Vegetation Types in Yosemite Valley Vegetation Type Mixed

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-1900 s 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 $1 M property damage

Yosemite Lodge 1975 Root disease centers outlined

Yosemite Lodge 1997 Root disease centers outlined

Public safety

Disturbance in Yosemite Valley

15 Mean = 1007 m 2 Median = 459 m 2 10 5 Range = 81 - 8300 m 2 0 0 60 0 12 00 18 00 24 00 30 00 36 00 42 00 48 0 54 0 00 60 0 66 0 00 72 00 78 00 84 00 90 00 Number of Gaps 20 N = 70 Gap size (m 2)

Heterobasidion annosum gaps all started at a stump or stumps They may continue to expand for about 30 years

Sentinel Beach Gap size = 4200 m 2 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

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 20 th 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 n=14 Old growth n=17 Stems/ha 20 cm dbh 536 324

Sierra San Pedro Martir, Baja, Mexico

Lake Tahoe Basin Sie Lat. 39 o 2000 -2600 m elevation 50 -100 cm annual ppt. rra Ne a vad lar su nin Pe Sierra San Pedro Martir, Baja d an ja Ba es ng Ra Lat. 31 o 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 SSPM, Baja 134 n=17 n=16

% 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

Pathogens and Conifer Hosts

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