Research Topics Ecosystem Processes
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Sierra Nevada Ecosystems
Sierra Nevada Ecosystems
About this Research:
Plumas/Lassen Administrative Study Vegetation Module Forest Restoration in the Northern Sierra Nevada: Impacts on Structure, Fire Climate, and Ecosystem Resilience.
Research Project Summary
A 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
We 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
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