Introduction
The Challenge Experimental Forest is located on the Plumas National Forest
surrounding the small community of Challenge. The forest was formally designated
in 1942 (but not activated until 1958) for experimentation in silvicultural
management of the young-growth forests at lower elevations on the west slope
of the Sierra Nevada. It was enlarged to its present boundaries in 1958.
Challenge occupies 1,446 ha in eastern Yuba County, California, at the western
edge of the Feather River District of the Plumas National Forest. Elevations
range from 730 to 1,130 m. Slopes on more than 80 percent of the Challenge are
less than 30 percent. All aspects are represented with west and south aspects
predominating.
Climate
The climate is Mediterranean in that summers are warm and dry, and winters
are cool and wet. Mean annual precipitation averages 1,727 mm, 98 percent of
which falls between October and May. The occasional snowfalls melt rapidly,
leaving the ground free of snow most of the winter. Annual temperatures normally
range from 6 °C in January to 21 °C in July.
Soils
The Challenge is located on a drainage divide of the Yuba River. Most of the
land is drained by tributaries of Dry Creek, which originates on the forest
and joins the Yuba River about 20 miles downstream. Most soils are old and deep.
Clayey, oxidic, mesic Xeric Haplohumults of the Challenge and Sites soil series
cover most of the forest.
Vegetation
Pacific ponderosa pine (SAF 245) is the major forest cover type. Sierra Nevada
mixed-conifer (SAF 243), California black oak (SAF 246), and Pacific ponderosa
pine-Douglas-fir (SAF 244) types are also present.
Long-Term Data Bases
Precipitation and maximum and minimum temperatures have been recorded at the
nearby Challenge Work Center, Plumas National Forest, since 1938. Soils have
been mapped both by the Cooperative Soil Vegetation Survey and the University
of Californian at Davis, in cooperation with Yuba County. The timber was inventoried
in 1938-39 and again in 1979.
Research, Past and Present
Early research at the Challenge sought answers to two major questions: how
to grow and harvest young-growth (80 to 100 years old) ponderosa pine to ensure
adequate regeneration, and how to dispose of logging slash to reduce fire hazard
and ensure adequate regeneration. Both even-age (clearcutting, seedtree, and
shelterwood) and uneven-age (group and single-tree selection) management systems
were studied. Management of native California hardwoods, field testing of hybrid
and introduced pine species, and amount and pattern of soil moisture depletion
by individual trees were other important early studies. In 1998, the first California
Long-Term Soil Productivity Experiment (LTSP) was installed as part of the North
American LTSP Research Network.
More recent investigations include:
- Determining the extent to which natural reproduction follows different methods
of cutting, logging, slash disposal, and site preparation.
- Evaluating the success of various direct seeding practices, including site
preparation and rodent control.
- Determining the magnitude, periodicity, and germination characteristics
of conifer and hardwood seedcrops.
- Determining the growth of the residual stand and changes in stand structure
under management.
- Determining the effect of timber harvest on the magnitude and duration of
leaching losses of soil nutrients.
- Determining the effect of soil compaction and loss of organic matter on
long-term soil productivity.
- Evaluating the effect of fertilization on soil chemistry and on the growth
and foliar chemistry of mixed-conifer species.
- Determining the effects of competition and fertilization on the growth,
biomass, and chemistry of ponderosa pine.
Results are directly applicable to more than 800,000 ha of low-elevation, highly
productive sites on the west slope of the northern Sierra Nevada.
Major Research Accomplishments and Effects on Management
Long-term results from a study of group selection demonstrated and quantified
the effect on regeneration of openings of different sizes on growth rates and
proportion of the mixed conifer species. Regeneration of five species of conifers,
three of hardwoods, and two of shrubs was evaluated for five different cutting
methods in terms of seedling stocking, density, and height growth. For ponderosa
pine, seed-tree and shelterwood methods produced the highest stocking and density.
Selection cutting methods were best for survival and establishment of sugar
pine, white fir, and Douglas-fir. For all species, seedling height growth increased
from single-tree selection to clearcutting. However, shrubs were particularly
dense after clearcutting and broadcast burning, and could be a major factor
in establishing adequate regeneration of rapid growth potential in this method.
A study of initial spacing and shrub competition on growth and development
of planted ponderosa pine showed that the influence of shrub competition in
restricting tree growth is short lived if trees are planted on a highly productive
site with intensive site preparation. After about 15 years of growth, trees
with a shrub understory begin to exceed that of trees free of shrubs. Stem volume
production is expected to be about the same with and without a shrub understory
in about 60 years.
Soil solution chemistry measured in the 17-year period following harvesting
and reforestation indicates that nitrate continues to leach below the rooting
zone until a continuous canopy of perennial vegetation develops (about 14 years).
However, losses are essentially equal to nitrogen inputs from precipitation.
Soil compaction led to sizable losses of tree growth through the first 5 years
of stand development (but the opposite is true on sandy soils). Loss of surface
organic matter had little influence of tree growth, but led to appreciable erosion
of the unprotected mineral soil (the effect was much greater than that of soil
compaction). Isotopic analyses of 13C in branch samples indicated periods of
physiological drought stress caused by treatment or by climate.
Collaborators
Staff from the Plumas National Forest, and scientists from the University of
California at Davis and Berkeley have worked on the Challenge.
Research Opportunities
The Challenge offers abundant opportunities for research into shrub-tree interactions,
stand dynamics of ponderosa pine from young plantations to 130-year-old natural
stands and long-term soil changes in soil productivity as a consequence of logging
operations.
Facilities
The Challenge has no facilities. General merchandise can be obtained in the
town of Challenge. The nearest accommodations are in Oroville (42 km northwest)
or Marysville (56 km southwest).
Lat. 39°28´ N, long. 121°13´ W
Contact Information 1
Challenge Experimental Forest
USDA Forest Service
Pacific Southwest Research Station
3644 Avtech Parkway
Redding, CA 96002
Tel: (530) 226-2530
http://www.fs.fed.us/psw/programs/ecology_of_western_forests/
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