Sierra Nevada Mixed-conifer Forest Research Articles

Fire decreases arthropod abundance but increases diversity: Early and late season prescribed fire effects in a Sierra Nevada mixed-conifer forest

Prior to fire suppression in the 20th century, the mixed-conifer forests of the Sierra Nevada, California, U.S.A., historically burned in frequent fires that typically occurred during the late summer and early fall. Fire managers have been attempting to restore natural ecosystem processes through prescription burning, and have often favored burning during the fall in order to mimic historical fire regimes. Increasingly, however, prescription burning is also being done during the late spring and early summer in order to expand the window of opportunity for needed fuel reduction burning. The effect of prescribed fires outside of the historical fire season on forest arthropods is not known. The objective of this study was to compare the short-term effects of prescribed fires ignited in the early and late fire season on forest floor arthropods. Arthropod abundance and diversity were assessed using pitfall trapping in replicated burn units in Sequoia National Park, California. Overall, abundance of arthropods was lower in the burn treatments than in the unburned control. However, diversity tended to be greater in the burn treatments. Fire also altered the relative abundances of arthropod feeding guilds. No significant differences in arthropod community structure were found between early and late season burn treatments. Instead, changes in the arthropod community appeared to be driven largely by changes in fuel loading, vegetation, and habitat heterogeneity, all of which differed more between the burned and unburned treatments than between early and late season burn treatments.

Tree mortality from fire and bark beetles following early and late season prescribed fires in a Sierra Nevada mixed-conifer forest

Over the last century, fire exclusion in the forests of the Sierra Nevada has allowed surface fuels to accumulate and has led to increased tree density. Stand composition has also been altered as shade tolerant tree species crowd out shade intolerant species. To restore forest structure and reduce the risk of large, intense fires, managers have increasingly used prescription burning. Most fires prior to EuroAmerican settlement occurred during the late summer and early fall and most prescribed burning has taken place during the latter part of this period. Poor air quality and lack of suitable burn windows during the fall, however, have resulted in a need to conduct more prescription burning earlier in the season. Previous reports have suggested that burning during the time when trees are actively growing may increase mortality rates due to fine root damage and/or bark beetle activity. This study examines the effects of fire on tree mortality and bark beetle attacks under prescription burning during early and late season. Replicated early season burn, late season burn and unburned control plots were established in an old-growth mixed conifer forest in the Sierra Nevada that had not experienced a fire in over 120 years. Although prescribed burns resulted in significant mortality of particularly the smallest tree size classes, no difference between early and late season burns was detected. Direct mortality due to fire was associated with fire intensity. Secondary mortality due to bark beetles was not significantly correlated with fire intensity. The probability of bark beetle attack on pines did not differ between early and late season burns, while the probability of bark beetle attack on firs was greater following early season burns. Overall tree mortality appeared to be primarily the result of fire intensity rather than tree phenology at the time of the burns. Early season burns are generally conducted under higher fuel moisture conditions, leading to less fuel consumption and potentially less injury to trees. This reduction in fire severity may compensate for relatively modest increases in bark beetle attack probabilities on some tree species, ultimately resulting in a forest structure that differs little between early and late season prescribed burning treatments.

Long-term surface fuel accumulation in burned and unburned mixed-conifer forests of the Central and Southern Sierra Nevada, CA (USA)

After nearly a century of fire exclusion in many central and southern Sierra Nevada mixed-conifer forests, dead and down surface fuels have reached high levels without the recurring fires that consume the accumulated organic matter. The effects of prescribed fires used to reduce fuel loads and restore fire have been monitored in Yosemite, Sequoia, and Kings Canyon National Parks for over 30 years. Ten years following prescribed fire treatments in Sequoia and Kings Canyon, mean total fuel loads accumulated to 84 percent of pre-fire levels in ponderosa pine forests, 83 percent in white fir-mixed conifer, and 66 percent in giant sequoia-mixed conifer forests. Thirty-one years after burning, mean fuel load of fine and sound woody fuels increased in ponderosa pine research plots in Yosemite. Most fuel accumulation appeared to occur within the first decade after fire, however the post-fire fuel complex was different than that pre-fire. In areas that have remained unburned, surface fuel accumulation appears to be relatively slow and may indicate that decay rates have approached accumulation rates for the mixed-conifer forest types. This long-term information has important implications for fire management planning, including scheduling fuel hazard reduction and subsequent maintenance treatments.

Wildfire burn patterns and riparian vegetation response along two northern Sierra Nevada streams

Riparian vegetation plays an integral role in the ecology of the streams it borders, and in many western US forests, is subjected to frequent wildfire disturbances. Many questions concerning the role of natural fire in the dynamics of riparian zone vegetation remain unanswered. This case study explores the relationships between wildfire burn patterns, stream channel topography, and the short-term response of riparian vegetation to fire along two creeks in the northern Sierra Nevada mixed-conifer forest. Post-fire sampling along 60, 3 m wide transects across riparian zones was used to document the topography, species distribution, sprouting response, and seedling recruitment 1 year after the Lookout fire in the Plumas National Forest, CA. Our results indicate that larger riparian zones acted as natural fire breaks, limiting the progression of the predominantly backing fire downhill toward the stream. On Fourth Water creek's steeper first terraces, where crown fires occurred, the percentage of burned plants that sprouted was higher than in the less-severely burned and more extensive first terraces of Third Water creek (93% versus 33%, P < 0.05). Total seedling recruitment was higher along Fourth Water creek (69 versus 35 seedlings, P < 0.05), while plant regeneration along Third Water creek was primarily vegetative. Along Fourth Water creek, the percent of burned hardwoods that sprouted increased with proximity to the water's edge from 33% on the slope above the riparian zone to 95% on the gravel bar, suggesting that moisture content plays a role in riparian species response to fire. An influx of white fir ( Abies concolor Gordon & Glend. (Lindl.)) seedlings on the second terraces of Third Water creek may indicate a shift in species composition if future fires are suppressed and regeneration trends do not change significantly in the next few years. These results contribute to the limited research on natural fire in riparian zones, and can inform management strategies designed to restore and maintain riparian vegetation in the fire-prone forests of the Sierra Nevada.

Fire and fire surrogate treatment effects on leaf litter arthropods in a western Sierra Nevada mixed-conifer forest

Frequent, low-intensity fires were a common feature of Sierran forest ecosystems, but suppression policies over the past century have left many forests at risk for catastrophic wildfires. Recent policies highlight the use of prescribed burning or harvesting as fire risk reduction tools, but few studies have investigated the impacts of these management practices on the leaf litter fauna of Sierran forests. This study examines how three fire and “fire surrogate” treatments, prescribed burning, overstory thinning with understory mastication, and combined thinning and burning, impact diversity and abundance of Coleoptera and other leaf litter arthropods. Pitfall trapping was used to collect litter arthropods before and immediately after treatments in replicated forest compartments. The diverse Coleoptera assemblage was dominated by only a few common species, with many rare species represented by only one or two individuals. Rank–abundance diagrams indicated that much of the change in the beetle assemblage due to the treatments was a result of changes in the numbers of rare species. Indicator species analysis showed several species closely allied with the treated compartments, but few with the untreated controls. Both NMS and CCA ordination show considerable change in overall assemblage structure on compartments treated with fire, but less change in the thinned compartments. Coleoptera species richness was slightly higher in burned compartments. Some common beetle species, families of beetles, and other common groups such as ants and spiders showed changes in abundance due to the treatments, but the changes were taxon-specific and showed no general pattern. Overall impacts of the treatments appear to be moderate, and the increased habitat heterogeneity at the compartment level may provide additional habitat for many rare species to coexist. We conclude that the use of fire and fire surrogate treatments in Sierran mixed-conifer forests is justified from the standpoint of their effects on leaf litter arthropods, but the history of management at the site and the scale of treatments must be carefully considered.

Comparative tree growth efficiency in Sierra Nevada mixed-conifer forests

Tree growth efficiency, or volume growth increment per unit leaf area, was calculated in multiaged mixed-conifer stands in the Sierra Nevada, California. Five conifer species with a range of tolerance to shade were sampled over a wide range of canopy positions and ages to detect species-specific patterns of growth efficiency (GE). All species showed non-linear, sigmoidal trends of volume increment with increasing leaf area. This pattern indicates slow increase in GE in shaded understory conditions and a peak in GE at intermediate tree size in mid-canopy positions. Larger trees showed lower GE than intermediate size trees. Species with lower shade tolerance had greater GE than species with greater tolerance. To test the effect of local light environment on tree volume growth, we used a light model to calculate the growing season average of absorbed light for individual trees. Models of volume increment as a function of weighted leaf area showed increased GE for small trees, indicating that the slow increase in volume increment in smaller understory trees was an effect of limited light conditions. Patterns of growth efficiency in multiaged mixed-species stands can be used to predict growth patterns of sub-canopy trees and judge relative shade tolerance.

Silvicultural and reserve impacts on potential fire behavior and forest conservation: Twenty-five years of experience from Sierra Nevada mixed conifer forests

Most administrators and ecologists agree that reducing the levels of hazardous fuels on forests is essential to restore healthy watersheds and protect adjacent human communities. The current debate over the appropriateness, technique, and timing of treatments utilized to restore vegetation structure and composition is currently on-going at local, state, and national levels. To provide information for these forums, the efficacy of seven traditional silvicultural systems and two types of reserves used in the Sierra Nevada mixed conifer forests is evaluated in terms of vegetation structure, fuel bed characteristics, modeled fire behavior, and potential wildfire related mortality. The systems include old-growth reserve, young-growth reserve, thinning from below, individual tree selection, overstory removal, and four types of plantations. These are the most commonly used silvicultural systems and reserves on federal, state, and private lands in the western United States. Each silvicultural system or reserve had three replicates and varied in size from 15 to 25 ha; a systematic design of plots was used to collect tree and fuel information. The majority of the traditional silvicultural systems examined in this work (all plantation treatments, overstory removal, individual tree selection) did not effectively reduce potential fire behavior and effects, especially wildfire induced tree mortality at high and extreme fire weather conditions. Overall, thinning from below, and old-growth and young-growth reserves were more effective at reducing predicted tree mortality.

Fuel treatment effects on snags and coarse woody debris in a Sierra Nevada mixed conifer forest

Snags and coarse woody debris are important elements of the structure and function of mixed conifer forests in the Sierra Nevada. In this paper, we report on the impacts of several replicated fuel treatments including, prescribed fire, commercial thinning (crown thinning and thinning from below) followed by rotary mastication of understory trees, mechanical followed by prescribed fire, and control, on snag and CWD quantity and structure. Post-treatment, the density of snags greater than 15 cm DBH in decay class 1 significantly increased in fire only and mechanical plus fire treatments compared with mechanical only and control treatments. Snag volumes (m 3 ha −1) were not significantly different between treatments for all decay classes. CWD (density, percent cover, volume) in decay classes 1 and 2 was not significantly altered by any treatment when aggregated across all diameter classes. Volume of CWD in decay class 3 was significantly reduced in the fire only treatment when compared to controls. Density and volume of CWD in class 4 was significantly reduced in mechanical plus fire and fire only treatments when compared with the controls and mechanical only treatments. Retention of large CWD levels may benefit some wildlife species short-term but increases in fire hazards and increased difficulties in fire control are the negative consequence. High overall fuel loads also increase the probability of snag and CWD consumption when an area inevitably burns. The influences of altering snag and CWD characteristics should be analyzed in the context of long-term forest management goals, including the reintroduction of fire as an ecosystem process and creation of forest structures that can incorporate wildfire without tree mortality outside a desired range.

Fuel reduction and coarse woody debris dynamics with early season and late season prescribed fire in a Sierra Nevada mixed conifer forest

Fire exclusion has led to an unnatural accumulation and greater spatial continuity of organic material on the ground in many forests. This material serves both as potential fuel for forest fires and habitat for a large array of forest species. Managers must balance fuel reduction to reduce wildfire hazard with fuel retention targets to maintain other forest functions. This study reports fuel consumption and changes to coarse woody debris attributes with prescribed burns ignited under different fuel moisture conditions. Replicated early season burn, late season burn, and unburned control plots were established in old-growth mixed conifer forest in Sequoia National Park that had not experienced fire for more than 120 years. Early season burns were ignited during June 2002 when fuels were relatively moist, and late season burns were ignited during September/October 2001 when fuels were dry. Fuel loading and coarse woody debris abundance, cover, volume, and mass were evaluated prior to and after the burns. While both types of burns reduced fuel loading, early season burns consumed significantly less of the total dead and down organic matter than late season burns (67% versus 88%). This difference in fuel consumption between burning treatments was significant for most all woody fuel components evaluated, plus the litter and duff layers. Many logs were not entirely consumed – therefore the number of logs was not significantly changed by fire – but burning did reduce log length, cover, volume, and mass. Log cover, volume, and mass were reduced to a lesser extent by early season burns than late season burns, as a result of higher wood moisture levels. Early season burns also spread over less of the ground surface within the burn perimeter (73%) than late season burns (88%), and were significantly patchier. Organic material remaining after a fire can dam sediments and reduce erosion, while unburned patches may help mitigate the impact of fire on fire-sensitive species by creating refugia from which these species can recolonize burned areas. Early season burns may be an effective means of moderating potential ecosystem damage when treating heavy and/or continuous fuels resulting from long periods of fire exclusion, if burning during this season is not detrimental to other forest functions.

Characterizing the light environment in Sierra Nevada mixed-conifer forests using a spatially explicit light model

The spatially explicit light model tRAYci was calibrated to conditions in multi-aged Sierra Nevada mixed-conifer forests. To reflect conditions that are important to growth and regeneration of this forest type, we sampled a variety of managed mature stands with multiple canopy layers and cohorts. Calibration of the light model included determining leaf area density for individual species with the use of leaf area – sapwood area prediction equations. Prediction equations differed between species and could be improved using site index. The light model predicted point measurements from hemispherical photographs well over a range of 27%–63% light. Simplifying the crown representation in the tRAYci model to average values for species and canopy strata resulted in little reduction in model performance and makes the model more useful to applications with lower sampling intensity. Vertical light profiles in managed mixed-conifer stands could be divided into homogeneous, sigmiodal, and continuous gradients, depending on stand structure and foliage distribution. Concentration of leaf area in the upper canopy concentrates light resources on dominant trees in continuous canopies. Irregular canopies of multiaged stands, however, provide more light resources to mid-size trees and could support growth of shade-intolerant species. Knowledge of the vertical distribution of light intensity in connection with stand structural information can guide regulation of irregular stand structures to meet forest management objectives.

Fire in Sierra Nevada Forests: A Photographic Interpretation of Ecological Change since 1849. By George E. Gruell. Missoula, Mont.: Mountain Press, 2001. xv + 238 pp. Illustrations, plates, glossary, bibliography, index. Paper $20.00

Previous articleNext article No AccessBook ReviewsFire in Sierra Nevada Forests: A Photographic Interpretation of Ecological Change since 1849. By George E. Gruell. Missoula, Mont.: Mountain Press, 2001. xv + 238 pp. Illustrations, plates, glossary, bibliography, index. Paper $20.00James FeldmanJames Feldman University of Wisconsin-Madison Search for more articles by this author PDFPDF PLUS Add to favoritesDownload CitationTrack CitationsPermissionsReprints Share onFacebookTwitterLinkedInRedditEmail SectionsMoreDetailsFiguresReferencesCited by Volume 8, Number 1January 2003 Published for the American Society for Environmental History and the Forest History Society Views: 1Total views on this site Citations: 2Citations are reported from Crossref Article DOIhttps://doi.org/10.2307/3985988 Views: 1Total views on this site Citations: 2Citations are reported from Crossref © 2003 American Society for Environmental History and Forest History SocietyPDF download Crossref reports the following articles citing this article:Gabrielle F. S. Boisramé, Timothy J. Brown, Dominique M. Bachelet Trends in western USA fire fuels using historical data and modeling, Fire Ecology 18, no.11 (Jun 2022).https://doi.org/10.1186/s42408-022-00129-4Asha Paudel, Michelle Coppoletta, Kyle Merriam, Scott H. Markwith Persistent composition legacy and rapid structural change following successive fires in Sierra Nevada mixed conifer forests, Forest Ecology and Management 509 (Apr 2022): 120079.https://doi.org/10.1016/j.foreco.2022.120079

Modeling the Effects of Fire Management Alternatives on Sierra Nevada Mixed-Conifer Forests

Decades of fire exclusion have led to hazardous fuel accumulations and the deterioration of fire-dependent ecosystems, particularly in the American West. Managers are striving to return the ecological role of fire to many ecosystems and would benefit from a much better understanding of how forest structure and composition might change if fires were reintroduced. We used a forest gap model, developed for forests in the Sierra Nevada, California, USA, that integrates climate, fire, and forest dynamics to investigate forest response to changes in the fire regime. The model simulates a spatially heterogeneous fuel bed that is responsive to changes in forest condition, making it well suited for examining alternative management approaches for restoring Sierra Nevada forests after a century of fire exclusion. Presuppression forest basal area, species composition, and spatial autocorrelation structure were restored quickly, if simulated disturbances that caused substantial tree mortality were reintroduced. Simulations of harvest induced the highest levels of mortality and, thus, most effectively restored forest structure and composition. However, prescribed fires were just as effective in restoring forest structure and composition if they were sufficiently severe.

Evaluation of the effects of silvicultural and fuels treatments on potential fire behaviour in Sierra Nevada mixed-conifer forests

Fire suppression has increased fuel loads and fuel continuity in mixed-conifer ecosystems, resulting in forest structures that are vulnerable to catastrophic fire. This paper models fire behaviour in a mixed-conifer forest and investigates how silvicultural and fuels treatments affect potential fire behaviour. The computer program FARSITE was used to spatially and temporally model fire growth and behaviour. Fire modelling was performed in the North Crane Creek watershed of Yosemite National Park. Treatments were simulated by adjusting fuel (total load, load-by-size class, depth), height-to-live crown base, tree height, and crown density parameters. Treatments modeled included prescribed burn, pile and burn, cut and scatter, thinning and biomass, thinning and biomass followed by prescribed burn, and salvage or group selection harvest with and without slash and landscape-level fuel treatment. The prescribed burn, thinning and biomassing followed by prescribed burn, and salvage or group selection with slash and landscape fuel treatments resulted in the lowest average fireline intensities, heat per unit area, rate of spread, area burned, and scorch heights. Cut and scatter, salvage or group selection treatments that do not treat slash fuels resulted in fire behaviour that is more extreme than the untreated forest. Restoration of mixed-conifer ecosystems must include an examination of how proposed treatments affect fuel structures. Combinations of prescribed fire and/or mechanical treatments can be used to reduce wildfire hazard.

Fire behavior, fuel consumption, and forest-floor changes following prescribed understory fires in Sierra Nevada mixed conifer forests

We quantified fire behavior, fuel consumption, and forest-floor responses to understory fires prescribed to cover a wide range and variable combination of fuel loadings, fuel moisture levels, and weather conditions in Sierra Nevada mixed conifer forests. At each of three locations, five burns were conducted under each of four different prescriptions (n = 60). Fuel loads ranged from 75–102 Mg/ha on an open site of low productivity to 125–177 Mg/ha on more productive sites. Total fuel consumption ranged from 15% in early spring burns to 92% in early fall burns. Organic horizons (litter and duff) composed 62–84% of the total fuel load. Consumption of the organic horizons ranged from 11 to 94%. Lower duff (Oa horizon) moisture content was 135% in the former and 15% in the latter. Consumption of the organic horizon was significantly correlated with lower duff moisture content (r2 = 0.51). More than 72% of all fuel was consumed after the flaming front had passed over an area. Postburn mineral soil exposure was strongly influenced by fuel consumption, season of burn, and characteristics of the site. In the late spring burns at one site, bare ground exposure was 3% before fire, 60% two months after fire, and 8% thirteen months after fire. There were slight to nonexistent increases in bare ground exposure from burns prescribed during early spring or late autumn (i.e., burns of low to moderate consumption).

Foraging and Habitat Relationships of Insect-Gleaning Birds in a Sierra Nevada Mixed-Conifer Forest

-Foraging habits and relative abundances of 12 birds comprising the insect-gleaning guild in a Sierran mixed-conifer forest were studied during two breeding seasons to determine: (1) foraging habitat preferences, (2) the extent to which species differ in their use of various components of the foraging niche, (3) patterns of relative abundance vs. niche breadth, and (4) differences between resident and migrant species. Comparisons of proportional availability and bird use of foliage height classes and tree species showed that tree species and, to a lesser extent, heights were used selectively by the guild. Incense-cedar (Calocedrus decurrens) was consistently avoided by all species; other tree species were generally used in a complementary manner by different birds. Of four measured components of foraging niche, the use of foraging site (consisting of air or tree part) showed the greatest difference between species, followed by tree species, foraging techniques, and foraging height. We found no correlation between niche breadth and species abundance for all guild members; however, a significant positive correlation existed for the five resident species. Resident and migrant species groups showed few fundamental differences in foraging patterns, except that migrants tended to use a greater proportion of deciduous foliage than residents. Our results suggest that to provide for this guild, land managers should maintain natural levels of tree species diversity in the mixed-conifer forest type. Many studies have shown that syntopic insectivorous birds differ in their methods of foraging. In forested habitats, birds tend to use different foraging techniques, foraging sites, tree species, and heights. The relative importance of these foraging niche components in distinguishing species has received less attention. Differences in importance of niche components reported in previous studies may reflect either real differences that exist in different habitats and geographic areas (Balda 1969), or incomplete analysis of all potentially relevant factors (Holmes and Robinson 1981). In particular, the importance of differential plant species use has not been fully appreciated (Holmes and Robinson 1981). Additional information is needed before sound generalizations can be made regarding patterns of differentiation among insect-gleaning birds. In managed forests, plant species composition and vegetation structure may be altered by logging, other silvicultural activities, and disruption of natural fire regimes (Kilgore 1971, Franzreb and Ohmart 1978, Szaro and Balda 1979). Identification of habitat preferences of forest birds can suggest recommendations to mitigate impacts of manipulation. The mixed-conifer forest of the Sierra Nevada, California, supports a large number of arboreal insectivorous birds (Verner and Boss 1980). We studied foraging substrate preferences, foraging behaviors and relative abundances of the 12 most common members of an insect-gleaning guild. We sought to: (1) compare the guild's use of tree species and vertical foliage layers with the availabilities of these habitat components; (2) describe and compare foraging sites and techniques used by each species; (3) evaluate the relative importances of foraging heights, tree species use, foraging sites, and foraging techniques in differentiating species ecologically; (4) determine if resident and migrant species differed in their foraging and abundance patterns; and (5) suggest management recommendations to mitigate the effects of habitat manipulation on members of the insect-gleaning guild.