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Teresa J. Brennan

Publications and source records attributed to Teresa J. Brennan.

13 recordsLinked to original sources
USGS2023

A decade-long study of repeated prescription burning in California native grassland restoration

Native bunchgrass communities dominated by Stipa pulchra are widely distributed in California but share dominance with non-native annual grasses. Restoration of these grasslands focuses on altering the balance of native to non-native grasses to favor the former. This study investigated the impact of burning on vegetation recovery. In the first postfire year burning showed a 70% reduction in cover of non-native annual grasses ( Bromus diandrus exhibited the greatest reduction) and minimal impact on S. pulchra recovery. In the following 3 years, S. pulchra recovered to levels comparable to controls, whereas the annual grasses remained below control levels until the fifth year. Also, in response to reduced annual grass cover on burned sites several species of non-native Erodium increased from 10 to 30% relative cover, however, the low growth form of these forbs presented a less competitive threat to bunchgrasses than the non-native annual grasses, and by the third postfire year returned to near control levels. The rare native geophyte Brodiaea kinkiensis was present throughout these grasslands and was not inhibited by burning treatments. To document the reliability of these patterns a second prescription burn was conducted on these sites 5 years after the first burn and vegetation recovery followed for the subsequent 4 years. Patterns observed after the first burn were duplicated following the second burn. The cover of S . pulchra varied in response to precipitation, with the 95% credible intervals of precipitation parameters overlapping zero, however, the cover of non-native grasses varied greatly with precipitation and had similar trajectories in unburned and burned plots.

California↗
USGS2020

Protecting the wildland-urban interface in California: Greenbelts vs thinning for wildfire threats to homes

This study utilized native chaparral and sage scrub shrubs to evaluate the impact of light summer irrigation on live fuel moisture content (LFMC) and predicted fire behavior. As to be expected LFMC varied markedly throughout the year being over 100% in winter in all species and treatments but differed markedly by treatment in the summer and fall. For most species lightly irrigated plants had the highest LFMC in the summer and fall, followed by thinned treatments and controls. These differences in moisture content coupled with structural differences in the vegetation contributed to expected differences in flame length and rate of spread. Lightly irrigated native shrubs planted around homes can reduce fire hazard and at the same time increase faunal diversity and other desirable features of utilizing native vegetation.

California↗
USGS2019

Postfire population dynamics of a fire-dependent cypress

Tecate cypress ( Hesperocyparis forbesii ) is a rare species restricted to four metapopulations in southern California, USA and a few isolated stands in northern Baja California, Mexico. It is a closed-cone, fire-dependent tree of conservation concern due to an increase in human-caused wildfires that have shortened the interval between fires in many of their populations. In 2003 the Mine/Otay Fire burned 70% of the Tecate cypress on Otay Mountain in San Diego County, California providing an opportunity to evaluate the immaturity risk of this species and to examine its recruitment, survivorship, and reproductive maturity over a 14-year period from 2004 to 2017. Sixteen plots were established in burned stands of Tecate cypress with prefire ages that ranged from 7 to 53 years old. After 14 years the overall density of Tecate cypress was still higher than before the fire, however the areal extent of the species decreased due to the loss of locations where either there was low cone production or fire intensity was too high. The immaturity risk for this species, while a function of prefire stand age, is confounded by other factors including the reproductive capacities of trees based on their density and size and the climatic variables affecting their growth over time. The future management of Tecate cypress and other fire-dependent species requires a knowledge of all factors impacting their immaturity risk, as well as an understanding of the potential fire-climate interactions that may impact their persistence in a future of climate change and altered fire regimes.

California↗
USGS2018

Drivers of chaparral type conversion to herbaceous vegetation in coastal Southern California

Aim In Southern California, native woody shrublands known as chaparral support exceptional biodiversity. However, large‐scale conversion of chaparral into largely exotic herbaceous cover is a major ecological threat and serious conservation concern. Due to substantial uncertainty regarding the causes and extent of this vegetation change, we aimed to quantify the primary drivers of and map potentially vulnerable locations for vegetation type conversion from woody into herbaceous cover. Location Santa Monica Mountains National Recreational Area, Southern California, USA. Methods We used air photograph image interpretation to quantify the extent to which chaparral shrublands transitioned to herbaceous cover from 1943 to 2014 across nearly 800 randomly located plots. Comparing plots that remained chaparral to those that converted to herbaceous cover, we performed hierarchical partitioning to quantify the independent contribution of a range of explanatory variables, and then used classification trees to explore variable interactions. We also developed a spatial model to create a seamless map delineating relative probability of type conversion. Results Of the original plots that were chaparral in 1943, 284 (36%) changed cover by 2014, with 79 completely converting, and 142 mostly converting to herbaceous cover. The primary mechanism behind shrubland decline and replacement was short intervals between fires (<=10 years), and type conversion was most likely to occur in arid parts of the landscape with low topographic heterogeneity and close proximity to trails and roads. Predictive maps delineated several hotspots with environmental conditions similar to those of type‐converted plots. Main conclusions Chaparral type conversion is a widespread conservation concern, and results here suggest that short‐interval fire and landscape disturbance are the most likely factors to exacerbate it, particularly in water‐limited portions of the landscape where chaparral is subject to greater physiological stress and slower recovery. Reducing fire ignitions and mapping vulnerable areas may be important strategies for prevention.

California↗
USGS2017

Impacts of mastication fuel treatments on California, USA, chaparral vegetation structure and composition

Mechanical fuel treatments are a primary pre-fire strategy for potentially mitigating the threat of wildland fire, yet there is limited information on how they impact shrubland ecosystems. Our goal was to assess the impact of mechanical mastication fuel treatments on chaparral vegetation and to determine the extent to which they emulate early post-fire succession. Mastication treatments significantly reduced the height and cover of woody vegetation and increased herbaceous cover and diversity. Non-native cover, density, and diversity were also significantly higher in masticated treatments. Comparisons with post-fire data from two studies showed that certain ephemeral post-fire endemics were absent or of limited occurrence from masticated plots in comparison to their abundance on adjacent burned plots. Structurally, masticated sites differed in the dense woody debris cover, whereas burned sites had little such ground cover. Regional comparison of masticated plots to previously published post-fire studies found that burned sites had greater cover, density, and diversity of native species. However, masticated sites and burned sites were broadly similar in distribution of different growth forms. Results from our study show that the use of mastication fuel treatments in chaparral are not in alignment with some resource conservation goals, but in some cases it is recognized that such sacrifice of natural resources may be an acceptable tradeoff to potentially mitigating fire hazard.

California↗
USGS2017

The importance of building construction materials relative to other factors affecting structure survival during wildfire

Structure loss to wildfire is a serious problem in wildland-urban interface areas across the world. Laboratory experiments suggest that fire-resistant building construction and design could be important for reducing structure destruction, but these need to be evaluated under real wildfire conditions, especially relative to other factors. Using empirical data from destroyed and surviving structures from large wildfires in southern California, we evaluated the relative importance of building construction and structure age compared to other local and landscape-scale variables associated with structure survival. The local-scale analysis showed that window preparation was especially important but, in general, creating defensible space adjacent to the home was as important as building construction. At the landscape scale, structure density and structure age were the two most important factors affecting structure survival, but there was a significant interaction between them. That is, young structure age was most important in higher-density areas where structure survival overall was more likely. On the other hand, newer-construction structures were less likely to survive wildfires at lower density. Here, appropriate defensible space near the structure and accessibility to major roads were important factors. In conclusion, community safety is a multivariate problem that will require a comprehensive solution involving land use planning, fire-safe construction, and property maintenance.

International Journal of Disaster Risk Reduction↗
USGS2015

Effect of mastication and other mechanical treatments on fuel structure in chaparral

Mechanical fuel treatments are a common pre-fire strategy for reducing wildfire hazard that alters fuel structure by converting live canopy fuels to a compacted layer of dead surface fuels. Current knowledge concerning their effectiveness, however, comes primarily from forest-dominated ecosystems. Our objectives were to quantify and compare changes in shrub-dominated chaparral following crushing, mastication, re-mastication and mastication-plus-burning treatments, and to assess treatment longevity. Results from analysis of variance (ANOVA) identified significant differences in all fuel components by treatment type, vegetation type and time since treatment. Live woody fuel components of height, cover and mass were positively correlated with time since treatment, whereas downed woody fuel components were negatively correlated. Herbaceous fuels, conversely, were not correlated, and exhibited a 5-fold increase in cover across treatment types in comparison to controls. Average live woody fuel recovery was 50% across all treatment and vegetation types. Differences in recovery between time-since-treatment years 1&ndash;8 ranged from 32&ndash;65% and exhibited significant positive correlations with time since treatment. These results suggest that treatment effectiveness is short term due to the rapid regrowth of shrubs in these systems and is compromised by the substantial increase in herbaceous fuels. Consequences of not having a full understanding of these treatments are serious and leave concern for their widespread use on chaparral-dominated landscapes.

International Journal of Wildland Fire↗
USGS2014

The role of defensible space for residential structure protection during wildfires

With the potential for worsening fire conditions, discussion is escalating over how to best reduce effects on urban communities. A widely supported strategy is the creation of defensible space immediately surrounding homes and other structures. Although state and local governments publish specific guidelines and requirements, there is little empirical evidence to suggest how much vegetation modification is needed to provide significant benefits. We analysed the role of defensible space by mapping and measuring a suite of variables on modern pre-fire aerial photography for 1000 destroyed and 1000 surviving structures for all fires where homes burned from 2001 to 2010 in San Diego County, CA, USA. Structures were more likely to survive a fire with defensible space immediately adjacent to them. The most effective treatment distance varied between 5 and 20 m (16&ndash;58 ft) from the structure, but distances larger than 30 m (100 ft) did not provide additional protection, even for structures located on steep slopes. The most effective actions were reducing woody cover up to 40% immediately adjacent to structures and ensuring that vegetation does not overhang or touch the structure. Multiple-regression models showed landscape-scale factors, including low housing density and distances to major roads, were more important in explaining structure destruction. The best long-term solution will involve a suite of prevention measures that include defensible space as well as building design approach, community education and proactive land use planning that limits exposure to fire.

International Journal of Wildland Fire↗
USGS2012

Housing arrangement and location determine the likelihood of housing loss due to wildfire

Surging wildfires across the globe are contributing to escalating residential losses and have major social, economic, and ecological consequences. The highest losses in the U.S. occur in southern California, where nearly 1000 homes per year have been destroyed by wildfires since 2000. Wildfire risk reduction efforts focus primarily on fuel reduction and, to a lesser degree, on house characteristics and homeowner responsibility. However, the extent to which land use planning could alleviate wildfire risk has been largely missing from the debate despite large numbers of homes being placed in the most hazardous parts of the landscape. Our goal was to examine how housing location and arrangement affects the likelihood that a home will be lost when a wildfire occurs. We developed an extensive geographic dataset of structure locations, including more than 5500 structures that were destroyed or damaged by wildfire since 2001, and identified the main contributors to property loss in two extensive, fire-prone regions in southern California. The arrangement and location of structures strongly affected their susceptibility to wildfire, with property loss most likely at low to intermediate structure densities and in areas with a history of frequent fire. Rates of structure loss were higher when structures were surrounded by wildland vegetation, but were generally higher in herbaceous fuel types than in higher fuel-volume woody types. Empirically based maps developed using housing pattern and location performed better in distinguishing hazardous from non-hazardous areas than maps based on fuel distribution. The strong importance of housing arrangement and location indicate that land use planning may be a critical tool for reducing fire risk, but it will require reliable delineations of the most hazardous locations.

California↗
USGS2012

Fire-driven alien invasion in a fire-adapted ecosystem

Disturbance plays a key role in many alien plant invasions. However, often the main driver of invasion is not disturbance per se but alterations in the disturbance regime. In some fire-adapted shrublands, the community is highly resilient to infrequent, high-intensity fires, but changes in the fire regime that result in shorter fire intervals may make these communities more susceptible to alien plant invasions. This study examines several wildfire events that resulted in short fire intervals in California chaparral shrublands. In one study, we compared postfire recovery patterns in sites with different prefire stand ages (3 and 24 years), and in another study we compared sites that had burned once in four years with sites that had burned twice in this period. The population size of the dominant native shrub Adenostoma fasciculatum was drastically reduced following fire in the 3-year sites relative to the 24-year sites. The 3-year sites had much greater alien plant cover and significantly lower plant diversity than the 24-year sites. In a separate study, repeat fires four years apart on the same sites showed that annual species increased significantly after the second fire, and alien annuals far outnumbered native annuals. Aliens included both annual grasses and annual forbs and were negatively correlated with woody plant cover. Native woody species regenerated well after the first fire but declined after the second fire, and one obligate seeding shrub was extirpated from two sites by the repeat fires. It is concluded that some fire-adapted shrublands are vulnerable to changes in fire regime, and this can lead to a loss of native diversity and put the community on a trajectory towards type conversion from a woody to an herbaceous system. Such changes result in alterations in the proportion of natives to non-natives, changes in functional types from deeply rooted shrubs to shallow rooted grasses and forbs, increased fire frequency due to the increase in fine fuels, and changes in carbon storage.

California↗
USGS2011

Comparing the role of fuel breaks across southern California national forests

Fuel treatment of wildland vegetation is the primary approach advocated for mitigating fire risk at the wildland–urban interface (WUI), but little systematic research has been conducted to understand what role fuel treatments play in controlling large fires, which factors influence this role, or how the role of fuel treatments may vary over space and time. We assembled a spatial database of fuel breaks and fires from the last 30 years in four southern California national forests to better understand which factors are consistently important for fuel breaks in the control of large fires. We also explored which landscape features influence where fires and fuel breaks are most likely to intersect. The relative importance of significant factors explaining fuel break outcome and number of fire and fuel break intersections varied among the forests, which reflects high levels of regional landscape diversity. Nevertheless, several factors were consistently important across all the forests. In general, fuel breaks played an important role in controlling large fires only when they facilitated fire management, primarily by providing access for firefighting activities. Fire weather and fuel break maintenance were also consistently important. Models and maps predicting where fuel breaks and fires are most likely to intersect performed well in the regions where the models were developed, but these models did not extend well to other regions, reflecting how the environmental controls of fire regimes vary even within a single ecoregion. Nevertheless, similar mapping methods could be adopted in different landscapes to help with strategic location of fuel breaks. Strategic location of fuel breaks should also account for access points near communities, where fire protection is most important.

California↗
USGS2011

Factors affecting fuel break effectiveness in the control of large fires on the Los Padres National Forest, California

As wildfires have increased in frequency and extent, so have the number of homes developed in the wildland-urban interface. In California, the predominant approach to mitigating fire risk is construction of fuel breaks, but there has been little empirical study of their role in controlling large fires.We constructed a spatial database of fuel breaks on the Los Padres National Forest in southern California to better understand characteristics of fuel breaks that affect the behaviour of large fires and to map where fires and fuel breaks most commonly intersect. We evaluated whether fires stopped or crossed over fuel breaks over a 28-year period and compared the outcomes with physical characteristics of the sites, weather and firefighting activities during the fire event. Many fuel breaks never intersected fires, but others intersected several, primarily in historically fire-prone areas. Fires stopped at fuel breaks 46% of the time, almost invariably owing to fire suppression activities. Firefighter access to treatments, smaller fires and longer fuel breaks were significant direct influences, and younger vegetation and fuel break maintenance indirectly improved the outcome by facilitating firefighter access. This study illustrates the importance of strategic location of fuel breaks because they have been most effective where they provided access for firefighting activities.

California↗