what may have been an old homestead (pear and apple trees).  The next day I hiked down into the Santa Ynez Valley where I caught a ride with some hippies as far as the “Brotherhood of the Sun” commune.

Yerba Santa, Eriodictyon californicum
Courtesy of SageHeart.

By the time I got to my next campsite, mares tails were passing overhead, followed by thick dark clouds.  It rained that night “filling the air with the odor of yerba santa”, one of the many pungent plants of the chaparral.  

The next two days I hiked up the south flank of the San Rafael Mountains, the highest of the three ranges.  I continued to see the same patterns -- Riparian Woodland in drainage bottoms giving way to Foothill Woodland and then chaparral moving upslope into drier habitats.  But there were new indicators appearing with increasing frequency -- conifers.  Coulter pine and big-cone Douglas fir were the first, soon joined by white fir, sugar pine and incense cedar -- trees indicative of cooler wetter habitats at higher elevations.
California incense cedar.

Sure enough I awoke to frost, with hoarfrost needles on grass stalks and “socks washed yesterday hanging frozen on line”.  Later in the day “snowflakes started to fall” and as the cover of conifers increased going up the trail, the snowfall got heavier.  The trees were beautiful frosted with snow, especially the incense cedars.  At the crest (5800 feet above sea level) I  “bivouaced under small oaks -- rainfly with poncho as vestibule” and noted:  “This seems to be a regular blizzard -- in April?  No Fair!”

In the morning I was “awakened by a cheerful bird singing madly somewhere in the brush.”  The sun came out as I put on my frozen pants and boots and optimistically headed off along the crest enjoying great views and majestic Douglas firs and incense cedars.  I hiked down into the canyon of the Sisquoc River, reached the bottom and got my rainfly set up just before the rain and hail started.  Enjoying a dinner of cookies I wrote “April showers bring May flowers.  This is a welcome storm even though it makes backpacking a little unpleasant.”

Continuing down the Sisquoc the next day I reached the Sisquoc Falls, spectacular in this dry country -- “3 or 4 large falls in a row, the largest 50-75 feet high” -- but by then it had been raining and hailing for an hour so I kept going.  That evening I noted:  “It would be nice to have one or two dry days to end the trip.”

The next day the rain finally stopped and my mood was high going up the south flank of the Sierra Madre, the last of range of my traverse.  It would be a long but gentle hike to my campsite once I reached the crest.  I saw several single-needle pinyons and then junipers soon after -- this was the beginning of pinyon-juniper woodland, a much drier vegetation type here in the rain shadow of the San Rafael Mountains.

At the summit was a fine example of a deterministic relationship between bedrock and vegetation.  Much of the Sierra Madre crest is underlain by the Miocene Branch Canyon and Monterey Formations -- a mix of nearshore and beach sandstones, and marine shales and mudstones.  I saw that the vegetation on the crest was a mosaic of grassland and hard chaparral, clearly correlated with changes in bedrock ... sweet!
Mosaic of grassland and shrubland on Sierra Madre crest; large grassland
is Salisbury Potrero (location of drawing below).  From ArcGIS online.
Areas underlain by sandstone supported chaparral dominated by scrub oak (Quercus dumosa) and bearbrush (Garrya fremontii).  The intervening areas of Monterey shales and mudstones were covered with grassland, including huge potreros (Spanish for field or pasture but used here for large grassy openings in the chaparral).  The explanation was straight-forward.  Fine soils derived from shales retain moisture near the surface and shallow-rooted grasses thrive.  Rockier sites support shrubs that send roots down cracks in the sandstone to trapped water.
Scanning electron micrograph of clay.
Clay particles have high surface area relative
to their small size.  Water adheres easily,
giving clay its absorptive properties.
Fascinating as this was, I came hate the grasslands.  Whenever the dirt road crossed grassland it was unbelievably gooey after four days of rain and snow.  I observed:  “grassland means clay soil which means slippery -- road is impossible to walk on.”  In contrast “in areas where scrub [oak and bearbrush] is present, the soil is rockier and better drained, and one can easily walk on the road.”  I had expected an easy walk but it was a hard trudge most of the way, and my boots had four inches of adobe on the bottoms at times.   I remember well my feelings when after walking through a stretch of scrub oak I would round a bend and see grassland ahead ... bummer!  I was reminded of the words of John Wesley Powell upon entering the Grand Canyon and seeing dangerous Precambrian hard rocks ahead: “The river enters the gneiss!” ... well ... maybe not quite that bad but pretty disappointing.

Finally I arrived at “a bucolic campsite -- Salisbury Potrero -- windmill, old wooden tank, table, fireplace, wood, & me all fenced in by barbed wire sitting in a large cow pasture.” The sky cleared completely, I built a roaring fire, took off my boots with “adobe bricks on the soles” and happily dried out my gear.    
At one point there was a loud cry sounding like “a dog mixed with a rooster” but it did not continue.  Then suddenly I sensed a bright light approaching from behind.  My heart raced and I turned to see “a huge glowing orange light ~ the Moon ... full tonight.  Outstanding!”  The Cuyama Valley to the north and the sparsely-vegetated Caliente Range beyond were spectacular and ghostly in the moonlight.  Two owls hooted back and forth, and Orion and his dogs marched off below the horizon.   I stayed up most of the night feeling very fortunate to be on Earth.

Transect across Transverse Ranges showing vegetation types, 1976.  Click to view.

Literature Cited


Onderdonk, NW.  2007.  Vertical-axis rotation controlled by upper crustal stress based on force balance analysis: A case study of the western Transverse Ranges of California.  Tectonophysics 436:1-8.

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