The Monterey Pine through geologic time
Less appreciated but just as important is that the past can be a key to better understanding the present. A splendid example of this is the Monterey Pine. Recent discoveries about its fossil history not only help us to better understand its present distribution, but also may help shape future conservation efforts.
I have long had a fondness for Monterey Pines. Several large Monterey Pines grew next door when I was a kid, so these were the first pines with which I became acquainted. Each was very tall, perhaps a hundred feet, with a craggy trunk three or four feet wide at the base. A carpet of brown needles blanketed the ground beneath them. In the highest branches, kestrels would perch, sometimes giving off a loud screech. On very hot days a rarity where I lived the trees emitted an eerie cracking sound as some of the cones opened.
The cones of this species are distinctive and one of the easiest to recognize from fossils. They are asymmetrical with large, smooth, bulbous umbos (knoblike protuberances) on the scales. The scales do not have sharp prickles like some pine species.
Most of the Pleistocene cones are carbonized (also called coalified). Basically, the wood has turned into coal. In this process, the molecules that make up the cellulose begin to break down after prolonged burial. Hydrogen and oxygen atoms are released in the form of water, carbon dioxide, and methane gas. This leaves behind carbon hence the term carbonized.
The evolutionary history of the Monterey Pine (Pinus radiata) and its close relative, the Bishop Pine (Pinus muricata), first piqued the curiosity of paleontologists in the early 1900s. In the 1930s Herbert Mason suggested that the disjunct populations of these pines could be explained by Tertiary-age islands. He hypothesized that the populations had been isolated on offshore islands millions of years ago, and that after these islands became part of the mainland, the pine's insular distribution persisted. It was a reasonable theory at the time, but geologic studies in recent decades have found no evidence for such islands, at least in central and northern California. In addition, later discoveries show that the Monterey Pine was much more widely distributed only a few tens or hundreds of thousands of years ago, well after the end of the Tertiary Period.
In a series of studies during the last half of the 1900s, paleobotanist Daniel Axelrod used geologic, fossil, and associated floristic evidence to better piece together the evolutionary and geographic history of the Monterey Pine. According to Axelrod, the closed-cone pines originated in Central America from an ancestor related to the modern Pinus oocarpa. The closed-cone pines spread northward into California about 15 million years ago. By this time, Pinus radiata had already evolved into a distinct species. The Monterey Pine's widespread fossil distribution along the California coast led Axelrod to believe that it flourished throughout the Pleistocene. He theorized that it was not until a warm, dry period 4,000 to 8,000 years ago that it was driven to near extinction, surviving in the form of five small populations. He called this warm period the Xerotherm. Today it is more commonly referred to as the Climatic Optimum or early Holocene warm period.A recent hypothesis
A few years ago, Connie Millar, a scientist with the United States Forest Service, developed a revised theory of the pine's history based on new fossil evidence and a refined understanding of climate change during the Pleistocene.
Marine sediments and ice cores from the Atlantic, Pacific, and Arctic oceans have now given us high-resolution climate records going back hundreds of thousands of years. Paleotemperatures have been determined from the shells of fossil organisms and from gases trapped in the ice, sometimes datable to individual years. These scientific breakthroughs have enabled scientists to paint a much more detailed picture of climate change than was ever before possible.
These records show that there have been at least 11 ice ages over the past million years. Each lasted for about 90,000 years. Between the glacial periods were warm interglacial periods of about 10,000 years, like the one we live in now. The Climatic Optimum (Axelrod's "Xerotherm"), when temperatures were even warmer than at present, appears to have had analogs in other interglacial periods. This casts doubt on Axelrod's theory, which relied on the premise that the early Holocene warming was unique. The previous interglacial period (125,000 to 111,000 years ago), for example, apparently had peak temperatures at least two degrees centigrade warmer than the period 8,000 to 4,000 years ago.
Another important line of evidence has come from fossil pollen. Pollen is extremely durable and can last for millions of years. The tiny grains have distinctive morphologies and in many cases are identifiable to species. Fossil pollen samples collected from cores of lake, bog, and marine deposits can show what plants were living nearby and how the flora in a particular area changed over time with changes in climate.
For her study of the Monterey Pine, Millar drew upon pollen evidence from sediment cores in the Santa Barbara Channel. According to Millar, the Monterey Pine "was least abundant during full interglacials (i.e., the Holocene and previous interglacials), when oaks dominated coastal habitats, and was also uncommon during the cold periods of the glacials, when junipers dominated. Monterey Pine, as well as other coastal pines, increased dramatically in abundance and shifted in coastal location during climate periods intermediate between these extremes that is, at times such as the end of the ice ages (climate warming), during ‘interstadial periods’ (warmish intervals within the ice ages), and at the end of interglacials (climate cooling)."
She also found that, "Times of abundance of Monterey Pine correlated also with increases in charcoal abundances in the sediment cores, corroborating that fire plays an important role in dispersal and spread of Monterey Pines by opening cones and preparing seed beds."
Shifts in distribution and today's populations
Domesticated Monterey Pines are common in the United States as garden trees, and small ones are used for Christmas trees. Although the species is not likely to become extinct, it is important that the wild populations be preserved. In other parts of the world, domesticated Monterey Pines are a major forest tree. In fact, it is the world's most planted conifer. Over 10 million acres have been planted. There are 2 million acres in Australia, 3 million in New Zealand, and millions more in Chile, Uruguay, Argentina, Spain, South Africa, and Kenya. Selective breeding has produced fast-growing trees with straight trunks ideal for the forest industry. Most of these countries have active breeding programs to develop still better strains for lumber and paper manufacturing. DNA studies have shown that the material from which these foreign plantings were developed was not very diverse genetically. On the contrary, the native forests are genetically diverse. According to Millar, the diverse germplasm in these native populations have "inestimable value." It could, for example, help with the development of strains that are more resistant to certain insect pests and diseases.
Given the Pleistocene history of the species, she recommends expanding conservation efforts beyond the five native populations. For example, "neo-native" forests could be planted in areas where the pine lived only a few thousand years ago such as near Point Reyes, Point Sur, Santa Barbara, and San Diego. This would help ensure survival of genetically diverse forests. When Dr. Millar spoke to the Monterey Bay Paleontological Society a few years ago, she told the tragic story of a park along the northern California coast where all the Monterey Pines were cut down. The trees were perfectly healthy, but were removed because they were a "non-native" species. Yet, a few thousand years ago the trees might have grown there naturally.
One of the great values of fossils is that they enable us to see the present from the perspective of geologic time. As the Monterey Pine clearly shows, this perspective can give us a greater appreciation for the present and help us plan better for the future.
So the next time you visit the Monterey Peninsula or drive by the trees along Highway 1 near Año Nuevo State Reserve, take a closer look at the Monterey Pines and their remarkable cones. Consider their clever survival strategy. Think of them not as relicts of the Ice Age on the verge of dying out, but as trees lying in wait for the next shift towards a cooler climate.
Axelrod, D.I. 1980. History of the maritime closed-cone pines, Alta and Baja California. University of California Publications in Geological Sciences v. 120.
Axelrod, D.I., and F. Govean. 1996. An early Pleistocene closed-cone pine forest at Costa Mesa, southern California. International Journal of Plant Science 157(3):323329.
Lanner, R.M. 1999. Conifers of California. Los Olivos, California: Cachuma Press.
Millar, C.I. 1998. Reconsidering the Conservation of Monterey Pine. Fremontia 26(3):1216.
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Photo of Monterey Pines © 2004 Frank Perry; Photo of Monterey Pine branch with cones © 2004 Frank Perry; Illustration of Monterey Pine cone by C.L. Taylor, 1908
Understanding Evolution © 2016 by The University of California Museum of Paleontology, Berkeley, and the Regents of the University of California