SOS Forests

A Brief History of the Post-Cretaceous Forests of SW Oregon, Part I

Part I. Before the Holocene

Intro

A discussion of fire ecology in Southwest Oregon which recently came to my attention contained the following statement:

These forest ecosystems have been evolving for tens of thousands of years, thus their species composition, structure, and ecological processes are essentially a product of their ongoing evolutionary and physical environments (Agee 1991, Waring 1969).

Technically speaking, from a paleobotany perspective, that statement is counter-factual.

The forest ecosystems of SW Oregon are new to the area. The oldest “native” tree species have been in residence no more than 12,500 years or so, and some less than half that.

The species composition is new; different species mixes occurred in earlier interglacials.

What’s an interglacial? To explain that we first have to explain the Ice Ages. But maybe we should back up even farther to start.

Transitions in the Tertiary

The Tertiary Era consists of the Paleocene (65-54.8 mya), the Eocene (54.8-33.7 mya), the Oligocene, (33.7-23.8 mya), the Miocene, (23.8-5.4 mya), and the Pliocene (5.4-1.8 mya) Epochs.

Global temperatures reached their highest post-Cretaceous levels at about the Paleocene-Eocene transition, and they have been falling ever since. Global temperatures were perhaps 25 to 30 degrees F higher than now. Atmospheric carbon dioxide was perhaps 5-10 times denser than now, and oxygen concentrations a third more than current levels. Earth was a greenhouse bursting with life.

But then the temperatures started to fall. Hypotheses for the temp decline are many-fold. Pangea was fractured and the landmasses were drifting apart. With less bumping and grinding of continents, volcanic activity waned. Consequently CO2 levels dropped. Antarctica drifted toward the South Pole, and ice began to build up, lowering global sea levels.

The falling temperatures in the Eocene were accompanied by annual dry spells, which we know as seasons. Seasonality favored pre-adapted deciduous trees from the paleo-boreo-tropics [here]. Mega thermal (heat tolerant) plant species moved toward the equator, and micro thermal (cold tolerant) species proliferated in the northern Hemisphere.

Paleo-boreal conifers were well-adapted to drought and cold. The Eocene fossils from Republic, Washington, dated about 48 mya, include paleo-genera of yellow cedar (Chamaecyparis), red cedar (Thuja), ginko (Ginko), true firs (Abies), spruce (Picea), pines (Pinus), hemlock (Tsuga), yew (Taxus), and sequoia (Sequoia).

The angiosperms were evolving right along with the conifers. At the Clarno Nut Beds, dated 44 mya, the following angiosperm paleo-genera are found: maple (Acer), hazelnut (Corylus), dogwood (Cornus), magnolia (Magnolia), plum (Prunus), and hackberry elm (Celtis). With over 170 fossil species, there are lots of fruits and seeds in the Clarno assemblage, hence the name “nut beds”. The Clarno flora is considered tropical to paratropical, with many lianas (vines). Crocodile bones are in the mix.

Also in the mix is the earliest occurrence of the genus Quercus, the mighty oak. Fossil acorns appear in the Nut Beds, too.

At least five major extinction events occurred in the Eocene alone:

The Lutetian-Bartonian event (41 mya)

The Bartonian-Priabonian event (37 mya)

The Late Priabonian event (35 mya)

The Terminal Eocene event (33.5 mya)

The Late Rupelian event (30.5 mya)

All these extinction events were associated with reductions in global temperature, thinning CO2, declining rainfall, and falling sea levels. The story is reported with great scientific rigor by Dr. Alan Graham, Paleontologist Emeritus of Kent State, in his masterwork of paleobotany, Late Cretaceous and Cenozoic History of North American Vegetation, 1999, Oxford Univ. Press. Despite the dry tone and frequent scientific citations, the tale told by Dr. Graham is spellbinding:

[The Late Rupelian] event essentially eliminated most [oceanic] holdovers from the warmer Late Eocene and transitional early Oligocene as marked by the extinction of many cool-water foraminifera and nanoplankton, oxygen isotope records, and a fall in sea level. From ~32 mya through 30 mya… the largest sea-level drop in Tertiary history is recorded, and it likely was the result of the beginning of full continental glaciation on Antarctica. …

The events… culminated in a rapid change in European fauna known as the Grande Coupure (the great break). Sixty percent of the mammals of Europe became extinct at about 32.5 mya. … During [the same period] a number of archaic forms disappeared from North America, further reducing prior similarities with European fauna. … About 60% of Early Oligocene genera were new to North America.

After the Eocene (56.5-35.4 mya) came the Oligocene (35.4-29.3 mya). Global temperatures plunged to new lows, only 5 to 10 degrees F higher than they are now. One possible factor was the opening of Drake’s Passage, the gap between Antarctica and South America. With Drake’s Passage open, a circumpolar current developed around Antarctica, further isolating the growing southern icecap from moderating equatorial waters.

A significant portion of the Eocene fossil flora found in the Clarno Nut Beds is also found in Europe, and even more so in Asia. By the Miocene (29.3-6.7 mya) those links were broken. Greenland separated from Europe, and although Beringia still connected North America and Asia, cold and dry conditions blocked genetic exchange by trees.

In the Miocene temperatures plunged again. Sea levels, CO2 concentrations, and rainfall all dropped to new historical lows. For the first time since life crawled out of the sea hundreds of millions of years earlier, large tracts of lifelessness spread like rashes upon the land. Deserts, desiccated terrain, appeared on Planet Earth.

Grasses had evolved by the Miocene, and grass prairies and savannas arose for the first time, too. Deserts and savannas are dryer and more seasonally hot and cold than any prior vegetation types. Deserts and global cooling go together. Less heat causes less evaporation, forming fewer clouds, and producing less precipitation. Ergo, deserts and savannas.

Then it got even colder. Global temperatures fell and fell in the Late Miocene and Pliocene (5.2-1.6) mya. In non-equatorial latitudes the chilled air was dryer, and rainfall all but stopped.

Vast continental forests disappeared in the Pliocene, replaced first by sclerophyllous shrubs and chaparral, then by grasslands, then by barren, dust-swept deserts. Where conifer and deciduous hardwood forests and tropical cypress swamps once flourished, nothing grew but dusty dunes and gravel flats. Lowland boreal forest of modern aspect, with cold-stunted spruce, fir, and pines, first appeared about 4 mya. About 2.4 mya tundra took form, perched on another phenomenon new to Mother Earth, permafrost. Finally, to cap things off (so to speak), around 2 mya Arctic continental glaciation got going.

By 850,000 years ago a vast ice sheet extended from the Arctic Ocean down the Mississippi Valley all the way to Nebraska. The portion of North America that is Canada was completely buried under a mile or two of solid ice.

Images from the Global Warming Art Project [here]. Click for larger versions.

The Quaternary Ice Age

The Tertiary Period ended and the Quaternary Period began roughly 1.8 mya. The Pleistocene is the Ice Ages, which occupies all the Quaternary, with the exception of the Holocene (12,000 BP to present).

We live in the Holocene, an interglacial, a time of glacial retreat and global warming. The colder periods of the Ice Ages are called stadials, times of global cooling when great ice sheets grow in the northern latitudes.

There have been 18 to 20 of each (stadials and interglacials) in the Pleistocene. The stadials have lasted about 90,000 years each, while the interstadials have lasted about 10,000 years. They have happened like clockwork, in 100,000 year cycles which correlate very closely with cosmic oscillations in the Earth’s orbit and tilt (known as Milankovitch Cycles) that impact solar insolation (radiation striking the Earth).

Ice Age glaciations are devastating to most species. Giant ice sheets form in the arctic and spread south, burying the surface of the Northern Hemisphere in mile-thick glaciers as far south as the 45th Parallel. Katabatic winds coming off the ice sheets extend tundra conditions far south of that.

During the long glaciations Southwest Oregon was ice, tundra, and sagebrush steppe.
The ice was extensive in the mountains, and large glaciers formed along the cordillera to the southern end of the Sierra Nevada. Woolly mammoths, cold-weather animals, ranged at least as far south as the La Brea Tar Pits in now downtown Los Angeles.

New Plant Assemblages

The conifers and other plants common in SW Oregon forests today (we call them “native species”) were not there during the latest (Wisconsin) glaciation, but instead were confined to coastal refugia to the south, probably in patches along the now submerged continental shelf of Northern California.

During the brief interglacials like the Holocene, temperate plants rushed back into the country, only to be extirpated by the next 90,000-year-long glacial stadial. This cycle of destruction and recovery has happened 18 to 20 times over the last 1.8 million years.

Not since the Permian Ice Age of 250 million years ago has the Earth been so cold.

As a result, many plant (and animal) extinctions have occurred during the Pleistocene. The plant species that have survived (in refugia) the freezing glaciations are hardy and aggressive colonizers. Botanical ecologists call them “cosmopolitan” species.

Some species (redwoods, giant Sequoia, Monterey pine) have not spread far from their refugia, but others (Douglas-fir, spruces, lodgepole pine) moved rapidly into the freshly thawed Holocene landscapes.

The communities of plants found today in Southwest Oregon are thus made up of new invaders, hardy survivors of a score of extreme, all-but-extinction, 100,000-year-long, glacial stadials. The plant mixes in this interglacial are not the same mixes that occurred in prior interglacials, nor (in most respects) anything like the plant communities of the Miocene, the last time it was as warm (continuously) as today.

From Late Cretaceous and Cenozoic History of North American Vegetation by Dr. Alan Graham:

[T]he last decade has produced results that are both revolutionary and fundamental to our understanding of biotic history. … Realization of the temporal nature of communities has implications for such time-honored concepts as succession, climax, and geofloras. … [D]esignating the vegetation type characteristic of a given region loses much of its meaning, especially for the times of turbulent glacial-interglacial transitions.

It also reduces the value of a concept that envisions the movements of vegetation as large blocks. … The actual association of elements in a community, especially when they represent novel combinations based on a mixture of wind-blown pollen types, usually can only be assumed.

The forests of today are not mutualistic associations of interdependent plant species co-evolved over millions of years; rather they are chance combinations of competitive species filling temporary niches during a temporary break in the Ice Ages.

Dr. Chad Oliver, Pinchot Professor of Forestry and Environmental Studies, and Director of the Yale University Global Institute of Sustainable Forestry, wrote in his landmark book, Forest Stand Dynamics:

… competition implies that trees have not evolved a dependence on each other but actually can impede each other’s growth. Competition, not mutualism, is now considered the primary pattern of interaction among holarctic tree species, and probably among trees in other floristic realms.

Proofs of the Competition Theory (in Western NA forests, at any rate) are:

1. You can completely remove any one species from a forest, and the rest of the plants do not die. In fact, they often grow better.

2. You can remove all of the plants, by clearcutting, burning, herbicides, or all three, and native forest plants will eventually re-invade and re-colonize the site. Short of paving, and sometimes not even then, it’s difficult to keep the vegetation down. Far from fragile, continental, cosmopolitan forest plant species are tough, adaptive, and aggressive competitors.

3. Most of our native forest species, and all of the dominant ones, can be and have been grown in other locations in complete isolation from their putatively mutually-dependent, co-evolved plant community.

4. Forensic forestry and paleobotany reveal that current plant associations are temporary and artifacts of individual forest and stand disturbance histories rather than requisite groupings of co-evolved interdependents.

The forests of SW Oregon today were not pre-destined by evolution so much as they are novel and accidental assemblages of invasive, competitive plants that have come to dominate temporarily, due to periodic climatic disturbances on a global geologic scale.

What’s more, they are not even all that “natural”.

Next: Part II: The Forests of the Anthropocene