Structure and dynamics of whitebark pine
forests in the Warner Mountains
Pete Figura
California Department of Fish and Wildlife
Outline
• Provide brief overview of Warner
Mountains and study design
• Describe whitebark pine forest
structure and dynamics in the
study area
• Summarize change in the study
area over 18 years (1994-2012)
Warner Mountains
•
Fault-block range rising above the Modoc
Plateau in northeastern California
– Main ridgeline approximately 9500’
– Gentle west slope
– Steep eastern escarpment
•
Whitebark pine dominates above 7800’
•
Three whitebark-dominated vegetation
types have been described (Riegel et al.
1990)
– PIAL / Stipa califorrnica
– PIAL / Penstemon gracilentus
– PIAL / Arenaria aculeata
– Arrayed along elevation/soil moisture
gradient
Study objectives
• Investigate whitebark
stand structure and
dynamics across
elevational gradient
• Establish permanent plots
to facilitate long-term
monitoring
Study Design
• Permanent plots established on western slope in vicinity of
Eagle Peak in 1994
– Area of extensive whitebark pine forests
– Area where whitebark occupies greatest elevation range
• 400 m2 plots located at 91 m
(300 ft) elevation intervals along
six systematically-located
transects
• All standing conifer stems (>1.4
m) marked with numeric tags
Transect and Plot Locations
Variables inventoried within plots
• Initial visit - 1994
– Stem density, diameter, height class, age
• Aged all single-stemmed trees and the largest stem of each multistemmed cluster
– Habitat type
– Other physical characteristics (aspect, slope, canopy cover,
amount of downed wood, etc.)
• Revisits – 2006 and 2012
– Determined whether each tagged stem was live/dead
– Pathogen assessment
– Measured diameter of 15% of stems (randomly chosen within
plots)
2012
Results
0 100 200 300 400 500 600 700 800 900
age
Whitebark pine tree age by elevation
2290
2380
2470
2560
2650
27402830/2930
elevation (m)
Mean
62.3
48.8
83.4
133.7
152.6
144.4
126.7
Range
24-156
18-187
13-269
22-423
43-759
42-868
28-703
60
30
0
diameter (cm)
90
120
Whitebark pine tree diameter (cm) by elevation
2290
2380
2470
2560
2650
27402830/2930
elevation (m)
Mean
12.6
12.9
17.2
26.4
19.7
15.7
9.5
Range
0.7–
77.2
1.5 76.5
1.5 –
65.0
0.8 109.2
0.5 –
88.9
0.8 –
114.0
0.5 –
69.6
More structural data
Conclusion: three primary whitebark pine
stand types
Old, high density stands
Old, large tree stands
Young, invading stands
Photo w/ stand types highlighted
Old, high
density stands
Young, invading stands
Old, large tree stands
Young / invading stands
(below approximately 2470 m)
• Young trees
• Little downed wood, no
large snags
• Low density, with
sagebrush/Wyethia
understory
• Low basal area
• Relatively short, smalldiameter trees
• Gentle slopes, relatively
well-developed soils and
high soil moisture index
Dynamics of low elevation stands
• Stands seemingly have resulted from
expansion into sagebrush (and
perhaps aspen), at the lower end of
whitebark’s elevational range
– Invasion appears to have begun in the
late 19th century
– Most trees established between 19151965
• Similar events have been documented
for other medium and high-elevation
species
– White fir and yellow pine invaded
sagebrush at lower elevations in
Warners (Vale 1975, 1977)
– Limber pine and bristlecone pine
expanding downslope into “sagebrush
basins” in the White Mountains
(Millar 2006)
Sagebrush invasion – aerial photo comparison
Sagebrush invasion – aerial photo comparison
2011
1946
Old, large tree stands
(between approximately 2470 and 2650 m)
• Trees of all ages
• Relatively high amounts of
downed wood; presence
of large snags
• Medium stem density
• High basal area
• Trees of all heights and
sizes, with the tallest trees
in the study area
• Moderate slope
steepness, soil
development, and soil
moisture index
Old, high density stands
(above approximately 2650 m)
• Trees of all ages
• Relatively high amounts
of downed wood;
presence of large snags
• High density
• Moderate basal area
• Generally shorter and
smaller diameter trees
(vs. middle elevations)
• Steepest slopes, poor soil
development, low soil
moisture index
Dynamics of middle and high elevation stands
• Old, all-aged
• Recruitment seemingly continuous
(rather than episodic)
• Most standing stems in these
stands have been recruited in past
150 years
– Repeat photos suggest that density
and cover are increasing
• Current seedling and sapling
density is low
Growth
• Stems in low/young stands grew
faster than those in older stands
– 47% of those stems doubled in
diameter from 1994-2012
– Only 3% of stems in older stands
doubled
• Cone production observed in
stems as young as 36 years old
Mortality from 1994-2006
• Annual rate of mortality 0.22%
• No stems died in 57% of plots
• Self thinning?
– Trees that died were generally small
• Mean dbh and height class of dying stems
were smaller than those of surviving
stems (two-tailed T-tests, p<0.0001)
• Likelihood of dying (dead stems/total
stems) was highest at middle
elevations where basal area is
highest
Self-thinning within clusters?
• Dead stems within
clusters were most often
(58%) the smallest stem
present in the cluster
90
80
% stems in clusters
• Older stems are less likely
than younger stems to be
found in multi-stemmed
clusters
– All sampled stems
>460 years old are
single-stemmed
Percentage of whitebark pine stems occurring in multistemmed clusters, with respect to age class
70
71
60
70
63
50
54
45
40
30
20
10
0
0
<100
100-199
200-299
300-399
400-499
>499
Age
2
R = 0.673
Mortality from 2006-2012
• Annual rate of tree mortality increased over fivefold
• Mountain pine beetle became primary cause of
mortality
– 60% of dying trees had MPB evidence
– 5.4% of trees alive in 2006 either killed or infected with
MPB in 2012
• MPB widespread in study area
– All transects
– 23% of plots
• Within plots and within the
entire study area, infected
trees were larger than random
samples of uninfected trees
(p<0.001, p<0.01)
White pine blister rust
• 1994
– Liberally estimated at 6% infection rate
– No “textbook” WPBR cases were observed, but
marginal cases given the “benefit” of the doubt
– 95% of those stems were still alive in 2006
– I was incompetent!
• 2012
– Competent surveyor
present
– 6 stems (0.34%)
infected with WPBR
Fire
• 41% of plots have some sign of fire - fire
scars on trees, charred wood, etc.
• Only 2% of stems have fire scars
– Most of those stems are old (>200 years)
• From 1913-1993, only three fires in the
six PLS sections overlapping the study
area
– All less than 0.04 ha
• 136 files in the wilderness from 19131993
– 87% less than 0.4 ha
Summary
• Sagebrush invasion - whitebark pine has been invading for past ± 100150 years
– Whitebark trees at low elevations are growing rapidly
• Mortality between 1994-2006 was limited
– Few stems died (0.2% stems/year)
– Likely balanced by recruitment from sapling stage
– Self-thinning / intra-specific resource competition may have driven bulk of
mortality
• Mortality increased substantially between
2006-2012
– MPB infestation widespread and primary cause of
mortality
• White pine blister rust remains rare in the
south Warner range
Special thanks!
• 1994 Field Crew
– Karen Pope
– Jack (1992-2006)
• 2006 Field Crew
– Sport
• 2012 Field Crew
– Danny Cluck
– Krystina Smith
– L. Breck
McAlexander
– Leslie Forero