71 Sierra Nevada Ecosystem Project: Final report to Congress, vol. II, Assessments and scientific basis for management options. Davis: University of California, Centers for Water and Wildland Resources, 1996. A b s t r ac t



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71

Sierra Nevada Ecosystem Project: Final report to Congress, vol. II, Assessments and scientific basis for management options. Davis: University of California, Centers for

Water and Wildland Resources, 1996.

A B S T R AC T

The Tertiary period, from 2.5 to 65 million years ago, was the time of

origin of the modern Sierra Nevada landscape. Climates, geology,

and vegetation changed drastically in the Sierra Nevada during this

time, and analyses of this period provide both context for and insight

into vegetation dynamics of the current and future Sierra. During the

early Tertiary, warm-humid, subtropical to tropical conditions prevailed

on the low, rolling plains of the area now the Sierra Nevada. Fossil

taxa with tropical adaptations and affiliations were widespread

throughout the region. In the Sierra Nevada, ginkgo 

(Ginkgo biloba),

avocado 


(Persea), cinnamon (Cinnamomum), fig (Ficus), and tree

fern 


(Zamia) were common. At the end of the Eocene epoch, about

34 million years ago, global climates changed rapidly from warm-

equable to cool-seasonal temperate conditions. In response, vegeta-

tion also shifted enormously; cool-dry-adapted conifers and

hardwoods, which had been refugial during the early Tertiary in up-

land areas of the Great Basin–Idaho region, migrated into newly hos-

pitable habitats of the Sierra Nevada. These floras contained many

of the taxa now native to the Sierra, plus relicts from the subtropical

forests of the earlier Tertiary and species adapted to temperate con-

ditions with summer rain—mixes that seem incompatible under mod-

ern conditions. Until late in the Pliocene epoch (about 1 million years

ago), adequate but diminishing rainfall distributed through the year

supported many taxa now extinct in the Sierra Nevada. By the late

Tertiary, in response to continued drying, winter cooling, and increas-

ing summer drought, and to gradual uplift of the Sierra Nevada, re-

placement of early Tertiary floras by modern taxa and associations

had occurred. With the development of a Mediterranean climate by

the late Pliocene, floras of the Sierra Nevada became segregated

ecologically into elevational, latitudinal, and orographic zones.

An important message for ecosystem management from a study

of the Tertiary flora of the Sierra Nevada is that, although vegetation

has changed drastically over 65 million years, the rate has been very

slow. Human impacts in the Sierra are potentially of a similar magni-

tude to these evolutionary changes but can occur at rates many times

faster; such changes may be more rapid than plants are likely able to

5

Tertiary Vegetation History



C O N S TA N C E   I .  M I L L A R

Institute of Forest Genetics

U.S. Forest Service

Pacific Southwest Research Station

Albany, California

adapt to. Another management implication is that currently native

species in the Sierra Nevada have existed in the past under drasti-

cally different climatic and environmental conditions than at present,

have had very different distributions, and have occurred in mixes not

seen in the recent past. Thus, assumptions about the behavior of

native species in the future under unknown climates and/or novel

management regimes should not be based solely on the behaviors

of species in current environments. Unforeseen responses are likely,

whether “positive” (population health, expansion, productivity), “nega-

tive” (population decline, extirpation), or novel. The most appropriate

management action is to maintain diverse, healthy forests with con-

ditions favoring resilience to unpredictable but changing future cli-

mates and management regimes. Plans that require landscapes to

reach precise vegetation targets are likely to fail. Management pro-

grams that build flexibility, reversibility, and alternative pathways are

more likely to succeed in an uncertain future.

I N T R O D U C T I O N

The Tertiary period is a slice of Earth’s history, roughly defin-

ing the time between the extinction of the dinosaurs and the

beginning of Northern Hemisphere continental glaciation,

from 65 million years ago (denoted as 65 Ma) to about 2.5 Ma

(table 5.1). This was a period of major change in global cli-

mates and of significant mountain building, overall moving

from warm-mild and moist-equable regimes to seasonally dry

and cool climates. The Mediterranean dry summer typical of

California today was unknown until late in the Tertiary. The

Tertiary was the time of initial uplift of the Sierra Nevada

and volcanism in the Cascade Mountains. Accompanying

these physical changes were radical transformations in the

vegetation assemblages that covered the landscapes.

The human time scale for land management stretches 100–

200 years into the future at its most imaginative. Why would

Back to CD-ROM Table of Contents


72

V O L U M E   I I ,   C H A P T E R   5

SNEP look back 65 million years? The Tertiary provides an

important larger context for understanding modern landscape

relationships in the Sierra Nevada. The Tertiary was the time

of revolutionary development of the modern vegetation, cli-

mates, and landscape. At the onset of the Tertiary, there were

humid subtropical climes in California, typical of vast peri-

ods of time prior to the Tertiary. Species such as ginkgo,

avocado, figs, and palms dominated broad plains and low

mountains in the area that is now the Sierra Nevada. By the

mid-Tertiary, plants with affinities to modern taxa—pines, firs,

oaks, and cottonwoods—appeared to be more widespread in

the region of the developing Sierra Nevada. By the close of

the Tertiary, most modern species and many modern vegeta-

tion assemblages were present and stratified into elevational

zones. Although species and plant communities shifted in

response to fluctuating conditions of the Quaternary period,

which followed, these shifts were minor relative to the major

evolutionary and continental-scale dynamics of the Tertiary.

Thus the Tertiary sets the stage for the present.

The present flows seamlessly from the past. Knowing the

origins and broad context of our flora informs our understand-

ing and appreciation of the dynamics of current Sierra Ne-

vada ecosystems—why species grow where they do, under

what environmental and ecological conditions they have

grown, what relationships have existed among plant associ-

ates, how biota respond to environmental change, and what

potentials exist for rapid and dramatic natural vegetation

change. Since many of our current taxa first appeared in Cali-

fornia under different climates and evolved under very dif-

ferent environmental conditions, the past informs us about

ecological responses that we are not able to infer from present

dynamics.

O B J E C T I V E S

The purposes of this chapter are to:

• briefly review and assess the methods used to reconstruct

the Tertiary vegetation of the Sierra Nevada

• develop a chronological overview of Tertiary geology, cli-

mate, and vegetation for the Sierra Nevada

• present floral lists and maps from published reports on

Tertiary fossil deposits of the Sierra Nevada and neighbor-

ing regions and

• summarize points relevant to ecosystem management of

the Sierra Nevada

The time frame for this chapter is the Tertiary period, as I

define the boundaries from 65 Ma to 2.5 Ma (table 5.1), with

focus on the Miocene and Pliocene epochs. Although a thor-

ough understanding of the biogeographic and phylogenetic

origins of modern Sierra species requires studying their pres-

ence in fossil floras throughout western North America and

beyond, the focus here is on what was and what was not in

the Sierra Nevada during the Tertiary. Thus, the geographic

focus is the greater Sierra Nevada region and parts of west-

ern and central Nevada, specifically the area defined by the

fossil floras chosen for inclusion here (figure 5.1).



A S S U M P T I O N S

1. This review is not intended to be exhaustive or compre-

hensive. Literature citations to more in-depth analyses are

provided.

2. The focus is on plants and vegetation primarily, geology

and climate secondarily; animals are not considered.

3. Confidence in knowledge decreases as we look further into

the past; the biases of the fossil record and interpretation

are discussed. Historical reconstruction is fraught with

speculation.

4. Systematics and dating of the original interpretations of

fossils are accepted unless subsequent revisions specific

to the flora were published, or unless subsequent publica-

tions cast doubt on identifications. Other than these revi-

sions, no modernization of nomenclature or taxonomic

revision is attempted. Taxonomic revisions often lead to

significant reinterpretations of biogeographic and evolu-

tionary events. Examples of these are given to indicate the

tenuousness of interpretations and the dependence on ac-

curate taxonomy.

5. Detailed projections by original authors about paleoclimate

(especially specific temperatures) and paleoaltitudes are



T A B L E   5 . 1

Geological time chart for the Quaternary and Tertiary

periods of the Cenozoic Era, showing approximate ages

and durations of epochs (Odin 1982; Shackleton and

Opdyke 1977; Swisher and Prothero 1990; Woodburne

1987).


Period

Epoch

Millions of Years Ago (Ma)

Quaternary

Holocene

0–0.01 (last 10,000 years)

Pleistocene

0.01–2.5


Tertiary

Neogene


Pliocene

2.5–7


Miocene

7–26


Paleogene

Oligocene

26–34

Eocene


34–54

Paleocene

54–65


73

Tertiary Vegetation History

Tertiary Fossil Floras in Sierra Nevada and Adjacent Regions in Nevada map 1

F I G U R E   5 . 1

Distribution of Tertiary floras in the Sierra Nevada and adjacent regions of western and central Nevada. Floras are numbered

in approximate order of age (from young to old). 

Note: Floras 3–10 are pollen sites, interpretations are based on a very small

number of grains, and species identifications may be incorrect. These sites should be treated with question.


74

V O L U M E   I I ,   C H A P T E R   5

often not summarized here. New methods have cast doubt

on the validity of some specific interpretations. Those new

techniques have not yet been applied to Sierran Tertiary

floras. Thus, generalizations about climate and environ-

ment are conservatively given.

6. Knowledge gained by understanding the origins of mod-

ern vegetation in the Sierra Nevada is relevant to ecosys-

tem management of the ecoregion.

R E V I E W   O F   M E T H O D S   F O R

R E C O N S T R U C T I N G   T E R T I A R Y

V E G E TAT I O N

Time Periods

The classification of time into eras, periods, and epochs (the

geological time chart) is somewhat arbitrary, implying that a

continuous process, time, is divisible. “Since geological time

is not salami, slicing it up has no particular virtue” (Vita-Finza

1973). Despite this fundamental contradiction, historic events

do tend to occur more or less periodically, lending themselves

to description in pieces rather than as a continuum. Perio-

dicities of tectonic, climatic, and biotic events are not often

synchronous, however, from place to place or between plant

and animal events. Boundaries of time periods are thus spe-

cific to regions, to biotas, and to causes (climate, paleomag-

netic events, biotic changes).

Classification into geologic time periods was especially

important in early paleontologic interpretation. Before direct

dating methods were available, age of a fossil flora was as-

signed based on geologic stratigraphy and correlation to other

local fossil floras. Radiometric dating (Dalrymple and

Lanphere 1969; Steiger and Jager 1977) has relieved the temp-

tation to date by correlation, since—within tolerances and

errors—fossil floras can be directly dated. This both improves

the accuracy of assigning floras to periods in the geologic time

chart and relieves pressure for relying on those assignments

to periods, since many floras can be discussed by direct age

rather than by period. However, although radiometric meth-

ods have been available for several decades, not all of the

Tertiary fossil floras in the Sierra Nevada (originally dated

by stratigraphic correlation) have been confirmed radiometri-

cally, and many that have been confirmed were dated in the

early years of radiometry, when techniques were less accu-

rate than recent methods. Quaternary sites discussed in

Woolfenden 1996, by contrast, are almost all radiometrically

dated. With the increased availability of accurate radiometric

dating, discussion about stratigraphic definitions of bound-

aries, once a topic of intense debate, has subsided.

I have included radiometric dates in this report where avail-

able. I do not defend a strict view of the dates for boundaries

of epochs or eras, instead accepting that they are guidelines

for orientation in the past. For convention, I adopt the

Mesozoic:Cenozoic boundary at 65 Ma (Odin 1982), the be-

ginning of the Northern Hemisphere ice ages for the

Tertiary:Quaternary boundary at 2.5 Ma (Shackleton and

Opdyke 1977; references in Thompson 1991), and combined

North American floristic and land-mammal stages for the

Tertiary epochs (table 5.1) (Odin and Curry 1985; Swisher and

Prothero 1990; Wolfe 1981; Wood et al. 1941; Woodburne 1987).

Workers in the field, including several reviewers of this chap-

ter in manuscript, propose alternative dates for time periods.

This underscores the fact that boundaries depend on which

factors are considered significant in the history of the earth. I

do not include here a review of alternative dating for the ep-

ochs of western North America.



Biases in Historical Interpretation

Misinterpretations of vegetation history occur due to inher-

ent biases in the fossil record, errors in understanding the

record, and cumulative errors due to subsequent analysis.

Each of these is discussed in turn.

Biases in the Fossil Record

The single most frustrating reality about reconstructing past

events is that there are gaps in the fossil record. These occur

due to limited exposures in time and space of fossil-bearing

rocks and sediments of successively older ages. For the Si-

erra Nevada, exposure of Tertiary rocks is uneven. No fossil-

bearing deposits of the earliest Tertiary (Paleocene) are known;

Eocene and Oligocene fossil floras are limited in extent and

are present mostly in the northern Sierra; Miocene and

Pliocene records are somewhat more numerous. Even for the

middle to late Tertiary, however, much better representation

occurs in adjacent western and central Nevada. Fortunately,

these floras contain many species that later appear in the Si-

erra Nevada and thus provide important material from the

perspective of the Sierra Nevada.

Tertiary records in western North America are primarily

impression macrofossils, that is, imprints left when leaves,

twigs, or fruits (macro-organs) were deposited in wetland

sediments (lake bottoms, bogs, marine environments, or other

wetlands). Occasionally, petrified organs and tissues are

found. In these, chemical replacement of living tissues has

occurred, leaving a nearly identical replica of the internal and

external anatomy of the organ or tissue (usually wood or

cones). From a regional perspective, macrofossil deposits bias

the sampling in that they overrepresent wetland species (wil-

lows [Salix], cottonwoods [Populus], etc.) that are adjacent to

depositional sites and underrepresent upland species. Sam-

pling is assumed to be limited to plants growing about 1 km

(0.62 mi) from the site of deposition (Gregory 1994). Beyond

this distance, smaller leaves are preferentially preserved over

large leaves, as are thick, tough leaves over fragile ones. By

and large, conifer remains are readily preserved if they get



75

Tertiary Vegetation History

into a deposit. Their usual ecological position in the uplands,

however, may limit their representation in the deposit. For

all these reasons, the number of specimens of a single type

occurring in a fossil deposit is usually not correlated with its

abundance in the environment, and many contemporaneous

species may be left out of the deposit altogether. Several other

biases due to preservation of individual macrofossil speci-

mens, referred to collectively as taphonomic bias, distort the

sampling and recovery of species from macrofossil deposits

(Greenwood 1992; Spicer 1989; Wolfe and Upchurch 1986).

Macrofossils also occur in the arid parts of the Sierra Ne-

vada region in wood rat middens (Betancourt et al. 1990).

These do not date to the Tertiary and are not considered here.

The other important plant remains from the Tertiary are

pollen grains and other microfossils. Wind- and waterborne

pollen is preserved in wetland sediments of lakes and bogs.

Pollen in these sediments is usually recovered from long cores

bored through lake sediments. Sampling like this has a sig-

nificant advantage over macrofossil deposits in that a con-

tinuous stratigraphic record through time may be obtained,

with much better control on species mixes, stratigraphic ori-

entation, and changes over time than lakeside macrofossil

deposits can offer. Pollen samples infrequently have been

taken from solid exposed sediments rather than from a core,

a technique that eliminates or reduces the opportunity to ana-

lyze a continuous record.

Pollen sampling is a common method for Quaternary analy-

sis, but only recently has it been applied to Tertiary sediments.

The Tulelake core (Adam et al. 1990; Adam et al. 1989) is the

only published continuous core for the Sierra Nevada region

that extends into the Tertiary, although other deep cores are

currently under analysis. Most notable is the Owens Lake

study, which provides continuous analysis of a sediment core

into the early Pleistocene (Owens Lake Core Study Team 1995).

Pollen floras suffer different kinds of systematic biases from

those of macrofossil deposits. Species with abundant and

wind-borne pollen grains are disproportionately represented.

Of these, there is a bias related to distance, in that pollen trav-

els different distances depending on species. For example,

because of its size and shape, 95% of giant sequoia (Se-

quoiadendron giganteum) pollen falls within 500 m (1,500 ft) of

a native forest source (Anderson 1990), whereas pine (Pinus)

pollen can travel hundreds of kilometers and still be an abun-

dant type in a pollen sample. Biases due to size of the deposi-

tional basin also occur. Pollen grains of different species

degrade with time, and differential preservation is especially

important in old samples, such as Tertiary pollen cores. For

several of these reasons, pine, fir, and spruce may dominate

the pollen record in numbers disproportionate to their repre-

sentation in the original flora. Methods to calibrate these bi-

ases are routinely applied (Overpeck 1985; Prentice 1985).

Other kinds of microfossils are often identified along with

pollen in Quaternary samples. These include diatoms, chryso-

phyte cysts and scales, radiolarians, coccoliths, ostracods, and

occasionally foraminifera (in saltwater basins). Charcoal and

some macrofossils (leaf tissue) may also be included in lake

sediment cores. Charcoal can provide information about fire

occurrence.



Biases in Reading the Record

Analysis of any fossil flora hinges critically on accurate sys-

tematic interpretation of specimens. Opportunities for

misidentifying macrofossils are abundant, because of poor

preservation (e.g., only part of a leaf or cone was imprinted

or intact), changes in size, shape, or structure due to preser-

vation, distant relationship to modern taxa (there is no living

analog), hybridization, and natural variation in the species.

Because so much interpretation depends on correct identifi-

cation, old fossil floras have been reviewed and their system-

atics revised; these revisions have sometimes been as dramatic

as assigning a specimen to a different kingdom from that in

the original publication. Individual paleobotanists vary in

their willingness to make identifications, with some assign-

ing specimens confidently to species and others listing only

family or genus. Methods have been developed to assess

physiognomy of fossil remains independent of taxonomic

identification (described below), thus circumventing the de-

pendence on correct systematic identification for some kinds

of analysis.

Microfossils also may be misidentified, but the risk is lower

in part due to the lack of diagnostic characters for identifying

pollen to lower taxonomic levels and the reduced temptation

to try. Thus, pollen is often identified only to genus, some-

times even to a combined family level (e.g., TCT, Taxodiaceae-

Cupressaceae-Taxaceae). The lack of species diagnostics limits

the usefulness of pollen analysis in studies that require knowl-

edge of individual species.

Some fossil floras have been independently analyzed for

macrofossils and for pollen. These provide the opportunity

to compare information from the two data sets and assess the

relative effectiveness of one or the other method. The Chalk

Bluffs fossil flora near Nevada City (figure 5.1; table 5.2; ap-

pendix 5.1, list 3), originally described by an extensive mac-

rofossil list (MacGinitie 1941), was reevaluated for pollen taxa

by Leopold (1983, 1984). This analysis revealed the biases of

both approaches. Pollen did not diagnose individual species

and was unable to record taxa from four families found in

macrofossils, yet it added taxa from eight families not recorded

in the macrofossils. The additional taxa were mostly wind-

pollinated species. Despite the differences in representation

of individual taxa, the vegetation and climatic interpretation

of the flora was similar between the two methods, that is, that

this assemblage was a rich subtropical forest in a warm, moist

climate. A significant addition from the pollen was the pres-

ence of taxa from the pine family (pine [Pinus], fir [Abies],

spruce [Picea]), with implications discussed later. Other com-

parisons of Tertiary pollen and macrofossil floras have yielded

greater discrepancies (e.g., only 38% correlation of taxa among

methods for a Washington flora [Reiswig 1983], 18% correla-

tion for a northwestern California flora [Barnett 1983]).




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