Old-growth Mixed-conifer Forest Research Articles

Fuel dynamics after reintroduced fire in an old-growth Sierra Nevada mixed-conifer forest

BackgroundSurface fuel loadings are some of the most important factors contributing to fire intensity and fire spread. In old-growth forests where fire has been long excluded, surface fuel loadings can be high and can include woody debris ≥100 cm in diameter. We assessed surface fuel loadings in a long-unburned old-growth mixed-conifer forest in Yosemite National Park, California, USA, and assessed fuel consumption from a management-ignited fire set to control the progression of the 2013 Rim Fire. Specifically, we characterized the distribution and heterogeneity of pre-fire fuel loadings, both along transects and contained in duff mounds around large trees. We compared surface fuel consumption to that predicted by the standard First Order Fire Effects Model (FOFEM) based on pre-fire fuel loadings and fuel moistures. We also assessed the relationship between tree basal area—calculated for two different spatial neighborhood scales—and pre-fire fuel loadings.ResultsPre-fire total surface fuel loading averaged 192 Mg ha−1 and was reduced by 79% by the fire to 41 Mg ha−1 immediately after fire. Most fuel components were reduced by 87% to 90% by the fire, with the exception of coarse woody debris (CWD), which was reduced by 60%. Litter depth in duff mounds were within 1 SD of plot means, but duff biomass for the largest trees (>150 cm diameter at breast height [DBH]) exceeded plot background levels. Overstory basal area generally had significant positive relationships with pre-fire fuel loadings of litter, duff, 1-hour, and 10-hour fuels, but the strength of the relationships differed between overstory components (live, dead, all [live and dead], species), and negative relationships were observed between live Pinus lambertiana Douglas basal area and CWD. FOFEM over-predicted rotten CWD consumption and under-predicted duff consumption.ConclusionsSurface fuel loadings were characterized by heterogeneity and the presence of large pieces. This heterogeneity likely contributed to differential fire behavior at small scales and heterogeneity in the post-fire environment. The reductions in fuel loadings at our research site were in line with ecological restoration objectives; thus, ecologically restorative burning during fire suppression is possible.

Floating neighborhoods reveal contribution of individual trees to high sub-stand scale heterogeneity

Abstract Forest structure is typically characterized using approaches that fail to capture the sub-stand heterogeneity reflecting the processes shaping stand structure at neighborhood patch scales. We propose a focal-tree based approach that flexibly scales across natural tree neighborhoods and demonstrate it by quantifying small-scale structural heterogeneity using diameter distributions across multiple scales in an old-growth mixed conifer forest in western Oregon exhibiting a rotated sigmoid shape. Tree-centered delineation of neighborhoods via Delaunay triangulation (spatial tessellation) was used to connect natural tree neighbors. At the First-order neighborhood scale, each tree was connected to an average of 6 neighbors in 75 m2 of convex hull area, which increased to 118 neighbors covering 2542 m2 by the Fifth-order scale. Agglomerative clustering identified the ten most prevalent sub-stand diameter distribution types (DDTs). At the smallest scale, these ten DDTs were characterized by four different distribution shapes (negative exponential, concave, rotated sigmoid, and unimodal) and varied considerably in the abundance of trees in different size classes and species composition (Douglas-fir composing the larger trees and western hemlock and western redcedar the smaller). On average, each individual tree contributed to over 3 different DDTs at any given scale. Small-scale tree neighborhoods were tremendously heterogeneous, such that the stand-level diameter distribution represented an aggregation across extremely disparate sub-stand distributions. As expected, these distributions exhibited spatial smoothing (i.e., increasing homogeneity with increasing scale) as small, heterogeneous tree neighborhoods were expanded into larger neighborhoods with more similar average composition, which converged on a rotated-sigmoid shape by the Fifth-order scale. Structural complexity in the demonstration stand likely owes more to shade-tolerant ingrowth below residual trees than to gap processes. The diversity of structural contribution (e.g., the number of DDTs to which a tree contributes) may be a more informative measure of structural complexity than indices based on a fixed spatial perspective. The floating neighborhood delineation and scaling approach allows for the identification of natural neighborhoods, permits the assessment of multiple conditions for any given tree, and provides of a logical mechanism for exploring scale-dependent ecological phenomena.

Post-fire morel (Morchella) mushroom abundance, spatial structure, and harvest sustainability

Morel mushrooms are globally distributed, socially and economically important reproductive structures produced by fungi of the genus Morchella. Morels are highly prized edible mushrooms and significant harvests are collected throughout their range, especially in the first year after fire, when some morel species fruit prolifically. Few studies have quantified post-fire morel mushroom abundance, despite their widespread human use. The purpose of this study is to provide the first ever estimate of post-fire morel mushroom abundance in Sierra Nevada mixed-conifer forest. Specifically, we estimate the abundance and spatial variability of morel mushrooms across an intensively mapped and measured forest research site during the first growing season following fire.We conducted this study in the Yosemite Forest Dynamics Plot, a long-term forest research installation located in old-growth mixed-conifer forest of Yosemite National Park, California, USA. We surveyed for morel mushrooms in n=1119 contiguous circular 3.14m2 plots arranged along 2240m of permanently marked, georeferenced transects. We characterized the spatial correlation of morel plots using k category (multicolor) join count statistics. We analyzed spatial correlations at interplot distances up to 9.0m.There were 595 morel mushrooms in the 1119 plots we measured. Mushrooms occurred in 17.8% of plots. We estimated a mean standing crop of 1693morelsha−1 (SE=155.4morelsha−1). Morel-occupied plots were strongly and significantly spatially autocorrelated. Most of the spatial correlation among morel-occupied plots was apparent at scales up to 7.0m, and was strongest at scales <3.0m.This study is one of only four that provide unbiased estimates of post-fire morel abundance. Our morel abundance estimates are generally consistent with prior work in high-latitude North American conifer forests. The strong spatial autocorrelation of morel-occupied microsites at scales <7m indicates that key factors controlling post-fire morel productivity are heterogeneous at small spatial scales. We propose a simple conceptual model to explain this spatial structure that includes spatial variability of pre-fire Morchella colonies; pre-fire vegetation community and fuelbed; fire behavior, intensity, and effects; and soil moisture and temperature. Relatively liberal harvest limits for recreational and subsistence harvesters appear appropriate and sustainable, at least for coniferous forests in the first year following fire. However, intensive commercial harvest, in jurisdictions where it is allowed, may warrant monitoring to assess potential impacts to long-term morel productivity, conflict with recreational harvesters, and non-target effects.

Long-term demographic trends in a fire-suppressed mixed-conifer forest

In the western United States, forests are experiencing novel environmental conditions related to a changing climate and a suppression of the historical fire regime. Mixed-conifer forests, considered resilient to disturbance due to their heterogeneity in structure and composition, appear to be shifting to a more homogeneous state, but the timescale of these shifts is not well understood. Our objective was to assess the effects of climate and fire suppression on stand dynamics and demographic rates of an old-growth mixed-conifer forest in the Sierra Nevada. We used a Bayesian hierarchical analysis to quantify species and community rates of recruitment, growth, and mortality. Despite a warming climate, we found that stand density, basal area, and carbon have increased over 56 years. Fir recruitment and growth significantly exceeded the community-level median rates, whereas pine recruitment and growth was significantly lower than the community-level median rates. Shifts in species composition from a well-mixed stand to a more dense fir-dominated stand appear to be driven by low growth and recruitment rates of pines relative to firs. In forests such as these with consistent and relatively low mortality rates, we recommend that restoration and management activities be focused on promoting pine recruitment and growth.

A forest reconstruction model to assess changes to Sierra Nevada mixed-conifer forest during the fire suppression era

Fire suppression has resulted in dramatic changes to species composition and structural diversity in the Sierra Nevada mixed-conifer forests. We need a better understanding of how these forests have changed during the fire suppression era, but empirical historical datasets are rare and methodologies for developing new historical reference information are subject to limitations. We sought to develop historical reference information for the Yosemite Forest Dynamics Plot (YFDP), a research plot located in an old-growth mixed-conifer forest in Yosemite National Park. We performed a dendrochronologial fire history analysis to characterize the historical fire regime of the YFDP, resulting in an estimated pre-1900 point fire return interval of 29.5years. We then developed two different forest reconstruction models to estimate structural and compositional forest changes since 1900, the year the last widespread fire burned the YFDP, to the present. We explored the use of two different tree growth models—a regionally parameterized competition-dependent model and a parsimonious site-specific model—as well as a decay model based on published estimates of wood decay rates. The competition-dependent growth model predicted slightly higher stem densities (175 trees ha−1 vs. 112 trees ha−1 in 1900) and slightly lower basal area (20.9m2ha−1 vs. 24.1m2ha−1 in 1900) than the site-specific growth model. Predictions about dead trees, especially large diameter sugar pines, are potentially inaccurate, both in this study and other reconstruction studies in the Sierra Nevada, due to a lack of size-specific snag and log decay rate data. Our study highlights the need for more detailed decay data for Sierra Nevada mixed-conifer forests. While reconstruction models are constrained by the data used to parameterize them, they can still produce estimates of historical conditions that are useful for understanding the direction and magnitude of forest change, as well as for planning forest management. Our analysis demonstrates dramatic changes in forest conditions since fire-exclusion—a clear ecological basis for restoration. We encourage managers to focus on restoring key ecological processes that maintain ecosystem structure and function, in this case fire, rather than attempting tree-for-tree recreations of historical structure without reintroducing fire.

Topography, Fuels, and Fire Exclusion Drive Fire Severity of the Rim Fire in an Old-Growth Mixed-Conifer Forest, Yosemite National Park, USA

The number of large, high-severity fires has increased in the western United States over the past 30 years due to climate change and increasing tree density from fire suppression. Fuel quantity, topography, and weather during a burn control fire severity, and the relative contributions of these controls in mixed-severity fires in mountainous terrain are poorly understood. In 2013, the Rim Fire burned a previously studied 2125 ha area of mixed-conifer forest in Yosemite National Park. Data from 84 plots sampled in 2002 revealed increases in tree density, basal area, and fuel buildup since 1899 due to fire exclusion. A dendroecological fire history and reconstruction of forest structure in 1899 showed that this area historically experienced frequent, low-severity fire. In contrast with this region’s historical fire regime, burn severity from Landsat imagery showed that this area burned at mixed-severity in the Rim Fire, with 13% of plots classified as unchanged, 31% low severity, 32% moderate severity, and 24% high severity. A random forest model was used to identify the controls of fire severity in this portion of the Rim Fire, using daily area burned, daily fire weather, and fuels and vegetation data for the surface and canopy. Topography, tree species composition, and cover of forbs and shrubs best explained the fire severity. As an example of a re-entry burn, this study demonstrates how fire exclusion alters fire–vegetation interactions, leading to uncharacteristically severe burns and potentially new fire-vegetation dynamics.

The influence of prefire tree growth and crown condition on postfire mortality of sugar pine following prescribed fire in Sequoia National Park

Tree mortality is a vital component of forest management in the context of prescribed fires; however, few studies have examined the effect of prefire tree health on postfire mortality. This is especially relevant for sugar pine (Pinus lambertiana Douglas), a species experiencing population declines due to a suite of anthropogenic factors. Using data from an old-growth mixed-conifer forest in Sequoia National Park, we evaluated the effects of fire, tree size, prefire radial growth, and crown condition on postfire mortality. Models based only on tree size and measures of fire damage were compared with models that included tree size, fire damage, and prefire tree health (e.g., measures of prefire tree radial growth or crown condition). Immediately following the fire, the inclusion of different metrics of prefire tree health produced variable improvements over the models that included only tree size and measures of fire damage, as models that included measures of crown condition performed better than fire-only models, but models that included measures of prefire radial growth did not perform better. However, 5 years following the fire, sugar pine mortality was best predicted by models that included measures of both fire damage and prefire tree health, specifically, diameter at breast height (DBH, 1.37 m), crown scorch, 30-year mean growth, and the number of sharp declines in growth over a 30-year period. This suggests that factors that influence prefire tree health (e.g., drought, competition, pathogens, etc.) may partially determine postfire mortality, especially when accounting for delayed mortality following fire.

Canopy arthropod responses to thinning and burning treatments in old-growth mixed-conifer forest in the Sierra Nevada, California

We compared canopy arthropod responses to common fuels reduction treatments at Teakettle Experimental Forest in the south-central Sierra Nevada of California. We sampled arthropod communities among four dominant overstory conifer species and three dominant understory angiosperm species before and after overstory or understory thinning or no thinning treatments followed by burning or no burning treatments. Arthropods were sampled in overstory trees by climbing and bagging foliage-bearing branches and counting all arthropods by taxon in each sample. Understory plants were sampled similarly from the ground. Arthropod assemblages showed significant differences among tree species and seasons, but not among treatment combinations. Taxa showing significant differences in abundance among plant species likely reflected differences in foliage quality or other host-associated conditions among plant species. Some arthropods showed significant value as indicator species. Overall, our results indicated that the restoration treatments recommended for Sierra Nevada mixed-conifer forests have little effect on associated canopy arthropods. However, given the significant differences in arthropod assemblages among plant species, restoration treatments should ensure that the full range of plant species characterizing these forests is maintained in order to protect their associated arthropods.

Contrasting Spatial Patterns in Active-Fire and Fire-Suppressed Mediterranean Climate Old-Growth Mixed Conifer Forests

In Mediterranean environments in western North America, historic fire regimes in frequent-fire conifer forests are highly variable both temporally and spatially. This complexity influenced forest structure and spatial patterns, but some of this diversity has been lost due to anthropogenic disruption of ecosystem processes, including fire. Information from reference forest sites can help management efforts to restore forests conditions that may be more resilient to future changes in disturbance regimes and climate. In this study, we characterize tree spatial patterns using four-ha stem maps from four old-growth, Jeffrey pine-mixed conifer forests, two with active-fire regimes in northwestern Mexico and two that experienced fire exclusion in the southern Sierra Nevada. Most of the trees were in patches, averaging six to 11 trees per patch at 0.007 to 0.014 ha−1, and occupied 27–46% of the study areas. Average canopy gap sizes (0.04 ha) covering 11–20% of the area were not significantly different among sites. The putative main effects of fire exclusion were higher densities of single trees in smaller size classes, larger proportion of trees (≥56%) in large patches (≥10 trees), and decreases in spatial complexity. While a homogenization of forest structure has been a typical result from fire exclusion, some similarities in patch, single tree, and gap attributes were maintained at these sites. These within-stand descriptions provide spatially relevant benchmarks from which to manage for structural heterogeneity in frequent-fire forest types.

Long-term effects of prescribed fire on mixed conifer forest structure in the Sierra Nevada, California

The capacity of prescribed fire to restore forest conditions is often judged by changes in forest structure within a few years following burning. However, prescribed fire might have longer-term effects on forest structure, potentially changing treatment assessments. We examined annual changes in forest structure in five 1 ha old-growth plots immediately before prescribed fire and up to eight years after fire at Sequoia National Park, California. Fire-induced declines in stem density (67% average decrease at eight years post-fire) were nonlinear, taking up to eight years to reach a presumed asymptote. Declines in live stem biomass were also nonlinear, but smaller in magnitude (32% average decrease at eight years post-fire) as most large trees survived the fires. The preferential survival of large trees following fire resulted in significant shifts in stem diameter distributions. Mortality rates remained significantly above background rates up to six years after the fires. Prescribed fire did not have a large influence on the representation of dominant species. Fire-caused mortality appeared to be spatially random, and therefore did not generally alter heterogeneous tree spatial patterns. Our results suggest that prescribed fire can bring about substantial changes to forest structure in old-growth mixed conifer forests in the Sierra Nevada, but that long-term observations are needed to fully describe some measures of fire effects.

Fire regimes, forest change, and self‐organization in an old‐growth mixed‐conifer forest, Yosemite National Park, USA

Fire is recognized as a keystone process in dry mixed-conifer forests that have been altered by decades of fire suppression, Restoration of fire disturbance to these forests is a guiding principle of resource management in the U.S. National Park Service. Policy implementation is often hindered by a poor understanding of forest conditions before fire exclusion, the characteristics of forest changes since excluding fire, and the influence of topographic or self-organizing controls on forest structure. In this study the spatial and temporal characteristics of fire regimes and forest structure are reconstructed in a 2125-ha mixed-conifer forest. Forests were multi-aged, burned frequently at low severity and fire-return interval, and forest structure did not vary with slope aspect, elevation, or slope position. Fire exclusion has caused an increase in forest density and basal area and a compositional shift to shade-tolerant and fire-intolerant species. The median point fire-return interval and extent of a fire was 10 yr and 115 ha, respectively. The pre-Euro-American settlement fire rotation of 13 yr increased to 378 yr after 1905. The position of fire scars within tree rings indicates that 79% of fires burned in the midsummer to fall period. The spatial pattern of burns exhibited self-organizing behavior. Area burned was 10-fold greater when an area had not been burned by the previous fire. Fires were frequent and widespread, but patches of similar aged trees were < 0.2 ha, suggesting small fire-caused canopy openings. Managers need to apply multiple burns at short intervals for a sustained period to reduce surface fuels and create small canopy openings characteristic of the reference forest. By coupling explicit reference conditions with consideration of current conditions and projected climate change, management activities can balance restoration and risk management.

Structure and composition of corticolous epiphyte communities in a Sierra Nevada old-growth mixed-conifer forest

Fifty white fir, red fir, incense cedar, Jeffrey pine and sugar pine were surveyed for corticolous bryophytes and macrolichens in the Teakettle Experimental Forest. Epiphyte abundances were estimated by percent cover in 5 m strata from ground-level to tree-tops. Gradients of bark pH within tree species and stand-level vapor pressure deficit (VPD) were also measured. Mosses had a strong positive association with white fir and proximity to perennial water. Clustering of trees in macrolichen space resulted in four significant groups: white fir, red fir, incense cedar, and Pinus. The lichens Ahtiana sphaerosporella and Letharia indicated the red fir group, Hypogymnia imshaugii and Melanelia elegantula the white fir group, and Nodobryoria oregana the Pinus group. Two nitrophilous lichens, Xanthoria polycarpa and Candelariella efflorescens were strongly positively associated with white fir, and absent from the Pinus species. Bark pH distinguished presence/absence of moss and composition of macrolichens among tree groups, while increasing VPD with height best explained within-group community structure. To foster epiphyte richness and diversity in this mixed-conifer forest, a heterogeneous mix of mature tree species should be retained when thinning, and mature trees in the narrowly confined riparian zone should not be harvested.

Initial tree mortality and insect and pathogen response to fire and thinning restoration treatments in an old-growth mixed-conifer forest of the Sierra Nevada, California

Fire and thinning restoration treatments in fire-suppressed forests often damage or stress leave trees, altering pathogen and insect affects. We compared types of insect- and pathogen-mediated mortality on mixed-conifer trees 3 years after treatment. The number of bark beetle attacked trees was greater in burn treatments compared with no-burn treatments, and in some cases, larger pine trees were preferentially attacked. Restoration treatments are not expected to change the trajectory of spread and intensification of dwarf mistletoe. Thinning treatments may have provided a sanitation effect in which large leave trees have lower levels of dwarf mistletoe. Although thinning treatments are known to exacerbate root disease, &lt;12% of cut stumps were infected with root pathogens ( Armillaria gallica and Heterobasidion annosum ). Treatments increased Ribes (alternate host for white pine blister rust) frequency and abundance, which may have very localized impacts on white pine blister rust dynamics. In some instances, fire, insects, and pathogens appear to conflict with forest restoration goals by reducing the percentage of pine and producing proportionally higher rates of tree mortality in large-diameter size classes. To better understand the long-term effects of restoration treatments on pathogens and insects, continued monitoring over the course of varying climatic conditions will be needed.

Fire history and the structure and dynamics of a mixed conifer forest landscape in the northern Sierra Nevada, Lake Tahoe Basin, California, USA

The goal of this study was to understand how fire regimes promote fine- and coarse-grain vegetation patterns in an old-growth mixed conifer forest dominated landscape in the General Creek watershed on the west shore of Lake Tahoe, California. We quantified the structure (e.g., composition, age, and size) of old-growth mixed conifer stands located across a range of environmental settings. Fire histories were reconstructed using fire-scar dendrochronology, and the influence of regional climatic variability on fire occurrence was assessed by relating the fire record to regional climate reconstructions. Fire regimes parameters varied across topographic gradients at landscape scales promoting fine grain forest structural patterns. The timing and extent of fires was related to inter-annual and inter-decadal variation in drought which was linked to the El Niño-Southern Oscillation and the Pacific Decadal Oscillation. Coarse scale vegetation patterns where related to upper slope positions and relatively infrequent high severity fires. Fire regimes and forest structure have changed since EuroAmerican settlement with virtually no fires and structural shifts towards higher stand densities and a greater representation of fire intolerant species. At the landscape scale, fire regimes and forests patterns in mixed conifer forests are influenced by a variety of process operating at multiple spatial and temporal scales. Coarse scale heterogeneity related to topography and moderate to high severity fire is superimposed on fine scale variability related to topographic gradients and local variability in fuel and forest structural characteristics. Fire suppression has resulted in a more homogenous landscape particularly with regard to the loss of coarse scale heterogeneity.

Fire disturbance and forest structure in old-growth mixed conifer forests in the northern Sierra Nevada, California

Question: This study evaluates how fire regimes influence stand structure and dynamics in old-growth mixed conifer forests across a range of environmental settings. Location: A 2000-ha area of mixed conifer forest on the west shore of Lake Tahoe in the northern Sierra Nevada, California. Methods: We quantified the age, size, and spatial structure of trees in 12 mixed conifer stands distributed across major topographic gradients. Fire history was reconstructed in each stand using fire scar dendrochronology. The influence of fire on stand structure was assessed by comparing the fire history with the age, size, and spatial structure of trees in a stand. Results: There was significant variation in species composition among stands, but not in the size, age and spatial patterning of trees. Stands had multiple size and age classes with clusters of similar aged trees occurring at scales of 113 - 254 m 2 . The frequency and severity of fires was also similar, and stands burned with low to moderate severity in the dormant season on average every 9 - 17 years. Most fires were not synchronized among stands except in very dry years. No fires have burned since ca. 1880. Conclusions: Fire and forest structure interact to perpetuate similar stand characteristics across a range of environmental settings. Fire occurrence is controlled primarily by spatial variation in fuel mosaics (e.g. patterns of abundance, fuel moisture, forest structure), but regional drought synchronizes fire in some years. Fire exclusion over the last 120 years has caused compositional and structural shifts in these mixed conifer forests.

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.

Drought triggered tree mortality in mixed conifer forests in Yosemite National Park, California, USA

Tree mortality is an important process causing forest structural and compositional change. In this study, we investigate the influence of drought and topography on recent patterns of tree mortality in old-growth mixed conifer forests in Yosemite National Park, located in the Sierra Nevada Mountains of northern California, USA. The surveyed stands have experienced a century of fire exclusion and are dominated by associations of Pinus ponderosa, Calocedrus decurrens and Abies concolor. The average age of trees in the stands was 88 years. We sought answers to the following questions: (1) Do periods of high tree mortality correspond with drought? (2) Do spatial and temporal patterns of high tree mortality vary by slope aspect? and (3) Do different tree species exhibit similar temporal and spatial patterns of tree mortality? We identified temporal patterns of tree mortality on three north- and south-facing slopes by determining the death date of trees using dendrochronology. Tree death date frequency was then compared by slope aspect and to Palmer Drought Severity Index (PDSI), and April snowpack depth as measures of growing season water availability. The frequency of tree death dates was negatively correlated with annual and seasonal PDSI and April snowpack depth, and more trees died in years with below normal PDSI and snowpack. Correlations between tree mortality and drought were evident only for multi-year periods (2–5 years). Temporal patterns of tree death were similar on north- and south-facing slopes and among species, but the density of dead trees was higher on north than south slopes. Dense stand conditions caused by fire suppression, and the coincident outbreak of bark beetles during drought, may have limited any buffering effect of topography on tree mortality. Drought induced tree mortality in mixed conifer forests in Yosemite National Park highlights the importance of both historical legacies such as fire suppression and exogenous controls such as climate as drivers of vegetation change.

Patterns of Mortality in an Old-Growth Mixed-Conifer Forest of the Southern Sierra Nevada, California

Abstract Mortality patterns in an old-growth, mixed-conifer forest, in the absence of wildfire, were investigated at the Teakettle Experimental Forest from 2000 to 2002. We tested the hypothesis that after a century of fire suppression, pathogen- and insect-associated mortality (between episodic droughts) would be significantly greater on ingrowth trees (i.e., smaller-diameter, shade-tolerant species in high-density clusters). Using a survey of over 30,000 mapped trees, overall mortality, as measured by standing dead trees, was 8.7% of all stems ≥5 cm dbh. Mortality levels were proportional to the population size of the five dominant conifer species, white fir (Abies concolor), red fir (A. magnifica), incense cedar (Calocedrus decurrens), Jeffrey pine (Pinus jeffreyi), and sugar pine (Pinus lambertiana). There was also no significant difference in mortality between shade-tolerant and shade-intolerant species. All dead trees were clustered within plots. Mortality was significantly higher than expected for large-diameter trees (&amp;gt;100 cm dbh) with all conifer species combined and for each individual species, except Jeffrey pine. Small-diameter dead trees were grouped in high-density clusters. Mortality was less than expected among small-diameter trees (5–20 cm dbh) for all species combined, red and white fir, and sugar pine. Mortality for all conifers was higher than expected in areas of high stand density and lower in areas of low stand density. Mortality of small-diameter trees was clustered and particularly high in areas of high stand density. Our data suggest pathogen- and insect-associated mortality is significantly greater in areas of high stand density but it is not higher for shade-tolerant species. Furthermore, mortality is higher than expected for large-diameter trees, suggesting an acceleration of old-tree mortality under current fire suppression conditions. FOR. SCI. 51(3):266–275.

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.

Fuel loads, snag abundance, and snag recruitment in an unmanaged Jeffrey pine–mixed conifer forest in Northwestern Mexico

Management of downed woody fuels and snags (standing dead trees) is receiving increasing attention because of their ecosystem values and effects on potential fire behavior. Research has correlated the abundance of many wildlife species with snags and downed woody material but very little information exists of the abundance and arrangement of these forest structures, particularly in unmanaged forests. Conifer forests in northwestern Mexico have not experienced systematic fire suppression or harvesting making them unique in western North America. In 1998, average snag density in Jeffrey pine–mixed conifer forests in the Sierra San Pedro Martir (SSPM) National Park was 3.95 snags/ha but 35% of inventoried plots had no snags. In 2002, average snag density significantly increased to 5.10 snags/ha after a multiple-year drought. Average surface and ground fuel loads were 15.8 and 8.7 t/ha, respectively. High variability characterized all snag and fuel attributes measured in this forest. This high amount of variation is probably the result of the relatively intact frequent surface fire regime and because no harvesting has occurred in the sampled area. The patchy distribution of snags observed argues against the application of uniform targets for snag retention across similar forested landscapes. An improvement in management guidelines would be to manage for snag density and large fuels over moderate spatial scales (hundreds of hectares) instead of on a per hectare basis. Forest fragmentation and diverse ownerships in many western United States forests complicates this recommendation. Conservation of the forests in the SSPM is critical because it is the last landscape-scale, old-growth mixed conifer forest in western North America with a relatively intact frequent fire regime.