White Fir Research Articles

Post-fire resurveys reveal predictability of long-term conifer recruitment in severely burned California dry forests

High severity fire is far outpacing post-fire reforestation capacity across the American West, highlighting the need to better understand when and where natural regeneration is sufficient to meet short- and long-term reforestation goals. The vast majority of available post-fire data represents regeneration in the first few years following a fire, raising the question of how well recovery trajectories can be predicted from early post-fire snapshots. We utilize a unique dataset from seven wildfires and 78 plots surveyed twice following stand-replacing fire in two California ecoregions, the northern Sierra Nevada and Klamath Mountains, to ask (1) how are post-fire vegetation and conifer densities changing over time and (2) how well do relatively early post-fire sampling efforts and biophysical factors (availability of seed source, shrub competition, and microclimate) explain longer-term (12–23 year) conifer recovery in California dry forests. Change in conifer seedling density between survey periods was highly variable. Overall, densities of all conifer species during the first decade after fire provided reliable estimates of longer-term conifer recovery in northern California dry forests, though in different ways for different species. Early post-fire seedling density readily explained longer-term densities for less shade-tolerant species, yellow pine (Pinus ponderosa and Pinus jeffreyi) and Douglas-fir (Pseudotsuga menziesii). In contrast, early seedling density was a poor predictor for shade-tolerant species, predominantly white fir (Abies concolor), with site conditions better explaining longer-term density. In general, the probability of a site experiencing net seedling mortality between surveys was highest in sites with high shrub cover and the probability of net recruitment increased with decreasing heat load, suggesting a continued role of microclimate and competition in forest recovery long after the initial post-fire period. Our results lend support to the use of relatively early post-fire surveys to infer longer-term forest recovery trajectories for forest management planning and can help refine reforestation prioritization tools.

Ulcerative tumour-like disease of white fir in the Pokuttia Carpathians

The current state of dark coniferous forests on Earth is a bio-indicator of global environmental change, in particular, the process of gradual degradation of tree species of the genera Pine, Spruce and Fir, due to the negative impact of anthropogenic activities, abiotic and biotic factors. The purpose of the study is to establish the symptoms, prevalence and harmfulness of the causative agent of ulcerative tumour-like disease on the shoots and trunks of white fir. The research was conducted on the territory of the forest fund of the Pokuttia Carpathians, in particular, within the branch Kutske forestry of the State Specialised Economic Enterprise “Forests of Ukraine” in the Kosivske forest enterprise. The study uses classical phytopathological and mycological methods, which are used to investigate the aetiology of the disease, analyse typical symptoms (starting from the early stages), and perform microscopy of the affected parts of fir. The symptoms, aetiology, and pathogenesis of white fir ulcer-like disease, which tends to spread, were investigated. A new name for the disease is substantiated, in particular, it is proposed to use the name “ulcerative tumour-like disease” of fir instead of the current name “cancer”. It is shown that a typical sign of infection is the development of tumours, which eventually transform into ulcers. It was found that the tumour-like type of lesion prevails on the undergrowth of fir, and on plants of the upper tiers – ulcerative. It is noted that a typical sign of ulcerative tumour-like disease of white fir is also the presence of pathologies such as “witches’ broom”. It is proposed to remove affected trees with typical signs of infection with ulcerative tumour-like disease to reduce the infectious background, especially during lighting and cleaning, since affected trees are not released from infection. Therefore, assessing the risk of infection of white fir trees with ulcerative tumour-like disease at the level of specific stands and ensuring an appropriate response to potential infection risks is an important part of sustainable forest management

The effect of shrub cover on conifer water-use patterns, growth and response to precipitation variability in the southern Sierra Nevada.

As wildfires increase in size and severity, large areas of forest are undergoing substantial increases in shrub cover. In forests where water is the limiting resource, the partitioning of soil water between shrubs and young trees may determine how shrubs affect tree growth and water-stress. Here we evaluated juvenile trees (average age=32years) of two dominant conifer species in the southern Sierra Nevada of California (Abies concolor (white fir) and Pinus jeffreyi (Jeffrey pine)) growing in the presence or absence of shrubs. The two shrub species included Arctostaphylos patula and Ceanothus cordulatus, a nitrogen-fixing species. We analyzed the δ2H and δ18O values of xylem water for both tree and shrub species to assess how shrub cover affects the water-uptake patterns of conifers and whether there is niche partitioning between trees and shrubs. We found that growing near shrubs did not have a significant effect on the water source dynamics of either tree species, with similar source water contributions calculated for conifers growing in both the presence and absence of shrubs. Using a tree-ring analysis of growth and δ13C from 2016 to 2021, a period of high precipitation variability, we found that shrub cover had a positive effect on tree growth while decreasing carbon discrimination, which may be due to increased nitrogen availability from Ceanothus cordulatus. Overall, our results suggest that growing in the presence of shrubs does not alter the water uptake patterns of white fir and Jeffrey pine and instead may have a positive effect on the growth rates of these species during both wet and dry years.

CLIMATOGENIC INFLUENCE AND PREDICTION OF SEASONAL RHYTHM CHANGES IN THE MAIN FOREST-FORMING SPECIES OF THE NORTHEASTERN CARPATHIANS

The article is devoted to the study of the phenological phenomena of the main forest-forming species (European spruce - Picea abies, white fir - Abies alba and forest beech - Fagus sylvatica) within the temperate climate zone (600-800 m a.s.l.) on the territory of the Carpathian National Nature Park for the period 2000-2022. The study employed a comprehensive set of field, instrumental, laboratory, analytical, and mathematical-statistical research methods, along with certain approaches applied in phenology and meteorology. According to the results of the statistical analysis, a significant influence of precipitation in March on the beginning of bud swelling (Picea abies and Fagus sylvatica species), the beginning of flowering trees (Picea abies, Abies alba species) and the onset of fruit ripening (Picea abies, Fagus sylvatica species) was revealed. and the beginning of seed dispersal (Picea abies, Abies alba species). It should be noted that the minimum air temperatures in July significantly affect phenological events that begin and continue at this time and later, in particular, the beginning of fruit ripening (Abies alba, Fagus sylvatica species) and the onset of seeds dispersal (in all species). Strong and moderate correlations between January-February maximum temperatures and various phenological events (e.g., in Picea abies, dates of shoot growth and flowering initiation; in Abies alba, dates of flowering initiation and seed ripening initiation; in Fagus sylvatica, dates of bud swelling and flowering initiation) suggest a notable impact of underground water, originating from meltwater during this period, on plant seasonal rhythms. The most difficult for modeling was the phenomenon of the beginning of flowering of trees, the formation of which is influenced by all involved climate predictors - precipitation, minimum and maximum air temperatures. Based on the results of the research, the initial long-term forecasting of the response of local forest crops to probable climate changes was carried out; so, it was shown that even under the worst-case scenario (RCP 8.5), the dates of occurrence of all phenological phenomena of model tree species will be within the scope of the sample of actual data for the observation period of 2000-2022.

Utjecaj ekspozicije krošnje na morfološka svojstva iglica devet četinjača

The aim of this research was to investigate if the crown exposure of some conifers influenced to needle properties. The leaf morphological traits of sixty-six trees of nine conifers: Atlas cedar, Austrian pine, Blue spruce, Douglas fir, European spruce, European yew, Serbian spruce, Silver fir, and White fir, from six Belgrade parks, were analyzed. Five needles were measured from each of the four main crown exposures. Length, width, area and perimeter of needles were investigated. Species, parks in which they were found, as well as crown exposures, differed mostly in needle length and needle width. Correlations between measured needle traits were determined by linear regression analysis. Strong positive correlations were found between the length, perimeter, and area of needles. The differences among the species in terms of light requirement determine species for individual planting as light-loving or partial shade species (Atlas cedar, European spruce, Serbian spruce, Blue spruce, Austrian pine and Douglas fir), or for group planting as shade-loving species (Silver fir, White fir and European yew).

Predicting snag fall in an old-growth forest after fire

BackgroundSnags, standing dead trees, are becoming more abundant in forests as tree mortality rates continue to increase due to fire, drought, and bark beetles. Snags provide habitat for birds and small mammals, and when they fall to the ground, the resulting logs provide additional wildlife habitat and affect nutrient cycling, fuel loads, and fire behavior. Predicting how long snags will remain standing after fire is essential for managing habitat, understanding chemical cycling in forests, and modeling forest succession and fuels. Few studies, however, have quantified how fire changes snag fall dynamics.ResultsWe compared post-fire fall rates of snags that existed pre-fire (n = 2013) and snags created during or after the fire (n = 8222), using 3 years of pre-fire and 5 years of post-fire data from an annually monitored, 25.6-ha spatially explicit plot in an old-growth Abies concolor–Pinus lambertiana forest in the Sierra Nevada, CA, USA. The plot burned at low to moderate severity in the Rim Fire of 2013. We used random forest models to (1) identify predictors of post-fire snag fall for pre-existing and new snags and (2) assess the influence of spatial neighborhood and local fire severity on snag fall after fire. Fall rates of pre-existing snags increased 3 years after fire. Five years after fire, pre-existing snags were twice as likely to fall as new snags. Pre-existing snags were most likely to persist 5 years after fire if they were > 50 cm in diameter, > 20 m tall, and charred on the bole to heights above 3.7 m. New snags were also more likely to persist 5 years after fire if they were > 20 m tall. Spatial neighborhood (e.g., tree density) and local fire severity (e.g., fire-caused crown injury) within 15 m of each snag barely improved predictions of snag fall after fire.ConclusionsLand managers should expect fall rates of pre-existing snags to exceed fall rates of new snags within 5 years after fire, an important habitat consideration because pre-existing snags represent a wider range of size and decay classes.

Thinning and prescribed burning increase shade-tolerant conifer regeneration in a fire excluded mixed-conifer forest

Fire exclusion and past management have altered the composition, structure, and function of frequent-fire forests throughout western North America. In mixed-conifer forests of the California Sierra Nevada, fire exclusion has exacerbated the effects of drought and endemic bark beetles, resulting in extensive mortality of fire-adapted pine species. Thinning and prescribed fire are widely used in these forests to reduce fuels, moderate fire behavior, and restore ecosystems. Tree regeneration influences future forest composition and structure, and therefore future resilience to disturbances, but long-term effects of thinning and prescribed burning on tree regeneration after prolonged fire exclusion are poorly understood. We measured tree regeneration one year prior to, and periodically for 16 years following thinning and prescribed burning in a mixed-conifer forest in the Sierra Nevada, California, USA. We asked three questions. How did the composition and density of tree regeneration change after thinning and prescribed burning? Did pretreatment vegetation types influence conifer regeneration density after treatments? Did planting after overstory thinning increase regeneration density of native pine species?Sixteen years after treatments, combined natural regeneration of shade-tolerant white fir (Abies concolor) and incense-cedar (Calocedrus decurrens) averaged 2,032 trees per hectare (tph) after understory thinning, and 7,745 tph after understory thinning combined with prescribed burning, increases of 37 % and 146 % from pretreatment densities. In contrast, combined natural regeneration of white fir and incense-cedar averaged 497 tph after overstory thinning, 780 tph after overstory thinning with prescribed burning, 113 tph after prescribed burning alone, and 807 tph in untreated controls, all of which were declines from pretreatment densities. Natural regeneration of white fir and incense-cedar was consistently an order of magnitude greater than Jeffrey pine (Pinus jeffreyi) and sugar pine (Pinus lambertiana), whose combined densities 16 years after treatments averaged 37 tph across treatments and did not significantly respond to thinning and/or prescribed burning. Natural conifer regeneration after treatments varied by pre-treatment vegetation type (closed canopy, Ceanothus cordulatus shrub dominated, and open sparse), with large increases of natural regeneration after understory thinning in closed canopy and Ceanothus shrub vegetation types. Planting increased sugar pine regeneration density after overstory thinning, marginally increased Jeffrey pine regeneration after overstory thinning combined with prescribed burning, and increased white fir regeneration after overstory thinning with and without burning. No treatments reduced white fir and incense-cedar natural regeneration while simultaneously increasing natural pine regeneration, suggesting new thinning, burning, and planting approaches may be required to meet regeneration restoration objectives.

Ability of seedlings to survive heat and drought portends future demographic challenges for five southwestern US conifers.

Climate change and disturbance are altering forests and the rates and locations of tree regeneration. In semi-arid forests of the southwestern USA, limitations imposed by hot and dry conditions are likely to influence seedling survival. We examined how the survival of 1-year seedlings of five southwestern US conifer species whose southwestern distributions range from warmer and drier woodlands and forests (Pinus edulis Engelm., Pinus ponderosa Douglas ex C. Lawson) to cooler and wetter subalpine forests (Pseudotsuga menziesii (Mirb.) Franco, Abies concolor (Gord. & Glend.) Lindl. Ex Hildebr. and Picea engelmannii Parry ex Engelm.) changed in response to low moisture availability, high temperatures and high vapor pressure deficit in incubators. We used a Bayesian framework to construct discrete-time proportional hazard models that explained 55-75% of the species-specific survival variability. We applied these to the recent climate (1980-2019) of the southwestern USA as well as 1980-2099 CMIP5 climate projections with the RCP8.5 emissions pathway. We found that the more mesic species (i.e., P. menziesii, A. concolor and P. engelmannii) were more susceptible to the effects of hot and dry periods. However, their existing ranges are not projected to experience the conditions we tested as early in the 21st century as the more xeric P. edulis and P. ponderosa, leading to lower percentages of their existing ranges predicted to experience seedling-killing conditions. By late-century, extensive areas of each species southwestern range could experience climate conditions that increase the likelihood of seedling mortality. These results demonstrate that empirically derived physiological limitations can be used to inform where species composition or vegetation type change are likely to occur in the southwestern USA.

Species-Specific Level Variation in Polyamines in Coniferous and Deciduous Woody Plant Species in Urban Areas

Urban heat islands (UHIs) and global warming will unavoidably have a negative impact on human health in urban areas, making urban forests much more susceptible to the risk of heat waves than forests. It is pivotal for urban forest management to understand tree species’ adaptation mechanisms by focusing on the species-dependent variability of polyamines (PAs), significant players in the amelioration of biotic and abiotic stress in plants, to mitigate the negative effects of UHIs and global warming on human health. Based on this background, the content of major polyamines (PAs) (putrescine, spermidine, and spermine) and total phenolics and the corresponding antioxidant capacities were determined and analyzed in the 24 most prevalent deciduous and coniferous tree species found in urban areas, namely Futoški Park in Novi Sad (Serbia). High-performance liquid chromatography (HPLC) coupled with fluorometric detection (HPLC-FD) was used to separate and quantify major PAs from tree species. Results showed a species-specific level variation in polyamines, total phenolic, and antioxidant capacity in coniferous and deciduous woody plant species in inspected urban areas. In terms of total PA content, the most notable deciduous tree species were Betula pendula, Junglans regia, and Quercus rubra, while the coniferous tree species Thuja occidentalis, Taxodium distichum, Pinus nigra, and Abies concolor stand out. The most dominant foliar PA in most of the inspected species was putrescine (ranging from 527.67 to 10,049.3 nmol g−1 DW), followed by spermidine (from 250.56 to 2015.92 nmol g−1 DW) and spermine (from 168.8 to 718.41 nmol g−1 DW). Furthermore, significant intra-genus variability in terms of PA content was recorded within the genera Pinus, Thuja, and Picea. This study demonstrated that the PA and phenolic compounds, in combination with antioxidant assays, can serve as reliable and trustworthy criteria and descriptors for the selection of adaptable tree species in the context of urban climate–smart forestry.

Niche conservatism and strong phylogenetic signals in climate, soil, and morphological variation of Neotropical firs (Abies, Pinaceae)

AbstractInterspecific trait divergence may reflect adaptation and reproductive isolation, particularly after the rapid differentiation that may follow the colonization of new environments. Although new lineages are generally expected to be morphologically and ecologically similar to their ancestors, environmental forces can also drive adaptive differentiation along specific phenotypic axes. We used climate niche models and comparative analyses based on a previously inferred phylogeny to examine the history of ecological and morphological divergence of Neotropical firs (Abies Mill., Pinaceae), a group of conifers that have recently colonized and diversified in the mountains of Mexico and northern Central America. We inferred past secondary contact zones by comparing current and past climate niche projections and looked for evidence of recent interspecific gene flow using genomic data. Neotropical firs have similar niches to each other and show a strong phylogenetic signal for most evaluated morphological traits. Analyses based on individual variables suggested a random walk model of differentiation. However, early adaptation to tropical conditions is inferred in the ancestor of the southernmost firs, as all modern southern taxa are differentiated climatically from Abies concolor, the northernmost species. In addition, observed autapomorphic traits for soil properties and the number of resin ducts in needles are consistent with possible species‐specific adaptations. Thus, a combination of nonadaptive and adaptive processes along different phenotypic axes, some related to the environment, likely operated after the southward migration of this plant lineage from North America and its subsequent radiation in the Neotropics.

Differences in regeneration niche mediate how disturbance severity and microclimate affect forest species composition

Climate change is altering forest composition through species-specific responses to fire and drought. Future forest composition will depend on how the different regeneration niches of co-occurring species align with current environmental conditions, especially after fire, which can promote germination by exposing mineral soil. Few studies, however, have examined the effects of disturbance severity and microclimate on post-fire regeneration to define and compare the regeneration niches of co-occurring tree species. We used seven years of annual demography and microenvironment data from a 25.6-ha fully censused, stem-mapped forest dynamics plot in California, USA, to examine how disturbance severity, snow duration, and temperature extremes affect the survival of Abies concolor and Pinus lambertiana seedlings that germinated naturally after a low- to moderate-severity fire. We defined disturbance severity at the microsite level, based on characteristics of the substrate, and at the neighborhood level, based on tree mortality. Both disturbance severity and snow duration had species-specific effects on seedling survival, but these differed by life stage. During the germination year, later snow disappearance was associated with a 0.5 increase in survival probability for A. concolor but hardly affected P. lambertiana; in contrast, higher neighborhood disturbance severity increased survival of both species. After the germination year, higher substrate burn severity was associated with a 0.8 increase in survival probability for A. concolor but hardly affected P. lambertiana; higher neighborhood disturbance severity and later snow disappearance increased annual survival of both species, but maximum summer temperature had minimal effect. Overall, available seed, higher substrate burn severity, higher neighborhood disturbance severity, and later snow disappearance promoted natural regeneration. However, lower substrate burn severity and earlier snow disappearance in the germination year disadvantaged A. concolor seedlings, increasing the relative abundance of P. lambertiana seedlings compared to the local tree population. Our results indicate that natural post-fire compositional shifts toward drought-tolerant Pinus species–and away from less drought-tolerant Abies species–are possible in the Sierra Nevada, with potential benefits for forest persistence under climate change. Broadly, we show that species differences in regeneration niches shape how disturbance severity and microclimate affect forest species composition.

A comprehensive resilience assessment of Mexican tree species and their relationship with drought events over the last century.

The resilience of forests to drought events has become a major natural resource sustainability concern, especially in response to climate change. Yet, little is known about the legacy effects of repeated droughts, and tree species ability to respond across environmental gradients. In this study, we used a tree-ring database (121 sites) to evaluate the overall resilience of tree species to drought events in the last century. We investigated how climate and geography affected the response at the species level. We evaluated temporal trends of resilience using a predictive mixed linear modeling approach. We found that pointer years (e.g., tree growth reduction) occurred during 11.3% of the 20th century, with an average decrease in tree growth of 66% compared to the previous period. The occurrence of pointer years was associated with negative values of the Standardized Precipitation Index (SPI, 81.6%) and Palmer Drought Severity Index (PDSI, 77.3%). Tree species differed in their resilience capacity, however, species inhabiting xeric conditions were less resistant but with higher recovery rates (e.g., Abies concolor, Pinus lambertiana, and Pinus jeffreyi). On average, tree species needed 2.7 years to recover from drought events, with extreme cases requiring more than a decade to reach pre-drought tree growth rates. The main abiotic factor related to resilience was precipitation, confirming that some tree species are better adapted to resist the effects of droughts. We found a temporal variation for all tree resilience indices (scaled to 100), with a decreasing resistance (-0.56 by decade) and resilience (-0.22 by decade), but with a higher recovery (+1.72 by decade) and relative resilience rate (+0.33 by decade). Our results emphasize the importance of time series of forest resilience, particularly by distinguishing the species-level response in the context of legacy of droughts, which are likely to become more frequent and intense under a changing climate.

Spatial patterns of vigor by stand density across species groups and its drivers in a pre-harvest ponderosa pine-dominated landscape in northern California

Information on spatial patterns of forest health and stand vigor in historical forests is quite limited, particularly for forests of North America, which have been subsequently harvested. We used forest inventory data from 1934, which pre-dated early 20th century timber harvest, to reconstruct patterns of vigor in overstory trees. Across 4,000 ha of ponderosa pine-dominated forests in the Blacks Mountain Experimental Forest (BMEF), we described the spatial patterns of pre-harvest (1934) forests using a stand density index (SDI) and accounting for different species and vigor classes. We then identified topo-edaphic variables associated with pre-harvest SDI. To do so, we first calculated SDI for two species groups: 1) ponderosa pine (Pinus ponderosa Laws.) and Jeffrey pine (Pinus jeffreyi Grev. & Balf.), and 2) incense-cedar (Calocedrus decurrens (Torr.) Florin) and white fir (Abies concolor (Grod. and Glend.)) as well as for seven vigor classes within these two groups. Second, using Moran’s I, we found four spatially aggregated clusters (Moran’s I = 0.35): 1) ponderosa pine-high vigor, 2) ponderosa pine-low vigor, 3) mixed species-high vigor, and 4) mixed species-low vigor. Most of the pre-harvest landscape consisted of high vigor ponderosa pine clusters with low SDI (relative density (RD) < 0.25). High elevation sections consisted of mixed species clusters of both high and low vigor with high SDI (RD > 0.35). Using classification and regression tree analysis, we identified elevation (m), surface-soil depth (cm), and sub-soil depth (cm) were related to the clusters. A multinominal logistic regression model (kappa = 0.44; classification accuracy = 64.4 %) identified surface stoniness, available water capacity at 150 cm depth and its interaction with sub-soil depth as important variables that were related to clusters. Within ponderosa pine-dominated forests, managers consider dense stands of low vigor trees to be at-risk of insect, disease, and fire. Thus, our results can be used by managers as representative baselines for characterizing (pre-harvest) stand conditions to guide management treatments to support high vigor and low density forest conditions.

Dendroclimatic Analysis of Sierra Nevada Mixed Conifer Species between Different Diameter Size Classes

Climate change is expected to lead to higher temperatures in the Mediterranean region of northern California in the Sierra Nevada. Dendroclimatic studies typically focus on large, old trees, but there relatively limited understanding on how climatic sensitivity can vary with trees of different size classes. We collected tree increment cores and compared radial growth responses of small (20.32 to ≤40.64 cm), medium (40.64 to ≤60.96 cm), and large (&gt;60.96 cm) diameter mixed conifer species in the Sierra Nevada to different climate variables (temperature, precipitation, and climate moisture index (CMI)). The most common tree species encountered were white fir (Abies concolor (Gord. &amp; Glend.) Lindl.), followed by sugar pine (Pinus lambertiana Dougl.), ponderosa pine (Pinus ponderosa Dougl. Ex P. &amp; C. Laws), and incense cedar (Calocedrus decurrens (Torr.) Florin). One of the most coherent responses from all diameter groups and across all species was the positive response to increasing minimum winter temperatures. All diameter groups and species also responded positively to precipitation and CMI at some point in the analysis period, which is the seasonal window of April of the prior year to October of the current year of ring formation. Perhaps the most notable difference when comparing the three diameter groups to climate was the higher occurrence of negative responses to the temperature of the previous year from the largest diameter group, as well as the higher number of negative responses to temperature in general. These results suggest that larger trees may be more sensitive to future climate projections compared with smaller trees and they may carry those effects into the next year. The use of dendroclimatology to assess how mixed conifer species in the Sierra Nevada responded to past climate is a key resource that can be used to infer how trees may respond to a future changing climate.

Too hot, too cold, or just right: Can wildfire restore dry forests of the interior Pacific Northwest?

As contemporary wildfire activity intensifies across the western United States, there is increasing recognition that a variety of forest management activities are necessary to restore ecosystem function and reduce wildfire hazard in dry forests. However, the pace and scale of current, active forest management is insufficient to address restoration needs. Managed wildfire and landscape-scale prescribed burns hold potential to achieve broad-scale goals but may not achieve desired outcomes where fire severity is too high or too low. To explore the potential for fire alone to restore dry forests, we developed a novel method to predict the range of fire severities most likely to restore historical forest basal area, density, and species composition in forests across eastern Oregon. First, we developed probabilistic tree mortality models for 24 species based on tree characteristics and remotely sensed fire severity from burned field plots. We applied these estimates to unburned stands in four national forests to predict post-fire conditions using multi-scale modeling in a Monte Carlo framework. We compared these results to historical reconstructions to identify fire severities with the highest restoration potential. Generally, we found basal area and density targets could be achieved by a relatively narrow range of moderate-severity fire (roughly 365-560 RdNBR). However, single fire events did not restore species composition in forests that were historically maintained by frequent, low-severity fire. Restorative fire severity ranges for stand basal area and density were strikingly similar for ponderosa pine (Pinus ponderosa) and dry mixed-conifer forests across a broad geographic range, in part due to relatively high fire tolerance of large grand (Abies grandis) and white fir (Abies concolor). Our results suggest historical forest conditions created by recurrent fire are not readily restored by single fires and landscapes have likely passed thresholds that preclude the effectiveness of managed wildfire alone as a restoration tool.

Fire-caused mortality within tree neighborhoods increases growth of Pinus lambertiana more than growth of Abies concolor

Global increases in large-tree mortality and multiple disturbances such as fire, drought, and pestilence increase the importance of understanding how large-tree growth responds to changes in competitive neighborhoods. We used 34,175 spatially mapped trees within 25.6 ha that burned at low to moderate severity in 2013 to investigate how changes to tree neighborhoods affected growth of 3,652 surviving trees of two species, Abies concolor (white fir) and Pinus lambertiana (sugar pine). Using diameter measurements taken in 2014 and 2019, we estimated post-fire radial growth of each surviving tree. We modeled annual basal area increment as a function of diameter and extracted residuals to yield relative growth rate (RGR). We used a multi-model approach and AIC to compare the species-specific effects on RGR of all post-fire tree mortality within a neighborhood radius versus mortality broken down by cause. To represent density reduction around each surviving tree, we computed crowding metrics for neighborhood radii up to 20 m for all tree mortality combined and for each mortality cause. The best-fit model included terms for density reduction within 10 m of the surviving tree due to 1) fire or mechanical damage and 2) bark beetle mortality, as well as their interactions with species and diameter. Density reduction due to fire or mechanical damage had a positive effect on RGR that was nearly twice as strong for P. lambertiana as A. concolor. Tree diameter interacted with density reduction for just P. lambertiana. Small-diameter (10-cm) P. lambertiana had a fourfold larger increase in basal area increment than large-diameter (60-cm) P. lambertiana as density reduction due to fire or mechanical damage ranged from 0 to 100 %. The strong growth response of medium-diameter P. lambertiana to density reductions could eventually increase the abundance of large-diameter (≥60 cm) P. lambertiana. In conjunction with mortality rates, which were lower for large-diameter P. lambertiana (0.97 %) than A. concolor (3.31 %), this could increase the relative abundance of large-diameter P. lambertiana from 51.2 % in 2020 to 62.6 % by 2050. Changes to competitive neighborhoods can have species-specific effects on large-tree growth, an important consideration for long-term management of fire-prone forests.

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 resistance to drought and bark beetle-associated mortality following thinning and prescribed fire treatments

Long-term trends show increased tree mortality over the last several decades, coinciding with above-average temperatures, high climatic water deficits, and bark beetle outbreaks. California’s recent unprecedented drought (2012–2016) highlights the need to evaluate whether thinning and prescribed fire can improve individual tree drought resistance and reduce bark beetle-associated mortality. Using a thinning and prescribed fire study on the Stanislaus-Tuolumne Experimental Forest in the central Sierra Nevada implemented prior to the drought (2011–2013), we used dendrochronological methods to estimate metrics of tree vigor (i.e., growth and resin ducts) of sugar pine (Pinus lambertiana Douglas) and white fir (Abies lowiana [Gordon & Glend.] A. Murray bis) among treatments, as well as between trees that died from bark beetle-associated mortality and their paired counterparts that survived. We used tree vigor to estimate drought resistance as the ratio between growth during drought (2012–2016) and pre-drought (2007–2011) for both species. For sugar pine, we also created analogous ratios for multiple resin duct characteristics to evaluate defense during drought. Our findings indicate that lower competition increased growth resistance of white fir, while prescribed fire had negligible impacts on growth. This translated to lower mortality, with live white fir showing higher growth resistance than those that died. While competition did not strongly affect sugar pine growth, greater growth resistance was noted for trees that lived than trees that died. However, reduced competition and prescribed fire increased defense resistance and resin duct density and relative resin duct area were negatively associated with sugar pine mortality. Live sugar pine showed greater defense resistance than dead counterparts particularly under higher levels of competition. These findings suggest thinning can promote or maintain growth during severe drought conditions and prescribed fire can be applied with negligible costs to tree growth while also producing the additional benefit of stimulating defense systems in sugar pine, which may enable them to better survive bark beetle outbreaks. Therefore, susceptibility to bark beetle-associated mortality may be ameliorated through increasing tree vigor with a combination of forest thinning and prescribed fire.

Rust Disease Caused by Uredinopsis osmundae on Balsam Fir, Canaan Fir, White Fir, and Sensitive Fern Reported from Pennsylvania, United States

Pennsylvania is the fourth-largest Christmas-tree-producing state in the United States, with annual sales of more than $22 million. During the summers of 2018 and 2019, a rust disease was observed on the leaves of balsam fir, Canaan fir, white fir, and sensitive fern in a Christmas tree crop in Pennsylvania. The specimens were sent to the Arthur Fungarium at Purdue University for further identification and long-term deposit. Based on morphological characteristics and sequence analysis, the rust species was identified as Uredinopsis osmundae. To our knowledge, this is the first report of U. osmundae on balsam fir, Canaan fir, white fir, and sensitive fern in Pennsylvania.

Predictive accuracy of post-fire conifer death declines over time in models based on crown and bole injury.

A key uncertainty of empirical models of post-fire tree mortality is understanding the drivers of elevated post-fire mortality several years following fire, known as delayed mortality. Delayed mortality can represent a substantial fraction of mortality, particularly for large trees that are a conservation focus in western US coniferous forests. Current post-fire tree mortality models have undergone limited evaluation of how injury level and time since fire interact to influence model accuracy and predictor variable importance. Less severe injuries potentially serve as an indicator for vulnerability to additional stressors such as bark beetle attack or moisture stress. We used a collection of 164,293 individual tree records to examine post-fire tree mortality in eight western USA conifers: Abies concolor, Abies grandis, Calocedrus decurrens, Larix occidentalis, Pinus contorta, Pinus lambertiana, Pinus ponderosa, and Pseudotsuga menziesii. We evaluated the importance of fire injury predictors on discriminating between surviving trees versus immediate and delayed post-fire mortality. We fit balanced random forest models for each species using cumulative tree mortality from 1 to 5-years post-fire. We compared these results to multi-class random forest models using first-year mortality, 2-5-year mortality, and survival 5-years post-fire as a response variable. Crown volume scorched, diameter at breast height, and relative bark char height, were used as predictor variables. The cumulative mortality models all predicted trees that died within 1-year of fire with high accuracy but failed to predict 2-5-year mortality. The multi-class models were an improvement but had lower accuracy for predicting 2-5-year mortality. Multi-class model accuracies ranged from 85% to 95% across all species for predicting 1-year post-fire mortality, 42%-71% for predicting 2-5-year mortality, and 64%-85% for predicting trees that lived past 5-years. Our study highlights the differences in tree species tolerance to fire injury and suggests that including second-order predictors such as beetle attack or climatic water stress before and after fire will be critical to improve accuracy and better understand the mechanisms and patterns of fire-caused tree death. Random forest models have potential for management applications such as post-fire harvesting and simulating future stand dynamics.