07 – Ponderosa Pine Forest and Woodlands (includes Eastside
Oak)
Rex C. Crawford & Jimmy Kagan
Geographic Distribution. This habitat occurs in much of eastern
Washington and eastern Oregon, including the eastern slopes of the Cascades, the
Blue Mountains and foothills, and the Okanogan Highlands. Variants of it also
occur in the Rocky Mountains, the eastern Sierra Nevada, and mountains within
the Great Basin. It extends into south-central British Columbia as well.
In the Pacific Northwest, ponderosa pine-Douglas-fir woodland
habitats occur along the eastern slope of the Cascades, the Okanogan
Highlands, and in the Blue Mountains. Ponderosa pine woodland and
savanna habitats occur in the foothills of the Blue Mountains, along
the eastern base of the Cascade Range, the Okanogan Highlands, and
in the Columbia Basin in northeastern Washington. Ponderosa pine
is widespread in the pumice zone of south-central Oregon between
Bend and Crater Lake east of the Cascade Crest. Ponderosa pine-Oregon
white oak habitat appears east of the Cascades in the vicinity of
Mt. Hood near the Columbia River Gorge north to the Yakama Nation
and south to the Warm Springs Nation. Oak-dominated woodlands follow
a similar distribution as Ponderosa Pine-White Oak but are more
restricted and less common.
Physical Setting. This habitat generally occurs on
the driest sites supporting conifers in the Pacific Northwest. It is widespread
and variable, appearing on moderate to steep slopes in canyons, foothills, and
on plateaus or plains near mountains. In Oregon, this habitat can be maintained
by the dry pumice soils, and in Washington it can be associated with serpentine
soils. Average annual precipitation ranges from about 14 to 30 inches (36 to 76
cm) on ponderosa pine sites in Oregon and Washington, and often as snow. This
habitat can be found at elevations of 100 ft (30m) in the Columbia River Gorge
to dry, warm areas over 6,000 ft (1,829 m). Timber harvest, livestock grazing,
and pockets of urban development are major land uses.
Landscape
Setting. This woodland habitat typifies the lower tree line zone forming
transitions with Eastside Mixed Conifer Forest and Western Juniper and Mountain
Mahogany Woodland, Shrub-steppe, Eastside Grasslands, or Agriculture habitats.
Douglas-fir-ponderosa pine woodlands are found near or within the Eastside Mixed
Conifer Forest habitat. Oak woodlands appear in the driest, most restricted,
landscapes in transition to Eastside Grasslands or Shrub-steppe.
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Structure. This habitat is typically a woodland or savanna with tree
canopy coverage of 10-60%, although closed- canopy stands are possible. The tree
layer is usually composed of widely spaced large conifer trees. Many stands tend
towards a multilayered condition with encroaching conifer regeneration. Isolated
taller conifers above broadleaf deciduous trees characterize part of this
habitat. Deciduous woodlands or forests are an important part of the structural
variety of this habitat. Clonal deciduous trees can create dense patches across
a grassy landscape rather than scattered individual trees. The undergrowth may
include dense stands of shrubs or, more often, be dominated by grasses, sedges,
or forbs. Shrub- steppe shrubs may be prominent in some stands and create a
distinct tree-shrub-sparse-grassland habitat. Composition.
Ponderosa pine (Pinus ponderosa) and Douglas-fir (Pseudotsuga
menziesii) are the most common evergreen trees in this habitat. The
deciduous conifer, western larch (Larix occidentalis), can be a
co-dominant with the evergreen conifers in the Blue Mountains of Oregon, but
seldom as a canopy dominant. Grand fir (Abies grandis) may be frequent in
the undergrowth on more productive sites, giving stands a multilayer structure.
In rare instances, grand fir can be co-dominant in the upper canopy. Tall
ponderosa pine over Oregon white oak (Quercus garryana) trees form stands
along part of the east Cascades. These stands usually have younger cohorts of
pines. Oregon white oak dominates open woodlands or savannas in limited areas.
The undergrowth can include dense stands of shrubs or, more often, be
dominated by grasses, sedges, and/or forbs. Some Douglas-fir and ponderosa pine
stands have a tall to medium-tall deciduous shrub layer of mallowleaf ninebark
(Physocarpus malvaceus) or common snowberry (Symphoricarpos
albus). Grand fir seedlings or saplings may be present in the undergrowth.
Pumice soils support a shrub layer represented by green-leaf or white-leaf
manzanita (Arctostaphylos patula or A. viscida). Short shrubs,
pinemat manzanita (Arctostaphylos nevadensis) and kinnikinnick (A.
uva-ursi) are found across the range of this habitat. Antelope bitterbrush
(Purshia tridentata), big sagebrush (Artemisia tridentata), black
sagebrush (A. nova), green rabbitbrush (Chrysothamnus
viscidiflorus), and in southern Oregon, curl-leaf mountain mahogany
(Cercocarpus ledifolius) often grow with Douglas-fir, ponderosa pine
and/or Oregon white oak, which typically have a bunchgrass and shrub-steppe
ground cover. Undergrowth is generally dominated by herbaceous species,
especially graminoids. Within a forest matrix, these woodland habitats have an
open to closed sodgrass undergrowth dominated by pinegrass (Calamagrostis
rubescens), Geyer’s sedge (Carex geyeri), Ross’ sedge
(C. rossii), long-stolon sedge (C. inops), or blue wildrye
(Elymus glaucus). Drier savanna and woodland undergrowth typically
contains bunchgrass steppe species, such as Idaho fescue (Festuca
idahoensis), rough fescue (F. campestris), bluebunch wheatgrass
(Pseudoroegneria spicata), Indian ricegrass (Oryzopsis
hymenoides), or needlegrasses (Stipa comata, S. occidentalis). Common
exotic grasses that may appear in abundance are cheatgrass (Bromus
tectorum), and bulbous bluegrass (Poa bulbosa). Forbs are common
associates in this habitat and are too numerous to be listed.
Other Classifications and Key References. This habitat
is referred to as Merriam’s Arid Transition Zone, Western ponderosa
forest (Pinus), and Oregon Oak wood (Quercus) in Kuchler,136
and as Pacific ponderosa pine-Douglas-fir, Pacific
ponderosa pine, and Oregon white oak by the Society of American
Foresters. The Oregon Gap II Project126 and
Oregon Vegetation Landscape-Level Cover Types127 that
would represent this type are ponderosa pine forest and woodland,
ponderosa pine-white oak forest and woodland, and ponderosa pine-lodgepole
pine on pumice. Other references describe elements of this habitat.45,
62, 88, 117, 118, 121, 122, 123, 144, 148, 209, 212, 221, 222
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Natural Disturbance Regime. Fire plays an
important role in creating vegetation structure and composition in this habitat.
Most of the habitat has experienced frequent low- severity fires that maintained
woodland or savanna conditions. A mean fire interval of 20 years for ponderosa
pine is the shortest of the vegetation types listed by Barrett et
al.22 Soil drought plays a role in maintaining an open
tree canopy in part of this dry woodland habitat. Succession and
Stand Dynamics. This habitat is climax on sites near the dry limits of each
of the dominant conifer species and is more seral as the environment becomes
more favorable for tree growth. Open seral stands are gradually replaced by more
closed shade-tolerant climax stands. Oregon white oak can reproduce under its
own shade but is intolerant of overtopping by conifers. Oregon white oak woodlands are
considered fire climax and are seral to conifers. In drier conditions,
unfavorable to conifers, oak is climax. Oregon white oak sprouts from the trunk
and root crown following cutting or burning and form clonal patches of trees.
Effects of Management and Anthropogenic Impacts. Before
1900, this habitat was mostly open and park like with relatively few undergrowth
trees. Currently, much of this habitat has a younger tree cohort of more shade-
tolerant species that gives the habitat a more closed, multilayered canopy. For
example, this habitat includes previously natural fire-maintained stands in
which grand fir can eventually become the canopy dominant. Fire suppression has
lead to a buildup of fuels that in turn increase the likelihood of
stand-replacing fires. Heavy grazing, in contrast to fire, removes the grass
cover and tends to favor shrub and conifer species. Fire suppression combined
with grazing creates conditions that support cloning of oak and invasion by
conifers. Large late-seral ponderosa pine, Douglas-fir, and Oregon white oak are
harvested in much of this habitat. Under most management regimes, typical tree
size decreases and tree density increases in this habitat. Ponderosa pine-Oregon
white oak habitat is now denser than in the past and may contain more shrubs
than in presettlement habitats. In some areas, new woodlands have been created
by patchy tree establishment at the forest-steppe boundary. Status
and Trends. Quigley and Arbelbide181 concluded
that the Interior Ponderosa Pine cover type is significantly less in extent than
pre-1900 and that the Oregon White Oak cover type is greater in extent than
pre-1900. They included much of this habitat in their Dry Forest potential
vegetation group,181 which they concluded has departed
from natural succession and disturbance conditions. The greatest structural
change in this habitat is the reduced extent of the late-seral, single-layer
condition. This habitat is generally degraded because of increased exotic plants
and decreased native bunchgrasses. One third of Pacific Northwest Oregon white
oak, ponderosa pine, and dry Douglas-fir or grand fir community types listed in
the National Vegetation Classification are considered imperiled or critically
imperiled.10
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