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Forest Ecology and Management 235 (2006) 143–154
www.elsevier.com/locate/foreco
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Andrew E. Scholl, Alan H. Taylor *
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Regeneration patterns in old-growth red fir–western white pine
forests in the northern Sierra Nevada, Lake Tahoe, USA
Department of Geography, The Pennsylvania State University, University Park, 302 Walker Building, PA 16802, USA
Received 5 January 2006; received in revised form 1 August 2006; accepted 2 August 2006
Abstract
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Red fir (Abies magnifica) forests in the Sierra Nevada are known to demonstrate both shade tolerant and disturbance related regeneration
making it difficult to understand the role of disturbances in the regeneration dynamics of the forests. Four stands with different structural
characteristics were selected for intensive sampling in order to capture the observed range of structural variability (e.g. composition, age, size and
spatial pattern) in an old-growth red fir–western white pine (Pinus monticola) forest in the northern Sierra Nevada. We used detailed stem mapping,
stand structural analysis and cross-dated fire scar samples to identify the relationships between disturbances and stand structure. All trees >5 cm
dbh within four 0.5-ha plots were aged and mapped. The species composition of the plots was similar but the density and basal area of the tree
populations varied among the plots. Red fir density and basal areas are greater than that of western white pine. The age structure indicated
continuous, but variable recruitment and there were few seedlings and saplings. The mean point fire return interval was 76 years (range 25–175
years) for the 400-ha study area. Most fires scarred only single samples suggesting that burns were small and patchy, but pulses of recruitment
suggest that some fires were moderate in severity. Regeneration pulses coincided with the dates of several fires (e.g. 1636, 1770). Moran’s I, a
measure of spatial autocorrelation, indicated that red fir and western white pine exhibited positive spatial autocorrelation at short (3–12 m) and
intermediate (36–75 m) distances. Groups of similar age trees were spatially discrete and groups of different ages tended to overlap, resulting in an
all aged forest. The spatial pattern of tree ages and the record of disturbance indicate that infrequent moderate severity fires have a lasting influence
on the structure and development of old-growth red fir forests.
# 2006 Elsevier B.V. All rights reserved.
Keywords: Abies magnifica; Pinus monticola; Forest dynamics; Disturbance; Fire history; Age structure; Size structure; Spatial patterns; Moran’s I; Regeneration
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1. Introduction
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Natural disturbance plays a critical role in mediating oldgrowth forest dynamics, and disturbances vary widely in type,
scale, and effect on stand structure (Henry and Swan, 1974;
White, 1979; Pickett and White, 1985; Pickett et al., 1989). In
upper montane red fir forests (Abies magnifica A. Murr), a
forest type nearly endemic to California (Oosting and Billings,
1943; Barbour and Woodward, 1985), windthrow and fire are
important disturbances that affect forest structure and
composition (Taylor and Halpern, 1991; Agee, 1993; Taylor,
2000; Taylor and Solem, 2001). Yet, little research has
examined the effects of natural disturbance on tree regeneration
patterns and stand development in these forests (Pitcher, 1987;
Taylor and Halpern, 1991; Chappell and Agee, 1996).
* Corresponding author. Tel.: +1 814 865 1509; fax: +1 814 863 7943.
E-mail address: [email protected] (A.H. Taylor).
0378-1127/$ – see front matter # 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.foreco.2006.08.006
Identifying how disturbances influence stand structure and
development is central to understanding the long-term
dynamics of red fir forests.
An outstanding feature of red fir stands is their intricate
horizontal structure (Hallin, 1957; Gordon, 1979). In oldgrowth stands, red fir trees of similar size typically occur in
patches, and overlap between patches creates complex stand
mosaics. Patches or groups of trees in the stand mosaic range
from a few hundred m2 to tens of ha in size (Hallin, 1957;
Gordon, 1979; Taylor and Halpern, 1991; Taylor, 1993). The
groups of similar size trees are thought to be trees similar in age,
and related to punctuated establishment of red fir in canopy
openings made by fire, disease, insect attacks, or windthrow
(Hallin, 1957). Disturbances that produce a mineral seedbed are
particularly favorable for regeneration of red fir and associated
tree species (Gordon, 1970a; Laacke, 1990), and punctuated
establishment after fire (Chappell and Agee, 1996) and logging
(Gordon, 1970b; Barbour et al., 1998) has been observed. Yet,
red fir also regenerates in partial shade in small openings made
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A.E. Scholl, A.H. Taylor / Forest Ecology and Management 235 (2006) 143–154
by tree-falls or death of single or small groups of canopy trees
(Ustin et al., 1984; Selter et al., 1986; Taylor and Halpern,
1991). Consequently, regeneration of red fir may not be
dependent on large-scale disturbance. Few studies in red fir
forests have emphasized detailed age structure analysis
(Pitcher, 1987; Taylor and Halpern, 1991). Instead, collection
of tree age or size data has often occurred over large areas
(Barbour and Woodward, 1985; Parker, 1992). Composite age
or size structure of forests from widely different sites do not
provide an understanding of local disturbance and how the
temporal and spatial arrangement of age classes developed in a
particular place (Stewart, 1986).
In this study, we identify the disturbance history and analyze
the age and size structure of trees in old-growth red fir forests in
four different stands to determine how stand structure and
development is influenced by the type and scale of natural
disturbances. We place particular emphasis on the spatial
analysis of tree ages since patch age structure is thought to
strongly reflect the impact of disturbance on tree regeneration
and long-term development of red fir forests.
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Our study was conducted in the largest uncut red fir forest on
the east shore of Lake Tahoe. The old-growth forest covers
400 ha on the upper slopes of the Carson Range on a north–
northwest facing slope between 2300 and 2500 m. The forest
was strongly dominated by red fir (Abies magnifica), and
western white pine (Pinus monticola). White fir (Abies
concolor) and lodgepole pine (Pinus contorta) were also
present in small numbers. Live and dead standing trees >1.0 m
dbh, snags, and logs on the forest floor in varying stages of
decay were characteristic features of the old-growth forest.
Evidence of fire disturbance in the form of fire scarred trees and
charcoal on logs on the forest floor was ubiquitous. Shrub and
forb cover was low, and pinemat manzanita (Arctostaphylos
nevadensis A. Gray.), a dwarf shrub, was the most abundant
understory species. After a thorough reconnaissance of the oldgrowth forest, four stands with different structural characteristics were selected for intensive sampling to capture the
observed range of structural variability (e.g. composition, age,
size and spatial pattern) in the 400 ha old-growth forest. A
0.5 ha (100 m 50 m) plot was established in each stand to
sample forest structure. Although we used large plots in our
study, plot size does impose limits on the maximum spatial
scale of inference for analyses of spatial pattern (Upton and
Fingleton, 1985). However, coincident groups of trees in the
same age-classes among plots permit inference on the influence
of larger scale events or processes that affect stand structure and
development (e.g. Hemstrom and Franklin, 1982).
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Old-growth red fir forests were studied on the western slope
of the Carson Range, on the east shore of Lake Tahoe, in the
northern Sierra Nevada. The climate is characterized by warm,
dry summers and cold, wet winters. Average monthly
temperatures at South Lake Tahoe, CA (1820 m) range from
1 8C in January to 18 8C in July, and annual precipitation is
78.4 cm, with 86% falling as snow between November and
April. April snow-pack depths above 2300 m frequently exceed
2 m. Thunderstorms occur in the dry season and lightning is a
common source of ignition in the red fir zone (Manley et al.,
2000). Terrain in the study area is complex and rock outcrops
and several perennial streams interrupt connectivity of ground
fuels, which may inhibit the spread of fire. Soils are shallow
(<1 m) loamy coarse sand derived from Mesozoic aged granite,
excessively drained and medium in acidity (Rogers, 1974).
People have been present in the Lake Tahoe Basin for a long
time, at least since the early Archaic period (ca. 7000 years)
(Lindström, 2000). Native Americans (Washoe) used the Lake
Tahoe Basin, seasonally, and their use may have modified local
vegetation patterns. Washoe people burned forests to drive
game and to increase production of certain plants for food and
fiber (Lindström, 2000). Euro–Americans arrived in the basin in
1844, but settlement was limited until the 1860s. Forests in
most of the basin were cut between 1873 and 1900 to meet
demand for wood in the Comstock silver mines in Virginia City,
Nevada (Lindström, 2000; Taylor, 2004). Although logging was
extensive there are tracts of uncut forest on both the east and
west side of Lake Tahoe (Manley et al., 2000). Local grazing,
especially of montane meadows, began in the mid 1850s and
grazing peaked between 1920 and 1930. Land use changed
again when lands in the Carson Range became part of the
Toiyabe National Forest in 1907. Early management emphasized fire suppression and the regulation of grazing (Strong,
1984; Lindström, 2000).
2.1. Stand selection
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1.1. Study area
2. Methods
2.2. Stand structure
The structural characteristics of the forest were determined by
mapping and measuring trees in each 0.5 ha plot. Each plot was
divided into a measured grid of 10 m 10 m cells. Stem location
in each grid cell (x, y coordinates) was determined by recording
distance from the cell origin (0,0) to the nearest 0.1 m with a
metric tape. The diameter and species of each live and standing
dead tree (5 cm dbh) was then recorded. Stems <5 cm dbh
were also identified to species and they were mapped as either
seedlings (0.2–1.4 m tall) or saplings (>1.4 m tall and <5 cm
dbh). The stem base, bole position, and direction of fall (azimuth)
for all downed trees in each plot were mapped. Forest canopy
cover for intermediate and taller trees above each cell was
visually estimated as being open (<33% cover), intermediate
(33–66% cover), or closed (>66% cover). Surface conditions
were characterized by estimating the cover of mineral soil, rock,
shrubs, forbs, and litter in each cell in each plot into one of six
cover classes: (<1, 1–5, 6–25, 26–50, 51–75, 76–100%).
The age structure of trees in each plot was identified by coring
all live trees (5 cm dbh) to the pith at either 30 cm (5–85 cm
dbh) or 100 cm (>85 cm dbh) above the soil surface with an
increment borer. Cores were sanded to a high polish, their annual
growth rings were visually cross-dated (Stokes and Smiley, 1968)
with an established tree-ring chronology (Holmes et al., 1986),
A.E. Scholl, A.H. Taylor / Forest Ecology and Management 235 (2006) 143–154
2.3. Disturbance history
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The history of disturbance in each plot was reconstructed
using three types of data: (1) dates of fires recorded in firescarred trees; (2) variation in radial growth patterns in cored
trees; and (3) the age structure of tree populations. Although
evidence of past fire in the form of charcoal and charred logs on
the forest floor was present throughout the forest, few trees had
basal fire scar wounds. In red fir forests, low intensity surface
fires may not scar trees and wounds may heal over completely
since the last fire, eliminating external evidence (Taylor, 1993).
Moreover, sapwood decay of red fir where it is injured by fire is
common (Agee, 1993) so fire scar lesions are usually not well
preserved in tree rings. Consequently, only a limited number
(n = 6) of partial cross-sections were available to reconstruct
the fire history in or near the plots. All samples were extracted
from live western white pine with visible fire scars using a
chainsaw (Arno and Sneck, 1977) and were located within
150 m of the four plots. Fire dates were identified by first
sanding each cross-section to a high polish and then crossdating the annual growth rings using standard dendrochronological techniques (Stokes and Smiley, 1968). The calendar
year of each tree ring with a fire scar in it was then recorded as
the fire date. The season each fire burned was also estimated by
recording the position (Baisan and Swetnam, 1990) of each fire
scar lesion within the annual ring.
We also identified disturbances in our plots by examining
radial growth patterns in each core. Disturbances are often
recorded in the growth patterns of trees and analyses of the time–
frequency distribution of radial growth variation can provide
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important insights on disturbance regimes and the role of
disturbance in long-term stand development (Lorimer, 1985).
When disturbances improve growing conditions for individuals
by killing nearby competitors, surviving trees may exhibit a
sudden increase in radial growth (release) and the date of
disturbance corresponds with the onset of the sudden radial
growth increase. On the other hand, if a disturbance damages a
tree, its radial growth may suddenly decrease (suppression) and
the date of onset of reduced growth may correspond to the year of
the disturbance (Barrett and Arno, 1988; Fritts and Swetnam,
1989). Frequent small-scale disturbances, such as tree-fall
display a temporal pattern of releases and suppressions which are
relatively constant through time (Lorimer, 1985; Runkle, 2000).
In contrast, in forests that experience more severe, but less
frequent, large-scale disturbances, such as windthrow or perhaps
fire, the temporal patterns of releases or suppressions is episodic
and not constant through time (Lorimer, 1985; Taylor, 1990;
Taylor and Halpern, 1991). To identify when canopy disturbances occurred, we identified the calendar date of the onset of
all releases (200% increase in radial growth for 5 years compared
to the previous 5 years) and suppressions (200% decrease in
radial growth for 5 years compared to the previous 5 years) in the
cores in each plot. To determine if forest canopy disturbances
were punctuated and relatively severe or small-scale and constant
through time, we developed a decadal time-scale frequency
distribution of growth releases and suppressions for each plot.
The disturbance index was expressed as the percentage of aged
trees in each decade that were alive on the date of the radial
growth release and suppression (Lorimer, 1985).
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and tree age was assigned based on the calendar year of the inner
most ring. For cores that missed the pith but had a complete arc
(33%), an annulus of concentric rings was used to estimate the
number of missing rings (Applequist, 1958). In all of these cases,
only 1–5 years were added to core age. Because red fir and
western white pine seedlings have variable growth rates (Gordon,
1970a; Pitcher, 1987) depending on canopy conditions (i.e. gap,
closed) we did not add an estimate of the number of years for trees
to grow to coring height to estimate dates of tree establishment.
Ages for all trees are reported as age at coring height.
Some trees (4%) could not be aged because their stems
contained rot or they were too large to extract a complete core.
We estimated the ages of these trees in the following way. First,
we developed a regression equation between core length and
tree diameter for all red fir (r2 = 0.93, n = 309) and western
white pine (r2 = 0.90, n = 83) that were cored to the pith.
Second, we measured the first 20 years of radial growth and
calculated the average number of rings/cm for each species (red
fir: 5 rings/cm, western white pine: 6 rings/cm). Third, the
missing radius for trees with incomplete cores was predicted
using the core-length dbh regression and the difference between
the predicted radius and core length was calculated. Finally, we
added the number of years represented by the missing length to
the age of each incomplete core, unless the predicted core
length was shorter than the collected core. In these cases, the
counted age was used as tree age.
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2.4. Spatial analysis
We identified the spatial characteristics of tree ages in each
plot in several ways. First, we calculated Moran’s I, which is a
measure of spatial autocorrelation (Moran, 1950). Spatial
autocorrelation is the property where entities (trees) with
similar characteristics (age) are found closer to each other than
entities with different characteristics (Upton and Fingleton,
1985). We calculated Moran’s I for 3 m distance classes (d)
from 1–100 m, using software developed by Duncan (1990), to
identify spatial autocorrelation over a range of spatial scales.
Since the regeneration requirements of species vary, we
calculated Moran’s I(d)) for each species independently. We
transformed values of Moran’s I(d) to standard deviates [z(d)],
such that a z(d) value of zero indicates no spatial autocorrelation (random), a positive value indicates that individuals with
similar characteristics occur near each other, and a negative
value indicates that individuals with different characteristics
occur near each other.
We also identified the spatial characteristics of tree ages
using cluster analysis to determine if tree groups were relatively
even-aged and different in age from spatially adjacent groups
(Duncan and Stewart, 1991). Groups of trees of similar age and
spatial location were identified by clustering species age and
location using relative Euclidean distance and Ward’s method.
Ward’s method of cluster analysis minimizes within group
variance relative to between group variance (Gauch, 1982; van
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A.E. Scholl, A.H. Taylor / Forest Ecology and Management 235 (2006) 143–154
Table 1
Plot characteristics of old-growth red fir–western white pine forests in the northern Sierra Nevada, Lake Tahoe
Site conditions
Plot 1
Plot 2
Plot 3
Plot 4
Aspect (degrees)
Elevation (m)
Slope (%)
Stand characteristics
314
2376
3.5
330
2442
15
330
2430
15.5
310
2478
19
ABMA
2
Basal area (m /ha)
Density (stems/ha)
Live
Standing dead
30.8
234
16
PIMO
23.1
76
4
ABMA
34.9
242
50
Plot 3
PIMO
21
84
22
PICO
ABMA
3.2
58.1
30
1
138
24
Plot 4
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Plot 2
PIMO
ABCO
15
0.1
74
18
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Plot 1
2
0
ABMA
PIMO
44.5
16.4
504
194
112
56
ABCO
0.2
2
0
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Densities and basal area are for stems >5 cm dbh. ABMA = A. magnifica; PIMO = P. monticola; ABCO = A. concolor; PICO = P. contorta.
Fig. 1. Size-class distribution of live and dead standing red fir (Abies magnifica) and western white pine (Pinus monticola) in four plots of old-growth red fir–western
white pine forest, northern Sierra Nevada, Lake Tahoe. Seedlings are 0.2–1.4 m tall, and saplings are >1.4 m tall and <5 cm dbh. The upper bound for each 10 cm size
class is reported on the x-axis.
A.E. Scholl, A.H. Taylor / Forest Ecology and Management 235 (2006) 143–154
147
Overall, the coefficients of determination for regression
equations of age on dbh in the plots were similar for red
fir (range, 0.64–0.85) and western white pine (range, 0.58–
0.75).
Tongeren, 1995). Groups of similarly aged patches of trees
were identified if the groups were different in age from spatially
adjacent groups.
3. Results
3.3. Age structure
3.1. Stand characteristics and size structure
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There was a positive relationship (P < 0.001) between tree
diameter (dbh) and age for each species in each plot (Fig. 2).
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3.2. Size–age relationships
Red fir and western white pine occurred in a wide range of
age-classes and there were trees 50–400 years old in each plot
(Fig. 3). The oldest western white pine (623 years) exceeded
the age of the oldest red fir (520 years) by >100 years. There
were differences in the age structure of red fir and western
white pine tree populations in the plots. First, the form of the
age–class distributions for the two species were different
(P < 0.05, Kolmogorov–Smirnov two-sample test) and red fir
had a more positively skewed pattern than western white pine
(Fig. 3). Second, the average age of red fir was usually younger
(plots 1–4 means = 185, 168, 252 and 185, respectively) than
western white pine (plots 1–4, means = 292, 285, 283 and 243,
respectively). In plots 1, 2 and 4, red fir had a unimodal age
structure (Fig. 3), while in plot 3 it was multi-modal. There
was also a common peak in red fir recruitment 150–200
years ago in all of the plots. Western white pine had a multimodal age–class distribution in each of the plots, and in three
of the plots there was a pulse in recruitment 150–200 years
ago.
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The species composition of the plots was similar and the
average density and basal area in the plots was 375 trees ha 1 and
61.8 m2 ha 1, respectively (Table 1). The forest canopy was
sparse and >70% of the cells in a plot were open, while 6%
were closed. The ground cover in the plots was mainly bare
mineral soil (mean = 75%) and shrub or forb cover was low.
Red fir and western white pine occurred in a wide range of
diameter classes in each plot suggesting that their populations
are self-replacing (Fig. 1). Yet, neither red fir nor western white
pine had the reverse-J size-class distribution that is typical of a
continuously regenerating shade tolerant species (Hett and
Loucks, 1976). Seedling and sapling density of both species in
the plots was low (Fig. 1).
Fig. 2. Diameter (dbh) vs. age for red fir (Abies magnifica) and western white pine (Pinus monticola) in four plots of old-growth red fir–western white pine forest, in
the northern Sierra Nevada, Lake Tahoe. The coefficient of determination (r2) is for a linear regression between diameter and age for each species in each plot.
A.E. Scholl, A.H. Taylor / Forest Ecology and Management 235 (2006) 143–154
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Fig. 3. Age-class distribution of all red fir (Abies magnifica) and western white pine (Pinus monticola) >5 cm dbh in four plots of old-growth red fir–western white
pine forest, in the northern Sierra Nevada, Lake Tahoe. Not shown is one P. monticola of 623 year in plot 2. The arrows mark fire dates and the upper bound for each 20
years age class is reported on the x-axis.
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3.4. Evidence of disturbance
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Fourteen fires were identified between 1580 and 1853 in
the six fire scar samples, indicating that the old-growth forest
had experienced recurring fire for at least 400 years (Table 2).
The average interval between fires calculated from a
composite of all fire dates in the six samples was 21 years
(range, 9–50 years). The average point fire interval (PFI), or
average period between fires on individual samples, was
longer at 76 years (range, 25–175 years). Only two fires
(1636, 1657) were recorded on more than one fire scar
sample. No fires were recorded after 1853. All of the recorded
fires occurred in the dormant season after trees stopped radial
growth for the year.
There were peaks in releases and/or suppressions in the plots
(Fig. 4) that coincided with dates of fires in the fire scar
samples, suggesting that the fires burned in the plots. Fire dates
that coincided with peaks include: 1580 (plot 2), 1600 (plot 1),
1636 (plot 1, 3 and 4), 1645 (plot 2), 1710 (plots 2–4), 1747
(plot 1), 1770 (plot 3), and 1820 (plots 1 and 4). Fires in other
years (e.g. 1680, 1853) may have caused suppressions or
releases, but not at a high frequency.
Table 2
Fire dates in six fire scar samples in old-growth red fir–western white pine
forests, northern Sierra Nevada, Lake Tahoe
Sample
# Scars
Fire dates
Range of fire
intervals
TRF6
3
1580
1645
1820
65–175
TRF22
2
1636
1670
34
TRF13
3
1657
1747
1853
90–106
TRF2
3
1590
1657
1682
25–67
TRF15
2
1600
1636
36
TRF19
3
1611*
1710
1770
*
Due to rot, scar could only be determined 1 year.
60–99
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A.E. Scholl, A.H. Taylor / Forest Ecology and Management 235 (2006) 143–154
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Fig. 4. Frequency (%) of aged trees exhibiting radial growth releases (solid) and suppressions (open) by decade out of all trees alive in each decade in four plots of
old-growth red fir–western white pine forest in the northern Sierra Nevada, Lake Tahoe. The solid line is sample depth and the upper bound for each decade class is
reported on the x-axis.
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Radial growth releases and suppressions also occurred in
decades with no evidence of fire suggesting that disturbances
other than fire also influenced stand development (Fig. 4).
Releases and suppressions were recorded with similar
frequency in the plots in each decade during the last 200
years. Over this period, the average percentage of trees with
releases and suppressions was 3.8% (range, 2.8–4.3%) and
4.1% (range, 2.7–5.8%), respectively.
Disturbance, especially fire, can generate distinct age–classes
in forests stands. In our plots, there was some correspondence
between disturbances recorded as radial growth suppressions and
releases, and peaks in the age-structure of red fir and western
white pine populations. In all of the plots, there was a sustained
pulse of red fir recruitment that began 200–220 years ago and the
onset of this wave of regeneration corresponds with the
occurrence of fires in 1770 and 1820 (Fig. 3). The 1820
recruitment pulse was evident in the western white pine
population in three of the plots. Overall, peaks in the age–class
distribution of western white pine in the plots corresponded more
closely than did those of red fir with the dates of fire disturbance.
Pulses of western white pine recruitment were associated with
fire dates in 1611 (plots 3 and 4), 1636 (plots 1–4), 1710 (plots 1,
3 and 4), and 1820 (plots 2–4).
3.5. Spatial analysis
The correlograms for red fir and western white pine show an
alternating pattern of significant (P < 0.05) positive and negative
values indicating patchiness in the spatial distribution of tree ages
(Fig. 5). For red fir, in three plots (1, 3 and 4), and western white
pine in two plots (2 and 3), there was significant positive
autocorrelation for distances of 3–12 m indicating that trees of
similar age occurred together at small spatial scales. Significant
peaks for red fir at distances of 39–57 m (plot 4), 36–75 m (plot
2), and western white pine at distances of 48 m (plot 1), 99 m
(plot 3), and 84 m (plot 4) represent distances between patches of
A.E. Scholl, A.H. Taylor / Forest Ecology and Management 235 (2006) 143–154
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Fig. 5. Spatial correlograms of red fir (Abies magnifica) and western white pine (Pinus monticola) tree ages in four plots of old-growth red fir–western white pine
forest in the northern Sierra Nevada, Lake Tahoe. The plots size is 100 m 50 m. Dashed lines indicate statistical significance (P < 0.05). Points above the upper
dashed line indicate distances of significant positive autocorrelation of tree ages while points below the dashed line indicate distances of significant negative
autocorrelation of tree ages.
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similarly aged trees. On the other hand, significant negative
values for red fir at distances of 30–42 and 75 m (plot 1), 27–30,
45–48, and 81–87 m (plot 2), 45–84 m (plot 3), 24–36 and 60–
72 m (plot 4), and western white pine at distances of 30–36 m
(plots 1–3), 90–93 m (plot 1), 63 m (plot 2) and 78–90 m (plot 3)
represent distances between patches of different age.
In each of the plots, the cluster analysis identified 4–6 tree-age
groups (Table 3; Fig. 6). Groups were relatively discrete and the
mean ages of trees in each group in each plot were different
(P < 0.01). Yet, there was overlap in the range of tree ages among
groups and the average age range in a group was 116 years
(Table 3). Similarly, trees in each age group varied widely in size.
The average dbh range of trees in an age group was 73.5 cm
(Table 3). Some tree age groups occupied large portions of a plot
resulting in spatial overlap among groups (Fig. 6).
4. Discussion
Diameter and age distributions for red fir and western white
pine in our old-growth stands indicate that populations of both
species are self-perpetuating, a pattern identified for modal sites
elsewhere in the range of red fir (Oosting and Billings, 1943;
Barbour and Woodward, 1985; Parker, 1992; Taylor, 2000).
However, the strength of the age–dbh relationship for red fir and
western white pine in the plots was not always strong, and weak
age–dbh relationships have been identified in other red fir
forests (Pitcher, 1987; Taylor and Halpern, 1991; Taylor, 1993).
Thus, the size structure of red fir and western white pine
populations is not sufficient to assess species’ persistence, but
population age structures are needed to make inferences about
stand dynamics and the effect of disturbances on stand
development.
Canopy mortality caused by the death of single trees or small
groups of trees, and fire, were the principal disturbance agents
affecting our old-growth stands. The frequency of radial growth
releases and suppressions per decade was similar in the four
plots, and relatively constant over the last 200 years. There was
no evidence of a synchronous pattern of high releases or
suppressions among plots indicative of large-scale episodic
disturbances such as windstorms (e.g. Taylor and Halpern,
A.E. Scholl, A.H. Taylor / Forest Ecology and Management 235 (2006) 143–154
151
Table 3
Mean, range, and standard deviation of groups of similar age trees identified by cluster analysis of tree age and location in old-growth red fir–western white pine
forests, northern Sierra Nevada, Lake Tahoe
Group #
Number
of trees
Age (years)
1
2
3
4
65
39
40
11
179.4
115.6
283.7
482.0
1
2
3
4
5
42
60
30
32
34
1
2
3
4
1
2
3
4
5
6
Mean
Size (cm dbh)
S.D.
Range
Mean
S.D.
20.1
23.1
46.2
39.0
135–230
64–156
199–369
412–523
28.3
13.1
58.9
88.1
16.2
7.8
29.8
25.8
113.4
169.3
243.6
360.0
120.6
22.2
13.1
32.6
61.9
28.7
57–138
144–200
195–304
291–623
52–163
14.8
26.7
45.1
67.6
20.7
17.4
25.1
24.6
26.2
20.4
9
31
29
38
158.1
373.0
284.0
178.3
158.1
26.6
219–33
33.1
140–184
320–425
219–332
118–235
29.6
99.5
55.3
17.0
56
64
24
73
74
18
150.6
185.4
116.3
175.7
252.0
345.5
14.7
15.1
21.2
13.2
25.4
24.1
120–181
155–225
49–143
145–204
200–304
305–394
11.3
21.7
9.6
21.5
41.1
24.1
Range
Max. distance
between trees (m)
Plot 1
89.4
98.8
83.6
46.7
5.3–92.9
5.3–151
12.5–118.4
34.7–128
5.8–122.3
59.0
57.7
98.6
95.8
54.3
15.3
28.9
24.6
14.3
10.1–61.3
42.5–151.9
13.2–107.1
5.2–80.7
16.0
77.5
97.9
51.8
5.7
12.4
6.0
13.5
19.5
21.2
5.3–27.2
5.7–53.2
5.5–28.1
5.9–60.6
8.4–84.3
41.8–119.4
61.5
46.5
61.8
70.2
103.3
89.5
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on
Plot 4
al
Plot 3
co
Plot 2
py
6.1–89.3
5.4–36
7.1–116.5
52–125.1
The mean, range, and standard deviation of tree sizes (dbh) for each age group and the maximum distance between trees in each group are also given.
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1991) or insect outbreaks (e.g. Veblen et al., 1991). This
suggests that events, such as tree-falls, are a frequent and
important small-scale disturbance in red fir–western white pine
forests that influence old-growth stand development.
Fourteen fires over a 273 year period were identified in the
limited fire scar record. The limited fire record suggests that
most fires that burned in the stands were small, low intensity
burns. Only two fires were recorded by more than one sample.
The current cover of ground fuel on the forest floor is not
conducive to fire spread. Much of the forest floor (>70%) in the
plots was bare ground and low surface fuel connectivity in
forest stands greatly impedes spread of fire from a point of
ignition (Albini, 1976; Rothermel, 1983). Fire spread is further
impeded by the dense short-needle fuel beds that occur under
red fir trees (van Wagtendonk et al., 1998; Fonda et al., 1998).
However, surface fires may have burned through the stands,
leaving little tree ring evidence of fire. Low intensity fires may
not scar trees and fire scar wounds may heal completely leaving
little external evidence of fire on trees in a stand (Taylor, 1993).
In our plots, there was a correspondence between sudden radial
growth changes in trees and fire years recorded in the fire scar
samples. The combination of fire scar and radial growth
evidence of fire suggests that the 1645 and 1770 fires burned in
one of the plots, the 1820 fire in two of the plots, and the 1710
fire burned in three of the plots.
The effects of fire disturbance on forest age structure are
highly variable and they are related, in part, to fire severity
(Chappell and Agee, 1996; Taylor and Skinner, 1998). Fires
often burn across a landscape with variable severity, killing
many trees in some areas and few or none in others. Even-aged
stands that have a uni-modal age structure are characteristic of
forests that burned at high severity, while multi-aged stands
reflect moderate severity fire that kill parts of a stand. Low
severity burns, in contrast, may not generate distinct fire related
age–classes (Taylor and Skinner, 2003). Our old-growth stands
had experienced repeated burning, they were multi-aged, and
they contained trees > 400 years old. This suggests that the
fires that burned in our stands were low or moderate severity
burns. Despite the low or moderate severity nature of past
burns, however, important features of contemporary old-growth
forest structure were related to past fire.
The 1770 fire had a strong influence on the age structure of
our forest. In each of the plots, a large pulse of red fir
recruitment followed the fire and restocking continued for 60–
80 years. Red fir establishes well on burned, compared to
unburned, surfaces (Chappell and Agee, 1996) due to exposure
of mineral soil, reduction in below ground competition for
nutrients and soil moisture, reduction in fungal populations, or
increased root access to mycorrhizal fungi (Gordon, 1970a;
Laacke, 1990). Red fir establishment, post-fire, is also more
successful under partial canopy cover (Chappell and Agee,
1996) where shadier and cooler conditions increase red fir
seedling survivorship compared to open areas (Selter et al.,
1986; Ustin et al., 1984). Red fir and western white pine trees in
each of the plots survived the 1770 fire. Post-fire restocking in
the plots took decades, suggesting that harsh site conditions
A.E. Scholl, A.H. Taylor / Forest Ecology and Management 235 (2006) 143–154
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152
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Fig. 6. Maps of groups of trees of similar age identified from cluster analysis of ages and locations of trees in four 100 m 50 m plots of old-growth red fir–western
white pine forest, in the northern Sierra Nevada, Lake Tahoe.
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(e.g. temperature, water stress), or competition with forbs or
shrubs (Gordon, 1970a,b; Nagel and Taylor, 2005) reduced tree
seedling establishment and recruitment. Long periods for postfire restocking may be common in red fir forests. Tree seedling
establishment spanning periods of 50 or more years have been
reported following fire in red fir forests in the southern Sierra
Nevada (Pitcher, 1987), and southern Cascade Range (Taylor
and Halpern, 1991).
Other minor pulses of recruitment, especially of western
white pine were associated with fire occurrence in the plots. In
red fir–western white pine forests in the southern Cascades,
seedling establishment of western white pine is much greater on
burned than unburned substrates (Chappell and Agee, 1996).
Optimal conditions for western white pine regeneration occur
on bare mineral soil seedbeds in proximity to unburned residual
trees that serve as a seed source (Graham, 1990). This
combination of conditions for seedling establishment may be
responsible for the stronger link between western white pine
recruitment and fire occurrence in our plots than for red fir.
A hallmark of red fir forests is their intricate horizontal
structure (Oosting and Billings, 1943; Hallin, 1957; Taylor and
Halpern, 1991). Stands are thought to be composites of
relatively even-aged patches of trees that establish after largescale disturbances such as fire or small canopy gap disturbances
caused by treefall. Spatial dependence in the ages of trees at
small to intermediate spatial scales supports a view of patchy
regeneration, but trees within a patch were not even-aged.
Groups of trees of similar age occurred together at small spatial
scales. However, groups that were of similar age were separated
from each other by groups of trees with a different similar age.
This suggests that regeneration occurred in larger patches
within a mosaic of trees that survived a disturbance event and
not in single tree gaps caused by the death of individual canopy
trees. The spatial structure of trees in our plots is probably the
A.E. Scholl, A.H. Taylor / Forest Ecology and Management 235 (2006) 143–154
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Acknowledgements
co
5. Conclusion
Red fir forests in the northern Sierra Nevada are structurally
complex. The highly variable nature of disturbance effects, and
the ability of red fir to regenerate under a wide range of
conditions, contributes to this complexity. Fire and tree-falls
were the main disturbances affecting old-growth red fir–
western white pine stand development. Infrequent, moderate
severity fires that burned unevenly through the forest strongly
shaped stand structure by promoting regeneration in widely
scattered patches within mosaics of surviving trees. The strong,
but complex, influence of moderate severity fire on stand
structure in our red fir forests highlights the important role of
infrequent disturbance on the development of spatial heterogeneity in old-growth forests.
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py
result of moderate severity fires that burned unevenly through
stands and created overlapping mosaics of dead and surviving
canopy trees. Regeneration would then occur throughout a
stand of existing trees that survived the fire, resulting in a multiaged forest. The temporal and spatial patterns of tree
regeneration after the 1770 fire is an example of such a fire
and the 230 year period since that burn suggests moderate
severity burns are infrequent, at least in the forests we studied.
r's
pe
This research could not have been completed without the
assistance of many individuals. J. Swanson and M. Johnson
provided important administrative and logistic support during the
field phase of this work. J. Balmat, R.M. Beaty, E. Heithoff, and
T. Schmitz assisted in the field, R.M. Beaty and S.P. Norman
assisted with laboratory work, and R.M. Beaty, S.P. Norman, and
A. Guarin provided comments on an earlier draft of this paper.
This research was supported by a cost-share agreement between
The Pennsylvania State University and the USDA Forest Service
Lake Tahoe Basin Management Unit (PA-05-98-19-030).
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