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Climate change is shifting forest tree species distributions across elevational and latitudinal gradients, and these changes are often pronounced at ecotones where species meet their climatic bounds and are replaced by other species. Using an extensive ecotone composed of lower‐montane white fir (Abies concolor var. lowiana) and upper‐montane red fir (Abies magnifica var. magnifica) in the central Sierra Nevada range of California, USA, we (1) examined how the demographics of the ecotone have responded to recent climate using a field observational study and a historical dataset, (2) quantified climate drivers across species life stages using contemporary demographic data, and (3) tested the potential impacts of future climate on species‐specific seedling survival and growth in a fully factorial growth chamber experiment that varied temperature, growing season length, and water availability. A re‐examination of the ecotone midpoint after 35 yr suggested a reduction in A. concolor sapling and tree densities and a rise in A. magnifica proportional dominance between surveys. Seedling abundances across the ecotone indicated that A. magnifica tends to dominate the regeneration layer and currently forms an important component of the seedling community at elevations below those where A. magnifica saplings or trees begin to co‐dominate stands. Observational and experimental assessments suggest that temperature and precipitation serve as important drivers, differentiating A. concolor vs. A. magnifica distributions, and are primary stressors at the seedling stage. Seedlings of both species were adversely affected by experimental climate treatments, although A. concolor exhibited greater survival and a more conservative growth strategy under extreme climatic stress than A. magnifica. Projections indicate that historical climate conditions will rise by an amount greater than the ecotone's current elevational extent by the end of the 21st century. Differential drivers of species abundances suggest that the projected climate will expand conditions that promote A. concolor abundance and impede A. magnifica abundance across the ecotone; however, disturbance activity and microclimatic conditions will also influence regeneration and overstory tree dynamics. Our study demonstrates the importance of quantifying species‐specific responses to climate and indicates that widespread regeneration failure may be one possible consequence in which species exhibit strong sensitivity to projected climate conditions. [ABSTRACT FROM AUTHOR]

Tree spatial patterns in dry coniferous forests of the western United States, and analogous ecosystems globally, were historically aggregated, comprising a mixture of single trees and groups of trees. Modern forests, in contrast, are generally more homogeneous and overstocked than their historical counterparts. As these modern forests lack regular fire, pattern formation and maintenance is generally attributed to fire. Accordingly, fires in modern forests may not yield historically analogous patterns. However, direct observations on how selective tree mortality among pre‐existing forest structure shapes tree spatial patterns is limited. In this study, we (a) simulated fires in historical and contemporary counterpart plots in a Sierra Nevadan mixed‐conifer forest, (b) estimated tree mortality, and (c) examined tree spatial patterns of live trees before and after fire, and of fire‐killed trees. Tree mortality in the historical period was clustered and density‐dependent, because trees were aggregated and segregated by tree size before fire. Thus, fires maintained an aggregated distribution of tree groups. Tree mortality in the contemporary period was widespread, except for dispersed large trees, because most trees were a part of large, interconnected tree groups. Thus, postfire tree patterns were more uniform and devoid of moderately sized tree groups. Postfire tree patterns in the historical period, unlike the contemporary period, were within the historical range of variability identified for the western United States. This divergence suggests that decades of forest dynamics without significant disturbances have altered the historical means of pyric pattern formation. Our results suggest that ecological silvicultural treatments, such as forest restoration thinnings, which emulate qualities of historical forests may facilitate the reintroduction of fire as a means to reinforce forest structural heterogeneity. [ABSTRACT FROM AUTHOR]

Topography, weather, and fuels are known factors driving fire behavior, but the degree to which each contributes to the spatial pattern of fire severity under different conditions remains poorly understood. The variability in severity within the boundaries of the 2006 wildfires that burned in the Klamath Mountains, northern California, along with data on burn conditions and new analytical tools, presented an opportunity to evaluate factors influencing fire severity under burning conditions representative of those where management of wildfire for resource benefit is most likely. Fire severity was estimated as the percent change in canopy cover (0-100%) classified from the Relativized differenced Normalized Burn Ratio (RdNBR), and spatial data layers were compiled to determine strength of associations with topography, weather, and variables directly or indirectly linked to fuels, such as vegetation type, number of previous fires, and time since last fire. Detailed fire progressions were used to estimate weather (e.g., temperature, relative humidity, temperature inversions, and solar radiation) at the time of burning. A generalized additive regression model with random effects and an additional spatial term to account for autocorrelation between adjacent locations was fitted to fire severity. In this fire year characterized by the relative absence of extreme fire weather, topographical complexity most strongly influenced severity. Upper- and mid-slopes tended to burn at higher fire severity than lower-slopes. East- and southeast-facing aspects tended to burn at higher severity than other aspects. Vegetation type and fire history were also important predictors of fire severity. Shrub vegetation was more likely to burn at higher severity than mixed hardwood/conifer or hardwood vegetation. As expected, fire severity was positively associated with time since previous fire, but the relationship was non-linear. Of the weather variables analyzed, temperature inversions, common in the complex topography of the Klamath Mountains, showed the strongest association with fire severity. Inversions trapped smoke and had a dampening effect on severity within the landscape underneath the inversion. Understanding the spatial controls on mixed-severity fires allows managers to better plan for future wildfires and aide in the decision making when managing lightning ignitions for resource benefit might be appropriate. [ABSTRACT FROM AUTHOR]

Timber is frequently salvage-logged following high-severity stand-replacing wildfire, but the practice is controversial. One concern is that compound disturbances could result in more deleterious impacts than either disturbance individually, with mechanical operations having the potential to set back recovering native species and increase invasion by non-native species. Following the 2002 Cone Fire on the Lassen National Forest, three replicates of five salvage treatments were applied to 15 units formerly dominated by ponderosa pine, covering a range of disturbance intensities from unsalvaged to 100% salvaged. Understory species richness and cover data were collected every two years between 2006 and 2012. Richness of both native and non-native species did not differ among salvage treatments, but both showed strong changes over time. While cover of forbs and graminoids did not differ with salvage treatment, cover of shrubs was significantly reduced at the higher salvage intensities. The three main shrub species are all stimulated to germinate by fire, potentially leaving seedlings vulnerable to any mechanical disturbance occurring immediately postgermination. Many other native perennial species emerge from rhizomes or other deeply buried underground structures and appear to be less affected by salvage harvest. Over time, the plant community in all salvage treatments shifted from dominance by shrubs and forbs to shrubs and grasses. Most of the grasses were native, except Bromus tectorum (cheatgrass), which was found in 4% of measurement quadrats in 2006 and 52% in 2012. Our results indicated that understory vegetation change 4-10 years post-high- severity wildfire appeared to be influenced more strongly by factors other than salvage logging. [ABSTRACT FROM AUTHOR]

We studied vegetation composition and structure in a mixed conifer-oak ecosystem across a range of fire severity 10 years following wildfire. Sample plots centered on focal California black oaks (Quercus kelloggii) were established to evaluate oak and neighboring tree and shrub recovery across a gradient of fire severity in the southern Cascade Range, USA. Shrub and oak resprouting was strongest around focal oaks where conifer mortality was greatest. Linear modeling revealed negative relationships between California black oak sprout height or basal area and residual overstory tree survival, primarily white fir {Abies concolor). The two dominant competing species, California black oak and white fir, showed opposite responses to fire severity. Sprouting California black oak and associated shrubs dominated in severely burned areas, while surviving, non-sprouting white fir maintained dominance by its height advantage and shading effects in areas that burned with low fire severity. Our results indicate that high-severity fire promotes persistence and restoration of ecosystems containing resprouting species, such as California black oak, that are increasingly rare due to widespread fire exclusion in landscapes that historically experienced more frequent fire. We present a conceptual model based on our results and supported by a synthesis of postfire resprouting dynamics literature. Our results and conceptual model help illuminate long-term postfire vegetation responses and the potential ability of fire to catalyze formation of alternate vegetation community structures that may not be apparent in studies that evaluate postfire effects at shorter time-since-fire intervals or at coarser scales. [ABSTRACT FROM AUTHOR]

Pervasive warming can lead to chronic stress on forest trees, which may contribute to mortality resulting from fire-caused injuries. Longitudinal analyses of forest plots from across the western US show that high pre-fire climatic water deficit was related to increased post-fire tree mortality probabilities. This relationship between climate and fire was present after accounting for fire defences and injuries, and appeared to influence the effects of crown and stem injuries. Climate and fire interactions did not vary substantially across geographical regions, major genera and tree sizes. Our findings support recent physiological evidence showing that both drought and heating from fire can impair xylem conductivity. Warming trends have been linked to increasing probabilities of severe fire weather and fire spread; our results suggest that warming may also increase forest fire severity (the number of trees killed) independent of fire intensity (the amount of heat released during a fire). [ABSTRACT FROM AUTHOR]

Whether ignited by lightning or by Native Americans, fire once shaped many North American ecosystems. Euro-American settlement and 20th-century fire suppression practices drastically altered historic fire regimes, leading to excessive fuel accumulation and uncharacteristically severe wildfires in some areas and diminished flammability resulting from shifts to more fire-sensitive forest species in others. Prescribed fire is a valuable tool for fuel management and ecosystem restoration, but the practice is fraught with controversy and uncertainty. Here, we summarize fire use in the forests and woodlands of North America and the current state of the practice, and explore challenges associated with the use of prescribed fire. Although new scientific knowledge has reduced barriers to prescribed burning, societal aversion to risk often trumps known, long-term ecological benefits. Broader implementation of prescribed burning and strategic management of wildfires in fire-dependent ecosystems will require improved integration of science, policy, and management, and greater societal acceptance through education and public involvement in land-management issues. [ABSTRACT FROM AUTHOR]

Temperatures well in excess of the lethal threshold for roots (60°C) have been measured in forest soils when woody fuels are burned. Whether this heat pulse is strongly moderated by soil moisture or soil texture is not fully understood, however. We measured soil heat profiles during 60 experimental burns, identifying changes in maximum soil temperature and heat duration above 60°C as a function of soil moisture and soil texture. Experimental treatments included a factorial combination of soil moisture (∼5, 15, 30, and 45% v/v) and soil textural (sandy loam, sandy loam-pumice, loam, clay loam) gradients, with a surface fuel load comprised of a dense layer of masticated wood. Soil moisture had a strong influence on heat transfer. A volumetric moisture content of 20% or greater quenched the heat pulse in all soils at depths of 2.5 cm and lower. In comparison, soil temperatures in dry soil far exceeded the lethal threshold to a depth of 10 cm. Differences in heating characteristics among the four soil types were minor despite their dissimilarities in texture, porosity, bulk density, and presumed thermal conductivity. It was also shown that intact soil cores were required to produce accurate heat profiles during burning, as maximum soil temperatures in the surface 5cm were overestimated by 40 to 100°C using disturbed soil (sieved and packed). The empirical results along with a simple predictive model of soil heating show that burning of woody fuels when underlying soils have 20% volumetric moisture or greater is an effective means for limiting lethal heating in a variety of soils. [ABSTRACT FROM AUTHOR]

Questions: What influence does mechanical mastication and other fuel treatments have on: (1) canopy and forest floor response variables that influence understory plant development; (2) initial understory vegetation cover, diversity, and composition; and (3) shrub and non-native species density in a second-growth ponderosa pine forest. Location: Challenge Experimental Forest, northern Sierra Nevada, California, USA. Methods: We compared the effects of mastication only, mastication with supplemental treatments (tilling and prescribed fire), hand removal, and a control on initial understory vegetation response using a randomized complete block experimental design. Each block ( n=4) contained all five treatments and understory vegetation was surveyed within 0.04-ha plots for each treatment. Results: While mastication alone and hand removal dramatically reduced the midstory vegetation, these treatments had little effect on understory richness compared with control. Prescribed fire after mastication increased native species richness by 150% (+6.0 species m2) compared with control. However, this also increased non-native species richness (+0.8 species m2) and shrub seedling density (+24.7 stems m2). Mastication followed by tilling resulted in increased non-native forb density (+0.7 stems m2). Conclusions: Mechanical mastication and hand removal treatments aided in reducing midstory fuels but did not increase understory plant diversity. The subsequent treatment of prescribed burning not only further reduced fire hazard, but also exposed mineral soil, which likely promoted native plant diversity. Some potential drawbacks to this treatment include an increase of non-native species and stimulation of shrub seed germination, which could alter ecosystem functions and compromise fire hazard reduction in the long-term. [ABSTRACT FROM AUTHOR]

In this article the author examines the risks faced by most Northern Spotted Owls (NSO) in fire-prone forests. The authors suggest that the development of conservation plans for NSO in such a landscape should consider a thorough evaluation of multiple threats and careful consideration of the consequences of management actions. They also cites a study in which the researchers underestimated the area of high-severity fire due to incorrect application of the Miller's method.

The article evaluates the effects of Fire and Fire Surrogate (FFS) forest stand treatments on vegetation structure, fuels and fire severity in western U.S. coniferous-forest. It states that the site replicated a framework for drawing broad generalizations about the effectiveness of prescribed fire and mechanical treatments on surface fuel loads. According to researchers, canopy cover was significantly lower than controls at three of five FFS sites with mechanical-only treatments. Based on the results, they concluded that mechanical plus fire, fire-only, and mechanical-only treatments using whole-tree harvest systems are all effective at reducing potential fire severity.

The article evaluates the effects of fuel reduction methods on forest vegetation structure and fuels in the U.S. The researchers evaluated the changes in vegetation and fuels from measurements taken before and after fuel reduction treatments at 12 Fire and Fire Surrogate (FFS) forest sites. Based on the results, they concluded that mechanical treatments were more effective at reducing overstory tree density and basal area. They also found out that prescribed fire treatments were more effective at killing seedlings, reducing surface woody fuels and elevating height to live crown.

Concerns about the use of genetically appropriate material in restoration often focus on questions of local adaptation. Many reciprocal transplant studies have demonstrated local adaptation in native plant species, but very few have examined how interspecific competition affects the expression of adaptive variation. Our study examined regional scales of adaptation between foothill and coastal populations of two California native bunchgrasses ( Elymus glaucus and Nassella pulchra). By combining competitive manipulations with reciprocal transplants, we examined the importance of the vegetation at a site as a selective factor in the process of local adaptation. By monitoring survival and reproduction of reciprocally transplanted populations over the course of 3 years, we also studied the effect of life history stage on the expression of local adaptation. For most of the fitness components we measured, local adaptation was detected and interspecific competition consistently amplified its expression. Expression of local adaptation was especially apparent in the more inbreeding species E. glaucus and suggests that with weaker gene flow, selection may be more effective in creating ecotypes within this species. Local adaptation was detected at all life history stages but was most strongly expressed in traits associated with adult reproduction and the viability of seeds produced by the transplants. Taken together, our results indicate that the importance of local adaptation will become more apparent in the later stages of a restoration project as the plants at a site begin to reproduce and as they experience greater interspecific competition from the maturing vegetation at the site. [ABSTRACT FROM AUTHOR]

Decimation of the California (U.S.A.) prairie has increased interest in the conservation and ecological restoration of grasslands composed of native perennial species. Widespread plantings are being attempted, and the potential for contaminating existing remnant populations with maladapted germplasm has led to concerns about the use of nonlocal seed sources. We evaluated regional patterns of isozyme and quantitative trait variation in the native perennial grass Nassella pulchra for the purpose of developing recommendations about the spatial scales over which seed can be translocated. Seed was collected from 10 remnant native populations. Progeny from all 10 populations were scored for isozyme variation with 11 stains, and progeny from 8 populations were planted in a common garden to evaluate variation for 11 quantitative (polygenic) traits. The correspondence between isozyme and quantitative trait variation and the relationships of both types of variation to geographic distance and climate were explored by Mantel test regressions. Populations were strongly differentiated for both isozymes and quantitative traits, but cluster analysis based on each type of data did not result in the same population groupings. This lack of congruence was further demonstrated by the nonsignificance of the regression of Hedrick's distances for isozymes on Mahalanobis distances for quantitative traits. Quantitative trait variation was strongly associated with climatic variables, whereas isozyme variation was not. This suggests that the relative importance of genetic drift and selection in shaping patterns of genetic differentiation may depend upon the type of trait evaluated. Quantitative traits are potentially better indicators of adaptation to regional and local environmental variation, and thus the usefulness of isozymes for making recommendations about the spatial scales over which seed of N. pulchra can be translocated may be limited. Unfortunately, obtaining data on patterns of quantitative trait variation is often time‐ and labor‐intensive. The close association of quantitative trait variation with regional climatic variables indicates that an index based on readily obtainable climatic information might aid restorationists in making rapid decisions about appropriate spatial scales for translocating native grasses. Compariciones de Isozimas y Rasgos Cuantitativos para Evaluar Patrones de Variación Genética en el Pasto‐Aguja Morado (Nasella pulchra) La disminución de la pradera de California (USA) ha incrementado el interés en la conservación y restauración ecológica de pastizales compuestos por especies perennes nativas. La dispersión amplia de siembras se ha intentado y el potencial de contaminación de poblaciones remanentes con germoplasmas mal adaptados ha generado preocupaciones sobre el uso de fuentes de semilla no locales. Evaluamos patrones regionales de variación de isozimas y variación cuantitativa de rasgos en el pasto perenne nativo Nasella pulchra con el propósito de desarrollar recomendaciones sobre las escalas espaciales en las cuales las semillas pueden ser movidas. Las semillas fueron colectadas a partir de 10 poblaciones remanentes nativas. Las progenies de las 10 poblaciones fueron evaluados para variación de isozima usando 11 colorantes y las progenies de 8 poblaciones fueron plantadas en un jardín común para evaluar la variación de 11 rasgos (poligenéticos) cuantitativos. La correspondencia entre variaciones de isozimas y de rasgos cuantitativos y las relaciones de ambos tipos de variación con distancia geográfica y clima fue explorada usando pruebas de regresión de Mantel. Sin embargo, los análisis de conjunto basados en cada tipo de datos no dió como resultado las mismas agrupaciones de poblaciones. Esta carencia de congruencia fue demonstrada más adelante por la no significancia de la regresión de distancias de Hendrick para isozimas en distancias Mahalanobis para rasgos cuantitativos. La variación en rasgos cuantitativos estuvo fuertemente asociada con las variables climáticas, mientras que la variación de isozimas no lo estuvo. Esto sugiere que la importancia relativa de la deriva génica y de la selección en patrones de forma de diferenciación genética pueden depender del tipo de rasgo evaluado. Los rasgos cuantitativos son potencialmente mejores indicadores de adaptación a la variación ambiental local y regional, por lo tanto la utildad de isozimas para hacer recomendaciones sobre escalas espaciales en las cuales las semillas de N. pulchra pueden ser movidas puede verse limitada. Desafortunadamente, la obtención de datos de patrones de variación de rasgos cuantitativos require frecuentemente de tiempo y esfuerzo intensivos. La asociación cercana de variación de rasgos cuantitativos con variables climáticas regionales indica que un indice basado en información climática accesible podría ayudar a los restauradores a tomar decisiones rápidas sobre escalas espaciales apropiadas para mover pastos nativos. [ABSTRACT FROM AUTHOR]

Interest in using native grass species for restoration is increasing, yet little is known about the ecology and genetics of native grass populations or the spatial scales over which seed can be transferred and successfully grown. The purpose of this study was to investigate the genetic structure within and among populations of Elymus glaucus in order to make some preliminary recommendations for the transfer and use of this species in revegetation and restoration projects. Twenty populations from California, Oregon, and Washington were analyzed for allozyme genotype at 20 loci, and patterns of variation within and among populations were determined. Allozyme variation at the species level was high, with 80% of the loci polymorphic and an average expected heterozygosity (an index of genetic diversity) of 0.194. All but two of the populations showed some level of polymorphism. A high degree of population differentiation was found, with 54.9% of the variation at allozyme loci partitioned among populations (F[SUBst] = 0.549). A lesser degree of genetic differentiation among closely spaced subpopulations within one of the populations was also demonstrated (F[SUBst] = 0.124). Self-pollination and the patchy natural distribution of the species both likely contribute to the low level of gene flow (Nm = 0.205) that was estimated. Zones developed for the transfer of seed of commercial conifer species may be inappropriate for transfer of E. glaucus germplasm because conifer species are characterized by high levels of gene flow. Limited gene flow in E. glaucus can facilitate the divergence of populations over relatively small spatial scales. This genetic differentiation can be due to random genetic drift, localized selective pressures, or both. In order to minimize the chances of planting poorly adapted germplasm, seed of E. glaucus may need to be collected in close proximity to the proposed restoration site. [ABSTRACT FROM AUTHOR]

Investigates the genetic consequences of a population bottleneck in Trifolium amoenum (Fabaceae) species. Alloxyme variation within populations; Genetic variation within the populations; Implication for reintroducing the species.