Richard A. Minnich

Fire Mosaics in Southern California and Northern Baja California

In spite of suppression efforts, severe wildfires burn large areas of southern California grassland, coastal sage scrub, and chaparral. Such large burns may not have been characteristic prior to the initiation of fire suppression more than 70 years ago. To compare controlled with uncontrolled areas, wildfires of southern California and adjacent northern Baja California were evaluated for the period 1972 to 1980 from Landsat imagery. Fire size and location, vegetation, year, and season were recorded. It was found that suppression has divergent effects on different plant communities depending on successional processes, growth rates, fuel accumulation, decomposition rates, and length of flammability cycles. These variables establish feedback between the course of active fires, fire history, spatial configuration of flammable vegetation, and fire size. Suppression has minimal impact in coastal sage scrub and grassland. Fire control in chaparral reduces the number of fires, not burned hectarage; fires consequently increase in size, spread rate, and intensity and become uncontrollable in severe weather conditions. The Baja California chaparral fire regime may serve as a model for prescribed burning in southern California.

Spatial distribution and properties of ash and thermally altered soils after high-severity forest fire, southern California

After a century of fire suppression, dense forests in California have fueled high-severity fires. We surveyed mixed conifer forest with 995-1178 trees ha −1 (stems > 10 cm diameter at breast height), and nearby pine- oak woodland having 175-230 trees ha −1 , 51 days after a severe burn, to contrast the spatial extent and properties of thermally altered soil at sites with different tree densities. Water-repellent soils were more extensive in forest than woodland. Deposits of white ash, composed largely of calicite, covered at most ∼25% of the land surface, in places where large fuel items (e.g. logs, branches, exfoliated oak bark) had thoroughly combusted. At least 1690 kg ha −1 of CaCO3 in ash was deposited over the forest, and at least 700 kg ha −1 was added to the woodland. Combustion of logs and large branches also reddened the underlying yellow-brown soil as deep as 60 mm (average 8 mm), and over ∼1-12% of the land surface. The reddened soils have magnetic susceptibilities that are three to seven times greater than surrounding unreddened soils within the burn, indicating thermal production of maghemite. Such fire-altered conditions persist over spatial and temporal scales that influence soil genesis in Mediterranean-type climate regions.

Mapping probability of fire occurrence in San Jacinto Mountains, California, USA

An ecological data base for the San Jacinto Mountains, California, USA, was used to construct a probability model of wildland fire occurrence. The model incorporates both environmental and human factors, including vegetation, temperature, precipitation, human structures, and transportation. Spatial autocorrelation was examined for both fire activity and vegetation to determine the specification of neighborhood effects in the model. Parameters were estimated using stepwise logistic regressions. Among the explanatory variables, the variable that represents the neighborhood effects of spatial processes is shown to be of great importance in the distribution of wildland fires. An important implication of this result is that the management of wildland fires must take into consideration neighborhood effects in addition to environmental and human factors. The distribution of fire occurrence probability is more accurately mapped when the model incorporates the spatial term of neighborhood effects. The map of fire occurrence probability is useful for designing large-scale management strategies of wildfire prevention.

Extreme southwestern margin of late Quaternary glaciation in North America: Timing and controls

Well-preserved latero-frontal moraines in the eastern San Bernardino Mountains of southern California provide evidence for several glacial advances during the late Quaternary and mark the southwesternmost limit of glaciation in the Western Cordillera. Using geomorphology and 10Be cosmogenic radionuclide dating, a succession of moraines from three glaciated valleys is dated to 18–20 ka (Last Glacial Maximum), 15–16 ka (Heinrich Event 1), 12–13 ka (Younger Dryas Stade), and 5–9 ka (early-middle Holocene). These ages substantiate the view that glaciation throughout the American Cordilleras was synchronous during the late Quaternary. Furthermore, these data show that glacial advances in southern California occur when a significant decrease in summer temperature is coupled with an increase in moisture flux producing high winter snowfall. This allows for perennial snow accumulation that may, under appropriate conditions, persist to form glacial ice.

Land of chamise and pines : historical accounts and current status of northern Baja California's vegetation

In marked contrast to Californias landscape of urban sprawl, expansive agriculture, and wildlands altered by protectionist management systems, many landscapes in neighboring Baja California would still be recognizable to the first European explorers. This book shows that the vegetation of present-day Baja California is remarkably similar to that observed in the 18th and 19th centuries, and that historical fire and grazing management has done little to alter the regions resilient mediterranean-type shrublands and forests.

Chapter 17 Air Pollution Increases Forest Susceptibility to Wildfires: A Case Study in the San Bernardino Mountains in Southern California

Many factors increase susceptibility of forests to wildfire. Among them are increases in human population, changes in land use, fire suppression, and frequent droughts. These and other factors have been exacerbating forest susceptibility to wildfires over the past century in southern California. We report on the significant role that air pollution has had on increasing forest susceptibility to wildfires, based on a 1999–2003 case study in the San Bernardino Mountains. Air pollution, specifically ozone (O3) and wet and dry deposition of nitrogenous (N) compounds as a by-product of fossil fuel combustion, has significantly increased since urbanization and industrialization of the region after 1945. Ozone and elevated N deposition cause specific changes in forest tree carbon (C), N, and water balance that enhance individual tree susceptibility to drought, bark beetle attack, and disease, and when combined, contribute to whole ecosystem susceptibility to wildfire. For example, elevated O3 and N deposition increase leaf turnover rates, leaf and branch litter, and decrease decomposability of litter, creating excessively deep litter layers in mixed-conifer forests affected by air pollutants. Elevated O3 and N deposition decrease the proportion of whole tree biomass in foliage and roots, thereby increasing tree susceptibility to drought and beetle attack. Because both foliar and root mass are compromised, carbohydrates are stored in the bole over winter. Elevated O3 increases drought stress by significantly reducing plant control of water loss. The resulting increase in canopy transpiration, combined with O3 and N deposition-induced decreases in root mass, significantly increases tree susceptibility to drought stress, likely contributing to successful host colonization and population increases of bark beetles. Phenomenological and experimental evidence is presented to support the role of these factors contributing to an increase in the susceptibility of forests to wildfire in southern California.

Snow levels and amounts in the mountains of southern California

In producing dimension stone, an actuated cutter is progressively directed against a surface of a mineral body to remove material and form a V-shaped indentation which constitutes a rectilinear parting extending inwardly for an appreciable distance along a desired plane of separation. The mineral body is thereafter engaged at points above and below the parting by pressure operated cutting means to form dimension stone pieces. The parting, especially when employed at two opposite sides of the mineral body, controls the cutting action to form the pieces with vertical corners each of whose surfaces intersect the V-shaped indentation to provide arrises which are characterized by sharply defined rectilinearity.

EVIDENCE, EXAGGERATION, AND ERROR IN HISTORICAL ACCOUNTS OF CHAPARRAL WILDFIRES IN CALIFORNIA

For more than half a century, ecologists and historians have been integrating the contemporary study of ecosystems with data gathered from historical sources to evaluate change over broad temporal and spatial scales. This approach is especially useful where ecosystems were altered before formal study as a result of natural resources management, land development, environmental pollution, and climate change. Yet, in many places, historical documents do not provide precise information, and pre-historical evidence is unavailable or has ambiguous interpretation. There are similar challenges in evaluating how the fire regime of chaparral in California has changed as a result of fire suppression management initiated at the beginning of the 20th century. Although the firestorm of October 2003 was the largest officially recorded in California (approximately 300,000 ha), historical accounts of pre-suppression wildfires have been cited as evidence that such a scale of burning was not unprecedented, suggesting the fire regime and patch mosaic in chaparral have not substantially changed. We find that the data do not support pre-suppression megafires, and that the impression of large historical wildfires is a result of imprecision and inaccuracy in the original reports, as well as a parlance that is beset with hyperbole. We underscore themes of importance for critically analyzing historical documents to evaluate ecological change. A putative 100 mile long by 10 mile wide (160 x 16 km) wildfire reported in 1889 was reconstructed to an area of chaparral approximately 40 times smaller by linking local accounts to property tax records, voter registration rolls, claimed insurance, and place names mapped with a geographical information system (GIS) which includes data from historical vegetation surveys. We also show that historical sources cited as evidence of other large chaparral wildfires are either demonstrably inaccurate or provide anecdotal information that is immaterial in the appraisal of pre-suppression fire size. Since historical evidence is inadequate for reconstructing a statistical distribution of pre-suppression fire sizes to compare with post-suppression data, other more propitious methods of evaluating change are discussed.

The introduction and naturalization of Schinus molle (pepper tree) in Riverside, California

Abstract In the late nineteenth century, an exotic tree from Peru, Schinus molle , was introduced for use in street planting at Riverside, California. Trees have since become naturalized in wildlands surrounding Riverside in areas with persistent moisture and bare soil, including riparian habitats, washes and springs, sites downslope from pavement which amplifies run-off, and irrigated citrus groves. The establishment of S. molle including seed viability and germination, seedling establishment, and phenology was evaluated from historical sources, field observations, and experimental tests in a lath house and the Agricultural Operations plot at the University of California, Riverside. Recruitment and migration of S. molle beyond the urban fringe has been slow, but cumulative because rare establishment is compensated by enduring survival. Phenotypic plasticity enables S. molle to adjust its phenology to a Mediterranean climate. Flowers and fruit develop continuously. Seeds are widely dispersed by birds, mammals, and water. Germination time varies inversely with temperature. Once well established seedling mortality was rare. Growth is greatest in the warm season until soil moisture is depleted. With continuous seed production and reliable dispersal the major bottleneck to establishment appears to be germination and seedling survival. While warm weather germination restricts establishment to occasional late spring and summer rains, it allows seedlings to flourish without competition from winter-active shrubs and grasses. S. molle maintains a continuous seed supply for germination whenever conditions are favorable. Autonomous development, free from interspecific interference, enables more efficient site occupation, allowing S. molle to grow as a tree in a shrubland.

Water Balance at the Southern Limit of the Californian Mixed-Conifer Forest and Implications for Extreme-Deficit Watersheds

This study estimates the soil water balance at forest/chaparral, upper forest, and wet-meadow sites in the Sierra San Pedro Mártir (SSPM), Baja California, Mexico, and compares the results to previously published data for similar sites in neighboring California, USA. Changes in soil water storage (0-80cm) were determined by using a neutron probe, deep drainage (> 80cm) was measured in minilysimeters, and runoff was estimated in 1m2 plots. Average annual precipitation was 714 - 226, 674 - 137, and 551 - 112 mm at the lower forest, upper forest, and wet-meadow sites, respectively. Evapotranspiration (ET) was 390 - 122, 478 - 87, and 446 - 122 mm, or 55%, 71% and 81% of precipitation. The relationship between average precipitation and average ET for SSPM was similar to published long-term ET data for extreme-deficit watersheds in southern California. Regression analyses of streamflow and ET data in review studies revealed that, as water deficit increases, the relationship between precipitation and ET becomes increasingly stronger, while that between precipitation and streamflow becomes concomitantly weaker. The slope of the regression line gives the proportion of precipitation lost to streamflow or ET, while the intercept indicates the amount of soil water recharge required annually before streamflow can occur. Available data from long-term studies in extreme-deficit watersheds revealed a near-zero intercept, and imply that 69% of precipitation is evapotranspired from the surface soil, Also, 15% is apparently transpired by deep roots extracting water below the solum, and the remaining 16% goes to streamflow.