Vegetation Types in Yosemite Valley Vegetation TypeHectares% Mixed Conifer95049 Meadow1478 Riparian20811 Black oak985 Live oak41021 Other1286

Yosemite Valley 1899

Yosemite Valley 1961

Early to mid-1900s Tree felling for buildings, campgrounds, and bark beetle control

Stump creation and subsequent infection by Heterobasidion annosum

Ponderosa pineIncense cedar

Yosemite Lodge complex 1972 cabin crushed by tree with rotted roots since 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

Number of Gaps Gap size (m 2 ) N = 70 Mean = 1007 m 2 Median = 459 m 2 Range = m 2

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

10 meters Sentinel Beach Gap size = 4200 m 2

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

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

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 %

No wildland fire > 800 ha has occurred since 1908 Lake Tahoe fires Century in the 20th Century 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 n=14 n=17

Sierra San Pedro Martir, Baja, Mexico

Lake Tahoe Basin Sierra Nevada Peninsular and Baja Ranges Sierra San Pedro Martir, Baja Lat. 39 o Lat. 31 o m elevation 65 cm annual ppt m elevation 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 SSPM Lake Tahoe and Baja years years

Stand Characteristics Lake Tahoe and Baja Stems/ha  20 cm dbh Seral536 Old growth324 SSPM, Baja134 n=16 n=14 n=17

Distribution of live trees by size class I = cm; II = cm; III > cm % stems in each DBH size class Seral n=5009 Old growth n=3718 SSPM n=1528

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 Year # dead trees Seral Old growth SSPM

Lake Tahoe Basin 1996

Distribution of dead trees by size class I = cm; II = cm; III > cm SeralOld growthSSPM % stems in each DBH size class

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