The Foxtail Pines of California

Pines are notorious for enduring very tough conditions: nutrient deficient mineral soils, steep slopes at cold exposures, steep slopes at hot exposures, and of course that ever persistent high wind. Sometimes they even endure all of these conditions at a single site. They are amazingly resilient in that capacity.

California is home to several of the world’s more resilient pines, including some of the most beautiful and resilient of them all: the Great Basin Bristlecone Pine (Pinus longeava) and two species of Foxtail Pines (Pinus balfouriana spp. balfourianaPinus balfouriana spp. austrina). The Great Basin Bristlecone Pine grows at the upper reaches of timberline atop mountain peaks in the western portion of the Great Basin Province. Specific to California, the Great Basin Bristlecone Pine grows in the White and Panamint Mountains.  

As for the two species of Foxtail Pines, the Klamath Foxtail Pine (Pinus balfouriana spp. balfouriana) occurs in the Klamath Mountains and the Sierra Foxtail Pine (Pinus balfouriana spp. austrina) occurs in the southern Sierra Nevada. Both of these species grow where where cold and dry conditions prevail.

Pines are known for their intolerance of shade, which sometimes drives them to seek out remote and inhospitable environments. Quite often they inhabit sites drenched in intense sunlight. Their tolerance of extreme conditions has allowed them to persist through eons of change and climate variability. As extreme conditions thinned out other vegetation types, it offered opportunities for these pines to stake claim in a generally less competitive habitat. 

Genetically, the Great Basin Bristlecone Pine and both species of Foxtail Pines are like distant cousins. They are descendants from a shared common relative. The Great Basin Bristlecone Pine and both species of Foxtail Pines are 5 needle pines under the subgenus of Strobus pines, which includes many of the white pines that grow throughout western North America’s subalpine forests. Under the Strobus subgenus, the Great Basin Bristlecone Pine and Foxtail Pines are part of the Balfourianae subsection. This Balfourianae subsection also includes the Rocky Mountain Bristlecone Pine (Pinus aristata), which, as it sounds, grows in the Rocky Mountains. The alliance of these pines within the Balfourianae subsection points to their genetic proximity to a more distant but commonly shared relative.

It’s believed the Balfourianae subsection of pines evolved from a common ancestor named Pinus crossii. Pinus crossii migrated into western North America during a time (~ 100 mya) when western North America was warm and received lots of rain year round. But over time that climatic condition also fluctuated between periods of cold and dry and periods of warm and wet. Many of the pine species moved around in response to the changing climatic conditions, which ultimately affected the diversity of pine species throughout western North America. Some species would move north or south in latitude and some species inhabited different habitat niches where more favorable growing conditions prevailed. Those that inhabited different habitat niches sometimes resulted in isolated populations that lead to speciation events. It's believed the ancient relative of the Balfourianae subsection emerged from such an event somewhere between 65 and 40 million years ago. And the story continued on from there. 

As the climate changed throughout western North America, so too did the topography. By about 17 mya, geologic activity had created the Basin and Range Province (a large intermountain region of alternating north-south trending mountains and desert valleys). Then, around 7 to 5 mya, a large rain shadow formed over western North America as the Sierra Nevada mountains took on their eventual formation.

These topographic changes lead to progressively colder and drier conditions throughout western North America. Once again, the diversity of pine species was rearranged. Some species migrated south towards the subtropics, some species migrated toward the coastlines where temperate conditions remained, and some species simply died off. However, the Balfourianae subsection persevered during this time. 

The peak of cold and dry came during the Pleistocene (2.58 mya to 11,700 years ago), when portions of western North America became periodically covered in snow and ice. Owing to their tolerance for cold and dry conditions, the ancestral Foxtail Pines and Bristlecone Pines enjoyed a competitive advantage that enabled them to expand throughout western North America. But the Pleistocene eventually came to end. It was succeeded by an era of climatic warming known as the Halocene (11,700 years ago to present). During the Halocene, cold temperatures were no longer a barrier to other species and the matrix of pine diversity within western North America shifted once more. It ultimately led to a range contraction for the ancestral Foxtail and Bristlecone Pines. They moved upward in elevation where colder and drier conditions allowed them to prevail under a less competitive environment. They came to inhabit the subalpine habitats of the intermountain west, which remain some of the coldest and driest conditions in western North America today. They have remained there ever since. 

Today, the Foxtail Pines and Great Basin Bristlecone Pine each have highly disjunct populations. These disjunct populations are likely the result of continued range contraction and isolation throughout the latter stages of the Pleistocene. 

In the case of the Foxtail Pines, isolated reproduction pools resulted in two distinct subspecies or varieties: Pinus balfouriana var. austrina Pinus balfouriana var. balfouriana. The balfouriana variety is endemic to the eastern Klamath Mountains while the austrina variety is endemic to the southern Sierra. In California, the Great Basin Bristlecone Pine (Pinus longeava) is found in the Panamint and White Mountains of eastern California. 

For all three of these species, their tolerance for cold and dry climatic conditions has definitely aided their survival, but such factors are not alone in their influence. These species are also known to tolerate more extreme soil conditions, like that of serpentine and dolomite. 

The Klamath Foxtail Pine (Pinus balfouriana var. balfouriana) can be found on exposures of serpentine soils in the Klamath Mountains. Serpentine soils are high in magnesium and other heavy metals (like nickel) that make it hard for other plant species to grow. Thus, serpentine soils become a restrictive edaphic feature.  

In the White Mountains, the Great Basin Bristlecone Pines are famous for being one of the few trees growing on dolomite slopes. Dolomite is a type of limestone with a highly alkaline PH level that is restrictive to the growth of most plants. While the Great Basin Bristlecone Pines do not select for dolomite, their tolerance of it has allowed their genetic offspring to enjoy a competitive free environment. Tolerance of dolomite is one adaptive trait allowing the Great Basin Bristlecone Pines to circumvent increased competition from other highly resilient plant species, like the shockingly tough-as-nails Great Basin Sagebrush (Artemisia tridentata). 

Wherever it grows, the Great Basin Bristlecone Pine is often one the few trees able to grow at the upper limits of timberline. It has a phenomenal ability to withstand bitter cold conditions. Likewise, it also has an exceptionally high tolerance for drought. To grow in these conditions, the trees must retain carbohydrate reserves through long periods of dormancy, whether that dormancy be due to scant moistures reserves during the summer months or long freezing winters when water is locked up in ice. It also means they must persist on a very lean diet, which has been critical to their ability to inhabit sites too extreme for most other species.  

A young Great Basin Bristlecone Pine (Pinus longeava) growing in the dolomite of the White Mountains. The green in the backdrop is Great Basin Sagebrush (Artemisia tridentata). Note how Great Basin Sagebrush occurs on a darker soil type and then stops a the transition to the white dolomite. 

Both the Foxtail Pines and Great Basin Bristlecone Pine are emblematic of the cold, the dry and the remoteness that is the arid west. These pines have evolved to stare down changing climate patterns by selecting some of the more extreme habitats that occur today. To that end, these species are also emblematic of the changes that have transpired throughout western North America. They evolved by way of an ability to adapt to extremely cold and dry conditions. They existed long before California’s present climate regime or even long before the arid climate that hangs over the Great Basin. They are feats of exceptional endurance and when you walk their groves, the feat of their success is apparent. Despite knowing how or why, it’s apparent that they exist by some amazing element of survival. Their story is worn.

Additional Photos

Additional Reading

A Garden of Bristlecones - Tales of Change in the Great Basin by Michael P. Cohen

Bailey, D. K. “Phytogeography and Taxonomy of Pinus Subsection Balfourianae.” Annals of the Missouri Botanical Garden, vol. 57, no. 2, 1970, pp. 210–249. JSTOR, Accessed 26 Jan. 2021.

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