Vegetation Types in Yosemite
Valley
Vegetation Type
Mixed Conifer
Meadow
Riparian
Black oak
Live oak
Other
Hectares
950
147
208
98
410
128
%
49
8
11
5
21
6
Yosemite Valley 1899
Yosemite Valley 1961
1899
1961
1866
1961
Early to mid-1900s
Tree felling for buildings,
campgrounds, and bark beetle
control
Stump creation and subsequent infection
by Heterobasidion annosum
Ponderosa pine
Incense cedar
Yosemite Lodge complex
1972
cabin crushed by tree with
rotted roots
since 1973
7 fatalities
19 serious injuries
Over $1M property damage
Yosemite Lodge 1975 Root disease centers outlined
Yosemite Lodge 1997 Root disease centers outlined
Public safety
Disturbance in Yosemite Valley
Area surveyed
Area in gaps
Mean gap size
17.4 ha
(~ 5 % of total area)
5.5 ha
(31% of surveyed area)
2
1007 m
2
(81-12,500 m )
Number of Gaps
20
N = 70
Mean = 1007 m2
15
Median = 459 m2
10
Range = 81 - 8300 m2
5
0
Gap size (m2)
Causal agent
Heterobasidion annosum & bark beetles
Mean area (m2)
% of Gaps
(range)
30.5
1342 (155-5075)
Armillaria mellea & bark beetles
30.5
491(94-1425)
Combination diseases (mostly H. annosum,
A. mellea, or Phaeolus schweinitzii)
12.5
2147 (169-8325)
7
181(81-269)
12.5
728 (113-1688)
7
1629 (325-4500)
Other diseases
Bark beetles alone
Other causes (fire, high water tables, etc.)
Heterobasidion annosum
gaps all started at a
stump or stumps
They may continue to
expand for about 30 years
Sentinel Beach
Gap size = 4200 m2
10 meters
Armillaria mellea infection center
Infects all tree species
Gap started at an infected
black oak.
Ponderosa Pine killed by
western pine beetle,
Dendroctonus brevicomis
Heterobasidion annosum + Armillaria mellea gap
Many gaps with very little regeneration and have not closed in
Change in gap area 1972-1999
Year
Area in gaps
(m2)
Percent in gaps
1972
6125
3.5
1999
53,981
31
El Capitan Picnic Area 1972
El Capitan Picnic Area 1997
Yosemite Valley as it used to be?
Prescribed
burn
Yosemite
Valley
Opportunities for restoration?
Pollution regulations
Pollen records suggest that conifers may have dominated
Yosemite Valley prior to the Miwok
Lake Tahoe Basin
Lake Tahoe Basin
Old Growth
Total coniferous forest = 48,620 ha
1998 area in old growth = ~2138 ha
Percent in old growth = ~4 %
Historic old growth area = 26,740 ha
(estimated)
Historic percent old growth = 55 %
(estimated)
Changes in forest
composition over 150 yrs
White fir and incense cedar
have doubled in importance
Jeffrey pine has declined
by 50 %
Lake Tahoe fires
in the 20th
Century
No wildland fire > 800 ha
has occurred since 1908
Between 1974 and 1996, there
were only 9 fires > 4 ha
Effective fire suppression and
the high elevation environment
(i.e., short fire season) have
kept large fires to a minimum
Stand Characteristics
Lake Tahoe
Seral
Stems/ha
 20 cm dbh
536
n=14
Old growth
n=17
324
Sierra San Pedro Martir, Baja, Mexico
Lake Tahoe Basin
Lat. 39o
2000-2600 m elevation
50-100 cm annual ppt.
Sierra San Pedro
Martir, Baja
Lat. 31o
2200-2900 m elevation
65 cm annual ppt.
Tree Species
Lower Montane
Pinus jeffreyi, Abies concolor,
Calocedrus decurrens, P. lambertiana
Upper Montane
Abies magnifica, A. concolor,
Pinus contorta, P. monticola
Subalpine
Pinus albicaulis, P. contorta,
P. monticola, Tsuga mertensiana,
Abies magnifica,
Fire Return Intervals
Lake Tahoe and Baja
Lake Tahoe
12-55 years
SSPM
13-52 years
Stand Characteristics
Lake Tahoe and Baja
Seral
Stems/ha
 20 cm dbh
536
n=14
Old growth
324
n=17
SSPM, Baja
n=16
134
% stems in each DBH size class
Distribution of live trees by size class
90
80
I = 20-50 cm; II = 50.1-100 cm; III > 100.1 cm
n=5009
n=3718
70
60
n=1528
50
40
30
20
10
0
Seral
Old growth
SSPM
Fir engraver beetle,
Scolytus ventralis
Heterobasidion annosum
in fir stump
Bark Beetles and Conifer Hosts
Mountain pine beetle
D. ponderosae
sugar, lodgepole,
western white pine
Jeffrey pine beetle
D. jeffreyi
Jeffrey pine
Fir engraver beetle
Scolytus ventralis
white fir, red fir
Pathogens and Conifer Hosts
Annosus root disease
Heterobasidion annosum
white fir, red fir
Dwarf mistletoes
Arceuthobium species
white fir, red fir, Jeffrey
pine
True mistletoes
Phoradendron species
white fir, incense cedar
White pine blister rust
Cronartium ribicola
sugar pine, western white
pine, whitebark pine
Mortality curves for mixed-conifer
species
# dead trees
600
500
400
Seral
Old growth
SSPM
300
200
100
0
60 64 68 72 76 80 84 88 92 96
Year
Lake Tahoe Basin 1996
% stems in each DBH size class
Distribution of dead trees by size class
90
I = 20-50 cm; II = 50.1-100 cm; III > 100.1 cm
80
70
60
50
40
30
20
10
0
Seral
Old growth SSPM
Changing role of pathogens and insects:
Lake Tahoe:
Pathogens and insects responsible
for most mortality
Baja:
Pathogens and insects most
important on older trees
Fire mosst important om smaller
trees
Future Threats To California Forests
Catastrophic fire
Air pollution
Urbanization
Introduced pests
Management
Prescribed fire
Thinning
Sequoia National Park
Annosus root disease in giant sequoia
Conclusions
• Human management of forests can change
historical roles of pathogens and insects
• These role changes and their consequences may
not become apparent for decades
• Current “restoration” activities (e.g.,
prescribed fire) generally do not take into
account pathogens and insects
• Returning forest to historical stand densities
and processes will not necessarily return
pathogens and insects to historical roles