Teakettle Experimental Forest Research Articles

Managing fire‐prone forests in a time of decreasing carbon carrying capacity

Changing climatic conditions are increasing overstory tree mortality in forests globally. This restructuring of the distribution of biomass is making already flammable forests more combustible, posing a major challenge for managing the transition to a lower biomass state. In western US dry conifer forests, tree density resulting from over a century of fire‐exclusion practices has increased the risk of high‐severity wildfire and susceptibility to climate‐driven mortality. Reducing dead fuel loads will require new approaches to mitigate risk to the remaining live trees by preparing forests to withstand future wildfire. Here, we used data from the Teakettle Experimental Forest in California to evaluate different prescribed fire burn frequencies and their impact on accumulated dead fuels after a 4‐year drought. Increasing burn frequency could reduce surface fuel build‐up but comes with additional challenges that will require creativity and experimentation to overcome.

An efficient and accurate deep learning method for tree species classification that integrates depthwise separable convolution and dilated convolution using hyperspectral data

ABSTRACT Addressing accuracy and computational complexity challenges in hyperspectral image classification for small sample and multi-species scenarios, we developed DSC-DC, a lightweight convolutional neural network. This model is based on Depthwise Separable Convolution and Dilated Convolution and was trained using the Teakettle Experimental Forest dataset (USA). In this study, DSC-DC achieved an overall accuracy (OA) of 99.83%, average accuracy (AA) of 99.64%, and Kappa coefficient of 0.9996. Compared to Support Vector Machine and K-Nearest Neighbors, it demonstrated markedly higher OA (3.88% to 7.55%) and AA (30.71% to 34.09%). Compared to Inception-V3, ResNet50, and MSR-3DCNN, DSC-DC marginally outperformed in accuracy (OA: 0.06% to 0.31%; AA: 0.32% to 3.64%) while reducing the training time by 3.5, 5, and 35 times, and the prediction time by 2, 3, and 17 times, respectively. Moreover, DSC-DC exhibits slightly superior accuracy and efficiency compared to a 5-layer optimal structure of the 3D-CNN model. The application of the DSC-DC model to the hyperspectral dataset from the Jiepai branch of the Gaofeng State Owned Forest Farm in the Guangxi province, China, further demonstrated the reliability, versatility, and practical potential of this model. This study provides a reliable and efficient reference solution for small-sample and multi-tree classification tasks.

Enhancing individual tree mortality mapping: The impact of models, data modalities, and classification taxonomy

Tree mortality is rapidly increasing as a result of more frequent and extensive droughts and forest fires across the globe. The increasing pace and scale of disturbance and resulting mortality necessitates the study of tree mortality at the scale at which it occurs (individual trees) to develop effective management strategies. Achieving this objective requires high-resolution aerial or satellite remote sensing data supported by field validation and efficient modeling. However, the optimal remote sensing data modalities and models for mapping overstory tree mortality remain uncertain and there is a lack of benchmark datasets for reproducible research, comparison, and model improvement. In this research, we propose a framework to generate tree mortality maps at the scale of individual trees and a labeled benchmark dataset using open-source high-resolution remote sensing data. Our benchmark dataset consists of over 15,000 image crops that have been manually labeled via multi-annotator majority for live, dead, mixed, and other classes in National Ecological Observatory Network (NEON) RGB, NEON hyperspectral, and National Agriculture Imagery Program (NAIP) RGBNIR imagery. Specifically, we present a multi-class classification framework that categorizes lidar-derived tree segments as live, dead, mixed, or other. We compared the performance of different machine learning models, remote sensing data modalities and their associated spatial resolution, spectral depth, and orthorectification using our benchmark dataset. Among various models, we found Convolutional Neural Network (CNN) models outperformed all other models with the highest accuracies and F1 scores across all data modalities. Similarly, among remote sensing data modalities, NEON RGB data at 0.1 m resolution performed best due to its high spatial resolution and high quality orthorectification. The overall accuracy of 87.27% with a macro F1 score of 0.76 was achieved using the RGB data with the CNN model. Our framework can be applied to any forest ecosystem in which lidar and raster imagery are available. We applied the framework using the best performing model and data modality to produce an individual tree mortality map over the Teakettle Experimental Forest in the southern Sierra Nevada. The framework and high-resolution mortality map can be valuable resources for forest managers and provide an invaluable basis for studying potential mortality drivers of dominant trees, forest inventory, selective logging, fire disturbance, and succession modeling.

Repeated burns fail to restore pine regeneration to the natural range of variability in a Sierra Nevada mixed‐conifer forest, U.S.A.

Fire‐exclusion has acted as a major perturbation on dry conifer forests in the western United States, increasing tree density and, in mixed‐conifer forests, the dominance of shade‐tolerant species. Restoration efforts aim to reverse these effects by reducing stand density, restoring relative proportions of tree species, and reintroducing recurrent fire, but there are limited long‐term data on the effects of repeated burning on tree regeneration. We analyzed two decades of seedling and overstory data from the Teakettle Experimental Forest in the southern Sierra Nevada, California, United States to determine how thinning and repeated burning affect seedling establishment and overstory recruitment. Across treatments, pine seedling densities remained much lower than shade‐tolerant seedling densities. We found repeated burns led to modest increases in sugar pine (Pinus lambertiana) and substantial increases in incense‐cedar (Calocedrus decurrens) seedling densities 4 years postburn. No significant differences in seedling densities among repeated burning treatments were detected for Jeffrey pine (P. jeffreyi) or white fir (Abies concolor). Estimates of natural midstory recruitment were much higher among white fir and incense‐cedar than pines, even following treatments. However, postharvest planting increased rates of pine midstory recruitment in overstory thinned treatments. Our results suggest that fire‐exclusion may have shifted the ecosystem out of its initial domain of attraction, creating a forest dominated by shade‐tolerant species that exhibits hysteresis by resisting a return to a natural range of variability even after restoring structure and process. Planting pine species may be effective at overcoming this resistance to restore the forest to a pine‐dominated state.

Tree growth responses to extreme drought after mechanical thinning and prescribed fire in a Sierra Nevada mixed-conifer forest, USA

An estimated 128 M trees died during the 2012–2016 California drought, largely in the southern Sierra Nevada Range. Prescribed burning and mechanical thinning are widely used to reduce fuels and restore ecosystem properties, but it is unclear if these treatments improve tree growth and vigor during extreme drought. This study examined tree growth responses after thinning, prescribed burning, and extreme drought at the Teakettle Experimental Forest, a historically frequent fire mixed-conifer forest in the southern Sierra Nevada of California, USA. Mechanical thinning (no thin, understory thin, and overstory thin) and prescribed burning (unburned, fall burning) were implemented in 2000–2001. Using annual growth data from increment cores, over 10,000 mapped and measured trees, and lidar-derived metrics of solar radiation and topographic wetness, we had two primary questions. First, what were the growth responses to thinning and prescribed burning treatments, and did these responses persist during the 2012–2016 drought? Second, what tree-level attributes and environmental conditions influenced growth responses to treatments and drought?Thinning increased residual tree growth and that response persisted through extreme drought 10–15 years after treatments. Growth responses were higher in overstory versus understory thinning, with differences between thinning types more pronounced during drought. Species-specific growth responses were strongest with overstory thinning, with sugar pine (Pinus lambertiana) and incense-cedar (Calocedrus decurrens) having higher growth responses compared to white fir (Abies concolor) and Jeffery pine (Pinus jeffreyi). For individual trees, factors associated with higher growth responses were declining pretreatment growth trend, smaller tree size, and post-treatment low neighborhood basal area. Growth responses were initially not influenced by topography, but topographic wetness became important during extreme drought. Mechanical thinning resulted in durable increases in residual tree growth rates during extreme drought over a decade after thinning occurred, indicating treatment longevity in mitigating drought stress. In contrast, tree growth did not improve after prescribed burning, likely due to fire effects that reduced surface fuels, but had little effect on reducing tree density. Thinning treatments promoted durable growth responses, but focusing on stand-level metrics may ignore important tree-level attributes such as localized competition and topography associated with higher water availability. Mechanical thinning was effective at improving growth in trees that had been experiencing declining growth trends, but was less effective in improving growth responses in large old trees of higher ecological importance.

Plant community response to thinning and repeated fire in Sierra Nevada mixed-conifer forest understories

Fire suppression in the western United States has significantly altered forest composition and structure, resulting in a higher risk of stand-replacing fire, large-scale drought mortality, and bark beetle outbreaks. Mechanical thinning and prescribed fire are common treatments designed to reduce high-severity fire risk, but few studies have tracked long-term understory plant community response to these treatments with repeated fire application that emulates historic fire regimes. We evaluate changes in understory plant diversity and composition, as well as light availability, soil moisture, and litter depth over two decades following a factorial field experiment crossing thinning and two applications of prescribed fire at the Teakettle Experimental Forest (TEF) in the southern Sierra Nevada. We compare responses in experimental fuels treatments against those in nearby old-growth, mixed-conifer forests with restored low-severity fire regimes in Yosemite and Kings Canyon National Parks. We hypothesized that 1) understory plant richness, evenness, and beta diversity would increase with each burn event; 2) repeated fire after initial thinning would produce the highest understory plant diversity; 3) the second burn would reduce shrub cover, especially in the initially thinned treatments that demonstrated vigorous Ceanothus cordulatus growth after initial treatments; 4) the second burn would increase light availability and bare ground while reducing litter depth; and 5) treatments with initial thinning followed by multiple prescribed burns would show the greatest similarities to reference forests in diversity metrics, shrub cover, and environmental conditions. Although initially local (10 m2-scale) understory plant richness and diversity increased most following thinning combined with prescribed fire, this treatment did not generate understory conditions similar to those in nearby reference forests over the long term. Vigorous shrub growth resulted in low understory evenness and beta diversity over time, which a secondary burn treatment did not alter. Burning without thinning retained a more heterogeneous understory following initial treatment. Two years after the second burn treatment, this burning without thinning resulted in high understory richness and evenness similar to reference forest understories. Our results suggest management treatments may need to focus on creating heterogeneity in burn effects to limit shrub cover and foster more diverse understory communities.

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.

Fuel treatment effects on soil chemistry and foliar physiology of three coniferous species at the Teakettle Experimental Forest, California, USA

A full factorial design crossing overstory (O) and understory (U) thinning and prescribed burning (B) was started at Teakettle Experimental Forest, California, in 2001 with the aim of achieving shifts in species composition to favor fire-resistant pines over fir. The goal of the present study was to evaluate the use of metabolic changes as early indicators for predicting the effects of these treatments on species-specific growth and long-term productivity of Pinus jeffreyi, Pinus lambertiana, and Abies concolor. Foliar polyamines, amino acids, soluble proteins and chlorophyll were quantified and related with foliar and soil chemistry. Most changes in soil chemistry occurred only with combined treatments (BU or BO). Changes in foliar metabolism were not driven by soil chemistry. In general, the pines were more sensitive to water stress as compared to fir as indicated by proline content. Understory thinning had no effect on any species. Both O and BO had species-specific effects on foliar amino acids, chlorophyll, soluble proteins, and inorganic ions. In addition, both O and BO were the most extreme thinning treatments for pines as they greatly increase the time needed to replenish carbon stocks. With the exception of some amino acids, B by itself had no major effects on foliar inorganic ions, chlorophyll content, and polyamines for any of the three species while BU caused a decrease in chlorophyll in white fir, suggesting a decline in photosynthetic capacity. These data point to BU as the best management option to specifically decrease the growth of fire-sensitive white fir without a major impact on pines.

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.

Prescribed burning and mechanical thinning effects on belowground conditions and soil respiration in a mixed-conifer forest, California

Soil respiration ( R S) is a major carbon pathway from terrestrial ecosystems to the atmosphere and is sensitive to environmental changes. Although commonly used mechanical thinning and prescribed burning can significantly alter the soil environment, the effect of these practices on R S and on the interactions between R S and belowground characteristics in managed forests is not sufficiently understood. We: (1) examined the effects of burning and thinning treatments on soil conditions, (2) identified any changes in the effects of soil chemical and physical properties on R S under burning and thinning treatments, and (3) indirectly estimated the changes in the autotrophic soil respiration ( R A) and heterotrophic soil respiration ( R H) contribution to R S under burning and thinning treatments. We conducted our study in the Teakettle Experimental Forest where a full factorial design was implemented with three levels of thinning, none (N), understory thinning (U), and overstory thinning (O; September to October 2000 for thin burn combination and June and July 2001 for thin only treatments) and two levels of burning, none (U) and prescribed burning (B; fall of 2001). R S, soil temperature, soil moisture, litter depth, soil total nitrogen and carbon content, soil pH, root biomass, and root nitrogen (N) concentration were measured between June 15 and July 15, 2002 at each plot. During this period, soil respiration was measured three times at each point and averaged by point. When we assumed the uniform and even contribution of R A and R H to R S in the studied ecosystem without disturbances and a linear relationship of root N content and R A, we calculated the contributions of R A to R S as 22, 45, 53, 48, and 45% in UU, UO, BN, BU, and BO, respectively. The results suggested that after thinning, R S was controlled more by R H while after burning R S was more influenced by R A. The least amount of R S variation was explained by studied factors under the most severe treatment (BO treatment). Overall, root biomass, root N concentration, and root N content were significantly ( p < 0.01) correlated with soil respiration with correlation coefficients of 0.37, −0.28, and 0.29, respectively. This study contributes to our understanding of how common forestry management practices might affect soil carbon sequestration, as soil respiration is a major component of ecosystem respiration.

Species-specific response to climate reconstruction in upper-elevation mixed-conifer forests of the western Sierra Nevada, California

Dendrochronology climate reconstruction studies often sample dominant, open-grown trees to reduce competition effects and isolate annual climate influences on radial increment growth. However, there has been no examination of how species respond as stand densities increase or which species in mixed-conifer forests provide a better record of past climate. We sampled 579 trees representing five upper montane mixed-conifer species at the Teakettle Experimental Forest in California’s southern Sierra Nevada to determine species-specific responses to annual climatic fluctuations. Using the Kalman filter, we examined the affect of local stand density on growth response and whether the growth–climate relationship improved with a time lag. The Kalman filter iteratively calculates error for predicted versus actual radial growth and accounts for this variation in the corrector equation. Under current high-density conditions, shade-tolerant white fir ( Abies concolor (Gord. &amp; Glend.) Lindl.) provided the best model for climate reconstruction. Shade-intolerant Jeffrey pine ( Pinus jeffreyi Grev. &amp; Balf.) had a lagged response to annual climatic fluctuations, possibly because its roots may tap water reserves in granitic bedrock fissures. Open-grown trees provided more accurate records of climate. Changes in forest density in this forest may have resulted in changes in species-specific response to annual climatic fluctuations.

Microhabitat Associations of Northern Flying Squirrels in Burned and Thinned Forest Stands of the Sierra Nevada

Prescribed burning and mechanical thinning are used to manage fuels within many western North American forest ecosystems, but few studies have examined the relative impacts of these treatments on forest wildlife. We sampled northern flying squirrels (Glaucomys sabrinus) and microhabitat variables in burned, thinned and control stands of mixed-conifer forest of the southern Sierra Nevada at the Teakettle Experimental Forest. We used this information to determine the effects of burning and thinning on the microhabitat associations of flying squirrels. Across pretreatment stands, the probability of flying squirrel capture increased with decreasing distance to a perennial creek and increasing litter depth. The probability of flying squirrel capture also was greater with increased canopy cover in thinned stands and increased litter depth in burned stands. Greater canopy cover may provide protection from predators, thicker litter layers may harbor a greater abundance of truffles, a primary food of northern flying squirrels, and creeks may provide squirrels with food sources, drinking water and nest trees. Results from this study underscore the need for more information on the effects of forest management on northern flying squirrels near the southern extent of the species' geographic range.

Lichens in relation to management issues

The central and southern Sierra Nevada are subjected to high levels of ozone, high and increasing nitrogen deposition, and unknown quantities of pesticides such as organophosphates from agricultural emissions in the Central Valley. Fire regimes have changed greatly over time, from relatively frequent fire historically, to fire exclusion, to its reintroduction as prescribed fire. Parts of the Sierra parks have been grazed by livestock, and some of this persists today. On top of these factors, climate is likely to change rapidly. Although a large literature exists on human impacts on lichens, almost nothing is known on this topic in the Sierra Nevada specifically. We are largely ignorant of the biodiversity, ecology, and ecological roles of lichens in the Sierra Nevada Park system (the “Sierra parks”: Sequoia and Kings Canyon National Park, Yosemite National Park, and Devils Postpile National Monument ). This paper synthesizes existing data, written reports and other information about lichens in and near the Sierra Nevada parks, as a first step toward developing better baseline data and assessing lichen populations or communities as potential indicators of ecosystem change. Lichens are diverse in their ecosystem roles and functional significance. Organizing the hundreds of lichen species present in the Sierra parks into functional groups helps us to understand, interpret, inventory, and monitor the diversity of lichens. We therefore divided lichens of the Sierra parks into the following functional groups: forage lichens, nitrogen fixers, acidophiles, wolf lichens, crustose lichens on rock, crustose lichens on bark and wood, biotic soil crusts, aquatic, other green algal macrolichens, and pin lichens (calicioids). Management issues that relate to lichens include biodiversity, air quality, water quality, fire, grazing, and the possibility of draining Hetch Hetchy Reservoir. Existing lichen data from the west slope of the Sierra Nevada, of relevance to the management of Sierra parks include: extensive lichen community data from the Forest Inventory and Analysis program (FIA), a few floristic studies focused on the parks, photo points on prescribed fire transects, lichen biomass estimates from four locations, elemental analysis of lichens from a small number of locations, lichen communities in relation to various nitrogen species in Kings River watershed, the Western Airborne Contaminants Assessment Project, Bryoria fremontii studies at Teakettle Experimental Forest, and herbarium databases. We recommend the following short list for future inventory and monitoring work: population status and trend of Bryoria fremontii , macrolichen community monitoring, revise and update the inventory of lichen biodiversity, and preliminary surveys of lichens in neglected habitats.

Fungi in the diets of northern flying squirrels and lodgepole chipmunks in the Sierra Nevada

The diets of a fungal specialist, northern flying squirrel (Glaucomys sabrinus (Shaw, 1801)), and a dietary generalist, lodgepole chipmunk (Neotamias speciosus (Merriam, 1890)), were examined in the old-growth, mixed-conifer forest at the Teakettle Experimental Forest in California's southern Sierra Nevada. Spores of fungi were identified from fecal pellets collected from both species during spring and summer of 1999 through 2002. Frequency of fungi in the diets of both squirrel species was consistently high across all seasons and years of study. Overall, G. sabrinus diets contained about 30% greater richness and evenness of fungal taxa than N. speciosus diets. There were no seasonal differences in richness and evenness of fungal taxa in squirrel diets. Richness of fungal taxa in diets was positively correlated with hypogeous sporocarp biomass and rainfall from June through August for N. speciosus but not for G. sabrinus. Dietary overlap between G. sabrinus and N. speciosus was high with respect to the most frequently consumed fungal taxa, although Gautieria and Gastroboletus were consumed in greater proportions by G. sabrinus than N. speciosus. Our results indicate that in the southern Sierra Nevada both G. sabrinus and N. speciosus were frequent consumers of a similar, diverse assemblage of fungal taxa and that consumption was proportional to seasonal differences in availability. For the more strongly mycophagist G. sabrinus, however, diet had a greater proportion of select fungal taxa and avoidance of less desirable taxa.

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.

Canopy Arthropod Assemblages in Four Overstory and Three Understory Plant Species in a Mixed-Conifer Old-Growth Forest in California

Abstract We compared canopy arthropod assemblages among overstory conifer and understory angiosperm species at Teakettle Experimental Forest in the Sierra Nevada in California during 1998–2000. Arthropods were sampled from upper, middle, and lower crown levels of one overstory tree of each of the four dominant conifer species (Jeffrey pine, sugar pine, white fir, and incense cedar), and from three understory plants of each of the major understory species (California black oak, manzanita, and white-thorn ceanothus) in each of five replicate plots during June and Aug. in each of the 3 years to represent seasonal and annual variation in abundances. Many taxa differed significantly in abundance among plant species, with one to five taxa being significant indicators for each plant species. Five to eight taxa on each plant species showed significant differences in abundance among years. Aphids and scale insects, predaceous mirids, and some detritivores showed peak abundances in 1999, a particularly dry year, whereas most other taxa showed lowest abundances during 1999 or declining abundances during this period, suggesting association with wetter conditions. Nonmetric multidimensional scaling (NMS), supported by multi-response permutation procedure (MRPP), showed that arthropod assemblages differed significantly among the seven plant species, especially between overstory conifers and understory angiosperms, and among the 3 years. These data indicate that the diversity and structure of arthropod communities depend on vegetation structure and/or condition, perhaps as modified by annual variation in weather conditions. FOR. SCI. 51(3):233–242.

Forest Stand Structure and Pattern of Old-Growth Western Hemlock/Douglas-Fir and Mixed-Conifer Forests

Abstract With fire suppression, many western forests are expected to have fewer gaps and higher stem density of shade-tolerant species as light competition becomes a more significant influence on stand pattern and composition. We compared species composition, structure, spatial pattern, and environmental factors such as light and soil moisture between two old-growth forests: Pacific Northwest western hemlock/Douglas-fir at the Wind River Canopy Crane Research Facility exhibiting gap-phase replacement and southern Sierra Nevada mixed conifer at the Teakettle Experimental Forest after 135 years without a fire. We hypothesized that fire suppression at Teakettle would create a current tree composition and distribution more like Wind River where light is an important influence on stand dynamics. Wind River has nearly continuous canopy cover and a high foliage volume that severely reduces understory light and stratifies the canopy composition by shade tolerance. Large trees are regularly spaced from 0 to 15 m and shade-tolerant and intolerant species are “repelled.” In contrast, Teakettle's canopy cover is discontinuous, foliage volume is one-fifth that of Wind River, and understory light is 15 times higher. Trees at Teakettle are significantly clustered in groups containing a mix of shade-tolerant and -intolerant species, separated by large gaps. Although Teakettle's gaps have higher moisture and a thinner litter layer than tree groups, regeneration in gaps is scarce. Fire suppression has increased stem density at Teakettle but it has not filled in gaps, stratified the canopy by shade tolerance, or produced a composition consistent with patterns at Wind River. Teakettle's distinctly clustered stem distribution may result from a minimum canopy cover threshold needed for tree establishment. If high temperatures produced by direct sunlight inhibit stem patterns, traditional stand management that reduces canopy cover to release regeneration should be applied with caution in the southern Sierra Nevada. FOR. SCI. 50(3):299–311.

Dwarf Mistletoe-Host Interactions in Mixed-Conifer Forests in the Sierra Nevada

ABSTRACT We determined the spatial pattern of dwarf mistletoe (Arceuthobium spp.) associated with two different conifer hosts, white fir (Abies concolor) and Jeffrey pine (Pinus jeffreyi), in forests around the Lake Tahoe Basin and at the Teakettle Experimental Forest, both located in the Sierra Nevada. We also examined a number of host variables and bark beetle incidence to determine how these factors might be involved in the Arceuthobium-conifer interaction. There was no significant relationship between dwarf mistletoe-infected trees and associated bark beetles. We found the highest incidence of dwarf mistletoe on Jeffrey pine in Lake Tahoe (87%), followed by dwarf mistletoe on white fir in Lake Tahoe (30%), with the lowest incidence on white fir at Teakettle (27%). Dwarf mistletoe incidence on white fir in our Lake Tahoe grid was not correlated to density but the dwarf mistletoe rating (DMR) was positively correlated to host size. At the Teakettle Forest, dwarf mistletoe incidence on white fir was not correlated with host density but the DMR was correlated with host size. Dwarf mistletoe incidence and DMR on Jeffrey pine were correlated with host density. Individuals, of both conifer species, in all diameter size classes were susceptible to dwarf mistletoe, with the lowest infection rate in the seedling-10-cm-diameter class. Arceuthobium on white fir in Lake Tahoe showed spatial dependence to a range of 20 m. However, Arceuthobium on Jeffrey pine in Lake Tahoe and on white fir at Teakettle showed no clear pattern of spatial structuring. The degree of infection and stand history appear to be important in the spatial dynamics of Arceuthobium spp.