True Fir Research Articles

Genome sequence of Heterobasidion occidentale, a fungus that causes annosus root and butt rot among conifer trees in North America.

We report an annotated draft genome of Heterobasidion occidentale, a fungus (Basidiomycota, Agaricomycetes) that has pathogenic and saprophytic lifestyles. This fungus belongs to the H. annosum (Fr.) Bref. sensu lato species complex that comprises several root rot pathogens. Heterobasidion occidentale causes annosus root and butt rot primarily in true fir (Abies spp.) and spruce (Picea spp.) species throughout western North America.

A 20-Year Ecotone Study of Pacific Northwest Mountain Forest Vulnerability to Changing Snow Conditions

(1) Background: Global climate change is expected to significantly alter growing conditions along mountain gradients. Landscape ecological patterns are likely to shift significantly as species attempt to adapt to these changes. We evaluated the extent to which spatial (elevation and canopy cover) and temporal (decadal trend and El Niño–Southern Oscillation/Pacific Decadal Oscillation) factors impact seasonal snowmelt and forest community dynamics in the Western Hemlock–True Fir ecotone region of the Oregon Western Cascades, USA. (2) Methods: Tsuga heterophylla and Abies amabilis seedling locations were mapped three times over 20 years (2002–2022) on five sample transects strategically placed to cross the ecotone. Additionally, daily ground temperature readings were collected over 10 years for the five transects using 123 data loggers to estimate below-canopy snow metrics. (3) Results: Based on validation using time-lapse cameras, the data loggers proved highly reliable for estimating snow cover. The method reported fewer days of snow cover as compared to meteorological station-based snow products for the region, emphasizing the importance of direct under-canopy field observations of snow. Snow season variability was most significantly impacted temporally by cyclical ENSO/PDO climate patterns and spatially by differences in canopy cover within the ecotone. The associated seedling analysis identified clear sorting of species by elevation within the ecotone but reflected a lack of a long-term trend, as species dominance in the seedling strata did not significantly shift along the elevation gradient over the 20-year study. (4) Conclusions: The data logger-based approach provided estimates of snow cover at ecologically significant locations and fine enough spatial resolutions to allow for the study of forest regeneration dynamics. The results highlight the importance of long-term, understory snow measurements and the influence of climatic oscillations in understanding the vulnerability of mountain areas to the changing climate.

Snag decomposition following stand‐replacing wildfires alters wildlife habitat use and surface woody fuels through time

AbstractHigh‐severity wildfires create pulses of snags that serve a variety of functions as they decompose over time. Snag‐related benefits (and hazards) are often linked to specific decomposition stages, but snag decomposition rates and pathways are not well understood in many forest types. We examined temporal patterns of snag decomposition, wildlife cavity creation, and surface woody fuel dynamics in dry coniferous forests of the interior Pacific Northwest region of North America by sampling 159 forest stands within a 39‐year chronosequence of stand‐replacing wildfires in dry coniferous forests dominated by ponderosa pine and Douglas‐fir. We found that most snags broke or fell during the first 15 years after wildfire; small‐diameter snags mostly broke off at or near ground level, while many large‐diameter snags initially broke off above a height of 2 m and then remained standing for an extended period. Ponderosa pine and lodgepole pine snags fell earlier than Douglas‐fir and true fir snags of comparable diameter classes. Wildlife cavities were most common in stands surveyed 8–20 years after fire and in snags with broken tops but were not limited to large diameter snags. Cavity snag diameters ranged from 17 to 98 cm, with 64% of cavity snags having diameters between 30 and 60 cm and 22% of cavity snags having diameters >60 cm. Surface woody fuels increased as snags broke and fell, reaching maximum levels in all size classes 15–20 years after fire. The percentage of large diameter rotten woody fuels increased steadily beginning about 15 years after fire, with implications for fuel management and subsequent wildfire behavior and severity. Our study supports the proposition that fire‐killed snags represent a transient resource pulse in which the relative contribution of snags to different ecological functions varies with snag diameter, species, time since fire, and landscape position.

Functional diversity affects tree vigor, growth, and mortality in mixed-conifer/hardwood forests in California, U.S.A, in the absence of fire

Mixed tree-species forest management can increase forest resilience by reducing the impacts of disturbances that disproportionately affect a single tree species or closely related groups of tree species. Beyond disturbance-risk reduction, tree-species diversification may foster functional-diversity effects (e.g., complementarity or facilitation) that alter the performance of a given tree species in mixed versus pure stands, potentially benefitting carbon sequestration and wildlife habitat. Tree species-mixture effects have been explored to only a limited degree in western US forests and, particularly, in California. Establishing whether vigor, growth, and mortality of common tree species vary with stand composition would help inform restoration and modeling of these forests under climate change. Using data from USDA Forest Service Forest Inventory and Analysis (FIA) plots from California, we examined how individual-tree vigor, as indicated by live crown ratio (LCR), periodic basal area increment (BAI), and mortality odds varied with functional dissimilarity (FDis). We quantified FDis using an index based on 11 traits related to resource acquisition, competition, environmental tolerances, and fire ecology. We classified major tree species into ponderosa pine (Pinus ponderosa), Jeffrey pine (Pinus jeffreyi), incense-cedar (Calocedrus decurrens), true firs (Abies spp.), Douglas-fir (Pseudotsuga menziesii), live oaks (Quercus spp.), and deciduous oaks (Quercus spp.) response groups. We tested for the main effects of FDis on tree responses, as well as for interactions with tree, site, stand, and climate factors. We found that initial tree height modulated the effects of FDis on ponderosa pine, Jeffrey pine, incense-cedar, and true fir LCR, whereas FDis interacted with climate to alter live oak and deciduous oak LCR. FDis decreased BAI in ponderosa pine and increased BAI in live oaks. FDis interacted with tree size to influence BAI for Jeffrey pine, Douglas-fir, and true firs. We found no evidence that climate or site quality modulated FDis effects on BAI for any species group. Tree mortality was not responsive to FDis, except for the true firs, where both initial tree height and competition interacted with FDis to increase and decrease mortality odds, respectively. FDis effects commonly shifted from positive to negative along gradients of stand structure and site quality, indicating that these effects vary with site and stand conditions. Our results have implications for balancing the ecosystem benefits of mixed stands, such as disturbance risk, carbon sequestration, and habitat during forest restoration projects in the region, as well as for more accurate modeling of complex stands.

Changes to relative stand composition after almost 50 years of Heterobasidion root disease in California true fir and pine forests

AbstractThe Agaricomycete Heterobasidion annosum sensu lato (s.l.), a species complex of fungal pathogens, causes root and butt rot on conifers throughout the northern hemisphere, thereby shaping structure, composition, and evolution of vast and diverse forest ecosystems. We analyze forest change 48–49 years following Heterobasidion root disease emergence in 63 permanent plots first established in 1970–1972 and measured at least once every decade through 2020. We use this dataset to infer the long‐term consequences of Heterobasidion root disease in a set of common forest types that reflect some of the most important hosts for this set of diseases in western North America. We contrasted three host–pathogen systems located within the Sierra Nevada, southern Cascades Mountain Range and the Modoc Plateau: (1) H. irregulare in host communities dominated by several Pinus (pine) species and Calocedrus decurrens (incense cedar) in Yosemite Valley, on the Sierra Nevada Range western slope; (2) H. irregulare in a largely single species (Pinus jeffreyi) host disease system on the eastern slopes of the Sierra Nevada and southern Cascades as well as the Modoc Plateau; and (3) H. occidentale—also on the western slope of the Sierra Nevada—in stands with a mixture of susceptible Abies (true fir) and a diversity of non‐hosts for this Heterobasidion species. Approximately 50 years after disease emergence, relative basal area and stem density were significantly reduced within disease centres in all three pathosystems, but changes to forest composition and relative species dominance were determined by pre‐disease host and non‐host diversity. In the western‐slope H. occidentale system, the disease increased the dominance of non‐susceptible species, Pinus species and C. decurrens. In the multi‐genus Yosemite pathosystem, H. irregulare did not significantly shift species dominance, and as expected, species shifts did not occur in the largely single‐genus eastern‐slope forests. In these widespread California forest ecosystems, two factors appeared to determine forest conditions almost 50 years after Heterobasidion root disease monitoring: the size of the disease centre and the initial diversity of non‐hosts. Along with pathogen species, these factors appear to affect the local severity of disease as well as the degree of species shifts relative to initial plot compositions, reaffirming host susceptibility classifications associated with these forest types.

Characterizing Balsam Woolly Adelgid Infestations and Associated Tree Mortality in Idaho

Abstract Balsam woolly adelgid, Adelges piceae (BWA), is a nonnative insect that has invaded much of the true fir range in North America, reducing host tree growth and causing mortality. Here, we describe a field study to characterize BWA infestation and effects on host trees in Idaho from the onset of infestation within twenty-eight sites with repeated surveys in 2008, 2013, and 2018. Declining tree health was categorized into five BWA damage classes based on the percent of the tree crown–expressing symptoms caused by BWA. Between 2008 and 2018, BWA presence increased across most sites regardless of host species, with an average of 48% (0.1%–100%) host trees per acre being infested. Severe damage or mortality of true fir from BWA occurred across thirteen sampled forest habitat types, including those representing harsh, high elevations. Although a significantly greater proportion of subalpine fir died than grand fir by 2018, BWA caused grand fir mortality. All diameter size classes became infested with BWA, including seedlings, and BWA caused mortality regardless of tree diameter. This assessment provides baseline data for a deeper analysis of insect, host, stand, and environmental interactions that may elucidate factors driving severe stand mortality. Study Implications Balsam woolly adelgid (BWA) is a cryptic, nonnative, and chronically lethal insect of true firs that is frequently overlooked until tree health substantially declines. Therefore, it is often not considered a serious damage agent during forest management planning until after severe damage has occurred across a stand. The BWA damage classification, modified from Spiegel et al. (2013), may be applied during stand exams. This simple and effective five-tiered classification system can be used to categorize BWA damage within individual trees or stands based on the collective symptoms caused by the insect. Feeding by BWA can ultimately result in stand-level mortality of both over- and understory fir and may require artificial regeneration to restore forest function. Fir regeneration was abundant at most BWA-damaged sites; however, seedling-sized firs were often heavily gouted from BWA to levels that will prevent them from growing into seed-bearing trees. Reported levels of mortality caused by BWA within 10 years of infestation and forest habitat type associations can be referenced for making management decisions for not-yet or recently infested stands.

Persistent composition legacy and rapid structural change following successive fires in Sierra Nevada mixed conifer forests

Mixed conifer forests in the Sierra Nevada, California, USA, face threats from frequent high-severity reburns caused by fuel accumulation and increasingly dry, hot conditions associated with climate change. Stand replacing fire and establishment of expansive shrub patches may result in positive feedbacks causing landscapes to convert to persistent shrubland or grassland. Our objective was to examine changes in mixed conifer forest density and species composition after successive fires across a range of initial and reburn severities. We resampled 134 field plots 5–6 years after the Chips Fire reburned stands initially burned by the Storrie and Rich fires on the Lassen and Plumas National Forests. Generalized Linear Models and post-hoc margins analyses were used to examine the interaction effects of initial and reburn fire severities on changes in conifer tree and seedling densities, percent cover of shrubs, and changes in species composition evaluated using fire tolerance index. We found that successive fires with severities less than high-severity significantly reduced tree densities to values within or close to the natural range of variation for mixed conifer stands. Despite these changes in density, species composition did not shift toward increasing dominance of fire tolerant species; mature trees and seedlings of true firs and other fire intolerant species remained dominant after successive low and moderate severity fires. Seedling establishment pulses were associated with sampling years with abundant precipitation, and unburned, unchanged, and low-severity plots in the reburn sample had significant increases in seedling densities across the range of initial burn severities. Shrub cover increased significantly with only a few reburn severity interactions, but declined significantly with repeated high-severity fire. Although two low- to moderate-severity fires may reduce tree densities to within the natural range of variation, they may not shift species composition towards historical patterns. Selective thinning in areas that burned at low- to moderate-severity may be necessary to restore historical species composition in Sierra Nevada mixed conifer forests.

Efficacy of Chemical and Biological Stump Treatments for the Control of Heterobasidion occidentale Infection of California Abies concolor.

We conducted an experimental evaluation of treatments to limit Heterobasidion occidentale infection of white fir (Abies concolor) stumps and wounds in California mixed conifer forests. We tested the efficacy of urea, borate, and a mixture of two locally collected Phlebiopsis gigantea strains in preventing pathogen colonization of fir stumps and separately, urea and borate as infection controls on experimental stem wounds. These were paired with a laboratory test on ~100 g wood blocks with and without a one-week delay between inoculation and treatment. Urea, borates, and Phlebiopsis treatments all significantly reduced the stump surface area that was colonized by H. occidentale at 84%, 91%, and 68%, respectively, relative to the controls. However, only the borate treatments significantly lowered the number of stumps that were infected by the pathogen. The laboratory study matched the patterns that were found in the stump experiment with a reduced area of colonization for urea, borates, or P. gigantea treatments relative to the controls; delaying the treatment did not affect efficacy. The field wound experiment did not result in any Heterobasidion colonization, even in positive control treatments, rendering the experiment uninformative. Our study suggests treatments that are known to limit Heterobasidion establishment on pine or spruce stumps elsewhere in the world may also be effective on true firs in California.

White-headed woodpecker nesting habitat at multiple spatial scales: Are habitat preferences adaptive?

Ecological theory predicts that animals will select habitats for breeding that confer an adaptive advantage through increased reproductive success. We therefore expect to see congruence between habitat preferences and measures of fitness. We studied abundance, nest site selection, and nest survival in white-headed woodpeckers (Picoides albolarvatus) in four forest types along an elevational gradient in the southern Sierra Nevada, California. Our primary objective was to ascertain what habitat features white-headed woodpeckers were selecting for nesting and at what scale, and whether those choices translated into increased reproductive success. We examined sets of models that included abiotic factors and habitat at three spatial scales: nest site (0.04 ha), local (~1 ha), and home range (~125 ha). White-headed woodpeckers were more abundant at higher elevations, with low abundance in low-elevation ponderosa pine forest, moderate abundance in mid-elevation mixed conifer forest, and reached their highest abundance in true fir habitat before becoming rare in high-elevation lodgepole pine forest. They both selected for and nested more successfully at higher elevations in true fir forest. White-headed woodpecker nesting ecology in the Sierra Nevada demonstrated a mixed pattern of congruence between characteristics important to nest habitat selection and nest survival. We found congruence for abiotic and home range scale models (agreement between preferences and nest survival), indicating adaptive selection. At the landscape scale, White-headed woodpeckers selected closed-canopy conifer forests with abundant edges. Habitat preferences at the nest site scale were negatively related to nest survival, suggesting maladaptive habitat selection at this scale. White-headed woodpeckers selected open forest with higher basal area of snags compared to available sites, but nest survival was higher in forest with higher canopy closure and lower basal area of snags. Local scale models were weak and did not help clarify habitat relationships. Our results highlight the importance of considering multiple spatial scales when examining habitat preferences. The primary management considerations for white-headed woodpecker habitat include retention of large, suitable snags and the need to consider forest heterogeneity. The close association with ponderosa pine forests and large-seeded pines found over most of their range did not hold in the Sierra Nevada, where white-headed woodpeckers were most abundant in true fir forests and selected and nested more successfully in true fir forests. Habitat models developed outside the area for which they were developed should be validated before they are applied to other locations.

Discovering Douglas-Fir Woodlands in the Historical Forests of Umatilla National Forest, Eastern Oregon and Washington

We discovered unique Douglas-fir open woodlands in the Umatilla National Forest using historical surveys. Historical ponderosa pine forests of the western United States are transitioning to denser forests comprised of a greater proportion of fire-sensitive species, including true firs. We used historical (1879 to 1887) surveys to quantify the composition and structure of the Umatilla National Forest in eastern Oregon and Washington and provided contemporary forest information for comparison. We also modeled fir and pine distributions using environmental predictors and the random forests and extreme gradient boosting classifiers. Historically, ponderosa pine and Douglas-fir comprised about 80% of all trees, with western larch relatively abundant at 10% of all trees. Currently, ponderosa pine and Douglas-fir are about 40% of all trees, while grand fir and lodgepole pine increased from rare species to about 40% of all trees. Historical density was about 165 trees/ha (trees > 12.7 cm in diameter). The wetter north unit of steep slopes and predominantly Douglas-fir was about 120 trees/ha, or open woodlands, whereas the drier, flatter south units of predominantly ponderosa pine were about 210 trees/ha, and densities of 160 and 190 trees/ha occurred on flat and gentle slopes, respectively, with predominantly ponderosa pine. Currently, Umatilla National Forest averages about 390 trees/ha; the north unit of grand fir and Douglas-fir tripled in density to 365 trees/ha, whereas the south units of ponderosa and lodgepole pines doubled in density to 410 trees/ha. Douglas-fir woodlands are an unusual combination of a relatively fire-sensitive tree species with an open structure, which may result from surface fires that remove tree regeneration, resulting in one layer of trees over an understory of herbaceous and shrubby vegetation. We interpreted that a spatially and temporally variable fire return interval favored Douglas-fir, but fires were frequent enough to allow herbaceous vegetation and shrubs to out-compete trees, maintaining the balance between trees and other vegetation in woodlands. Fire exclusion has resulted in forest-type transition and also an information deficit about circumstances under which relatively fire-sensitive Douglas-fir instead of fire-tolerant ponderosa pine would establish at low densities over large extents.

Understory responses to restoration in aspen‐conifer forests around the Lake Tahoe Basin: residual stand attributes predict recovery

The removal of conifers from aspen (Populus tremuloides) stands is being undertaken throughout the western United States to restore aspen for local‐ and landscape‐level biodiversity. Current practices include mechanically removing conifers or hand thinning, piling, and burning cut conifers in and adjacent to aspen‐conifer stands. To evaluate the effectiveness of restoration treatments, we examined tree regeneration and herbaceous vegetation cover in thinned, thinned and pile burned, and non‐thinned control stands. Growth rates of small conifer saplings threatening to outcompete and replace aspen were also measured. Two to four years after pile burning, herbaceous vegetation cover within the footprint of burned piles (i.e. burn scars) was 35–73% of that in adjacent areas. Aspen was more likely to regenerate inside burn scars where fewer surrounding trees were true firs. Conifer seedlings were more likely to regenerate in burn scars where more of the surrounding trees were conifers (pine or fir) as opposed to aspen. Fir saplings had much slower growth than did aspen saplings. Overall, our findings show that restoration treatments are promoting desirable outcomes such as enhancing aspen regeneration but that follow‐up treatments will be needed to remove numerous conifer seedlings becoming established after restoration activities. Eliminating conifers, while they are small, growing slowly, and contributing little to fuel loads may be an economical way to prolong restoration treatment effectiveness.

A morphological comparison of Arceuthobium abietinum and A. campylopodum (Viscaceae) and nomenclatural changes for A. abietinum

Fir dwarf mistletoe (Arceuthobium abietinum, Viscaceae) parasitizes true firs (Abies spp.) in the Pacific Northwest through California, the Southwest, and into northern Mexico. At present, fir dwarf mistletoe consists of two special forms (formae speciales, f. sp.) and one subspecies that are morphologically similar, but exhibit a high degree of host specificity. However, A. abietinum has been treated as a subspecies of A. campylopodum or conspecific with it in some taxonomic treatments. Therefore, we undertook this study to compare the morphologies of these dwarf mistletoes; collecting morphological data from nearly 100 A. abietinum and 60 A. campylopodum populations collected across most of their geographic ranges and analyzing these data using univariate and multivariate statistical tests. Our results demonstrated that the special forms and subspecies of A. abietinum are morphologically distinct from A. campylopodum, thereby, supporting the continued recognition of A. abietinum as a separate species. Furthermore, our analysis found significant differences for several of the characters we examined between the special forms of A. abietinum. Therefore, we recombined the special forms as subspecies: A. abietinum subsp. abietinum (formerly f. sp. concoloris) and A. abietinum subsp. magnificae (formerly f. sp. magnificae). The previously described A. abietinum subsp. wiensii was morphologically distinct from subsp. abietinum and subsp. magnificae. The distinctiveness of these taxa was supported by their host affinities as well.

Mass mortality of Abies veitchii caused by Polygraphus proximus associated with tree trunk diameter in Japan

Bark beetles (Curculionidae: Scolytinae) can cause widespread tree mortality, and change the structure, composition, and function of forests; however, limited data is available on bark beetle infestation of true fir forests. Although Polygraphus proximus Brandford (Coleoptera: Scolytidae) causes rapid degradation of large areas of natural and planted fir forests in Russia, the species is not present in large numbers and does not cause mass mortality in central Japanese subalpine/subarctic forests dominated by Abies veitchii. Furthermore, the relationship between P. proximus infestation and tree trunk diameter has not yet been studied to determine if the insect has a preference for trees of a certain diameter, although tree trunk diameter is often a crucial parameter affecting bark beetle colonization and tree mortality. We investigated P. proximus attack on A. veitchii and the association between the resulting tree mortality and tree trunk diameter at two sites in Honshu and Hokkaido, Japan. Polygraphus proximus colonized and utilized standing A. veitchii trees and fresh-cut logs. Both live and standing dead A. veitchii trees were attacked with high frequency: 331 (70.0%) of 473 trees in Honshu, and 121 (88.3%) of 137 trees in Hokkaido. Abies veitchii mortality was 38.1% in Honshu and 59.1% in Hokkaido. The proportion of trees with entry holes was significantly higher in small-diameter trees than in large-diameter trees in Honshu. Small-diameter trees also had significantly more holes than large-diameter trees in Honshu. On the other hand, there were no significant relationships between tree trunk diameter, and the presence and the number of holes in Hokkaido. Large-diameter trees were better able to survive P. proximus attacks in both sites. This study suggests that the beetles attack small-diameter trunk trees in early phase of mass mortality and then gradually attack large-diameter trunk trees with the increasing beetle attack. Polygraphus proximus may be influencing forest structure of A. veitchii. Enhanced colonization of small-diameter trees by P. proximus may act as a trigger of mass mortality and result in higher tree mortality might pose a threat to the regeneration of A. veitchii forests in Japanese subalpine/subarctic areas.

Exotic Abies Species in Czech Provenance Trials: Assessment after Four Decades

Abstract The growth of seven exotic true fir (Abies) species and native Abies alba have been compared in three provenance trials in the Czech Republic, at the relatively advanced ages of 44, 38, and 35 years respectively. A clear differentiation is observable between the species. The closely related species group of A. alba and A. cephalonica appears rather heterogeneous in its phenotypic behavior. A. alba provenances show superiority, but also a high differentiation. Productivity of provenances of A. cephalonica fall behind A. alba; however A. cilicica and A. pinsapo provenances have shown total mortality. The high potential of A. grandis is confirmed by outstanding growth; provenances from the coastal plain in Washington State performed best. A. procera grows slower than A. grandis, but still faster than A. alba provenances. Health risks, extreme ecologic distances of transfer, trend shifts of growth rate, and rank change with age are uncertainties that require necessary caution when selecting provenances for importation. In recent years, public and institutional perceptions concerning the introduction of non-native tree species and provenances has shifted, and the practice is no longer seen as necessarily inappropriate.

Curtailing succession: Removing conifers enhances understory light and growth of young aspen in mixed stands around Lake Tahoe, California and Nevada, USA

Aspen is a keystone species that is relatively rare in the Sierra Nevada Mountains and is undergoing succession to conifers in the absence of disturbance. In the presence of more conifers, recruitment of young aspen to replace aging overstory aspen is failing and diverse understory plant communities are being lost. We studied vegetation attributes and understory light in nine aspen-conifer stands in the Lake Tahoe Basin in California and Nevada, USA. Within each stand we measured and mapped trees in a one-hectare study area and collected 27–30 hemispherical canopy images on a grid that were used to estimate forest canopy variables and the amount of light reaching the understory. We also measured percent cover of herbaceous vegetation on the same sample grid. After restoration treatments cut smaller conifer trees at six of the nine sites, we repeated canopy images and began monitoring the growth of young aspen adjacent to sample grid points. Less light was reaching the understory beneath areas of forest canopy dominated by true firs. Understory light was enhanced by removal of conifers (pine and fir species). Stand density and species composition of trees surrounding a grid point where each hemispherical image was taken were useful predictors of understory light and forest canopy variables, herbaceous vegetation cover, and the morphology and growth of young aspen. Less herbaceous vegetation was found in the vicinity of true firs and pines, particularly in areas with less light being transmitted to the forest floor. In areas where the overstory was pure aspen, herbaceous vegetation cover remained high across the range of understory light levels measured. Greater conifer removal treatment intensity, in terms of proportion of stand density cut, enhanced crown ratio and growth of young aspen. Height and diameter increment of young aspen was negatively impacted by the presence of true fir trees in the vicinity. If the objectives of forest management include promoting understory vegetation cover or vigorous aspen regeneration, then removing conifers (especially true firs) from aspen-conifer stands appears to be a viable management tool; using such an approach should focus on removing as many conifers as possible within approximately 8–11m of areas where an enhancement of understory light, herbaceous vegetation, and the growth and vigor of young aspen is desired.

A Severity Rating System for Evaluating Stand-Level Balsam Woolly Adelgid (Hemiptera: Adelgidae) Damage in Two Abies Species in Western North America

Severity rating systems are fundamental to understanding the impacts of disturbance agents in forest stands. The balsam woolly adelgid (BWA), Adelges piceae (Ratzeburg) (Hemiptera: Adelgidae), is an invasive forest pest in North America that infests and causes mortality in true fir, Abies spp. There is currently no single system for evaluating damage caused by BWA in the western United States. Because range expansion through favored habitat is inevitable, it is imperative to begin long-term monitoring using a unified approach to evaluate changing conditions and hasten management opportunities. We developed a new rating system for two western host species: grand fir and subalpine fir. Unlike other severity scales, our index describes stand-level damage rather than impacts on individual host trees alone. We sampled 57 sites across the current range of BWA in the western United States and compiled severity indices using 10 metrics of overstory and understory damage. We used analyses of variance to identify five discrete severity classes and translated the results into a descriptive table of damage characteristics for each class. This index is proposed as an improvement over existing rating systems for western North America because of its broader scope, demonstrated ability to distinguish between classes, and identification of the predominant indicators that will improve the efficiency and efficacy of field assessments. The adoption of this system will facilitate long-term monitoring through site resurveys that will be directly comparable over time, also allowing future studies to conduct risk assessments and target stands that face the greatest threat to forest health.

Tree mortality and structural change following mixed-severity fire in Pseudotsuga forests of Oregon’s western Cascades, USA

Mixed-severity fires are increasingly recognized as common in Pseudotsuga forests of the Pacific Northwest and may be an important mechanism for developing or maintaining their structural diversity and complexity. Questions remain about how tree mortality varies and forest structure is altered across the disturbance gradient imposed by these fires. Therefore, we sampled live and dead trees at 45–1.0ha plots, each with four 0.10ha nested subplots, stratified across an unburned, low, moderate and high-severity fire gradient. Burned plots were primarily sampled 10 and 22years post-fire, except three plots sampled four years post-fire. We estimated probability of mortality for fire-tolerant (Douglas-fir, incense-cedar, sugar pine) and fire-intolerant (western hemlock, western redcedar, true fir) trees from 5079 samples. The probability of mortality varied across all species and fire-severity classes, but the greatest difference was observed between fire-tolerant and fire-intolerant functional groups. Probability of mortality decreased with increasing DBH for all species except western hemlock that did not increase its fire resistance with increasing size. Some large, fire-tolerant trees survived high-severity fire but only in 31% of our plots. Snag fall and fragmentation was estimated for trees killed by the fire from 2746 sampled snags and logs 10 and 22years post-fire. The proportion of snags fallen decreased with increasing DBH for all species, and was positively correlated with fire severity, except for Douglas-fir that had a higher proportion felled following low-severity fire. Snag fragmentation rates were positively correlated with DBH and fire severity for all species. Individual tree and snag estimates were scaled to plot-level structural attributes and contrasted by their coefficient of variation within- and among-plots from unburned to moderate-severity, as well as across all sampled conditions. Structural attributes varied more within- than among-plots, likely a result of patchy mortality that increased with fire severity. Although vertical and horizontal structural diversity increased at sub-hectare scales, the highest variability occurred for all structural attributes when compared across all sampled conditions. We conclude that the range of fire effects imposed by mixed-severity fire is important in creating structural diversity in Pseudotsuga forests of the Pacific Northwest and should be promoted through progressive fire management programs aimed at maintaining this forest’s fire regime now and in the future.

Delphinella Shoot Blight on Abies lasiocarpa Provenances in Norway

Delphinella shoot blight (Delphinella abietis) attacks true firs (Abies spp.) in Europe and North America. Especially subalpine fir (A. lasiocarpa), one of the main Christmas tree species in Norway, is prone to the disease. The fungus kills current year needles, and in severe cases entire shoots. Dead needles become covered with black fruiting bodies, both pycnidia and pseudothecia. Delphinella shoot blight has mainly been a problem in humid, coastal regions in the northwestern part of Southern Norway, but, probably due to higher precipitation in inland regions during recent years, heavy attacks were found in 2011 in a field trial with 76 provenances of subalpine fir in Southeastern Norway. However, the amount of precipitation seemed less important once the disease had established in the field. Significant differences in susceptibility between provenances were observed. In general, the more bluish the foliage was, the healthier the trees appeared. The analysis of provenance means indicated that, at least for the southern range, the disease ratings were correlated with foliage color. This study also includes isolation, identification, a pathogenicity test, a seed test and electron microscopy of the wax layer on the needles. The fungus was identified based on the morphology of spores and by sequencing the Internal Transcribed Spacer (ITS) regions of the ribosomal DNA. Koch’s postulates were fulfilled. The fungus was found present on newly harvested seeds and may therefore spread via international seed trade. When comparing the wax layers on green and blue needles, those of the latter were significantly thicker, a factor that may be involved in disease resistance.

Tradeoffs between chilling and forcing in satisfying dormancy requirements for Pacific Northwest tree species.

Many temperate and boreal tree species have a chilling requirement, that is, they need to experience cold temperatures during fall and winter to burst bud normally in the spring. Results from trials with 11 Pacific Northwest tree species are consistent with the concept that plants can accumulate both chilling and forcing units simultaneously during the dormant season and they exhibit a tradeoff between amount of forcing and chilling. That is, the parallel model of chilling and forcing was effective in predicting budburst and well chilled plants require less forcing for bud burst than plants which have received less chilling. Genotypes differed in the shape of the possibility line which describes the quantitative tradeoff between chilling and forcing units. Plants which have an obligate chilling requirement (Douglas-fir, western hemlock, western larch, pines, and true firs) and received no or very low levels of chilling did not burst bud normally even with long photoperiods. Pacific madrone and western redcedar benefited from chilling in terms of requiring less forcing to promote bud burst but many plants burst bud normally without chilling. Equations predicting budburst were developed for each species in our trials for a portion of western North America under current climatic conditions and for 2080. Mean winter temperature was predicted to increase 3.2–5.5°C and this change resulted in earlier predicted budburst for Douglas-fir throughout much of our study area (up to 74 days earlier) but later budburst in some southern portions of its current range (up to 48 days later) as insufficient chilling is predicted to occur. Other species all had earlier predicted dates of budburst by 2080 than currently. Recent warming trends have resulted in earlier budburst for some woody plant species; however, the substantial winter warming predicted by some climate models will reduce future chilling in some locations such that budburst will not consistently occur earlier.

Effects of provenance on the survival, growth and stem form of European silver fir (Abies alba Mill.) in Britain

European silver fir (Abies alba Mill.) was the first true fir to be introduced to Britain in 1603. Despite early enthusiasm for the species, it has not been widely planted and work on the effects of silver fir woolly aphid (Adelges nordmannianae Eckstein) in the mid-twentieth century concluded that large scale use of the species could not be justified. This was interpreted as a research challenge and resulted in the establishment of a series of provenance experiments being established between 1966 and 1970. Results from these experiments after 46 years clearly show that European silver fir is a productive species and could be deployed in British forests as a component to diversify species composition and in the wider use of continuous cover forestry. In general, the trials show that good provenances survive and grow on a wide range of sites and it is not necessary to select different provenances depending on site location or climate. Provenances from Calabria in Italy performed very well and should be the first choice of origin for future planting in Britain. However, provenances from a wide range of the species natural distribution performed well demonstrating that European silver fir is a very adaptable species.