Pinus torreyana Parry ex Carrière, Traité Gén. Conif. 326 (1855).
C. Michael Hogan PhD
2008 September 11
Pinus torreyana (Torrey Pine) is a rare and endangered species restricted to two small populations in southern California, USA. Its distribution is among the narrowest of any world pine species, being listed by Businský (2008) as the 8th rarest pine species in the world; furthermore, ongoing land development pressure continues to threaten the Torrey Pine. A new theory of prehistoric colonization of the Santa Rosa Island population of P. torreyana by human-mediated dispersal is presented herein.
Distribution and varieties
The two extant populations of P. torreyana are situated (a) on the California mainland immediately north of San Diego and (b) on Santa Rosa Island 280 km to the northwest. These populations were first distinguished as subspecies by Haller (1976), and later reduced to varietal rank by Silba (1990) because of the very limited differences between them (Earle 2009).
The mainland colony consists of P. torreyana var. torreyana and is comprised of two recognizable components: (1) a coastal strip approximately 3.4 km long within the city of Del Mar and (2) separated by the Soledad Valley, a second southerly strip of about 1.9 km in length, most of which is within the Torrey Pines State Preserve. The width of each strip does not exceed 1.6 km. A third smaller site was mapped by Griffin & Critchfield (1976), but could not be found a decade later, due to intense land development in that area (Haller 1986). The mainland population holds approximately 3000-3400 mature trees growing amid a Southern California Coastal Scrub ecosystem on moderate to steep slopes of sandstone soil.
The Santa Rosa Island population, designated as the variety P. torreyana var. insularis (Haller) Silba, also consists of two distinct geometric units. On the northeast shore the larger stand extends for about one km along an east-west axis direction within a band 300 m along the shore (Farjon et al. 1999). A smaller discrete stand is approximately 1.2 km to the southeast 800 m inland and extends for a further 500m. The entire island population occupies roughly 405 hectares, and contains about 1000-2000 mature trees.
Molecular genetic studies illustrate that 59 gene loci account for the isozyme variation within 25 distinct enzyme systems identified in P. torreyana (Ledig & Conkle 1983). Remarkably all of the individuals analyzed within the mainland population were identical at each of the 59 loci, with samples from all age groups and spatial regimes. The Santa Rosa Island population differs in two of the gene loci studied; however, the island population is also uniform within its individuals. These molecular studies are supported by morphological and growth trait observations within the populations. One should note that, since the electrophoresis methods used in the genetic studies cannot detect all allelic variation, it is likely that the two subspecies differ at other loci. In any case the lack of genetic diversity within each of the two populations suggests a population bottleneck for the mainland and a founder effect for the island occurrence.
For decades researchers have analyzed the puzzle of how two separate populations of Torrey Pine evolved. Using molecular genetic studies, it is considered that the island colonies were founded by transport of seeds from the mainland; possibly as long as 180,000 years ago (Ledig & Conkle 1983) but more likely as recently as 8,000 years before present, given the absence of any chloroplast DNA variation between the two populations (Waters & Schaal 1991). The most commonly cited mechanism is ocean current drift of seeds, necessarily from an extinct P. torreyana forest in Santa Barbara or San Luis Obispo County (since the prevailing ocean current is north to south). Although ocean levels were about 100 m lower and the Channel Islands only six km distant from the mainland shore in that early time, no one has worked out the viability of saline water sea transport and regrowth on a beach environment, since Torrey Pine requires high ground rather than beach or intertidal zones.
I suggest rather that the seed transport occurred by Chumash peoples who had an extensive network of settlements on the Channel Islands as well as the Santa Barbara and San Luis Obispo mainland. The Chumash were expert in plank canoe building and use thereof to navigate between islands and continent; moreover, pine nuts are known to be an important food to these Native Americans. The Chumash peoples were one of the earliest known North American groups, with settlements present at least 13,000 to 11,000 years before present and complex trading networks between coastal villages extending beyond the Millingstone Horizon (Hogan 2008). The timeline for Chumash transport is well within the margin of error of time estimates for island colonization derived from Nei's Standard Genetic Distance calculations. My suggestion would also explain the position of the second inland colony of P. torreyana, disjunct from the shore colony; in fact, the Chumash may have sown both colonies in an effort to enhance foodstocks in an island setting lacking in mast production.
The architecture of P. torreyana presents an open crown and broad structure, with mature height of seven to 15 meters. Needles are in groups of five, with a grayish color for var. torreyana, and bluish-gray for var. insularis. Typical cones have 60–130 edible large, tough-shelled seeds and may weigh roughly 110 grams (von Mueller 1895). Leaves (needles) are typically 19–35 cm long and borne in groups of five, and the sturdy cones range from 7 to 15 cm in length. Growth traits of Torrey Pine proximate to the Pacific Ocean manifest a severely backswept form, sculptured not only from prevailing westerly winds, but also the phenomenon of "salt pruning", whereby coastal woody vegetation suffers leaf attrition by saline sea spray bombarding the seaward side of the plant for decades of life. These forms provide the iconic shape associated with Torrey Pine.
Given the precarious population size and narrow range of this species, it is important that state and local governments provide maximum protection to this species. Heavy ongoing visitation to the Torrey Pine State Reserve coupled with land development encroachment pressures present the major threats. There are peripheral areas to each population that are candidates for protection, in that ecological buffering and potential expansion area could be advanced. Lesser attention has been given to the associated forest understory, chaparral and wildlife that is linked to the Torrey Pine forests. More study of these species is needed to understand what other narrow endemics may depend upon this unique habitat.
* Businský, R. (2008). The genus Pinus L., Pines. Acta Pruhoniciana 88: 3-126.
* Earle, C. J. (2009). Pinus torreyana. The Gymnosperm Database.
* Farjon, A., Page, C. N., & Brown, M. J. (1999). Conifers: Status Survey and Conservation Action Plan. IUCN, 121 pages ISBN 2831704650
* Griffin, J. R., & Critchfield, W. B. (1976). The Distribution of Froest Trees in California. USDA Forest Service Research Paper PSW-82/1972 (reprinted with Supplement, 1976).
* Haller, J. R. (1986). Taxonomy and relationships of the mainland and island populations of Pinus torreyana (Pinaceae). Systematic Botany 11: 39-50.
* Hogan, C. M. (2008). Morro Creek, The Megalithic Portal, ed. A. Burnham
* von Mueller, F. (1895). Select Extra-tropical Plants: Readily Eligible for Industrial Culture or Naturalisation, with Indications of Their Native Countries and Some of Their Uses. R. S. Brain, Govt. printer, 654 pages.
* Ledig, F. T., & Conkle, M. T. (1983). Gene diversity and genetic structure in a narrow endemic, Torrey Pine (Pinus torreyana Parry ex Carr). Evolution 37 (1): 79-85.
* Silba, J. 1990. A supplement to the international census of the coniferae 2. Phytologia 68: 7-78.
* Waters, E. R. & Schaal, B. A. (1991). No variation is detected in the chloroplast genome of Pinus torreyana. Canadian Journal of Forest Research 21: 1832-1835.