Date of Graduation
Davis College of Agriculture, Natural Resources and Design
Division of Forestry and Natural Resources
Forests in the western United States have experienced a shift from historical disturbance regimes in the past century. Many of these changes were induced by European settlers logging the forests and suppressing fires. In the past, the dry mixed conifer forests of California’s Sierra Nevada mountains experience frequent, low to mixed severity fires. This fire regime helped maintain a heterogeneous landscape comprised of groups of trees and openings. However, due to fire suppression and high grading logging, forest structure has changed; there are less openings and more small, fire-intolerant trees that can carry a fire into the forests crown. The new fire regimes resulting from this change in structure are large, high severity fires that kill a majority of the overstory trees. These novel regimes require novel approaches to regenerate the forest as they are not adapted to large, high severity fires. The United States Forest Service (USFS) will often plant trees after fires to aid with reforestation after large wildfires. A new technique being testing is clustering the trees into groups of two to four, instead of the traditionally evenly spaced plantations.
To evaluate these plantations, I compared growth and development in several post fire plantations and natural regenerating stands in the Eldorado National Forest in the north-central Sierra Nevada Mountains. I tested for growth and ecological differences between clustered and evenly spaced plantations, some with pre-commercial thinning (PCT) and some without, as well as comparing them to stands of naturally regenerating trees using mixed effects models. I compared diameter and height growth, along with tree density, shrub size, and understory species diversity. My results suggest that clustered plantations provide a slight facilitative effect when compared to the evenly spaced plantations. I also found high variability in tree stocking, highlighting the intense shrub competition these young plantations face.
I also forecasted growth and fire behavior 100 years into the future using the Forest Vegetation Simulation (FVS) and its Fire and Fuels Extension (FFE). In these simulations I tested combinations of different fuels treatments (mastication only, mastication with prescribed burning, and no fuels treatments) with different overstory thinning intensities (residual densities of 370SDI (stand density index), 495SDI, 618SDI (TPH), and no overstory thinning) on stand growth and potential fire behavior using three way analysis of variance. I compared growth and crowning index at the end of the simulation and the simulation age when the flame length, basal area mortality, and fire type reached low severity between fuel treatment, thinning intensity, and original management of stands (plantation with PCT, plantation without PCT, and natural regenerating stands). I found an overall pattern of decreasing crown fire occurrence and fire induced mortality across all simulations due to increasing canopy base height and decreasing canopy bulk density. Mastication with prescribed burning was the most effective treatment for quickly reducing fire behavior by consuming surface fuels, thus drastically lowing flame length. My results highlight the different stressor that post fire plantations experience and how different silvicultrual treatments interact with stand development over time to reduce fire risk. They also demonstrate the importance of treating stands early and the effectiveness of surface fuel treatments.
Allen, Iris C., "Evaluating Post-Fire Plantation Restoration in a Mixed Conifer Forest in the Sierra Nevada" (2019). Graduate Theses, Dissertations, and Problem Reports. 4091.