Vegetation Types in Yosemite Valley

Vegetation Type Hectares %Mixed Conifer 950 49Meadow 147 8Riparian 208 11Black oak 98 5Live oak 410 21Other 128 6

Yosemite Valley 1899Yosemite Valley 1899

Yosemite Valley 1961Yosemite Valley 1961

18991899 19611961

18661866 19611961

Early to mid-1900sEarly to mid-1900s

Tree felling for buildings,Tree felling for buildings,campgrounds, and bark beetlecampgrounds, and bark beetlecontrolcontrol

Stump creation and subsequent infection Stump creation and subsequent infection by by Heterobasidion annosumHeterobasidion annosum

Ponderosa pine Incense cedar

Yosemite Lodge complex1972

cabin crushed by tree with rotted roots

since 19737 fatalities

19 serious injuriesOver $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 17.4 ha (~ 5 % of total area)

Area in gaps 5.5 ha

(31% of surveyed area)

Mean gap size 1007 m2 (81-12,500 m2)

Nu

mb

er o

f G

aps

0

5

10

15

20

060

012

0018

0024

0030

0036

0042

0048

0054

0060

0066

0072

0078

0084

0090

00

Gap size (m2)

N = 70

Mean = 1007 m2

Median = 459 m2

Range = 81 - 8300 m2

Causal agent % of GapsMean area (m2)

(range)Heterobasidion annosum & bark beetles 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)

Other diseases 7 181(81-269)

Bark beetles alone 12.5 728 (113-1688)

Other causes (fire, high water tables, etc.) 7 1629 (325-4500)

Heterobasidion annosum gaps all started at a stump or stumps

They may continue to expand for about 30 years

10 meters

Sentinel BeachGap size = 4200 m2

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

YearArea 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 BasinOld 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 %

No wildland fire > 800 ha has occurred since 1908

Lake Tahoe fires in the 20th CenturyCentury

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

Old growth

Stems/ha 20 cm dbh

536

324

n=14

n=17

Sierra San Pedro Martir, Baja, Mexico

Lake Tahoe Basin

Sierra Nevada

Peninsular and Baja R

anges

Sierra San Pedro Martir, Baja

Lat. 39o

Lat. 31o

2200-2900 m elevation65 cm annual ppt.

2000-2600 m elevation

50-100 cm annual ppt.

Tree Species

Lower MontanePinus jeffreyi, Abies concolor, Calocedrus decurrens, P. lambertiana

Upper MontaneAbies magnifica, A. concolor, Pinus contorta, P. monticola

SubalpinePinus albicaulis, P. contorta, P. monticola, Tsuga mertensiana,Abies magnifica,

Fire Return Intervals

Lake Tahoe

SSPM

Lake Tahoe and Baja

12-55 years

13-52 years

Stand Characteristics

Lake Tahoe and Baja

Stems/ha 20 cm dbh

Seral 536

Old growth 324

SSPM, Baja 134n=16

n=14

n=17

Distribution of live trees by size classI = 20-50 cm; II = 50.1-100 cm; III > 100.1 cm

0

10

20

30

40

50

60

70

80

90%

ste

ms

in e

ach

DB

H s

ize

clas

s

Seral

n=5009

Old growth

n=3718

SSPM

n=1528

Fir engraver beetle, Fir engraver beetle, Scolytus ventralisScolytus ventralis

Heterobasidion annosumHeterobasidion annosum in fir stumpin 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 diseaseHeterobasidion annosum

white fir, red fir

Dwarf mistletoesArceuthobium species

white fir, red fir, Jeffreypine

True mistletoesPhoradendron species

white fir, incense cedar

White pine blister rustCronartium ribicola

sugar pine, western whitepine, whitebark pine

Mortality curves for mixed-conifer species

0

100

200

300

400

500

600

60 64 68 72 76 80 84 88 92 96Year

# d

ead

tre

es

Seral

Old growth

SSPM

Lake Tahoe Basin 1996

Distribution of dead trees by size classI = 20-50 cm; II = 50.1-100 cm; III > 100.1 cm

0

10

20

30

40

50

60

70

80

90

Seral Old growth SSPM

% s

tem

s in

eac

h D

BH

siz

e cl

ass

Changing role of pathogens and insects: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 smallertrees

Future Threats To California Forests

Catastrophic fireAir pollutionUrbanizationIntroduced 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