Monitoring ParkScapes Over Time: Plant Succession on the
Pumice Desert, Crater Lake National Park
By Elizabeth E. Horn, ParkScience,
Vol. 22, No. 1, Fall 2003
View scanned original article
Desert is a conspicuous natural feature in Crater Lake
National Park, Oregon. Contrasting sharply with the
surrounding lodgepole pine (Pinus contorta) forests, the
Pumice Desert is a rather barren-appearing, flat area of
about 5.5 square miles (14.2 square kilometers). The Pumice
Desert is about 4 miles north of the Rim Drive on the north
Park staff were unsure of the explanation
for this opening along the park’s north entrance road. Why
was plant succession so slow? What were the factors that
kept and maintained this opening? What kept lodgepole pine
from colonizing the area in greater numbers? Specific
answers are hard to find. Studies showed many factors were
interacting to hinder plant succession, including gopher
activity, infertile soils, soil temperatures, and seed
source. Few pine seedlings were found during field work,
suggesting a problem with recruitment. An interpretive sign
along the park roadway explains that the ancient Mount
Mazama had massive glaciers along its flanks. One of these
glaciers carved a deep valley north from the summit,
extending several miles beyond the park boundary. After the
glaciers melted, glowing avalanches of gaseous material
filled the valleys and depressions around Mt. Mazama. The
porous pumice soil was probably more than 100 feet deep in
the valley now occupied by the Pumice Desert.
As a graduate student and seasonal
ranger-naturalist during the summer of 1964-1965, I studied
this desert area as part of my master’s thesis. The study
was initiated in 1965 to (1) describe the existing
vegetation, (2) measure selected environmental factors such
as air and soil temperature, evaporation stress,
precipitation, soil nutrients and soil moisture; and (3)
establish photo points so that any changes in vegetation
could be tracked over the years.
Vegetation was sampled with 22 line
transects, each 200 feet (61 meters) in length and 10.9 feet
(3.3 meters) wide for a total transect area of 0.1 acre
(0.04 hectare). Coverage data were taken on the basis of
line interceptions. Density and frequency data were gathered
within the transect, and coverage data were measured on the
basis of interceptions along the centerline. Because only
three lodgepole pines were found in the 22 line strips,
another 100-acre (40.5-hectare) plot was set up to measure
only the lodgepole pine. The lodgepoles were measured by
height and basal diameter; they were not big enough for
diameter breast height measurements.
Summer climate data were gathered using a
recording hygrothermograph, rainfall was measured with a
totalizing gage, and environmental evaporative stress was
estimated with Livingston black and white bulb atmometer
spheres. Soil samples were taken to a testing laboratory.
Nine of the line transects were randomly
selected as permanent plots, although I purposefully
included examples of the three microhabitats in the area
(north and south-facing slopes and the flat central wash
with fine gravels). Each end was marked with an iron rod.
The large tree plot was also marked for future reference.
Photos were taken from each end of the strip plots and from
each corner of the tree plot.
Findings in 1965
The 1965 research showed only 14 species of
plants growing within the plots. The park is home to nearly
600 plant species, so the flora of the Pumice Desert is
quite sparse by comparison, with only 0.6 individuals per
square foot and plant coverage averaging 4.9%. Most plants
demonstrated one or more morphological characteristics
typical of alpine or desert plants. The 100-acre
(40.5-hectare) tree plot contained 27 trees with an average
height of 4.6 feet (1.4 meters). The tallest was 9.8 feet
tall (3.0 meters).
Climate data showed air temperatures
that were both higher and lower than those recorded at
park headquarters. The lack of vegetation probably
contributed to these differences. Most significant were
the high soil temperatures recorded on sunny days. For
instance, when air temperature over a six-hour period
did not exceed 83oF
the surface soil temperature averaged 102oF
soils were generally deficient in nutrients and in organic
matter. However, they were not significantly different from
other pumice soils in central Oregon where there was more
growth than in the Crater Lake Pumice Desert. Reasons for
these differences are yet to be studied.
Plot visits in 1977, 1995, and 2000
Vegetation plot in 1965 (top photo) with one
lodgepole pine along its edge. Right-hand photo
taken in 1995 showed the tree was dead; a few limbs
protrude into the photo. Background shows increased
number of trees invading from the northern forest
I revisited and surveyed the vegetation in
the permanently marked line strips in 1977 and in 1995.
Photos were taken from each end of the plots (see fig.1).
The results were fairly consistent each year. Although the
1977 tallies were slightly higher than in 1965 or 1995, the
percent cover for each year remained about 4.9% (see table
1). Although individual plant species numbers fluctuated
somewhat from year to year, the totals remained fairly
In 2000 the trees in the 100-acre tree plot
had increased dramatically, from 27 to 47 in number (see
figs. 2 and 3). The average height was 8.9 feet (2.7 meters)
tall, a substantial increase in size showing good growth
over the interim 35 years. The tallest tree was 49 feet
(14.9 meters). Nineteen of the trees measured in 2000 were
as tall or
taller than the tallest 1965 tree (see fig.
4). Many of the trees showed distinctive mounding at the
base, caused either by wind deposition or gopher activity.
More than half the trees had multiple stems or trunks, a
characteristic not uncommon in harsh environments. What
triggered the successional processes after what appeared to
be a long time of low productivity is unknown. One
possibility is a general warming of the climate, which would
favor a longer growing season.
Effect of visitors on vegetation—an
interesting side story
A small vehicle turnout with an interpretive
sign had been established in1958 along the north entrance
road at the south edge of the Pumice Desert. Visitors had
walked into the Pumice Desert from the interpretive pullout,
and the effect was obvious within an area of about 100 yards
(91.4 meters) from the pullout. To quantify this, a sampling
strip was laid out in 1965 and the results were compared to
the other plots. The total number of individuals counted in
this single plot was 1,037 compared with an average of 2,580
individuals in the other 1965 plots. The cover was 1.6%
compared with the 4.9% average of the other plots. This
reduction in plant cover occurred during a short seven-year
period (1958–1965). This data collection was only done in
Park and researcher contributions
Figure 2. Typical multi-stemmed
lodgepole pine photographed in the summer of 2000.
This tree has nine trunks and measured 19 feet in
The project was very much a joint effort
from its 1965 beginning. Crater Lake managers and staff
provided time and transportation for doing the research.
Purdue University provided the climate and recording
equipment. However, the single most important ingredient was
the participation of park staff. Park biologist Richard
Brown was the enthusiastic instigator. Staff donated their
time off to help tabulate plants on the plots and make
climate and soil recordings. The park had a darkroom, and
several staff developed photos of the plots. The maintenance
staff made the iron rods to mark the plots and devised a
coding system for their identification. It became everyone’s
project and provided a focus for the developing camaraderie
among the staff.
The case for long-range vegetation studies
Park settings are ideal places for
long-range vegetation studies. The 1965 study provided a
snapshot in time. It told what was on the Pumice Desert
during that particular summer. But it was a static picture.
Long-range monitoring has the following advantages:
1. Once a study is set up, it can be
monitored with little additional effort or cost. Each
additional visit to the site involved about five days each
for me and a few colleagues.
2. Photo points can speak volumes even if
vegetation is not tabulated. Photo points, once established,
are usually not difficult to find again.
3. Park areas are generally not disturbed by
human-made intrusions. The vegetation and the landscape are
for the most part protected from unnecessary human-caused
impacts. However, in this study, there were two artificial
intrusions on the Pumice Desert. The first was the route of
the Pacific Crest National Scenic Trail. Records show the
trail was probably routed on an old fire road through the
Pumice Desert in 1929. The trail was moved to a route east
of the Pumice Desert in 1976 to line up with the trail
around Mt. Thielson north of the park and to reduce plant
damage in the Pumice Desert. However, the track and the
vegetation changes can still be seen along the old route.
The second intrusion is the trampling that has occurred near
the interpretive sign. Although the changes near the
interpretive sign were not part of the study, both examples
are valuable because they illustrate the impact that can
occur on delicate plant communities.
4. The data from the 1965 tree plot showed
that few trees had populated the Pumice Desert. The 1977 and
1995 vegetation plot data showed little change in the
average plant cover or individuals per square foot.
Expectations would be that there was also little change in
the tree plot data. However, the 2000 data on the tree plot
showed that succession was indeed proceeding. Had we not set
up and followed up on an additional plot to only monitor
tree numbers, we would have assumed that little had changed
on the Pumice Desert. We had two sets of data (tree plot and
vegetation plot), and if we had stopped with only one set,
we would not have had a complete picture. Why the tree
numbers have increased while the vegetative numbers remained
fairly constant is another interesting question. The answer
may be as simple as normal population dynamics, which would
be more apparent in the shorter-lived herbaceous plants.
Spring moisture, for example, would have a definite
influence on the survival of seedling herbaceous plants with
shallow root systems. Patience and thoroughness are
necessities for long-range monitoring.
Figure 4. View from northwest corner of the tree plot, lower
photo taken in 1965; upper photo taken in 2000. Note the
increased number of trees in the middle ground in the 2000
photo. Note the large areas of barren ground. Pocket gopher
mounds show in the foreground of the 1965 photo.
Research during the past 35 years has shown
that many interacting factors undoubtedly combine to slow
plant succession on the Pumice Desert, including climate,
short growing seasons, sterile soils, hot soil temperature
in summer affecting seedling survival, and rodent or gopher
activity that aerated and mixed the soils (a positive thing)
but also negatively affected the vegetation when they ate
the roots. However, the number of trees that have become
established on the 100-acre tree plot has increased by 75%
between 1965 and 2000. Eventually, as succession proceeds,
conditions will become more hospitable for new seedlings.
The plots established in 1965 allow continued monitoring of
this progress. The surrounding lodgepole pine will
eventually infiltrate the area and the Pumice Desert will
cease to exist.
I greatly appreciate the assistance of the
staff of Crater Lake National Park in the many stages of
this research. I also would like to acknowledge the
assistance of plant ecologists Fred Hall and William Hopkins
in resurveying the vegetation plots and the cheerful
assistance of Kirk M. Horn throughout.
Applegate, E. 1939. Plants of Crater Lake
National Park. American Midland Naturalist 22(2): 225–314.
Horn, E. M. 1968. Ecology of the Pumice
Desert. Northwest Science 42(4):141–149.
Sternes, G. L. 1963. Climate of Crater Lake
National Park. Crater Lake Natural History Association,
Crater Lake, Oregon.
Williams, H. 1942. The Geology of Crater
Lake National Park, Oregon. Carnegie Institution of
Washington Publication 540,Washington, D.C.
About the author
Elizabeth L. Horn holds a B.A. from
Valparaiso University and an M.S. from Purdue University and
began this research as a seasonal ranger-naturalist at
Crater Lake National Park from 1964–1965. She retired from a
30-year career with the U.S. Forest Service and now resides
in West Yellowstone, Montana, and can be reached at
Figure 3. Author examines base of
multi-stemmed, bushy lodgepole pine. Multiple trunks are not
uncommon in harsh environments. There could be several
causes. Genetics can create a predisposition for multiple
stems, but the surrounding forest did not exhibit multiple
stems. Rodent seed caches could result in multiple stems.
However, examination of seedlings under 6 inches in height
determined that seedlings have single stems, while slightly
larger bushy-shaped trees clearly showed apical stem damage.
This would release the side stems to grow upright. The
damage could be caused by wind-borne pumice abrasion,
gophers, or winter drying from low snow levels. In some
cases, gopher gnawings were clearly present. Although the
researchers have some guesses, the cause of this phenomenon
would be an interesting future study.