ABSTRACT EFFECTS OF WILDLAND-URBAN INTERFACE FUEL TREATMENTS ON POTENTIAL FIRE BEHAVIOR AND ECOSYSTEM SERVICES IN THE CENTRAL SIERRA NEVADA MOUNTAINS OF CALIFORNIA Christopher C. Hamma For the past several decades, the wildland-urban interface (WUI) has been expanding in the low- to mid-elevation mixed-conifer belt of California’s Sierra Nevada mountain range. Concurrently, the effects of fire exclusion and shifting climatic patterns in this region have led to increases in wildfire size and severity, posing an ever-greater risk to life and property. As a result, the need for implementation of fuel treatments to reduce fire hazard is generally recognized to be urgent. However, by removing vegetation, these treatments may also diminish the ability of forest ecosystems to provide valuable ecosystem services to society. Forest managers, landowners, and other WUI stakeholders would therefore benefit from a better understanding of the effects of various fuel treatment types on both fire hazard reduction and ecosystem benefits. The present study examined the effects of four commonly-used fuel treatment types on stand-level forest structural characteristics, surface and canopy fuel loading, potential fire behavior, air pollution removal, and carbon sequestration and storage. Fuel treatments involving thinning and/or prescribed burning were largely successful at reducing live and dead fuel loading, with corresponding reductions in predicted fire behavior. The little-studied but increasingly popular practice of mastication (chipping or shredding small trees and brush and leaving the debris on the ground) was associated with significantly increased surface fuel loading, although deleterious effects on potential fire behavior were not found. Overall, the findings from the fire and fuels portion of the present research largely match those reported in other, similar studies in Sierra Nevada mixed-conifer forest. However, the current analysis found little in the way of significant treatment effects on stand-level air pollution removal or carbon dynamics. This study was affected by challenges including small sample size and high variability in the data; nonetheless, the results underscore the general validity of fuel treatment implementation in central Sierra Nevada WUI areas for moderating wildfire severity and effects, with the recognition that the efficacy of such treatments may be limited under extreme weather conditions.

Many fire-prone forests in western North America suffer from hazardous fuel conditions. Mechanical mastication is an increasingly common method of fuels treatment, but little is known regarding long-term effectiveness. A randomized block design at two sites (northern Sierras and southern Cascades) compared ladder fuels and overstory growth among treatments including mastication alone and mastication followed with prescribed fire or herbicide 10 or 11 years post-treatment. Subsequent herbicide application reduced ladder fuels in comparison to mastication alone or with prescribed fire. Prescribed fire further reduced ladder fuels at the southern Cascades site, however, in the northern Sierras postire ladder fuels were positively related to overstory absence. Mastication alone effectively released ponderosa pine at the southern Cascades site, whereas neither herbicide nor prescribed fire affected pine radial growth. This study demonstrates the feasibility of prescribed fire and herbicide for increasing treatment longevity of mastication, but also highlights potential limitations and important considerations.

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The Sierra Madre Occidental and neighboring Madrean Sky Islands span a large and biologically diverse region of northwest Mexico and portions of the southwestern United States. Little is known about the abundance and habitat use of breeding birds in this region of Mexico, but such information is important for guiding conservation and management. We assessed densities and habitat relationships of breeding birds across Sky Island mountain ranges in Mexico and adjacent portions of the Sierra Madre from 2009 to 2012. We estimated densities at multiple spatial scales, assessed variation in densities among all major montane vegetation communities, and identified and estimated the effects of important habitat attributes on local densities. Regional density estimates of 65% of 72 focal species varied significantly among eight montane vegetation communities that ranged from oak savannah and woodland at low elevations to pine and mixed-conifer forest at high elevations. Greater proportions of species occurred at peak densities or were relatively restricted to mixed-conifer forest and montane riparian vegetation likely because of higher levels of structural or floristic diversity in those communities, but those species were typically rare or uncommon in the Sky Islands. Fewer species had peak densities in oak and pine-oak woodland, and species associated with those communities were often more abundant across the region. Habitat models often included the effects of broadleaf deciduous vegetation cover (30% of species), which, together with tree density and fire severity, had positive effects on densities and suggest ways for managers to augment and conserve populations. Such patterns combined with greater threats to high-elevation conifer forest and riparian areas underscore their value for conservation. Significant populations of many breeding bird species, including some that are of concern or were not known to occur regionally or in mountain ranges we surveyed, highlight the importance of conservation efforts in this area of Mexico.

Management of natural landscapes requires knowledge of key disturbance processes and their effects. Fire and forest histories provide valuable insight into how fire and vegetation varied and interacted in the past. I constructed multi-century fire chronologies for 10 sites on six mountain ranges representative of the eastern Great Basin (USA), a region in which historic fire information was lacking. I also constructed tree recruitment chronologies for two sites. I use these chronologies to address three research foci. First, using fire-scar data from four heterogeneous sites, I assert that mean fire interval (MFI) values calculated from composite chronologies provide suitable estimates of point MFI (PMFI) when sample area size is ≈&frac; ha. I also suggest that MFI values for single trees can be used to estimate PMFI after applying a correction factor. Next, I infer climate effects on regional fire patterns using 10 site chronologies and tree-ring-based indices of drought and of El Niño Southern Oscillation (ENSO) and Pacific Decadal Oscillation ([PDO), Pacific Ocean surface temperature variability known to affect North American climate. Regional fire years (≥33% of recording sites) were synchronized by wet-dry cycles where the probability of occurrence was highest in the first year of drought following a wet phase and lowest when climate conditions transitioned from dry to wet. Regional fire probability was highest when ENSO and PDO were negative (Southwest pattern). Local fire years occurred under a broad range of conditions. Fire seasonality was bimodal with early and late-season fires dominant. I imply that Native American burning practices were responsible for differences in historic and modern fire seasonality. Lastly, I assess fire regime and tree recruitment variability within two fire-sheds. PMFI varied more than 10-fold within each site. A mixed-severity regime was dominant. A majority (>60%) of fires were small (<10 ha) but together accounted for a minor proportion of area burned. Recruitment pulses varied spatially from stand to landscape-scales and were often synchronous with multi-decade, fire-quiescent periods. I recommend that management strategies employ fire and fire-surrogate treatments to restore disturbance processes to these and similar landscapes at spatial and temporal scales consistent with the historic record.

The structure and composition of mixed-conifer forest (MCF) in central Oregon has been altered by fire exclusion and logging. The resulting increased density, spatial contagion, and loss of fire resistant trees decrease the resiliency of this ecosystem to fire, drought, and insects. The historical and current composition and structure of MCF are characterized by steep environmental gradients and a complex mixed-severity fire regime. This inherent variation makes it difficult to determine the magnitude of anthropogenic effects and set objectives for restoration and management. As a result, there is a lack of consensus regarding how MCF should be managed and restored across the landscape. My primary research objectives were to: (1) Characterize the current structure and composition of MCF and how these vary with environmental setting; and (2) Characterize establishment and tree growth patterns in MCF in different environmental settings. To address these objectives, I collected field data on structure and composition and increment cores across a range of environmental conditions in MCF of the eastern Cascades and Ochoco Mountains. I used cluster analysis to identify four stand types based on structure and composition in the eastern Cascades study area and four analogous types in the Ochoco Mountains study area. Variation in understory composition and the presence of large diameter shade tolerant species distinguish each type. Stand types occupied distinct environmental settings along a climatic gradient of increasing precipitation and elevation. At relatively dry PIPO sites understories were dominated by ponderosa pine. At wetter PIPO/PSME and PIPO ABGC sites understories were dominated by shade tolerant species, but ponderosa pine was dominant in the overstory. At the coolest and wettest PIPO/PSME/ABGC sites understories were dominated by grand fir and shade tolerant species were common in the overstory. In the eastern Cascades current density of all live trees and snags was 432, 461, 570, 372 trees per hectare (TPH) for the four stand types identified. Stand types in the drier Ochoco Mountains were currently less dense at 279, 304, 212, and 307 TPH. Current MCF densities in both areas are 2-3 times higher than densities estimated for the late 19th and early 20th centuries from other studies in those two areas. Reconstruction of cuts in each stand type indicates that the density of large diameter ponderosa pine has been reduced by approximately 50% in all stand types in both study regions. Age histograms demonstrate that current density and composition of MCF stand types is a product of abrupt increases in tree establishment following fire exclusion in the late 19th century. The number of trees established increased after 1900 in all stand types, but the timing and composition of changes in establishment varied with climate. At dry PIPO sites increases in establishment were delayed until the 1920s and 1930s and were composed of ponderosa pine. At PIPO/PSME and PIPO/ABGC sites with intermediate precipitation, establishment was dominated by ponderosa pine prior to 1900, but after 1900 establishment was dominated by a large pulse of Douglas-fir and grand fir. At the wettest PIPO/PSME/ABGC there was less evidence of changes in structure and composition over time. My results indicate that compared to dry pine and dry-mixed conifer sites, relatively productive moist mixed-conifer sites were characterized by large changes in structure and composition. Such sites could be considered more ecologically altered by lack of fire than drier forest types that had high fire frequencies but slower rates of stand development and less plant community change. Radial growth patterns of cored ponderosa pines differed between the eastern Cascades and Ochoco Mountains. In the eastern Cascades mean growth rates and variance decreased during favorable climatic periods after 1900. This is likely related to increased competition, and provides evidence that current stand density lacks a temporal analog in the 18th and 19th centuries. Sensitivity of growth to climate and harvest suggest competition for water in the denser forest of the eastern Cascades, and indicates thinning will increase the diameter growth rate of large old pines. In the Ochoco Mountains, ponderosa pine tree growth was less responsive to climate prior to fire exclusion in the late 1800s, and growth did not respond to fire events. This suggests competition among trees was historically low in this region. After fire exclusion growth became more responsive to wet and dry climatic cycles, which may indicate that increased density and competition made trees more responsive to climate variability. Patterns of slow and fast growth appeared to differ between study regions and likely differ at the sub-regional scale. Further analysis of the relationship between growth and climate in different environmental settings is needed to distinguish where stand development has been modified by disruption of fire regimes. / Graduation date: 2013