Plumas/Lassen Administrative Study Vegetation Module
Forest Restoration in the Northern Sierra Nevada:
Impacts on Structure, Fire Climate, and Ecosystem Resilience.
Research Project Summary
century of fire suppression, forest management practices, and a
warmer andmoisterclimate have left Sierran forests with many dense
stands of small diameter, shade-tolerant, fire-sensitive trees.
This change has shifted the fire regime from frequent low-intensity,
small-scale ground fires to large-scale, intense crown fires. These
fires pose a threat to rural communities and can convert forested
landscapes to persistent shrub fields. One approach to restoring
forests and reducing fire intensity is to use thinning and gap-creation
to increase the proportion of large fire-resistant trees and encourage
more shade-intolerant regeneration. We focus on the effects of fuel
treatments on forest structure, composition, understory microclimate,
and succession, because changes in these conditions will define
how fire and the forests responds to restoration.
Thinning alters stand structure, modifying the forest microclimate
and affecting whether trees, shrubs or grass grow back. The intensity
and quality of light determines whether fire tolerant pines, or
fire intolerant trees, grow back. There is surprisingly little information
on the effects of thinning on microclimate in Sierran forest. One
oft-cited reference on this topic conjectures that "the greater
the stand opening, the more pronounced the change in microclimate
is likely to be" (Weatherspoon 1996). Microclimate (e.g., temperature,
wind speed, and humidity) directly affects fuel moisture and fire
behavior, and is an important input to fire models. Understanding
how thinning and other forest restoration actions affect forest
succession across resource gradients in the northern Sierra landscape
is essential for determining the long-term effects of fuel treatments.
Improve understanding of the relationship between forest canopy
cover, fire climate, and understory plant dynamics in northern Sierran
mixed conifer forest.
Investigate the resilience of mixed conifer forest to harvest disturbance
across the landscape gradients of precipitation and soil type.
METHODS AND DESIGN
have a number of experimental efforts underway. We are conducting
a forest thinning experiment in which replicated 22-acre
blocks of mixed conifer forest will be thinned to 50% or 30% canopy
cover from their current level of 60-70% canopy cover. A wide variety
of data has been collected for several years in these plots prior
to thinning, including understory plant-species composition, fuel
moisture, fuel loading, stand structure (Forest Inventory and Analysis
protocol), small mammal species composition, wind speed, air temperature,
and humidity (continuous recording), moisture in duff and 0-0.1
m soil depths, irradiance at chest and ground height (10-m grid
spacing), and tree canopy cover and closure via canopy densitometer,
spherical densiometer, hemispherical photography, and moosehorn.
Changes in these factors after thinning goes forward will be interpreted
in terms of fire hazard and forest regeneration.
A second experiment concerns the growth and survival of Sierran
conifers and hardwoods with regard to light level and soil type.
Emphasis on managing Sierran forests for increased canopy openness
has created a need for a quantitative understanding of the relationship
between understory light levels and sapling performance. What level
of light tips the balance between regeneration of shade-tolerant
and shade-intolerant conifers, and does soil type change this level?
We are measuring the performance of individual trees across the
Plumas National Forest to answer this question.
The third experiment concerns ecosystem resilience to forest harvesting,
particularly group selection silviculture, across landscape gradients
of precipitation and soil type. Group selection silviculture
has been suggested as a one-size-fits all solution to harvesting
merchantable timber from Sierran forests, avoiding clear-cutting
but still promoting regeneration of shade-intolerant tree species.
Yet the effectiveness of group selection silviculture may vary with
site condition, and the removal of forest canopy may alter the abiotic
regeneration environment in unexpected ways. We are measuring microclimate,
soil moisture, and plant community development in group selection
openings, natural openings, and adjacent closed canopy stands in
East-side and West-side forest types. These data will help us understand
and predict forest resilience to harvest disturbance.
Finally, we facilitate the research of the owl, fire and fuels,
bird, and small mammal modules of the Plumas-Lassen Administrative
Study by helping them meet their needs for high-quality data on
forest structure and composition.
APPLICATION OF RESEARCH RESULTS
Our studies will investigate how measures of canopy cover and canopy
closure obtained with a variety of instruments relate to one another.
It will document how reductions in canopy cover will affect microclimate,
which will help managers anticipate the probability of successfully
regenerating desired species under the various treatments. Finally,
it will provide better understanding of how current thinning prescriptions
influence fire risk, successional processes, and forest resilience
across Plumas and Lassen National Forest landscapes.
Ponderosa pine, mixed conifer, and east-side pine forests within
the Plumas and Lassen National Forests.
1)) North, M., 1) Bigelow, S.
1) USDA Forest Service, PSW Research Station
Sierra Nevada Research Center
2121 Second Street, Suite A101
Davis, CA 95616
PUBLICATIONS AND REPORTS