DEHESA BEARGRASS

Dehesa Beargrass (Nolina interrata) is a rare and endangered plant (California Rare Plant Rank: 1B.1) found in southern California and northern Baja. San Diego County is home to roughly 10 occurrences of Dehesa Beargrass, with a large percentage of those occurrences being on or around McGinty Mountain. [The common name ‘Dehesa Beargrass’ is actually named after the small town of Dehesa within the Dehesa Valley, which is just north of McGinty Mountain]. Like a few other rare California plants that take refuge in San Diego County, this plant straddles a geopolitical border between Mexico and the United States.

Dehesa Beargrass is a scraggly and unassumingly beautiful plant. The foliage carries a bright grey-green shine to it that pops against the muted olive tone of its chaparral habitat. On occasion, the grey-green foliage is complimented by a stalk of cream colored inflorescence (something I always seem to miss). The plant does not bloom every year and primarily reproduces asexually from an underground stem, which aids its ability to survive wildfires. In the garden, the plant has both the bold texture and the striking flower thing going on. In its habitat, the plant is perfectly accentuated by the dark tones of its stony, rocky soil, which in itself is an interesting geologic story.

McGinty Mountain itself is a rather nondescript mountain in eastern San Diego County. It lies near the towns of Jamacha and Jamul just southeast of the city of El Cajon. The mountain is somewhat of a “green island” amidst San Diego County’s sprawling east county developments. Estate residential lots wrap around the mountain’s lower slopes and while McGinty Mountain remains a semi-popular hiking spot, most east county residents largely experience it as an olive green backdrop spanning their viewshed to the east. It’s probably safe to say that most residents of San Diego County do not know that McGinty Mountain is home to the rare Dehesa Beargrass.

The geology of McGinty Mountain is composed of Cuyamaca Gabbro, which is a type of mafic rock that originated deep within a segment of oceanic crust. (That segment of oceanic crust is also know as an ophiolitic suite). Generally speaking, gabbro forms when magma from within the Earth's mantle rises to the surface along a mid-ocean ridge. As that magma rises, it oozes through layers of the Earth's crust and then slowly cools into a dense layer within the lower portions of the ophiolitic suite. The "volcanic" or magmatic origins of gabbro means it forms much like basalt and then cools more like granite, where a slow cooling process allows for the formation of large visible crystals and a dense structure. 

Aerial View (left) and Geologic Overlay (right) of McGinty Mountain. Note the differing North orientation of the two images.

So how does a rock type with oceanic origins end up at McGinty Mountain? It's a long story of plate tectonics.

Long ago, as the Farallon Plate was moving eastward and subducting beneath the North American plate, a segment of ophiolitic suite formed along the mid-ocean ridge of the Pacific Ocean. As the Farallon Plate was moving eastward, so too was that segment of ophiolitic suite. As that segment moved eastward it eventually crashed into the western edge of the North American plate. Over time, the ophiolitic suites (and other oceanic sedimentary layers) piled up to form what is known as an accreted terrane. Ongoing movements of both the Farallon and North American plates placed compressive forces along the margin of the plates and pushed the land upward in elevation. As a result of ongoing compression and a continual rise of the accredited terrane, California was eventually born and with it came McGinty Mountain...so the story goes in what is a very understated and generalized portrayal of events.   

Erosion eventually exposed the gabbro at McGinty Mountain and weathering eventually created the gabbro derived soils that occur there. 

Gabbro soils (the plant is actually Hesperoyucca whipplei (Chaparral Yucca) and not a Nolina)

Weathered gabbro

Gabbro soils are rich in iron, magnesium and calcium. Some plants cannot tolerate that chemistry and, as a result, gabbro soils can become a restrictive edaphic feature for other plant species growing in that area. This is opportunistic for Dehessa Beargrass, who seems to tolerate it just fine. As the occurrences on McGinty Mountain would suggest, Dehessa Beargrass can tolerate gabbro. Whether or not that remains an advantageous trait is unknown (to my knowledge anyway). But its alliance for gabbro, a relatively rare soil type, does help explain Dehessa Beargrass's rare status - rare soils can often support rare plants.  

When it comes to rare plants, the ability to survive unique edaphic conditions is often a virtue. And quite often those virtues lead to interesting stories - stories that unfold into a wider portrayal of evolution, survival and adaptation. Stories that bring context to the perceived mundane. 

Nolina interrata (Dehesa Beargrass) - Post flowering

Nolina interrata (Dehesa Beargrass) at Regional Parks Botanic Garden (Tilden Botanic Garden) - Still in post flowering form

For a seemingly non-descript mountain in eastern San Diego County, the story of McGinty Mountain also becomes a story of a rare plant. It's a story of how rocks that formed deep within the Pacific Ocean over millions of years ago came to subsequently travel thousands of miles to eventually form eastern San Diego County. And it's a story of a rare plant who may very well have preserved its existence through an adaptive tolerance to elevated levels of iron, magnesium and calcium, in addition to its adaptive reproductive traits in response to fire. And now it's also a story of a rare plant who has clung to existence amidst the sprawl of residential development and estate lots. 

Oh, and by the way, McGinty Mountain is also home to another rare plant: Tetracoccus dioicus - also a beautiful and interesting plant that seems to prefer gabbro soils. So how about that for a seemingly non-descript mountain. 

Additional Resources

Accretionary Terrane: https://youtu.be/hJEa-aTZUJQ

Plate Tectonics in a Nutshell (Tanya Atwater):   https://youtu.be/IDTBY5WDELg

Map showing occurances of Dehesa Beargrass:   https://calscape.org/loc-California//Dehesa%20Beargrass%20(Nolina%20interrata)

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