63. Onion Creek
(Talley 1977b)
Location
This study area (dropped from RNA consideration in 1982) is
on the Tahoe National Forest, Placer County. It lies about 2
miles (3 km) S. of Norden. It lies within portions of sect. 36
T17N, R14E, sects. 1 and 12 T16N, R14E, sect. 29 T17N, R15E,
and sect. 32 T16N, R15E MDBM (39°19'N., 120°20'W.), USGS
Norden quad (fig. 128). Ecological subsections – Upper
Batholith and Volcanic Flows (M261Eh).
Target Elements
White Fir (Abies concolor) and Red Fir (Abies magnifica)
Distinctive Features
Tree Growth: A large body of data is presented on this topic.
To summarize: growth of red and white fir within closed
stands is essentially equal and linear between 5 and 30 years
after establishment. Fastest growth of both species occurs on gentle S.- to W.facing aspects. Trees growing in open sites attain breast height in 15 to 30 years.
Slowest red fir growth is on high, steep N.-facing slopes (70 years to breast
height), and slowest white fir growth is on glacial moraines on S.-facing slopes
(100 years to breast height).
Structure: Much of the present white fir forest was dominated by red fir until
150 years ago. Establishment of sugar pine (Pinus lambertiana) and Jeffrey pine (P.
jeffreyi) within this white fir forest has been gap-phase. Incense-cedar (Libocedrus
decurrens) is also largely gap-phase. Dry meadows formerly dominated by large
individuals of incense-cedar are now often characterized by forests of dense,
young white fir, indicating recent invasion of these habitats. Before an increase
in establishment about 1750 for white fir, other gently sloping meadow borders
were probably codominated by incense-cedar, white fir, sugar pine, and Jeffrey
pine, forming an open mixed conifer forest.
The steady upsurge of white fir in the past 350 years coincides with
increasing warmth, which was augmented by above-average precipitation 50100 years ago. Reduction in fire frequencies may also explain the recent white fir
invasion of dry meadows at elevations below 6562 ft (2000 m).
The increase in density and extent of white fir forest contrasts with the
situation for red fir, which shows stability over the past 400 years. Red fir forest
is regionally stable, but locally gap-phase in its reproductive strategy. In some
areas where wind-throw was extensive, there are large, even-aged, and dense
stands of young red fir. Western white pine (Pinus monticola) is much less
common than red fir, but it has a similar negative log-linear relationship in size
classes. In general, recent changes in densities of tree species in red fir forest
will probably not result in major changes in composition of this climax forest
over coming centuries. Lower-elevation sites experiencing upsurges in white fir
will combine with a red fir increase within upper-elevation white fir forest to
produce a broader overlap zone of red and white fir forest.
Physical Characteristics
This site encompasses 1035 acres (419 ha) between 6040 and 8382 ft (1840-2555
m) on the W. slope of the Sierra Nevada in the uppermost watershed of Onion
Creek, a tributary of the North Fork of the American River. S.-, SW.-, and W.facing slopes predominate, with slopes steepening to vertical cliffs and talus at
the upper elevations.
Figure 128—
Onion Creek ecological survey
area
Rocks are Miocene and Pliocene volcanic pyroclastic deposits. At Norden, 88
percent of the precipitation falls between November 1 and April 30, with 91
percent of all precipitation falling as snow. Mean annual precipitation is about 53
inches (1346 mm). Mean maximum summer temperatures are 64-75 °F (18-24
°C), whereas winter temperatures below -13 °F (-25 °C) are very rare.
Association Types
Eight 0.07-ha plots were sampled in white fir forest; nine were sampled in red fir
forest.
Rock Scree, Cliff, and Talus (91200): 400 acres (162 ha). This association forms a
topographic timber line above about 7220 ft (2200 m). There are occasional trees of
Jeffrey pine and Sierra (western) juniper (Juniperus occidentalis ssp. australis), but
the majority of the vegetation (usually <50 percent total cover) is herbaceous or
shrubby. The following species are typical: Eriogonum umbellatum ssp. polyanthum,
E. lobbii, Artemisia tridentata, Arctostaphylos nevadensis, Ceanothus cordulatus, C.
prostratus, Collinsia parviflora, Gayophytum ramosissimum, Helianthella californica,
Monardella odoratissima, Phlox diffusa, Sitanion hystrix, and Wyethia mollis.
Red Fir Forest (85310): 363 acres (147 ha). Red fir forest is restricted to higher
elevations and cold air pockets where drought stress is low. Red fir is strongly
dominant on N.-facing slopes above 6890 ft (2100 m) and along canyon
bottoms at lower elevations. Importance values are consistently high (227±23).
Density is variable, ranging from 101 to 1823 stems/ha, as is basal area (72±25
m2/ha), but cover is more stable (55±14 percent). Principal associates with red
fir are western white pine (high elevations) and white fir (low elevations).
Average importance value for white fir in this forest is 62 with a density of 125
stems/ha, 18 percent crown cover, and basal area of 19 m2/ha. Western white
pine occurs in low density at five plots, with an average importance value of
29, a density of 34 stems/ha, crown cover of 7.2 percent, and basal area cover
of 6 m2/ha. Sugar pine and Jeffrey pine also occur as isolated large individuals
at lower elevations. Mountain hemlock (Tsuga mertensiana) occurs in the
highest N.-facing slope stands.
Subordinate species (herbs and shrubs) for both red and white fir forest are
similar and include Asclepias cordifolia, Chimaphila umbellata, Corallorhiza maculata,
Hieracium albiflorum, Kelloggia galioides, Osmorhiza chilensis, Phacelia hydrophylloides, Pedicularis semibarbata, Pterospora andromedea, Pyrola picta, Ribes roezlii,
and Symphoricarpos vaccinoides. Species confined to low-elevation forests
(believed previously to have been mixed conifer) include mesophilic taxa such as
Goodyera oblongifolia, Adenocaulon bicolor, Smilacina stellata, and Thalictrum fendleri.
Arabis platysperma and Chrysopsis breweri are most typical of red fir forest.
White Fir Forest (84240): 250 acres (101 ha). White fir dominates all forest sites
sampled below 6400 ft (1950 m) and at higher-elevation sites with steep to
moderate S. or W. aspects. Average white fir importance value is 181. Basal area
is variable, averaging 91±81 m2/ha, and density averages 634 stems/ha.
Reproduction within all white fir sites is dense. White fir appears to be
proliferating at the expense of sugar pine, Jeffrey pine, and incense-cedar.
Invasions of incense-cedar stands by white fir are noted, but incense-cedar is still
reproducing at the ecotones between forest and meadows, and in such situations
it may be rapidly increasing in abundance. Jeffrey pine is not currently
reproducing within the main body of white fir forest and is generally scattered
as older individuals. Sugar pine is common on gentle topography below 6560 ft
(2000 m), especially around dry meadows where it may have been the principal
dominant 200 years ago. At present, intermediate and young sugar pines are
uncommon or absent. Red firs are abundant as saplings and seedlings in
portions of the white fir forest but uncommon as mature trees below 6400 ft
(1950 m). Red fir abundance within white fir forest is related to the closeness of
red-fir-dominated forests.
Before the increase of white fir and red fir, a more even mixture of the
principal species of the mixed conifer canopy in a semi-open forest is
hypothesized for most sites on gentle topography below 6400 ft (1950 m). White
fir cannot successfully compete with red fir at higher elevation primarily because
of its intolerance of high winter snow packs in the sapling stage.
Huckleberry Oak Scrub (37542, 37510): 79 acres (32 ha). Quercus vaccinifolia
forms a dense chaparral on S.-facing exposures. Other species present include
Allium campanulatum, Arctostaphylos nevadensis, A. patula, Carex sp., Calochortus
leichtlinii, Ceanothus cordulatus, C. prostratus, Poa sp., Melica sp., Pellaea bridgesii,
and Penstemon speciosus. Moderate exposures of this type are being invaded by
white fir and red fir with lesser numbers of Jeffrey pine and incense-cedar.
Invasion fronts of white fir are typical of ecotones between fir forest and
chaparral. The core of this association is resistant to invasion because long snowfree periods and resulting critically low-water potentials of summer and fall do
not permit conifer establishment, and because the huckleberry oak scrub adapts
to crown fires.
Dry Meadows (45100): 35 acres (14 ha). This association is characterized by an
abundance of Lotus nevadensis, Madia exigua, Poa sp., Trifolium sp., Verbascum
thapsus, Cirsium sp., etc. Encroachment by white fir and, to a lesser extent, Jeffrey
pine has significantly reduced dry meadow habitats below 6600 ft (2012 m).
Mountain Alder Thicket (63500, 61520): This riparian scrub is restricted to seepy
areas. Alnus tenuifolia usually dominates (75 percent cover), with scattered
specimens of Acer glabrum var. torreyi, and Salix scouleriana. There are many
mesophytic understory species (15 species listed in association table). A cluster
of aspen (Populus tremuloides) also occurs in the ecotone between dry meadow
and Alnus thickets. Typically, Acer glabrum occurs around the periphery of the
Alnus thickets.
Plant Diversity
One hundred eight taxa are listed for all habitats, but total diversity is
undoubtedly higher.
Conflicting Impacts
The area is part of the Onion Creek Experimental Forest, and as such, its
candidacy as an RNA was rejected.