Identification and Ecology of

Old Ponderosa Pine Trees in

the Colorado Front Range

Laurie Stroh Huckaby, Merrill R. Kaufmann,

Paula J. Fornwalt, Jason M. Stoker, and Chuck Dennis

United States

Department

of Agriculture

Forest Service

Rocky Mountain

Research Station

General Technical

Report RMRS-GTR-110

September 2003
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E-mail rschneider@fs.fed.us

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Rocky Mountain Research Station

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Fort Collins, CO 80526

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Fort Collins, CO 80526

Huckaby, Laurie Stroh; Kaufmann, Merrill R.; Fornwalt, Paula J.; Stoker, Jason M.; and

Dennis, Chuck. 2003. Identification and ecology of old ponderosa pine trees in the

Colorado Front Range. Gen. Tech. Rep. RMRS-GTR-110. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station. 47 p.

Abstract

We describe the distinguishing physical characteristics of old ponderosa pine trees in the

Front Range of Colorado, the processes that tend to preserve them, their past and present

ecological significance, and their role in ecosystem restoration. Photographs illustrate identifying features of old ponderosa pines and show how to differentiate them from mature

and young trees. The publication includes a photographic gallery of old ponderosa pine

trees growing on poor, moderate, and good sites. We illustrate trees growing under various forest conditions and with different injuries and histories. We discuss

dendrochronological methods of aging old trees and determining their fire history. The

companion field guide includes a condensed description of ponderosa pine ecology, distinguishing characteristics of old ponderosa pines, and a photographic gallery illustrating

their identifying features.

Keywords: Colorado Front Range, ponderosa pine, old growth, fire ecology

All illustrations and photos by Laurie S. Huckaby, unless otherwise noted.

The trees pictured from Cheesman Lake all burned in the Hayman fire in

June 2002.

On the cover: An old ponderosa pine growing on an open, south-facing slope in the

Arapaho-Roosevelt National Forest. It is more than 350 years old.
Contents

Introduction ....................................................................................................... 1

Ecological Zones of the Colorado Front Range ............................................... 1

Ponderosa Pine and the Montane Zone of the Front Range ................................ 4

How Old is Old? ............................................................................................ 4

The Concept of Old Growth and Ponderosa Pine ........................................... 4

Old-Growth Ponderosa Pine Stands in the Front Range.................................. 5

Fire Ecology of the Montane Zone.................................................................. 5

Old Trees in the Landscape .............................................................................. 10

Stand Structure ............................................................................................. 10

Historical Landscape .................................................................................... 12

Modern Landscape ....................................................................................... 14

The Urban Interface Problem ....................................................................... 14

Forest Restoration and Old Trees .................................................................. 16

Identification of Old Ponderosa Pine Trees in the Front Range ......................... 16

Appearance of Old Trees (>200 years).......................................................... 16

The Transition From Young to Old (Canopy Trees

150 to 250 Years Old)............................................................................... 20

Appearance of Young Trees (Canopy Trees <150 Years Old) ......................... 20

Tree Size ....................................................................................................... 23

Open-Grown Versus Forest-Grown .............................................................. 24

Types of Injuries ........................................................................................... 24

Common Locations of Old Trees .................................................................. 26

Gallery of Old Tree Photos............................................................................... 31

How We Found Trees for This Publication.................................................... 31

Old trees on poor sites (fig. 29 a, b, c, d, e, f) ............................................... 32

Old trees on moderate sites (fig. 30 a, b, c, d, e, f) ....................................... 35

Old trees on good sites (fig. 31 a, b, c, d, e, f) .............................................. 38

Conclusion ...................................................................................................... 41

Why Keep Old Trees? ................................................................................... 41

Postscript ......................................................................................................... 41

Acknowledgments ........................................................................................... 41

Bibliography .................................................................................................... 42

Appendix ......................................................................................................... 45

Dendrochronology ....................................................................................... 45

Glossary of Terminology ............................................................................... 46

USDA Forest Service Gen. Tech. Rep. RMRS-GTR-110. 2003. 1

Introduction

Old trees were historically a major component of

montane forests in the Colorado Front Range. They

were an integral part of the spatial and temporal

heterogeneity inherent in the ecosystem. Now they are

relatively scarce. Many were harvested during the

settlement period in the late 19th and early 20th centuries.

Century-old stumps of trees that were 400 to 500 years

old when cut remain like ghosts amid stands of trees

that have established during the last 100 years. Other

old trees were destroyed in extensive human-caused

fires during the same period. Surviving old trees are

now stressed by competition from dense ingrowth of

younger trees and are in danger from insect outbreaks

and stand-replacing fires (Biondi 1996).

Land managers are now aware that modern forest

structure differs from that of the pre-Euro-American

settlement landscape, and that change in structure is

precipitating a change in disturbance processes such

as fire and insect outbreaks. Urban encroachment into

the montane zone puts pressure on land managers to

mitigate the danger of wildfire. The best way to handle

this danger is to restore an ecologically sustainable

landscape structure in the montane zone of the Front

Range, one that resembles the patterns that existed

before Euro-American settlement—patterns that

encouraged the survival of old trees. Ecological

restoration requires that we understand what the

ecosystem was like in the past and what processes

regulated and maintained it. Old trees preserve much

of this information. The ecosystem records its own

history in the form of tree age structure, species

distribution, tree ring patterns, and fire scars. Historical

accounts from the settlement period, survey records,

old photographs, and early research provide a human

perspective on the ecosystems of the past.

Restoration of sustainable ecological conditions can

be accomplished by management activities that mimic

historical structure and processes through mechanical

thinning and prescribed burning, guided by scientific

studies that reconstruct the structure, composition and

processes of the historical landscape (Kaufmann and

others 1994, Swetnam and others 1999). Restoration

management based on historical ecology can protect

and restore old-growth stands. However, unless the old

trees are explicitly identified and protected, forest

restoration activities could inadvertently damage or

destroy many old trees. Old ponderosa pine trees are a

legacy of the pre-Euro-American settlement past—a

past not so distant in time and hopefully one that it is

still possible to recapture. In addition to the historical

information contained within their rings, old ponderosa

pine trees are resistant to surface fire, provide food and

habitat for wildlife, and are a source of genetic

continuity for the forest. They are aesthetically

attractive, and they are an intrinsic part of the beautiful,

dynamic landscape that draws thousands of people to

the Front Range every year.

This guide is written to help the reader identify old

trees and understand their past and present role as

components of forests in the lower montane zone. The

Appendix gives additional detail on dendrochronological methods used for determining ages of

trees and fire history. The Glossary defines some of

the more technical terminology used in this publication.

The Bibliography gives references and further reading

on ponderosa pine in the Front Range.

Ecological Zones of the Colorado Front

Range

For this publication, we define the Front Range as

the area south of the Wyoming border, east of the

Continental Divide, west of the Great Plains, and north

of Pikes Peak (near Colorado Springs; fig. 1a,b). Two

related environmental factors drive ecological

processes and the distribution of species in the Front

Range: elevation and moisture availability. Going up

in elevation has effects similar to going up in latitude.

Identification and Ecology of Old Ponderosa Pine Trees in the

Colorado Front Range

Laurie Stroh Huckaby, Merrill R. Kaufmann, Paula J. Fornwalt,

Jason M. Stoker, and Chuck Dennis
2 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-110. 2003.

accompanied by lodgepole pine and limber pine,

forming a transitional mixed conifer forest.

Above 9000 ft (2770 m), where there is usually a

persistent winter snowpack, the ponderosa pine and

Douglas-fir begin to drop out of the forest mix, and

are replaced by lodgepole pine, patches of aspen and

limber pine, subalpine fir and Engelmann spruce.

Above 10,000 ft (3077 m), the lodgepole pine and aspen

become scarcer, and forests are dominated by mixtures

of spruce and fir up to treeline at around 11,500 ft.

(3540 m). Bristlecone pine and limber pine occur in

isolated stands near treeline. Above 11,500 ft

(3540 m), the growing season is too short for trees,

and the vegetation is composed of low-lying tundra

plants similar to those found in the Arctic (Marr 1961).

Our focus is the montane zone, between 6000 and

9000 ft (1845 to 2770 m) elevation, below the level of

persistent snowpack. Most of this elevational zone is

dominated by forests of ponderosa pine mixed with

Douglas-fir, and interspersed with shrubs, grasses and

aspen.

Figure 1a. Overhead view of the Front Range showing the

elevational gradient. Light green designates the plains at

around 5000 ft (1540 m). The montane zone is shown as

dark green. The subalpine zone is shown as a gradient

of tan to redish brown; white areas are above 14,000 ft

(4300 m). Sampling sites are denoted by yellow stars.

(Map by J. Stoker.)

At higher elevations, the growing season becomes

shorter, temperatures are cooler, and precipitation is

greater. Fire and other disturbances become less

frequent at higher elevations. Changes in vegetation

composition along this gradient reflect these

environmental changes (fig. 2).

The lowest elevations, where conditions are the

warmest and driest, support shortgrass prairie that

grades into shrublands in the foothills. Between 5000

and 6000 ft (1540 to 1845 m) elevation, these

ecosystems gradually transition into open ponderosa

pine forest. The ponderosa pine commonly becomes

mixed with Douglas-fir and aspen at higher

elevations. The aspect of a slope has an important

effect on forest species composition at low and

middle elevations. Slopes that face south, west, and

southwest are exposed to intense sunlight and dry

prevailing winds. These warm, dry slopes support

stands dominated by ponderosa pine. Douglas-fir is

more common on slopes with a northerly component.

Above about 8000 ft (2460 m), these species are
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-110. 2003. 3

1b. Three-dimensional view of the Front Range, looking west from the plains. The montane zone (6000 to 9000 ft [1846 to

2770 m] elevation) is shown in blue. Maps by J. Stoker.

Figure 2. Changing vegetation communities along the elevational gradient of the Front Range, with approximate transitional

elevations. Elevation is shown in feet.
4 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-110. 2003.

Ponderosa Pine and the Montane Zone

of the Front Range

Ponderosa pine (Pinus ponderosa Dougl. ex Laws.)

is one of the most widespread tree species in the western

United States, ranging from northern Mexico into

southern Canada, and from California to Nebraska. The

Rocky Mountain variety is called scopulorum, to

distinguish it from the variety found in the Pacific

Northwest (Oliver and Ryker 1990) and from the

subspecies arizonica, found in the Southwest near the

Mexican border. In the Colorado Front Range (fig. 1b),

ponderosa pine grows from the border of the prairie

and the foothills, up to around 9000 ft (2770 m)

elevation, depending on topography. It may be found

as high as 10,000 ft (3077 m) on warm south-facing

slopes. It is the dominant tree in the montane zone (6000

to 9000 ft; 1845 to 2770 m), where it often occurs with

Douglas-fir (Pseudotsuga menziesii), especially on

north-facing slopes (Peet 1991). Ponderosa pine can

grow to be very large in other parts of the country, but

rarely exceeds 100 ft (30 m) in height in the Front

Range. It is tolerant of heat, drought, and fire (Oliver

and Ryker 1990).

Ponderosa pine trees have needles in fascicles of

two or three. Each tree produces both male and female

reproductive structures; the female strobili mature into

woody cones. Seeds are moderate in weight for tree

seeds, and may be widely dispersed by wind. Ponderosa

pine is susceptible to infestation by dwarf mistletoe

(Arceuthobium vaginatum) and to attack by several

insects, most notably the mountain pine beetle

(Dendroctonus ponderosae) (Oliver and Ryker 1990).

Ponderosa pine has long been an important tree for

timber, though logging has not been a major industry

in the Front Range in recent decades (Veblen and

Lorenz 1991).

How Old Is Old?

Ponderosa pine is a long-lived species. The oldest

known ponderosa pine trees are more than 700 years

old (843 years, Central Utah; 780 years, Mt. Rosa, CO;

742 years, NW Arizona; OLDLIST database). The

oldest known ponderosa pine trees in the Front Range

are a little over 600 years old (Huckaby and others

2001; C. Woodhouse, J. Lukas, P. Brown, personal

communication). However, ponderosa pines of that age

are uncommon in the Front Range, whether because

of past land use, the frequency of past natural

disturbances, or the physiological limits of the species

under local conditions. Trees between 300 and

500 years old are widespread though only locally

frequent, while trees more than 200 years old are fairly

common throughout the Front Range above about

6500 ft (2000 m) elevation.

The Concept of Old Growth and

Ponderosa Pine

A single old tree does not make an old forest. Even

a scattering of old trees does not necessarily define an

old-growth stand. Old growth is a stand condition that

develops over time. Old-growth stands have special

characteristics that result from long years of growth in

the absence of stand-replacing disturbance. Old-growth

forest structure varies with forest type, climate, site

characteristics, and disturbance regime, but it is

distinguished from younger growth by large,

physiologically old (for the species and local site

conditions) trees as stand dominants, variation in tree

size and spacing, accumulations of dead standing and

fallen trees relative to younger stands, decay, multiple

canopy layers, gaps, and understory patchiness

(Kaufmann and others 1992).

A workable definition of old growth cannot be

absolute in terms of some minimum age for the trees

or some maximum number of canopy layers. The

characteristics that constitute old growth vary with the

forest type, local site productivity, and the natural

disturbance regime. However, in all old-growth stands,

competition among dominant trees is minor.

Senescence and mortality are important processes,

since the oldest cohorts are reaching their natural

longevities as determined by site and environment.

Physiologically, the old trees have reached their

maximum heights for the site conditions and are no

longer growing taller. Their crowns are declining, and

their respiration equals or exceeds their productivity.

They produce secondary metabolic products that may

provide some resistance to insects and diseases. In an

old stand, net productivity is low, even zero or negative,

as newly produced biomass is offset on average by

respiration and mortality of trees or tree parts

(Kaufmann and others 1992).

Old growth is a temporary condition, as are all stages

of forest development. Stand replacement will

eventually occur, either by disturbance or as the stand

eventually deteriorates so that it no longer functions

as old growth, though this may take a very long time.

There are multiple pathways by which stands can

become old growth, even through disturbance. Species

typically considered seral can become old growth if

low intensity disturbance maintains their dominance,

such as surface fire in ponderosa pine (Moir 1992).
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-110. 2003. 5

Old growth forest does not necessarily have to be

undisturbed, even by human activity, as long as the

disturbance does not disrupt the stand structure and

function.

How much old growth existed in pre-Euro-American

settlement times? How much area did it cover? It is

impossible to know for certain, but scientific studies

reconstructing historical forests have suggested that old

growth historically constituted a significant component

of the landscape mosaic in ponderosa pine systems

(Arno and others 1995, Fulé and others 1997, Huckaby

and others 2001, Veblen and Lorenz 1991). The area

occupied by old-growth stands was determined by

historical disturbance regimes, and old growth may

have been more common in some forest types than

others. The proportion of old growth probably varied

over time as well. In the current landscape, old-growth

stands are usually small, fragmented, and isolated.

Scattered individual old trees are more common now

than are old-growth stands.

Old-Growth Ponderosa Pine Stands in the

Front Range

Old-growth ponderosa pine in the montane zone of

the Front Range differs from the usual perception of

old-growth forest, which is most commonly derived

from subalpine or Pacific Northwest models. Most

people think of stands of big trees as old growth, but

that is not necessarily true. Young trees on good sites

may be very large, and some of the oldest trees exist

on poor sites and are relatively small.

Some sources define old growth as any preEuro-American settlement forest (Morgan 2000), but

we prefer a more physiologically based definition.

Ponderosa pine trees begin to change their growth and

appearance at around 200 years of age (Kaufmann

1996). Stand structure begins to change around that

age, too. By the time the dominant trees are 300 to 400

years old, most stands have developed old-growth

characteristics, including coarse woody debris, snags,

a variety of tree ages and sizes, dominant trees at the

maximum size for the site conditions, and a disturbance

equilibrium (Kaufmann and others 1992).

The Front Range montane zone is a dry,

unpredictable environment and is relatively

unproductive in terms of plant growth and diversity.

Trees grow slowly and do not become very large.

Disturbance is inherent in ponderosa pine forests. Lowand mixed-severity fires helped maintain open

conditions and species composition over time. In the

case of ponderosa pine forests, exclusion of disturbance

such as fire is itself a disturbance, one that shifts species

composition away from ponderosa pine. Historically,

disturbances with variable effects created a complex

mosaic of stands of different ages across the landscape.

Without local stand-replacing disturbance, old stands

reached a kind of equilibrium and could persist for

centuries (Huckaby and others 2001).

Stands of old-growth ponderosa pine are now

relatively rare in the Front Range because of past

logging and wildfire. However, individual old trees are

not uncommon, and in many locations stands of trees

that were too small to be cut during the settlement

period are now around 200 years old, poised to become

the old growth of the future.

Fire Ecology of the Montane Zone

Historically, fire was the primary regulator of forests

in the Front Range. The fire regime of an area is the

pattern of fire severity and frequency over a period of

time. Fire regimes are influenced by climate, fuels,

topography, and ignition sources, which are

interrelated. Fire history reconstructions have shown

that before Euro-American settlement of the Front

Range (beginning with the influx of settlers in the gold

rush of 1859), a fire occurred in any given location in

the lower montane zone on average every 10 to 20 years

(Brown and others 2000, Brown and others 1999,

Donnegan and Veblen 2001, Goldblum and Veblen

1992, Veblen and others 2000). Larger fires occurred

at longer intervals, on the order of 30 to 60 years

(Brown and others 1999). The more frequent but less

intense surface fires probably thinned out dense stands

of young trees and ingrowth, maintaining an open forest

(fig. 3a, b). Occasional mixed-severity fires included

a stand-replacing component as well as surface fire.

These fires created openings (fig. 4a, b, c) by killing

all the trees in patches where the forest was dense or

when burning occurred under extreme weather

conditions, but they left many survivors. Most preEuro-American settlement fires in the montane zone

were of low or mixed severity (Brown and others 1999,

Heyerdahl and others 1994).

During regionally dry years such as 1851, fires

burned many thousands of acres throughout the west

(Swetnam and Baisan 1996). Fires with more severe

effects may have occurred very infrequently; carbon

14 dating of charcoal layers in an alluvial fan near

Buffalo Creek indicates that severe erosion following

fires occurred there at approximately 1000-year

intervals over the last 2900 years (Elliot and Parker

2001).
6 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-110. 2003.

Figure 3a. Surface fire in a ponderosa pine stand burns

the grass, shrubs, and small trees, but the larger trees

are protected by their thick bark and pruned-up crowns.

If the fire is hot enough to kill the cambium on one

side, mature trees may be scarred. This prescribed

fire on the Arapaho-Roosevelt National forest burned

in June 2001.

3b. A stand of large, older trees a few days

after a surface fire. The grasses, shrubs, and

small trees have burned, opening up the

stand, while the larger, older trees are mostly

undamaged. The understory vegetation will

regenerate within a few years. This area

burned as a surface fire in the Bobcat Gulch

fire in 2000, but it had also burned in 1993.

That fire reduced fuels and made the area

less susceptible to crown fire. Photo by M.

Kaufmann.
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-110. 2003. 7

Figure 4. When weather conditions are

hot, dry, and windy and fuels are

dense, fire in ponderosa pine forests

can be intense. Crown fire may kill

most of the trees in an area. Dense

young stands are susceptible to this

kind of fire behavior, and much of

the Front Range is now covered by

this type of forest. The Schoonover

fire (a) in May 2002 burned almost

4000 acres (1670 ha) in the South

Platte Basin near Trumbull,

Colorado. It made a wind-driven run

on the afternoon of May 22. The

Hayman fire overlapped the area

burned by Schoonover two weeks

later.

(c) A fire at Cheesman Lake in 1851

burned some patches intensely,

leaving behind openings that were only

beginning to fill in with trees 150 years

later when burned in the Hayman fire

(a by Paula Fornwalt; c by M.

Kaufmann).

The Bobcat Gulch

fire (b) in June

2000 burned

10,600 acres

(4417 ha), much

of it as a crown

fire.
8 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-110. 2003.

Figure 5a. Fire scar face on an old pine along Highway 7 at

Olive Park, CO. Its pith date is 1589.

5b. Fire scars on an old tree above Trumbull, CO, near the

South Platte River.

Mature ponderosa pines have thick bark that insulates

the cambium from heat. Seedlings and saplings lack this

thick bark and are susceptible to fire kill. Many old trees

have distinctive scars at the base from past fires that were

hot enough to kill the cambium on a portion of the tree’s

circumference (fig. 5a, b). If the cambium is killed all the

way around, the tree will die. If only part of it is killed, the

tree often survives and heals in subsequent years by

growing wood around the wound. This wood is often

soaked with pitch, which the tree secretes to repel insects

and fungi from the wound. The pitch is very flammable,

so a tree scarred by fire once is likely to scar again. These

scars show up as distinctive patterns in the tree rings, which

allow dating fires to a precise year, and sometimes to the

part of the growing season in which they occurred (fig.

6a, b, c, fig. 7b) (Arno and Sneck 1977, Brown and others

1999, McBride and Laven 1976, Stokes and Smiley 1968).

This method of cross-dating fire scars allows

reconstruction of historical fire regimes (see the Appendix

for details).

Fire behaves differently in different ecosystems. Fire

was historically more frequent at lower elevations,

especially on the plains and in the adjacent foothills (Mast

and others 1998). Fire frequency decreased and severity

increased with elevation. Above 9000 ft (2770 m) elevation

where lodgepole pine dominates forests, most fires were

stand-replacing crown fires that occurred at intervals of

150 to 300 years. Spruce-fir forests usually burned as

stand-replacing crown fires within 300 to 500 years or

more (Peet 1991). Humans have had much less influence

on these longer fire regimes through fire exclusion than

on the more frequent montane fire regime. The natural

fire regime of ponderosa pine forest in the Front Range

also differs from the well-known patterns in ponderosa

pine in the Southwest, where more predictable summer

rainfall and longer growing seasons allowed the forest to

be more productive. There, surface fires were more

frequent and more strongly driven by the El Niño climatic

cycle, occurring on the order of every three to seven years,

and most historical fires did not have a stand-replacing

component (Covington and Moore 1994, Woolsey 1911).
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-110. 2003. 9

In pre-1860 montane forests, grasses and shrubs in

the understory of open woodlands periodically provided

continuous fuels that allowed fire to spread across the

landscape. Periods of wet weather allowed fuels to

accumulate. Subsequent dry weather cured the fuels

and made them more flammable. When fuels had built

up over time and the conditions were right, larger, more

intense fires occurred that usually had mixed effects

(Brown and others 1999). Warm, dry periods, periods

of high year-to-year variability, and periods of intense

El Niño-Southern Oscillation (ENSO) activity

encouraged more frequent fires. Cool, wet periods and

periods of decreased variability and ENSO activity

allowed for longer intervals between fires, greater

buildup of fuels, and more intense fires when they did

occur (Donnegan and others 2001).

The role of Native Americans in the Front Range

fire regime is poorly documented and not well

understood. While Native Americans may have ignited

fires either intentionally or by accident, the historical

fire regime was strongly climate-driven, and it seems

unlikely that human ignitions altered the fire regime

significantly except on a local scale (Cassells 1983).

Most historical fires in the Front Range were probably

started by lightning immediately before or during the

growing season. Fire frequency increased in many (but

not all) montane locations during the settlement period,

especially around areas of mining and railroad activity

(Brown and others 1999, Donnegan and others 2001,

Veblen and others 2000). This was due in part to an

6c. A cross-section of a fire-scarred ponderosa pine. The

fire scar face is toward the top of the photo, and the fire

scars are marked with black arrows. This tree was cut in

a thinning operation near Trumbull in 1999. It has been

sanded and cross-dated. The pith date is 1690. The tree

was scarred by fires in 1717, 1765, 1820, 1860, and 1880,

and it sustained another injury in 1943. The scars in 1717

and 1765 had healed completely before the next fire

occurred.

Figure 6a. Sampling a fire-scarred stump by cutting a

cross-section.

6b. Fire scars can be sampled from living trees by cutting

out a partial section through the scar face.
10 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-110. 2003.

increase in human-set fires, but also to a change in

climate. The preceding period between 1790 and 1840

was one of less annual variability and decreased ENSO

activity, and this change affected fire regimes

throughout the Southwest and Southern Rocky

Mountains (Donnegan 2001). Fire was less frequent

during this period, allowing for a build-up of fuels.

After 1840, increasing fire frequency coincided with

greater year-to-year variation in precipitation and

drought as well as increased human ignitions due to

Euro-American settlement.

By the early 1900s, fire frequency in the montane

zone dropped dramatically. In some locations, heavy

livestock grazing over the preceding decade or two had

reduced surface fuels and retarded fire spread. Many

areas had been recently logged or burned or both, which

had also reduced fuels. However, organized fire

suppression became effective during the early 20th

century and has continued to the present.

Old Trees in the Landscape

The persistence of old trees is the result of the

coincidence and interaction of many factors, including

the history of land use at a site, the history of natural

disturbance, climate, topography, vegetation, and chance.

Because ponderosa pines may live for hundreds of years,

they are affected by patterns of climate and disturbance

over centuries, as well as by individual events. Each old

tree represents a single case, but old trees also have a role

in the larger landscape. Entire stands of old trees are

unusual, but a few old trees among younger trees in a

stand are a common occurrence.

Interactions across multiple scales in space and time

create a complex distribution of vegetation communities.

Variable frequency and intensity of disturbances

interacting with variability in climate over time and with

variable topography in space created a temporally shifting

mosaic of vegetation across the landscape. Old trees were

always present in the landscape, though locations of old

stands changed over the period of centuries. When looking

at the survival of old trees, one must consider the effects

of disturbance at the scale of a single tree, a stand (a group

of trees with a more or less uniform history), or a landscape

composed of many stands, and the effects of processes

across scales in time.

Stand Structure

Stand structure is the distribution of tree sizes, ages,

and species in a given area. All of these have changed in

the Front Range as the result of human land use, fire

exclusion, and possibly climate change. Many of the

largest and some of the oldest trees were cut during and

after the settlement period. When fire was successfully

suppressed, more seedlings survived than would have

under a natural fire regime. In the absence of fire over

long periods, ponderosa pine may lose dominance in a

stand to other, less fire-tolerant species such as Douglasfir (Oliver and Ryker 1990).

Because ponderosa pine seedlings require mineral soil

and full sun, logging and fires create favorable

opportunities for regeneration. While ponderosa pines

produce cones and seeds most years, not all of the seeds

are viable, and few of them are likely to germinate and

survive to maturity (W. Shepperd, personal

communication). When conditions are favorable, trees

produce a huge crop of seeds, increasing the odds that

some will survive to maturity. Because groups of trees

tend to establish during good conditions, the ages of trees

in a forest are usually clumped (Arno and others 1995,

Savage and others 1996). It is common to see dense groups

of young trees following one of these regeneration events.

Before Euro-American settlement, such groups were

thinned by fires that killed all but a few of the young trees

(fig. 7a,b, c, d). In the absence of fire, most of them survive

and create dense forest.

Figure 7a. A clump of young ponderosa pines thinned by

surface fire. This group of saplings was killed in an area

of surface fire in the Crosier Mountain prescribed burn in

1998. The largest one survived. Note the grass cover

under the trees.
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-110. 2003. 11

7c. Even very small trees can

survive cool surface fires.

This tree will probably

survive, but it may be

scarred by this prescribed

fire, which was intended to

reduce dead fuels on the

ground.

7b. Historical fires were

sometimes locally cool

enough to spare small trees.

Core samples from old trees

at Cheesman Lake show

completely healed fire scars

incurred when the trees

were very small. The top

core is from a tree scarred

by a fire in 1587, when the

tree was about 2.5 inches in

diameter and about 20 years

old. The bottom core is from

a tree scarred by a fire in

1631, when the tree was

only half an inch in diameter

and 16 years old. These

cores also show how slowly

trees can grow in a dry

environment.

7d. A group of seedlings that

germinated following the

Snowtop fire in 1993 was

scorched by surface fire during

the Bobcat Gulch fire in June

2000. Most of them will probably

die. Note the non-native thistles

on the left side of the picture that

invaded following the fire in 2000.
12 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-110. 2003.

Grazing by domestic animals removes grasses and

allows trees to invade meadows where they had

previously been excluded by frequent fire and

competition with grasses (Baisan and Swetnam 1997).

The warming of global climate may also be contributing

to increased seedling survival in recent years. Many of

these 20th century trees are now tall enough to create

“ladder” fuels—a fire burning along the ground may

burn from these small trees with branches close to the

ground into the crowns of the larger trees.

Douglas-fir is now more abundant in the Front Range

than it was historically (Kaufmann and others 2000a).

While Douglas-fir has always shared the montane zone

with ponderosa pine (fig. 8a), many more young

Douglas-fir survive now than when fire was more

frequent. Young Douglas-fir trees have thin bark and a

large proportion of live crown to total tree height (live

crown ratio) that make them more susceptible to fire

(fig. 8b). Old Douglas-firs, however, are quite resistant

to fire. They self-prune as ponderosa pines do, and they

have very thick bark. Many of the older trees also have

fire scars. The species composition of montane forests

is now shifting toward dominance by Douglas-fir even

in areas where ponderosa pine once existed in nearly

pure stands.

Historical Landscape

Before Euro-American settlement, the montane

landscape of the Front Range was a varied, shifting

mosaic of forest stands of different ages, densities, and

species compositions, mixed with grassy openings,

shrub fields, and riparian communities. Disturbances

had different effects in different stands. A fire might

Figure 8a.An old Douglas-fir dominates a west-facing slope

at Cheesman Lake where the young ponderosa pines

are infested with dwarf mistletoe.

8b. A young Douglas-fir grows amid a post-Euro-American

settlement ponderosa pine forest in Rist Canyon. Young

Douglas-fir trees have thin bark and branches that extend

all the way to the ground, making them susceptible to

burning. Trees like these are common in the understory

of ponderosa pine forests in the absence of surface fire.
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-110. 2003. 13

burn quickly and cool through an old, open ponderosa

pine stand on a south-facing slope, causing minimal

damage, but the same fire might burn much more

intensely in a dense young stand of mixed ponderosa

pine and Douglas-fir on an adjacent north-facing slope,

killing many trees and scarring some of the survivors.

Over the centuries, any given location might be

occupied by a shrub field in the years following a standreplacing fire, a dense young stand a century later, or

an open old stand that survived fire after fire until a

hot, wind-driven fire turned it back into an opening

again (Kaufmann and others 2001).

Based on studies done in rare places where there

was no logging, it seems that pre-Euro-American

settlement forests were much more open and spatially

variable than those of the present (Kaufmann et al

2001). They had a larger proportion of old trees and

fewer young trees than modern forests, except where

the occasional stand-replacing fire had burned. Regular

fires maintained open stands—usually less than 30%

canopy cover—and created non-forested areas

(Kaufmann and others 2001, Fornwalt and others

2002). Competition for light and water was minimal in

such stands, and danger of lethal fire to the older trees

was small. A few young trees survived to become large

enough to resist fire and became the next generation forest.

How common were old trees before Euro-American

settlement? There is no way to know for sure, but there

are ways to estimate. The few, rare stands that were

not logged preserve the historical age structure, though

they have also experienced fire exclusion, ingrowth,

and mortality. One of the best examples of an unlogged

landscape in the Front Range montane zone was the

8645 acre (3500 ha) forest around Cheesman Lake

(Kaufmann and others 2001). That area burned severely

in the Hayman fire, but studies done in preceding years

showed that of trees selected as the five oldest in

randomly selected stands, 30% were more than 400

years old, and 40% were between 200 and 400 years

old (Huckaby and others 2001).

Even in stands that were logged, evidence remains

of the pre-Euro-American settlement age structure.

Because the climate of the Front Range is cool and

dry, wood rots very slowly. The stumps of many trees

cut during logging operations in the late 19th century

still exist. Tree-ring dating can determine when these

trees died, and how old they were at that time. Historical

accounts (such as Jack 1900), government land surveys,

early studies, and old photographs (as in Veblen and

Lorenz 1991) give glimpses of what the landscape

looked like around the time of settlement (fig. 9).

Unfortunately, these sources are not quantitative and

do not reflect the period much before settlement.

Computer models can help visualize what the

pre-Euro-American settlement landscape looked like and

how disturbances behaved. Models use information on

Figure 9. This photograph was taken in

December 1896,

west of where the

Cheesman Lake dam

would be in a few

years. There is patchy

snow on the ground.

There had been no

logging or settlement

in the area before the

photograph, but scars

on trees recorded a

fire in 1880. Note how

open the forest is,

how many older trees

there are (designated

by yellow arrows), and

how many fallen logs,

probably trees killed in

the 1880 fire. The

foreground is now

under water (photo

courtesy of Denver

Water).
14 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-110. 2003.

climate, topography, and species characteristics to predict

where and how trees grow (Fornwalt and others 2002).

Modern Landscape

Much of the Front Range foothills and montane zone

was logged during the settlement period, from the late

1850s through the 1920s. Sporadic logging continued

through the 1960s. Many trees were cut during the

1970s and early 1980s in an attempt to control a

mountain pine beetle outbreak. Ponderosa pine was the

primary tree used in building homesteads, fences,

railroads, and mines, and for fuel in settlements and

railroads. Larger trees were preferred, particularly those

without scars or other damage. In some accessible

areas, no old trees remain. The same period saw an

increase in fire frequency and intensity, especially

around settlements and railroads, because of humancaused ignitions (Veblen and others 2000). Many

forests were destroyed in these fires. By the time the

US Forest Service was established in 1905, forests in

the Front Range had been devastated by human-caused

fire, logging, erosion, and overgrazing. The mission of

the Forest Service was to stop the abuses, enforce more

sensible land use, and put out fires. As Native American

tribes were confined on reservations, their influence on

fire regimes was removed (Veblen and Lorenz 1991).

Post-settlement land use changed the disturbance

regime of the landscape and the scale at which it

operated. The effects of human activities homogenized

the landscape (fig. 10). Forest regeneration following

widespread logging and human-caused fires created

large areas where the trees are now about the same age

and size. Many trees were planted during the 1930s to

combat erosion (Gary and Currie 1977, Pike National

Forest archives, Arapaho-Roosevelt National Forest

archives). Lack of subsequent disturbance has allowed

most of these trees to survive, creating dense,

continuous stands. One of the consequences of a

homogeneous landscape is that when disturbances do

occur, they have relatively homogeneous effects, such

as widespread insect epidemics or large fires. What

heterogeneity exists in the current landscape is driven

largely by variation in topography and land use. Artificial

land ownership and management boundaries have

imposed land use and management activities that now

limit the mosaic from shifting spatially as it once did.

Shade-tolerant Douglas-fir and shrubs such as Rocky

Mountain juniper (Juniperus scopulorum), mountainmahogany (Cercocarpus montanus), Gambel oak

(Quercus gambelii), and antelope bitterbrush (Purshia

tridentata) have grown up in the understories of

previously open ponderosa pine forests, creating heavy

fuel conditions and ladder fuels that can carry fire into

the crowns of large trees. Global warming may be

contributing to increased tree growth and survivorship

over the last 50 years as well. Current dense forest

conditions place montane forests at risk of standreplacing wildfires. Old trees that were well adapted

to survive fires in the historical ecosystem are now at risk.

The processes that preserved old trees have been altered.

How common are old trees now? Measuring the age

structure of forest stands on a large scale is impractical,

so there is probably no realistic way to know. It is nearly

impossible to differentiate old trees from remotely

sensed data like aerial photos and satellite images. The

best way to locate old trees is to look for them on the

ground. No one has mapped the forests based solely

on age, but given the land use history in the Front

Range, it is safe to say that old trees are not as common

as they once were (fig. 11).

The Urban Interface Problem

The human population of the Front Range montane

zone has grown considerably in recent years. Many

people have moved into the mountains adjacent to the

large urban centers (fig. 12). Roads fragment the

landscape, limiting species and processes from moving

around as they once did. Fragmented ownership makes

management of the land difficult, especially on the scale

at which most ecosystem processes operate. Fire is one

of the processes disrupted by human settlement.

Since the early 1900s, fire suppression has been very

effective. The montane zone now supports forests that

are younger, denser, and more continuous than those

Figure 10. The homogeneous modern landscape in the

Front Range. This ponderosa pine forest in the South

Platte basin west of Denver is composed of

regeneration following logging and fire in the late 1800s.

The trees are fairly uniform in size and age, and the

forest is dense and continuous over a large area. Photo

by M. Kaufmann.
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-110. 2003. 15

Figure 11. Old trees in the modern landscape. This group of old trees (designated by yellow

arrows) lives on a bench at the base of a west-facing slope along the Mt. McConnell trail off

the Pingree Park road at 7592 ft (2336 m) elevation. This is a better site than the sparse,

disturbed slopes above. Young ponderosa pines, Douglas-fir, and juniper are filling in the

understory in the absence of surface fire. The old trees have smooth orange bark, small live

crown ratios, open crowns with large branches and flattened tops, lightning scars, and fire

scars. We cored the tree designated by the orange arrow. Its DBH was 24.9 inches (63 cm).

Its pith date is 1642. It had a fire scar in 1707.

Figure 12. Urban development is expanding into the montane forest as the human population of the

Front Range grows. This is an area in the foothills west of Denver (photo courtesy of the Colorado

State Forest Service).
16 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-110. 2003.

that occurred historically. Such forests are at risk of

burning in large, intense wildfires, endangering human

lives and property. Dense continuous stands are also

more subject to the spread of disease, insect attacks,

and the stress of competition for light and water.

Several recent fires have illustrated the risk of urban

interface wildfire. The Buffalo Creek fire in 1996 burned

nearly 12,000 acres (5000 ha.) of ponderosa pine forest,

most of it as a stand-replacing crown fire. Fortunately it

consumed very few homes, but subsequent flooding killed

two people, and erosion had significant effects on Denver’s

water supply. The Bobcat Gulch fire and the Hi Meadow

fire in June 2000 each burned about 10,600 acres (4292

ha) and destroyed more than 60 homes. The Hayman fire,

the largest in recorded Colorado history, burned almost

138,000 acres (55,870 ha.) and 133 homes in the South

Platte basin in 2002. All were human ignitions. Fires of

this magnitude were unusual historically, maybe

unprecedented. Now, in addition to changing forest

structure, such fires endanger many thousands of people

who live in the montane zone. Efforts are now being made

in the Front Range to thin montane forests and to restore

fire in the form of prescribed fires, especially where the

risk of wildfire is great (Foster Wheeler Environmental

Corporation 1999).

Forest Restoration and Old Trees

The most ecologically sound way to mitigate the

risk of intense wildfire is to return the forest to a

structure similar to what existed before Euro-American

settlement. Because fire was the main regulating

process in pre-1860 forests, the logical way to do this

is through prescribed fire, but some areas are too

heavily populated for prescribed fire to be safe or

acceptable to residents. Prescribed fires create smoke

and are at risk of escaping containment should the

weather change unexpectedly. In some areas, fuels have

accumulated to such levels that a prescribed fire would

be more intense than desired, killing more trees than

intended. In such cases, a combination of mechanical

treatment and prescribed fire may be necessary.

Mechanical treatment for restoration purposes involves removing trees based on scientific reconstructions

of the historical forest. In general, pre-Euro-American

settlement trees are retained as well as some of the larger

young trees, to create a prescribed forest density and

species composition. Most of the young trees are cut,

possibly including some fairly large ones, to restore the

forest to a historical density. On a landscape scale, it is

desirable to re-create the historical mosaic of forest age

and size structure, including sizeable openings. This means

clear-cutting or burning intensely in some locations, while

leaving open forest in other locations and somewhat denser

forest in others. New harvest machines can cut individual

trees and remove them with minimal disturbance to the

soil. Some of them shred and broadcast the trunks,

branches and needles like mulch so they can decay or be

burned in a prescribed fire later. Where such activities

have been done in the Front Range, there has been a

positive response from the grasses and other understory

plants, and under non-extreme conditions, a moderation

of fire behavior.

Restoring Front Range montane forests to a historical

density is controversial. Environmentalists are concerned

that restoration will be an excuse to reintroduce

unregulated logging. Even in areas where it is clear that

thinning would reduce the risk of wildfire, diameter limits

have been proposed that would limit the trees that could

be cut to those less than some pre-determined size. This

method is designed to save older trees, but because the

relationship between tree size and age is very poor in the

Front Range, it could allow for cutting small old trees,

while keeping large young ones that maintain a density

greater than the target, and therefore not sufficiently

mitigating the risk of stand-replacing fire. There is

currently only a limited local market in the Front Range

for the small diameter material that will be cut in

restoration activities. That makes restoration very costly,

as the work must be subsidized by landowners or land

management agencies and little or no cost is recouped

through sale of the material.

Regardless of how restoration is accomplished, old trees

will benefit from a reduction of forest density in the

montane zone of the Front Range. Reducing forest density

will reduce stress due to competition for water and light

and will reduce the risk of insect epidemics and standreplacing fire. Identifying individual old trees for

preservation in restoration activities is essential. Longterm maintenance of restored forest conditions is critical.

Unless natural processes such as fire can be reintroduced

to the montane zone, the lower density forest will have to

be maintained with continued thinning and prescribed fire.

Identification of Old Ponderosa Pine

Trees in the Front Range

Appearance of Old Trees (>200 years)

At around 200 years of age, ponderosa pine trees

begin to take on distinctive physical characteristics as

the result of physiological changes (Kaufmann 1996).

Trees acquire these characteristics gradually as they

interact with their environment (fig. 13). Trees growing

in poor sites or those that are otherwise stressed, as by

dwarf mistletoe infestation, may look old earlier.
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-110. 2003. 17

Figure 13. Typical characteristics of old ponderosa pines. This classic old tree lived on a south-facing slope at Cheesman

Lake. The inside date on a cross-dated core from this tree was 1455, and that was about an inch from the pith, which

was rotten. Most of the other trees on this slope were around 300 years old. They all burned in the Hayman fire in 2002.

Spiky dead top

Small live

crown ratio;

branches

pruned higher

on uphill side

by fire

Sparse,

flattened crown

Roots exposed

by centuries of

erosion

Smooth, orange

bark

Fire scar at

base
18 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-110. 2003.

Crown shape is determined in part by the site where

the tree lives and by its history, but in general, as a tree

ages and stops growing taller, the crown appears to

flatten out on the top and to acquire a “bonsai” look

(fig. 14). This flattening may be more pronounced on

poor sites or in older trees. Older trees often have sparse

or very open crowns, as branches have died off over

the years, leaving a few large branches. Also, the

growing tips of the branches are farther from the trunk,

and the interior of the crown often has little foliage.

The proportion of the live crown to the height of the

tree becomes less, both through self-pruning and fire

pruning. As a tree gets taller and the crown becomes

longer, the lower branches become shaded by the upper

branches. When the lower branches cannot get enough

light to photosynthesize, they die and eventually fall

off. This has the added benefit of removing ladder fuels

from the lower part of the trunk, making it less likely

that a surface fire will get into the crown. Fires

themselves can have the same effect. A surface fire may

scorch the lower branches, killing them without killing

the tree. This gives the crowns of older trees on slopes

or along the edges of old burns a distinctive sloped

appearance (fig. 15 a, b).

Old tree trunks become more columnar over time

due to differential growth in the upper crown

(Kaufmann, unpublished data). As trees become larger,

they have a larger surface area over which to spread

growth. Old trees are usually not as vigorous as younger

trees, so they have less energy for growth. They put on

Figure 15a. An old pine on the Redfeather-Rustic road; its

pith date is 1550. Its branches have been pruned on the

uphill side by past fires, giving its crown a distinctive

sloped shape. Recent ingrowth at its base could carry

fire into the crown, endangering a tree that has resisted

fire for 450 years.

Figure 14. Crown characteristics of old trees. This old tree

lives on an open, south-facing slope along the RedfeatherRustic road at 8349 ft (2569 m) elevation. The shape of

its crown is typical of an old, open-grown tree: its top is

flattened, it has a sparse, open crown with few large

branches, and a small ratio of living branches relative to

the tree’s total height. The lower branches have died or

have been killed by past surface fires. It has a columnar

trunk and pale orange bark. Its pith date is 1643, meaning

that it germinated more than 350 years ago. It is a moderatesized tree, with a DBH of 20.3 inches (51.5 cm).
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-110. 2003. 19

15b. A ponderosa pine scorched on the uphill side by surface

fire in a prescribed burn. This type of scorching probably

explains the lopsided crown shape of some old trees and

shows how trees often become scarred on the uphill side.

Not only do fuels accumulate there, but fires generally burn

uphill unless they are driven by strong winds. Branches

already pruned by fire kept flames out of the crown.

wood in the upper crown first, and if there is any

carbohydrate left, they grow wood in the lower part of

the trunk. This is one reason why tree-ring samples

from old trees are likely to have missing rings in the

lower trunk, especially rings formed during stressful

years. The tops of old trees are often killed by lightning,

fungi, or insects, giving them a spiked appearance.

Trees with dead tops are often hollow because the injury

allows the entry of fungi.

As ponderosa pine trees age, their bark becomes

paler in color, smoother and with smaller flakes. The

exact color of the bark varies from location to location,

perhaps because of genetics. The old trees we found in

the northern Front Range tended to be more gray, while

those in the southern Front Range were more orange.

Sun exposure may also bleach the bark color. Old tree

bark lacks the deep fissures, large flakes and black color

of young tree bark (fig. 16 a, b).

Figure 16a. Typical bark on a young tree near Cheesman

Lake; it is dark in color, with deep fissures and big flakes.

16b. Typical bark on an old tree near Cheesman Lake; it is

mostly pale orange, smooth, with small flakes.
20 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-110. 2003.

The longer a tree lives, the more likely it is to be

damaged. Dead tops, broken branches, lightning scars,

fire scars, rot pockets, and burls are all signs of past

injury. A ponderosa pine with an exposed fire scar face

is probably at least 150 to 200 years old, since spreading

surface fires of the kind that cause such scars ceased in

most of the Front Range around 1900 (see fig. 5). See

table 1 for a list of identifying characteristics of trees

more than 200 years old.

The Transition From Young to Old (Canopy

Trees 150 to 250 Years Old)

Ponderosa pine trees around 200 years old are middleaged and their form begins to change about that time (fig.

17). They are still vigorous, but their upward growth is

slowing, so their tops start to flatten out as apical

dominance weakens. Lower branches have begun to die

off and self-prune, either as they are shaded by upper

branches or neighboring trees, or killed by old surface

fires. The crown begins to open as dense, small branches

on the inside die off close to the trunk. The remaining

branches become longer and larger in diameter, and the

needles are distributed more toward the outside of the

crown. The general shape of the crown becomes round or

oval rather than teardrop-shaped with a pointed top, as is

common in young trees. The deep fissures in the bark

begin to smooth, and the color becomes lighter, more

orange or gray than black. Middle-aged trees are old

enough to have a fire scar or two, but it has been long

enough in most locations since the last surface fire that

such scars may have nearly healed and are not readily

visible. A healed scar might appear as a deep fissure low

on the trunk, rather than the easily recognized “catface.”

Trees of this age are often the dominants in present Front

Range stands. See table 1 for a list of identifying

characteristics of trees 150 to 250 years old.

Appearance of Young Trees (Canopy Trees

<150 Years Old)

Ponderosa pine trees less than 150 to 200 years old

usually have pointed tops, because they are still growing

taller (fig. 18). Strong apical dominance allows young

trees to grow fast and to compete for light. Seedlings less

than 3 or 4 years old must compete with grasses and forbs,

and their survival is always in question. Once seedlings

are about six inches tall, they are considered established

and are more likely to survive. However, they are still

susceptible to many factors in their immediate

environment as opposed to larger-scale climatic factors.

When they are 3 to 6 ft (1 to 2 m) tall, they begin to interact

with the other trees around them, and are more likely to

reach the canopy. How long it takes a tree to reach this

size varies tremendously. It may take only 20 years or so

on a good site, but even tiny trees may be older than they

look, especially on poor sites. We have sampled trees that

were only three ft. (1 m) tall but were more than 90 years old.

The trunks of young trees are usually tapered. Branches

grow relatively low on the trunk and have usually not

begun to self-prune. The bark of young ponderosa pine

trees is dark gray or dark orange and black, with deep

Table 1—Identifying characteristics of ponderosa pine trees at different life stages.

Crown shape Live crown ratio Branches Trunk shape Bark Likely injuries

Old trees flattened, “bonsai” small; often few but large columnar smooth, small fire scars, dead

(>200 years) shape, sparse and fire-pruned flakes, pale tops, broken

open, may be orange or gray branches,

lopsided lightning scars, rot,

burls, exposed

roots

Transitional ovoid, flattening on moderate; fine branches beginning orange or gray relatively few;

trees the top, full and perhaps half in the interior to lose flakes with dark possibly healed

(150-250 years) rounded the trunk, of the crown taper edges, shallow or mostly healed

beginning to dying, longer fissures, fire scars, lightning

self-prune branches becoming scars, mistletoe

thickening smoother

Young canopy pointed top, large many fine tapered large, coarse very few; possible

trees “teardrop” or branches, flakes, deep mistletoe or

(<150 years) “Christmas tree” dense foliage fissures, dark lightning scars

shape, dense near the trunk gray or black with

foliage dark orange

○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○ ○○○○○○○○

○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○ ○○○○○○○○
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-110. 2003. 21

Figure 17. A middle-aged tree showing the transition in appearance to old tree characteristics. This tree, growing on a

gentle south-facing slope above Lily Lake in Rocky Mountain National Park, is around 200 years old. It is beginning to

take on old tree characteristics: its crown is still full but beginning to open up as fine branches die, and its top is ovoid

and beginning to flatten. Its branches are beginning to self-prune. Its bark is becoming smooth and orange, losing its

deep fissures.

Moderate live

crown ratio;

some pruning

Ovoid crown

shape, rounded

and flattening

on top

Fine branches

in the interior

of the crown

dying, longer

branches

thickening, but

crown is still

moderately full

Bark becoming

smooth, lighter

in color
22 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-110. 2003.

Figure 18. Typical characteristics of young ponderosa pines. This vigorous young tree grew on a west-facing slope at Cheesman

Lake. There were many other young trees in this stand, regeneration following a stand-replacing fire in 1851.

Pointed, live

top

Long live

crown relative

to total tree

height; this tree

is beginning to

self-prune

Lots of small,

live branches;

full, vigorous

crown

Fissured brown

bark, beginning

to turn orange
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-110. 2003. 23

fissures and thick, flaky layers. On some small trees the

bark appears almost all black; these slender youngsters

are called “blackjacks.” Most young trees do not have

fire scars. They may have mistletoe, but if trees are infected

while young, the mistletoe will be distributed throughout

the crown and may distort the shape of the branches. Young

trees on good sites may be very large, but they usually

have a distinctive pointed top and a full crown that extends

most of the length of the trunk. Crowns of healthy young

trees have many dense, small-diameter branches, most of

which bear live needles, so the crown looks full and thick.

They usually have few dead branches. Dense forests are

more likely to be composed of young trees. See table 1

for a list of identifying characteristics of canopy trees less

than 150 years old. Figure 19 shows an old tree and a

young tree side by side on a poor site.

Tree Size

Not all old trees are large, and not all large trees are

old. However, if a tree is by far the largest or tallest

tree in the stand, it is probably older than its neighbors

(fig. 20). Ponderosa pines in the Colorado Front Range

are usually limited by the availability of water. Trees

grow more quickly on sites where water is readily

available. Such sites include those along streams, even

intermittent streams where water may not flow on the

surface year around; flat, grassy areas where water

soaks into the soil; and at the base of rock outcroppings,

where water collects off the rocks. A 150-year-old tree

on a good site may be 30 inches (80 cm) in diameter.

Where water is less readily available, trees grow

more slowly. Steep slopes, slopes that face south and

Figure 19. An old tree and a young tree side by side. These

two trees live on a dry south-facing slope near the top of

a ridge along the Redfeather-Rustic road at 8453 ft.

(2601 m.) elevation. They are close to the same size, but

the tree on the left is 128 years older. Its pith date is

1761. Its DBH is 13.2 inches (34 cm). Its crown has been

pruned up to about a third of the length of the trunk, and

its bark is pale and smooth. The tree on the right has a

pith date of 1889. Its DBH is 12.4 inches (32 cm). Its

crown is sparse, but still pointed at the top, and its live

crown occupies more than two-thirds of its trunk. Its bark

is nearly black.

Figure 20. A case in which size does indicate an older tree:

this old tree towers above more recent ingrowth at the

base of a slope along the Pingree Park Road. Its pith

date is 1672 and it has a fire scar in 1846. The old tree

has the sparse, pruned crown and smooth orange bark

typical of old trees. Its smaller neighbors have full, pointed

crowns and dark, fissured bark.
24 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-110. 2003.

west and are exposed to sun and wind, ridgetops, rocks,

and places where the soil is very shallow or welldrained do not hold water. Trees on these sites must

depend on brief pulses of water from rain or snow

events. Small, old trees are likely on such sites. A

400-year-old tree might be as little as 12 inches

(30 cm) in diameter. On such sites, trees are usually

less subject to competition and to the spread of parasites

and diseases. The lack of water means that understory

species like grasses and shrubs are sparse, providing

little fuel for fires. Fires burning there are not likely to

be intense, so old tree survival is enhanced.

Open-Grown Versus Forest-Grown

Trees that have grown in the open tend to have a

different appearance from those that have grown in a

forest. Open-grown trees tend to have fuller, more

rounded crowns and a few long, large-diameter

branches (fig. 21 a). Trees that have grown in a more

closed environment will be relatively slimmer, with

shorter, thinner branches (fig. 21 b). Old trees can be

found in closed forest environments, but the oldest trees

are usually open-grown, possibly because they lack

competition, or because they are less likely to be

destroyed in a fire.

Ponderosa pines require full sunlight to grow well,

and when stands are dense and trees shade one another,

the shaded trees grow slowly, often develop poor form,

and may ultimately die. However, such suppressed trees

may be relatively old despite their small size (fig. 21

c). They can persist for hundreds of years under the

shade of other trees. If the neighboring trees are

eliminated (by fire or logging, for example), suppressed

trees may be released and start growing faster. This is

evident in the width of their annual rings (fig. 22 a, b,

c). Douglas-fir does not suffer as much from shading

as ponderosa pine. Young Douglas-fir trees benefit from

shaded conditions where the soil surface does not get

as hot as in the open.

Types of Injuries

Fire scars are common injuries found on old pines,

but trees accumulate scars from other sources as well.

Old trees often have distinctive scars from lightning

strikes (fig. 23 a). Lightning scars are usually long and

narrow and often spiral around the trunk. They usually

start in the crown, and they may or may not reach the

ground. Sometimes lightning kills the top part of the

tree. It may blow off branches or even split the trunk,

but trees can survive this kind of damage.

Porcupines often feed on the cambium of ponderosa

pines, and scars left by their gnawing are easily

mistaken for fire scars (fig. 23 b). However, porcupine

scars are generally irregular in shape and usually do

not reach the ground. Porcupines may sit on a branch

higher in the crown and gnaw a patch of the trunk above

the branch. Other injuries can create similar-looking

scars, including abrasion damage from logging, roadbuilding, and neighbor trees falling against the trunk.

These scars are usually irregular in shape.

Native Americans harvested the cambium of

ponderosa pine for food and medicine, and their activity

left detectable scars (Kaye and Swetnam 1999,

Martorano 1981, Swetnam 1984). Various tribes did

Figure 21a. Open-grown tree: This middle-aged tree lives

in a sage meadow near a south-facing ridgetop along

the Redfeather-Rustic road at 8453 ft (2601 m) elevation.

It is beginning to develop old-tree characteristics as well

as open-grown characteristics. Note the full, rounded

crown and long heavy branches. Its top is beginning to

flatten out, but the lack of surface fire during the 20th

century has allowed it to keep its lower branches. Its pith

date is 1811; its DBH is 21 inches (53.3 cm). The core

revealed a healed fire scar near the pith and relatively

rapid growth.
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-110. 2003. 25

this “peeling” throughout the range of ponderosa pine.

The Utes were known to practice the technique in the

Front Range until the late 19th century. Women used a

sharp instrument to cut into the bark from as high as

they could reach, down to about knee level. Then they

peeled off the bark and cambium in a strip a few inches

to a foot wide. Scars caused by peeling can be mistaken

for lightning scars or fire scars. They usually occur on

large, older trees, often in groups. The scars usually

occur at mid-trunk and do not reach the ground.

Many old trees have dead tops, caused by lightning

strikes, porcupine damage, or infestation by fungi. On

21b. Forest-grown tree: This tree grows in relatively dense

ponderosa pine forest on an east-facing slope along

Highway 7 at 8671 ft (2668 m) elevation. It is a dominant

forest-grown tree. Note its tall, straight trunk and narrow

crown with short lower branches. Its bark is beginning to

look smooth and pale. Its pith date is 1762; its DBH is

22.2 inches (56.4 cm).

21c. Suppressed tree: This tree inhabits a small, rocky ridge

in a relatively dense forest along Highway 7 near Olive

Park at 8349 ft (2569 m). It is rotten at the center, but the

inside date on the core was 1671, and narrow rings

indicated very slow growth. Its DBH is only

10.2 inches (25.9 cm). It is small, with twisted branches,

a sparse crown and a flattened, deformed top.

steep slopes or on the edges of washes, tree roots may

be exposed by centuries of erosion. Along streams and

washes, debris washing downstream during flash floods

can damage the cambium and cause scars that look

somewhat like fire scars. It is possible to date flood

events using these scars, which usually occur on the

upstream side of the trunk. Old trees may be infested

with dwarf mistletoe, a common parasite (fig. 23 c, d),

though young trees are also susceptible. Old trees may

survive for decades with mistletoe, with slower than

normal growth. Young trees are likely to be killed or

fatally weakened by an infestation. Mistletoe infestation
26 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-110. 2003.

Figure 22. These core samples show growth releases in trees following logging at Saloon Gulch along the South Platte

River. These cross-dated core samples come from old living trees that were probably too small to interest the loggers in

the 1880s. The bark is on the right side of the picture; the pith or inside of the sample is to the left. Each sample is from

a different tree. The growth releases are indicated by the sudden increase in the width of the annual rings, which look

like vertical stripes on the samples. The area was logged in the mid-1880s. When their neighbors were cut, reducing

competition for light and water, the remaining trees responded within a few seasons by growing more quickly for the

next 40 years or so. Their growth rate began to slow again in the 1920s and 1930s as regeneration again closed up the

stand.

can create old-growth crown characteristics in younger

trees (Hawksworth and Weins 1972, Hawksworth and

Geils 1990). Mistletoe infestation was locally common

prior to Euro-American settlement, but may have been

limited spatially by frequent fire as infected trees tend

to be more flammable than healthy ones.

Common Locations of Old Trees

Despite disruptions to the montane ecosystem in the

last 150 years, old trees are widespread in the Front

Range. Trees in inaccessible locations and on poor sites

often were not cut when logging was easier elsewhere.

Rock outcroppings, steep slopes, ridgetops (fig. 24 a),

and higher elevations (fig. 24 b) that were historically

less accessible by road, less populated, and less susceptible to fire provided havens for old trees. Rock

outcrops in meadows and the edges of meadows are

also havens for old trees, as they were protected from

intense fires.

We found relatively few old ponderosa pine trees

below about 6500 ft (2000 m) elevation. Whether this

is because trees at lower elevations were more accessible from settlements and were cut, or because there

were just fewer trees there originally is unclear. Frequent fires and drought at low elevations probably confined trees to ridgetops, watercourses and rock

outcrops, and historical photos bear this out (fig. 25).

The lower foothills are now covered with ponderosa

pine up to 120 years old, established since the beginning of fire exclusion. Above 6500 ft (2000 m), old

ponderosa pine trees become more common. The

higher elevations were historically less accessible by

road and were less heavily populated, and fire may have
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-110. 2003. 27

23d. Dwarf mistletoe infesting a ponderosa pine. This

parasite deforms tree crowns and may give a younger

tree an old tree crown shape prematurely.

23c. Dwarf mistletoe causes characteristic swellings on the

branches and may create tangled “witches’ brooms” in

the crown.

23b. Porcupine damage on a ponderosa pine. Porcupine

scars can look like fire scars, but they are usually more

irregular in shape and usually do not extend to the ground.

The aspen behind it was damaged by elk. Figure 23a. A lightning scar on an old tree at Cheesman

Lake. Lightning scars can sometimes look like fire scars,

but they are usually long and narrow, extending down

from the crown but sometimes not to the ground, and

often spiraling around the trunk.
28 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-110. 2003.

been less frequent. Rock outcrops in meadows and the

edges of meadows are often havens for old trees in

high valleys like Estes Park. Warm south-facing slopes

as high as 10,000 feet elevation support old ponderosa

pine trees, and older ponderosas are common in the

mixed conifer zone, where they occur with lodgepole

pine, limber pine, Douglas-fir, and aspen.

Old trees are often found on south- and west-facing

slopes (fig. 26 a, b), on and around rock outcrops, at

the edges of high meadows (fig. 27), and on the upper

third of slopes. Even in areas that were logged and now

support young forest, individuals with twisted or

24b. Though ponderosa pine forests are most common at

lower elevations, older ponderosas can be found on

sunny, south-facing slopes above their usual range. They

may be relicts from a past climate or fire regime. This

lone, old ponderosa pine lives on a south-facing slope at

10,000 ft (3077 m) elevation along the Old Flowers Road

trail. Note the clone of young aspen around the old pine.

Figure 24a. This old tree lives on a dry, rocky ridge along

Highway 7. It is small, deformed, and rotten at the center,

but it is more than 350 years old. It has a healed fire scar

and a lightning scar, smooth gray bark, a flattened top,

and a sparse open crown. Many of its neighbors are of

similar age and are stunted by the harshness of the site.

scarred trunks, rot, large limbs, and dead tops were

often passed over by loggers. Old trees often occur in

small groups, but it is unusual to find a stand in which

all the trees are old. Old-growth stands are usually

multi-aged. Of course, none of these suggestions is a

rule. We found old ponderosa pine trees alone, on northfacing slopes, near rivers, in fairly dense stands, and

right beside houses and campgrounds. Across a vista,

groups of old trees are apparent as taller, rough-looking, browner areas amid the smoother, greener, more

uniform-looking young forest.
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-110. 2003. 29

Figure 25. The lower Cache la Poudre River canyon in 1920. This picture was taken from Inspiration Point, looking

northwest across the river from the new Bennetts Creek auto road (now the Pingree Park Road) into the area beyond

Eggers. The road construction camp is in the left center. Most of the view ranges in elevation between 6000 and

7000 ft (1846 to 2154 m). Note how few trees were growing at this elevation. Most of the trees were along the river and

other drainages (photo by W.I. Hutchinson, courtesy of Arapaho-Roosevelt National Forest archives).
30 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-110. 2003.

Figure 26a. Old trees growing on steep, rocky south- and

west-facing slopes. These two old trees live on a rocky

west-facing slope along the Crosier Mountain trail at

around 7500 ft (2300 m) elevation. Both have smooth,

gray-orange bark, gnarled branches, live crowns pruned

up above the ground, and fire scars. The tree on the left

also has a lightning scar, and its top is dead, probably

killed by lightning. b. Old trees on open, grassy southand west-facing slopes. This old tree lived on a barren,

grassy-shrubby, south-facing slope at Cheesman Lake

at 7066 ft (2174 m) elevation; its center was rotten, but

its inside date was 1720, and its estimated pith date was

1550. It was typically open-grown, with a flattened top,

large, gnarled branches, and pale orange smooth bark.

It had a visible fire scar face with several scars.

Figure 27. A classic old tree along the Redfeather-Rustic

road at 8320 ft (2560 m) elevation; its pith date is 1569,

its DBH is 24.4 inches (62.1 cm). It lives on the west

edge of a rock outcrop in the midst of a meadow and has

the typical flattened top, smooth orange-gray bark, and a

fire scar.

a b

s
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-110. 2003. 31

Gallery of Old Tree Photos

How We Found Trees for This Publication

We searched for old trees along a north-south

transect of the Front Range to capture changes in soils

and weather conditions that vary along this latitudinal

gradient. We took four excursions, each with multiple

sampling locations: one along the dirt road from Rustic in the Poudre Canyon to Redfeather Lakes; one

along the Pingree Park and Crown Point roads above

the Poudre Canyon; one along the Glen Haven road

from Drake to Estes Park (which is full of old trees

that are all on private land), and then south along Highway 7 from Estes Park to Allenspark; and one in the

Cheesman Lake property in the South Platte basin,

which is owned by Denver Water. Based on other research from the Cheesman Lake property, we knew

old trees were common there because much of that area

was never logged. Except for Cheesman Lake, we did

all our sampling on U.S. Forest Service land. We deliberately avoided canyons, that are historical and modern travel corridors, and where fire probably behaved

differently. Our sampling locations are marked by yellow tree shapes in figure 1.

Old trees usually occurred in small groups. We selected

trees from which we felt we could get a good picture and

increment core, photographed them, and cored them with

an increment borer to determine their ages. Coring usually does not damage the tree. We selected trees in a variety of growing conditions, from harsh to moderate to good

sites. We were probably biased by taking pictures of trees

in relatively open conditions, where it was easier to get a

good picture. However, old trees seem more likely to occur in open conditions.

We grouped portraits of old trees by the site conditions

on which they occurred, because site condition, as well as

age, affect a tree’s appearance (fig. 28). We chose trees on

various slope positions and aspects and in both open and

closed forest (though more in open forest) to represent

the range of conditions under which old trees may be

found. We also included a few portraits of younger trees

under different site conditions for comparison. With each

tree portrait, we report the tree’s diameter at breast height

(DBH; standardized to 4.5 ft [1.4 m]) above the ground).

This is a general measure of a tree’s size and is included

to indicate their relative size under different conditions.

We did not measure the heights of the trees.

We also report the tree’s age, which we derived by

cross-dating cores taken low on the trunk. We crossdated the cores by matching ring-width patterns in the

samples with composite chronologies from the area.

When trees were rotten, we estimated their ages by

Figure 28. Idealized silhouettes of ponderosa pines of

various ages and on various site conditions in the Front

Range of Colorado (in the style of the Keen [1955] tree

vigor classification for ponderosa pine in Oregon). The

horizontal rows represent three age classes: top, the

youngest, less than 150 years; middle, transitional trees

150 to 250 years old; and bottom, the oldest, trees more

than 250 years old. The three site classes are represented

by the vertical columns: left, good sites such as meadow

edges and riparian areas; middle, moderate sites such

as east-facing slopes or gentle slopes; and right, poor

sites such as steep south-facing slopes or ridgetops

(drawing by Joyce Vandewater).

estimating the number of rings that would fill out the

remaining length of the tree radius and using a negative exponential curve to take into account age-related

growth trends (see the Appendix for details). Most of

the trees pictured are between 400 and 500 years old.
32 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-110. 2003.

Old trees on poor sites (fig. 29 a, b, c, d, e, f)

Figure 29a. DBH=18.4 inches (46.7 cm), pith date 1545.

This was a typically scraggly looking old tree on a poor,

rocky site. Its flattened, very sparse crown, small live

crown ratio, and smooth orange bark were exaggerated

by the harshness of the site. At 7319 ft (2252 m) elevation,

this mostly east-facing ridge on the Cheesman Lake

property is relatively low and dry, but young Douglas-fir

populated the understory.

29b. DBH=20.8 inches (52.9 cm), pith date 1574. This tree

lived in a common location for old pines: an open, southfacing slope just below a ridgetop at 7000 ft

(2154 m) elevation on the Cheesman Lake property. It

had a scar face with several fire scars, small live crown

ratio, a sparse, slightly flattened, open-grown crown, and

smooth orange bark.
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-110. 2003. 33

29c. DBH=18.8 inches (47.7 cm), pith date 1605. This small

old tree grows on a poor, rocky site at 8778 ft (2701 m)

elevation, relatively high for ponderosa pine. It has been

encroached by aspen and lodgepole pine. It has a

somewhat flattened top, a very sparse crown and tiny

live crown ratio, smooth orange-gray bark, and four fire

scars. It lives along Highway 7 at Meeker Park.

29d. DBH=15.2 inches (38.5 cm), pith date 1560. This group

of old trees lives on a rock outcrop in the middle of a

meadow along the Redfeather-Rustic road at 8307 ft

(2556 m) elevation. We cored the tree on the left. The

flattened tops, deformed crowns, small live crown ratios

and smooth orange-gray bark identify them as both old

and stressed.
34 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-110. 2003.

29f. DBH=12.5 inches (31.7 cm), pith date 1889. This

younger tree lives on a south-facing slope near ridgetop

at 8453 ft (2601 m) elevation along the Redfeather-Rustic

road. Its pointed crown, long live crown ratio, and darker

bark indicate that it is younger than some of its neighbors,

though its small size attests to the harshness of the site.

29e. DBH=21.4 inches (54.3 cm) inside date 1663, est. pith

date 1521. This tree lives near seven other old trees on

a steep, rocky, west-facing slope at 7592 ft. (2336 m.)

elevation along the Mt. McConnell trail. Evidence of local

disturbance includes many fallen trees. It and its

neighbors are infested with dwarf mistletoe. Nearby trees

have fire scars from 1851, 1707, and 1700. This tree has

typical smooth orange-gray bark, a flattened top, twisted

and pruned branches, and a big burl (caused by an injury)

beside a fire scar. It is rotten in the center.
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-110. 2003. 35

Old trees on moderate sites (fig. 30 a, b, c, d, e, f)

Figure 30a. DBH=19 inches (48.4 cm), pith date 1589. This

tree grows on the edge of a north-facing ridge at 8307 ft

(2557m) elevation along Highway 7 at Olive Park. It is

open-grown but not very tall. It has a flattened top, smooth

orange bark, large branches, and fire scars on the north

side. 30b. DBH=20.6 inches (52.3 cm), inside date 1744, est.

pith date 1626. This tree lived on an open east-facing

slope just below a ridge on the Cheesman Lake property

at 7319 ft (2252 m) elevation. The entire stand was old;

there were many fallen trees, perhaps killed by a past

fire. The tree was rotten in center, possibly because of its

dead top. It had very smooth, pale bark and a sparse,

open-grown crown.
36 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-110. 2003.

30c. DBH=25.3 inches (64.2 cm), pith date 1595. This opengrown tree lives in an east-facing meadow at

8710 ft (2680 m) elevation along Highway 7. It has classic

old-tree characteristics: the flattened, “bonsai” open crown

and the smooth orange bark. Though it lacks visible fire

scars on the trunk, its lower branches are charred from

past surface fires.

30d. DBH= 23.4 inches (59.5 cm), pith date 1769. This

middle-aged tree lives on a flat, rocky ridgetop along the

Pingree Park Road at 7293 ft (2244 m) elevation, in a

location that sees heavy recreation use and has much

ingrowth of Rocky Mountain juniper. It has a full, opengrown crown that is beginning to flatten at the top, fire

scars, wounds from bear scratches, and bark beginning

to shift from dark gray to orange.
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-110. 2003. 37

30f. DBH=15.7 inches (40 cm), pith date 1887. This tree

lives on a gentle east-facing slope at the edge of a

meadow along the Crown Point road at 8425 ft (2500 m)

elevation. It has the typical pointed top of a rapidly growing

tree, no visible scars, and a long, full crown. Its bark is

fissured, beginning to turn orange, and lower branches

are starting to self-prune.

30e. DBH=24.4 inches (62 cm), pith date 1527. This tree

inhabits a gentle, open, south-facing slope near the hilltop

along the Redfeather-Rustic road at 8453 ft (2601 m)

elevation. It has a typical flattened top and smooth orange

bark. Because it is on a moderate site, it has a fuller

crown than trees of similar age on poor sites. A core

revealed a rot pocket from a healed fire scar—perhaps

from the same fire that killed the short snag beside it?
38 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-110. 2003.

Old trees on good sites (fig. 31 a, b, c, d, e, f)

Figure 31a. DBH=27 in (68.5 cm), inside date 1718; est.

pith date 1581. This large tree lives on a grassy, shrubby,

gentle north-facing slope at 6409 ft (1972 m) elevation

above a river valley along the Glen Haven road. Its center

is rotten, probably because of its dead top. It is opengrown, with large heavy branches, a dead top, smooth

orange bark, a sparse crown, and a large burl near the

base.

31b. DBH=40.8 inches (103.8 cm), inside date 1729; est. pith

date 1486. This ancient tree grows on a flat bench above a

southeast-facing slope, just below the ridge at 8453 ft. (2601

m.) elevation, along the Redfeather-Rustic road. It is the

oldest and largest tree in the vicinity. It has a dead top,

smooth red-orange bark, small live crown ratio, and large,

heavy branches. No fire scars showed on the trunk, but it

had an axe scar from long ago. It has been marked by the

U.S. Forest Service for preservation as a wildlife habitat tree.

It was rotten in the center. This was the second-largest and

second-oldest tree we found.
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-110. 2003. 39

31c. DBH=31 inches (79.1 cm) inside date=1586; est. pith

date 1539. This tree lives in a flat sage meadow at the

abandoned townsite of Manhattan, 300 ft (100 m) above

a stream at 8460 ft (2603 m) elevation along the

Redfeather-Rustic road. The center was rotten. It is

typically open-grown and has smooth orange bark, but

because it is on a good site, its old tree crown characters

are subtle; the crown is more rounded and full than a

tree of similar age on a poor site.

31d. DBH=25.4 inches (64.6 cm), inside date 1600, est. pith

date 1462. This old tree, surrounded by a cluster of younger

ones, lives at 8125 ft (2500 m) elevation along the Crown

Point Road at the base of a gentle, east-facing slope near

the edge of a meadow. It was rotten in the center. It has a

deformed, dying top, a lopsided crown with lower branches

scorched on the underside, and a scar on the uphill side. If

the estimate of age is correct, this is the oldest tree that we

sampled.
40 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-110. 2003.

31f. DBH=31.9 inches (81.2 cm) pith date 1879. This large,

open-grown young tree was being encroached by younger

ingrowth of ponderosa pine and Douglas-fir. Its growth

rate was tremendous. It lived on the Cheesman Lake

property on a gentle west-facing slope not far from a

perennial stream at 7157 ft (2202 m) elevation. It had

dark, fissured bark, a long live crown, pointed top, and

no scars. It was one of the largest trees we sampled.

31e. DBH=42.5 inches (108 cm) inside date=1710, est. pith

date 1614. This tree also lives in the flat sage meadow at

the abandoned townsite of Manhattan, 300 ft (100 m)

above a stream at 8460 ft (2603 m) elevation along the

Redfeather-Rustic road. The tree was so large our borer

could not quite reach the center, so we estimated its pith

date. It is the largest tree we sampled. It is typically opengrown, and has smooth orange bark, but because it is on

a good site, its old tree crown characters are subtle.
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-110. 2003. 41

Conclusion

The pre-Euro-American settlement history of the

Front Range is not recorded in the form of ruins or

artifacts, but in the living monuments of old trees. Ecological restoration efforts intended to reconstruct the

landscape as it was before Euro-American influence

must conserve old trees, which were an integral part of

the landscape. To do that, workers must be able to identify old trees, define treatment prescriptions that preserve them, and create conditions that will allow them

to persist. Existing trees that are 200 years old or more

will form the basis for future old growth stands.

Why Keep Old Trees?

• Restoration to a more historical and ecologically

sustainable state. We know that old trees were more

common in the Front Range before Euro-American

settlement than they are now. They contributed to the

operation of natural processes that sustained the ecosystem. A sustainable montane ecosystem must contain old trees.

• History. Old trees preserve the history of climate

and fire in their rings, and they continue to record this

history as they grow. Older stands give us a better idea

of what the area looked like before settlement and how

natural processes operated in the absence of intensive

human influence.

• Wildlife benefit. Old trees provide habitat for wildlife in their dead branches and hollow trunks. They

produce seeds that are food for wildlife. Old, open

stands support diverse vegetation and provide suitable

cover for deer and elk. Rotting wood harbors fungi and

insects that provide food for bears and other animals.

• Fire resistance. Old trees are fire-resistant. They survived many fires before settlement, and if the forest is

thinned to reduce ladder fuels, they will likely survive

many more. Surface fires can burn through older stands

with historical densities without killing all the trees.

• Aesthetics. Many people find old trees to be beautiful. Their shapes are graceful and interesting. Older

stands, with open, grassy understories and large,

shapely old pines are very attractive, much more so

than a dense jungle of adolescent Douglas-fir. Beautiful scenery is part of what draws tourists to Colorado.

Old ponderosa pine trees are living embodiments of

the history of the Front Range. Despite dramatic

changes in human land use, stand structure, climate,

and disturbance regimes during their lifetimes, many

old trees survive, even adjacent to urban development.

The individual ponderosa pine trees pictured in this

book were part of the historical landscape. Some were

here when Coronado led his band of explorers into

Kansas in 1541; all of them had been here for centuries before Major Long’s expedition first sighted Long’s

Peak in 1820. These trees persisted through the gold

rush and intensive land use of the 19th century. The end

of the 20th century has left them in an environment

unlike any they have experienced, where the danger of

wildfire, competition, and disease is greater than ever

before.

As we move into a new century with a new, developing philosophy regarding our relationship to our

environment, old trees can assist in shaping the future

of the landscape with lessons from the past. Because

landscape-altering activity by humans is relatively recent in the Front Range, land managers have a better

chance here than in most locations to understand and

recapture the structure and function of the landscape

before it was dramatically altered by human activity.

To create a sustainable landscape where humans, old

trees, and the other elements of a healthy forest can

coexist, we must preserve the old trees as we restore

the ecosystem to a more sustainable and historical state.

Postscript

As of this writing (September 2002), the Hayman fire,

which started on June 8, burned almost 138,000 acres

(55,870 ha) over much of the montane zone of the South

Platte basin, destroying homes and businesses—and old

trees. The trees from Cheesman Lake pictured in the Gallery are all dead (fig. 32). A thousand-year-old log on the

south end of the property, which survived 17 fires as a

living tree and more after it was dead, is largely consumed.

The Hayman fire was human-caused, driven by high

winds, extreme drought, and dense, continuous fuels. The

fire season of 2002 started early and burned more acreage in Colorado than in the last 10 years combined. The

combination of altered stand structure, the worst drought

in decades, and an increased human population have created an unprecedented situation. Hopefully restoration

work in the montane forests of the Front Range will protect remaining old trees.

Acknowledgments

We wish to thank Bill Romme, Jeff Lukas, Mike

Battaglia, and Peter Brown for their reviews and helpful comments, and Denver Water for allowing us to

sample on their property.
42 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-110. 2003.

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Appendix

Dendrochronology

Dendrochronology is the science of using tree rings

dated to the exact year of their formation to analyze

temporal and spatial patterns of processes. It works

because in temperate regions, woody plants add a distinct layer of growth every season. The width of each

annual growth ring (the wood produced in that year) is

constrained by limiting factors in the plant’s environment. In semiarid environments like the ponderosa pine

zone of the Front Range, the variation in ring width is

usually caused by variation in water availability, especially early in the growing season (Woodhouse and

Brown 2001). At higher elevations, temperature is usually more limiting to tree growth.

The fundamental principle of dendrochronology is

crossdating—matching the patterns of ring width with

other tree ring series of known date in order to assign

an exact calendar year to each ring (Stokes and Smiley

1968). Crossdating allows dendrochronologists to positively date pieces of wood for which the year of death

or the pith date are not known. Without crossdating,

we could only count the rings, not be sure of their year

of formation. By analyzing multiple samples from an

area, we can minimize the noise created by the growing conditions experienced by individual trees and statistically filter out all but the effects of interest (such

as climate or disturbance).

This is important in places such as the Front Range

where growing conditions are harsh and samples are

likely to have false rings or missing rings. In some years

a tree may be unable to produce any wood at all in the

part of the stem that was sampled, and this condition

shows up as a missing ring in the sequence. Some of

the samples used in this publication were missing as

many as 20 rings. During some years water is available in the spring but conditions become very dry in

the middle of summer. The growth process slows and

the tree produces cells that look like the small, thickwalled latewood cells typical of the end of the growing season. If water becomes available again in the late

summer, the tree may begin to grow normally again,

and a false ring is formed.

Dendrochronologists select trees growing on sites

that are sensitive to some limiting environmental quantity. For example, sampling a tree growing along a

riverbank will not provide information about rainfall

patterns because the tree already has as much water as

it needs and its growth will not respond much to variation in precipitation. Trees are usually sampled with

an increment borer, a hollow instrument like an auger

that removes a pencil-sized sample of wood (see fig. 7

b; 22 a, b, c). This does not usually damage the tree.

Cores are glued to a strip of stronger wood and sanded

with fine sandpaper so the cells within the rings can be

seen under a stereomicroscope. Samples can then be

cross-dated visually. The widths of the rings can be

measured using a computer-based instrument and these

measurements can be statistically analyzed or combined

with others to create a chronology.

The principles of dendrochronology can be applied

to many fields of study. Tree rings can be used to date

the construction of archaeological ruins (Douglass

1929), to reconstruct past climate using the relationship between tree ring width and instrumental temperature and precipitation data (Fritts 1976), to reconstruct

the history of global climate events such as the El NiñoSouthern Oscillation (Woodhouse 1993), to detect the

effects of pollution (Cogbill 1977), and to detect and

date forest disturbances that affect tree growth (Veblen

and others 1991) including insect outbreaks (Swetnam

and Lynch 1993) and fires (Brown et al 1999).

Dendrochronology can be used to reconstruct fire

regimes by dating fire scars and determining forest age

structure. Samples are taken from fire-scarred faces on

living trees and remnant wood by cutting a partial or

complete section with a chainsaw. The wood is sanded

finely and cross-dated just as cores are. The fire scars

are localized within cross-dated rings on each sample.

Fire scar dates collected from the same geographic area

can then be statistically analyzed.

The mean fire interval (MFI) is the average number

of years between fire dates. It can be computed for a

single sample or for all samples from a site combined.

Because it is possible to have a minimum number of

years in an interval (1), but not necessarily a maximum interval, the data are often positively skewed. For

that reason, the Weibull Median Probability Interval

(WMPI) is sometimes used to express the average time

between fire events because it does not assume normal

distribution of the data as the MFI does. The minimum

and maximum intervals between fires at a given location are also important, as they give an idea of the range

of times between fires (Grissino-Mayer 2001).

Fire scar sampling gives a conservative estimate of

the number of fires that occurred in a location. Not

every tree is scarred by every fire that it experiences,

and two adjacent trees might record different fire dates,

or none at all, even though fires occurred there. Also,

fire scars only occur on trees that survived fires, which
46 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-110. 2003.

occurs when the fire at that location was a surface fire.

This can occur around the edges of a crown fire, as

well. The effects of stand-replacing fires are determined

by sampling the age structure of a forest. If most of the

oldest trees in a stand germinated around the same time

following a fire date confirmed by fire scars, it is likely

that the fire was stand-replacing in that location. Death

dates from logs that retain their outer rings under bark

can also confirm the date of a stand-replacing fire.

For excellent information on all aspects of dendrochronology, go to the Ultimate Tree-ring Web Pages,

http://web.utk.edu/~grissino/.

Glossary of Terminology

Canopy cover—a measure of forest density. For a given

area, it is the percentage of the ground that is covered by tree crowns. This is most often measured

from aerial photographs.

Cambium—the growing tissue of a tree immediately

under the bark; every growing season, the cambium

produces a layer of wood.

Community—an assembly of organisms that tends to

occur together under similar environmental conditions; usually considered to be a smaller spatial scale

than an ecosystem.

Cross-dating—the dendrochronological method of assigning a calendar year to a tree ring by matching

patterns of ring width to patterns in a composite

chronology of known dates.

Crown—the leaves and live branches of a tree.

Crown fire—an intense fire that consumes the crowns

of the trees. Active crown fire burns from tree crown

to tree crown, killing most of the trees. Passive crown

fire involves torching of individual trees. Crown fires

occur naturally in some ecosystems on an infrequent

basis (about every 150 to 300 years in lodgepole

pine, for example); it usually occurs under conditions of dense fuels, high winds, and severe drought.

DBH—(diameter at breast height) a measure of tree

size; the diameter of the trunk at 4.5 ft (1.4 m) above

the ground.

Dendrochronology—the science of precisely dating

tree rings based on patterns of annual ring width;

see Appendix for details.

Disturbance—a process that changes the structure of

a population; in ponderosa pine forests, this includes

fires of varying intensities, insect outbreaks, logging, grazing, windthrow, etc.

El Niño-Southern Oscillation (ENSO)—a climatic phenomenon characterized by weakening of trade winds

and sea surface warming in the eastern and central

equatorial Pacific Ocean which occurs ever 2 to 7

years and affects weather around the world. In the

American Southwest and West, El Niño conditions

are usually warmer and wetter than normal. Anomalously cold sea surface temperatures in the equatorial

Pacific causes La Niña conditions, which are usually cool and dry in western North America. The

Southern Oscillation Index (SOI) is the change in

the difference in tropical sea level pressure between

the eastern and western hemispheres that accompanies El Niño conditions. High SOI (large pressure

difference) is associated with strong trade winds and

La Niña conditions; low SOI (small pressure difference) is associated with weaker than normal trade

winds and El Niño conditions.

Front Range of Colorado—generally described as the

mountains and foothills of northern Colorado,

bounded on the west by the Continental Divide, on

the east by the Great Plains, on the north by the

Wyoming border, and on the south by Pikes Peak.

Fire intensity—the heat output from a fire.

Fire regime—the pattern of fire frequency and intensity in a given area over time.

Fire severity—the ecological effects of a fire.

Gradient—A gradual change in a quantity that occurs

with the change in a given variable; for example,

temperature declines with increasing elevation.

Live crown ratio—the percentage of the tree trunk that

supports living branches; a tree with 100% live

crown ratio has branches all the way to the ground.

Montane—refers to middle-elevations; in the Front

Range, between about 6000 and 9000 ft (1845 to

2770 m) in elevation, below the line of persistent

winter snowpack. This zone is dominated by coniferous forests of ponderosa pine and Douglas-fir, with

patches of aspen and openings that may be shrubby

or grassy or both.

Mixed severity fire—a fire event that has mixed effects, burning as a surface fire in some locations and

as a crown fire in others depending on fuel and

weather conditions.

Old growth—a forest defined by well-developed structures and often complex living interrelationships;

dominance by late successional species (which can

be disturbance-dependent species in the case of frequent low-intensity disturbance); and only minor
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-110. 2003. 47

competition among dominant trees. The age of the

dominant trees defines old growth, and it varies from

species to species and ecosystem to ecosystem.

Pith, pith date—the pith is the center of the tree; in a

core sample or a cross section, it represents the earliest year of growth for the sample. If the pith is not

present on a sample because of rot or an off-center

core, it is possible to estimate the number of rings

to the pith if the diameter of the tree is known.

Prescribed fire—a fire deliberately set by humans to

accomplish some planned alteration in community

structure, usually under specific, low-risk conditions.

Surface fire—a fire that burns primarily along the

ground, consuming grasses, shrubs and small trees,

but usually causing minimal damage to larger trees;

historically this was the common type of fire in

Southwestern ponderosa pine.

Succession—the gradual change in the species composition of an ecosystem over time in the absence

of major disturbance or other environmental change;

for example, Douglas-fir will gradually dominate

upper montane forests in the absence of fire, supplanting much of the ponderosa pine.
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