Impacts of treatments on forest structure and fire behavior in dry western forests

by Justin Ziegler

Institution: Colorado State University
Year: 2014
Record ID: 2043962
Full text PDF: http://digitool.library.colostate.edu:80/R/?func=dbin-jump-full&object_id=338726


Forest managers are increasingly using mechanical treatments in dry forests of the western US in order to produce stands with spatially complex structure while also reducing crown fire potential. However, there has been a lack of evaluation of these treatments on spatial patterns in dry forest types of the western US. In addition, the implications of heterogeneous fuels complexes on fire behavior are not well understood due to a lack of experimental data and the use of semi empirical models which cannot account for the structural complexity of fuel beds. The lack of well quantified studies on changes in spatial heterogeneity and limitations on quantifying the associated fire behavior suggest there are gaps in our knowledge regarding the implications of mechanical fuels treatments. The primary emphasis of this thesis is in Chapter 1. I comprehensively stem-mapped seven 4 ha plots, after treatment in dry, coniferous treated stands across the Southern Rockies and Colorado Plateau. Then, I estimated pre-treatment structure by constructing linear allometric regressions of tree characteristics and applying these to mapped stumps thus producing stem-maps before treatment. To investigate how these treatments altered structural complexity, I used spatial statistical analyses to assess spatial relationships of trees, before and after treatment, occurring at stand and within-stand scales as well as horizontal and vertical dimensions. Then, I assessed the cumulative effects of the reduction and spatial alterations of structure on potential fire behavior, measured by rate of spread, fireline intensity and percent of canopy consumed, across a range of open wind speeds using the Wildland urban-interface Dynamics Simulator (WFDS). WFDS is a physics-based model capable of representing the 3-D complexities of the fuels complex and captures fuel-atmosphere-fire dynamics through space and time. Results from this chapter suggest (1) treatments impact facets of structural complexity in varying ways, though avoided large-scale homogenization of forest structure, (2) within canopy wind speeds increase following treatments and, (3) fire behavior can be altered in two distinct manners following treatments. In most cases, the alterations in the fuels complex coupled with greater within canopy wind speeds resulted in an overall decrease in potential fire behavior and crown fire activity, especially at high open wind velocities. However, in two cases I examined there were increases in fire behavior following mechanical treatments. In these cases the increases were primarily associated with increased surface fire behavior. The results from this chapter suggest that these mechanical treatments may not always enhance, but can promote, a degree of structural complexity, and that mechanical treatments are effective if implemented strategically. Chapter 2 reports on litter bulk density values for use by managers to improve fuel loading assessments. Litter bulk density as a factor, in conjunction with litter depth, is used to estimate litter load…