by Frank Perry, Research Associate, Santa Cruz Museum of Natural History
reproduced from the Monterey Bay Paleontological Society Bulletin, July–September, 2004. © 2004 Frank Perry

“The present is the key to the past.” This well-known geological axiom was first championed by Scottish naturalist James Hutton in the late eighteenth century. Hutton proposed that modern-day Earth processes — such as erosion, sedimentation, volcanism, etc. — could, over long periods of time, leave behind the rock record we see today. It was not necessary to invoke extraordinary events or “catastrophes” to explain the past. Geologic processes that anyone could see during his or her lifetime had acted much the same way throughout earth history. This concept is more formally known as the Principle of Uniformitarianism. It mostly holds true, although now we know that very rare events, such as asteroid impacts, have also played important roles in the history of the Earth and of life.

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.

Closed-cone pines

The Monterey Pine, along with the Bishop and Knobcone Pines, belong to an informal taxonomic category known as the “California closed-cone pines.” In most species of pine, a set of cones matures annually, opening and dropping their seeds in the fall. With the closed-cone pines, however, many of the cones remain sealed with resin and attached to the branches. In the case of the Knobcone Pine, the cones can remain closed for decades. Monterey Pine cones may open after a few years, but remain on the tree.

Botanists call this closed-cone trait serotiny (sahr-ROT-in-ee). Most scientists believe that serotinous cones evolved as an adaptation to dry climates with occasional forest fires. The heat from fires sweeping through the crowns of the trees causes the resin seal to melt. In as little as a day after the fire, the cones open, at once releasing a seed supply saved up from multiple years. Although the burned mature trees usually die, many new trees soon sprout from the seeds, recolonizing the area more quickly than other tree species.

Distribution

Today, the Monterey Pine has a very restricted distribution. It is native to just five areas: the Año Nuevo-Swanton area (San Mateo and Santa Cruz Counties), the Monterey Peninsula and Carmel (Monterey County), Cambria (San Luis Obispo County), and Guadalupe and Cedros Islands off Baja California in Mexico. The species was more widely distributed at various times in the past. Fossil cones or needles of the Monterey Pine have been recorded from 20 localities in California, including the famous La Brea Tar Pits. Most of the localities are near the coast and range from Tomales Bay in the north to Chula Vista near San Diego and on the Channel Islands. The one inland locality is in Riverside County. The fossils range in age from middle Miocene through Pleistocene.

Fossil cones

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.

Evolutionary history: early hypotheses

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

There is no evidence that the Monterey Pine was ever continuously distributed along the California coast at any one time over the past two million years. Instead, its populations were always fragmented. The species expanded, shifted, or colonized new sites during periods of favorable climate. Its range contracted and some forest stands died out during periods of unfavorable climate.

Millar used this revised interpretation of the tree’s history to suggest a revised conservation strategy. Many biologists are concerned about the survival of the five native stands of Monterey Pine. On Guadalupe Island, for example, the population has long been threatened by goats. In 1964 only 320 trees remained. By 1992 the population had dwindled to 150. On the mainland, urbanization, fire suppression, genetic contamination, and pine pitch canker (a deadly fungal disease ravaging many stands) all threaten the trees.

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.