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Giant sequoia (Sequoiadendron giganteum) is an iconic conifer that lives in relict populations on the western slopes of the California Sierra Nevada. In these settings, it is unusual among the dominant trees in that it associates with arbuscular mycorrhizal fungi rather than ectomycorrhizal fungi. However, it is unclear whether differences in microbial associations extend more broadly to nonmycorrhizal components of the soil microbial community. To address this question, we used next-generation amplicon sequencing to characterize bacterial/archaeal and fungal microbiomes in bulk soil (0-5 cm) beneath giant sequoia and co-occurring sugar pine (Pinus lambertiana) individuals. We did this across two groves with distinct parent material in Yosemite National Park, USA. We found tree-associated differences were apparent despite a strong grove effect. Bacterial/archaeal richness was greater beneath giant sequoia than sugar pine, with a core community double the size. The tree species also harbored compositionally distinct fungal communities. This pattern depended on grove but was associated with a consistently elevated relative abundance of Hygrocybe species beneath giant sequoia. Compositional differences between host trees correlated with soil pH and soil moisture. We conclude that the effects of giant sequoia extend beyond mycorrhizal mutualists to include the broader community and that some but not all host tree differences are grove-dependent.

Conifers are the dominant plant species throughout the high latitude boreal forests as well as some lower latitude temperate forests of North America, Europe, and Asia. As such, they play an integral economic and ecological role across much of the world. This study focused on the characterization of needle transcriptomes from four ecologically important and understudied North American white pines within the Pinus subgenus Strobus The populations of many Strobus species are challenged by native and introduced pathogens, native insects, and abiotic factors. RNA from the needles of western white pine (Pinus monticola), limber pine (Pinus flexilis), whitebark pine (Pinus albicaulis), and sugar pine (Pinus lambertiana) was sampled, Illumina short read sequenced, and de novo assembled. The assembled transcripts and their subsequent structural and functional annotations were processed through custom pipelines to contend with the challenges of non-model organism transcriptome validation. Orthologous gene family analysis of over 58,000 translated transcripts, implemented through Tribe-MCL, estimated the shared and unique gene space among the four species. This revealed 2025 conserved gene families, of which 408 were aligned to estimate levels of divergence and reveal patterns of selection. Specific candidate genes previously associated with drought tolerance and white pine blister rust resistance in conifers were investigated.

Abstract Giant sequoia (Sequoiadendron giganteum) is an iconic conifer that lives in relict populations on the western slopes of the California Sierra Nevada. In these settings, it is unusual among the dominant trees in that it associates with arbuscular mycorrhizal fungi rather than ectomycorrhizal fungi. However, it is unclear whether differences in microbial associations extend more broadly to nonmycorrhizal components of the soil microbial community. To address this question, we used next‐generation amplicon sequencing to characterize bacterial/archaeal and fungal microbiomes in bulk soil (0–5 cm) beneath giant sequoia and co‐occurring sugar pine (Pinus lambertiana) individuals. We did this across two groves with distinct parent material in Yosemite National Park, USA. We found tree‐associated differences were apparent despite a strong grove effect. Bacterial/archaeal richness was greater beneath giant sequoia than sugar pine, with a core community double the size. The tree species also harbored compositionally distinct fungal communities. This pattern depended on grove but was associated with a consistently elevated relative abundance of Hygrocybe species beneath giant sequoia. Compositional differences between host trees correlated with soil pH and soil moisture. We conclude that the effects of giant sequoia extend beyond mycorrhizal mutualists to include the broader community and that some but not all host tree differences are grove‐dependent.

Societal Impact Statement Damage to white pine forests by white pine blister rust, caused by the non‐native invasive fungal pathogen Cronartium ribicola, is affecting people across western North America. The loss of white pine forests can impact economic opportunities as well as ecosystem and human well‐being. In this paper we quantify the durability, stability, and usability of natural genetic resistance to the disease in two white pine species using long‐term field trials. By combining quantitative genetic resistance breeding programs with data from field validation trials, land managers and policy makers will be able to better ensure healthy forests for the future. Summary Many tree species are susceptible to non‐native pathogens or pests. Their level of susceptibility can be so severe that they are extirpated, and their use for forest restoration or reforestation is curtailed. Programs to find, enhance, and utilize genetic resistance in tree species are underway. Because trees are long‐lived, resistance must be effective for decades to centuries to be useful. Resistance needs to be durable, stable, and present at a useful level. Field plantings provide the best opportunity to assess resistance durability and stability across a range of environments. In this paper we examine the durability and stability of resistance levels, previously identified in seedling screening trials, of two white pine species, Pinus monticola (western white pine) and P. lambertiana (sugar pine) to white pine blister rust, using eight 15 to 20‐year‐old field trials in Oregon and northern California. We found that resistance varies for each host species and in different environments. Major gene resistance (MGR) may have limited utility and the search for resistance should include all types of resistance, as quantitative disease resistance (QDR) appears to be more durable than MGR in the long term at many sites in our study. Our data provide encouragement and support for the use of long‐term field validation studies in combination with quantitative genetic resistance breeding programs. We advocate for an increased use of field trials to ensure that resistance is effective for restoration and recovery against invasive pathogens and pests.

Conifers are the dominant plant species throughout the high latitude boreal forests as well as some lower latitude temperate forests of North America, Europe, and Asia. As such, they play an integral economic and ecological role across much of the world. This study focused on the characterization of needle transcriptomes from four ecologically important and understudied North American white pines within the Pinus subgenus Strobus. The populations of many Strobus species are challenged by native and introduced pathogens, native insects, and abiotic factors. RNA from the needles of western white pine (Pinus monticola), limber pine (Pinus flexilis), whitebark pine (Pinus albicaulis), and sugar pine (Pinus lambertiana) was sampled, Illumina short read sequenced, and de novo assembled. The assembled transcripts and their subsequent structural and functional annotations were processed through custom pipelines to contend with the challenges of non-model organism transcriptome validation. Orthologous gene family analysis of over 58,000 translated transcripts, implemented through Tribe-MCL, estimated the shared and unique gene space among the four species. This revealed 2025 conserved gene families, of which 408 were aligned to estimate levels of divergence and reveal patterns of selection. Specific candidate genes previously associated with drought tolerance and white pine blister rust resistance in conifers were investigated.

Sugar pine trees from nine stands in two California study areas were assessed to determine the effects of pruning on the incidence and growth of white pine blister rust. Lower limbs up to 8 feet high were removed on alternate trees. Six years following treatment, the number of infections in pruned trees was reduced compared to unpruned trees at one study area, but no blister rust was found at the other area. The results suggest that artificial pruning of sugar pine may be part of an effective, integrated strategy to maintain this species in mixed-conifer California forests.

Societal Impact Statement: Damage to white pine forests by white pine blister rust, caused by the non‐native invasive fungal pathogen Cronartium ribicola, is affecting people across western North America. The loss of white pine forests can impact economic opportunities as well as ecosystem and human well‐being. In this paper we quantify the durability, stability, and usability of natural genetic resistance to the disease in two white pine species using long‐term field trials. By combining quantitative genetic resistance breeding programs with data from field validation trials, land managers and policy makers will be able to better ensure healthy forests for the future. Summary Many tree species are susceptible to non‐native pathogens or pests. Their level of susceptibility can be so severe that they are extirpated, and their use for forest restoration or reforestation is curtailed. Programs to find, enhance, and utilize genetic resistance in tree species are underway. Because trees are long‐lived, resistance must be effective for decades to centuries to be useful. Resistance needs to be durable, stable, and present at a useful level. Field plantings provide the best opportunity to assess resistance durability and stability across a range of environments.In this paper we examine the durability and stability of resistance levels, previously identified in seedling screening trials, of two white pine species, Pinus monticola (western white pine) and P. lambertiana (sugar pine) to white pine blister rust, using eight 15 to 20‐year‐old field trials in Oregon and northern California.We found that resistance varies for each host species and in different environments. Major gene resistance (MGR) may have limited utility and the search for resistance should include all types of resistance, as quantitative disease resistance (QDR) appears to be more durable than MGR in the long term at many sites in our study.Our data provide encouragement and support for the use of long‐term field validation studies in combination with quantitative genetic resistance breeding programs. We advocate for an increased use of field trials to ensure that resistance is effective for restoration and recovery against invasive pathogens and pests. [ABSTRACT FROM AUTHOR]

• Disturbance severity and snow duration have species-specific effects on seedlings. • Lower substrate burn severity disadvantaged Abies concolor seedlings. • Earlier snowmelt in the germination year decreased Abies concolor survival. • Post-fire compositional shifts toward drought-tolerant Pinus species are possible. 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. [ABSTRACT FROM AUTHOR]

Conifers are the dominant plant species throughout the high latitude boreal forests as well as some lower latitude temperate forests of North America, Europe, and Asia. As such, they play an integral economic and ecological role across much of the world. This study focused on the characterization of needle transcriptomes from four ecologically important and understudied North American white pines within the Pinus subgenus Strobus. The populations of many Strobus species are challenged by native and introduced pathogens, native insects, and abiotic factors. RNA from the needles of western white pine (Pinus monticola), limber pine (Pinus flexilis), whitebark pine (Pinus albicaulis), and sugar pine (Pinus lambertiana) was sampled, Illumina short read sequenced, and de novo assembled. The assembled transcripts and their subsequent structural and functional annotations were processed through custom pipelines to contend with the challenges of non-model organism transcriptome validation. Orthologous gene family analysis of over 58,000 translated transcripts, implemented through Tribe-MCL, estimated the shared and unique gene space among the four species. This revealed 2025 conserved gene families, of which 408 were aligned to estimate levels of divergence and reveal patterns of selection. Specific candidate genes previously associated with drought tolerance and white pine blister rust resistance in conifers were investigated. [ABSTRACT FROM AUTHOR]

Many forest ecosystems with a large pine component in the western United States have experienced environmental stress associated with climate change and increased competition with forest densification in the absence of fire. Information on how changes in climate and competition affect carbon allocation to tree growth and defense is needed to anticipate changes to tree vigor and, ultimately, stand structure. This study retrospectively examined the influence of annual climate and competition measures on the growth and defense of 113 large sugar pines (Pinus lambertiana) in a mixed-conifer forest of the central Sierra Nevada of California. We found that growth in large sugar pine was positively associated with higher January temperatures and lower intraspecific competition. Resin duct size was negatively associated with climatic water deficit and total competition, while resin duct area contrastingly showed a positive relationship with total competition. From 1979 to 2012, the rates of growth increased, while resin duct size decreased. Our results suggest that tree vigor measures can respond differently to climate and competition factors that may lead to separate growth and defense trends over time. Stress associated with warmer temperatures and higher competition may distinctly influence individual tree and stand-level vigor with potential implications for future forest dynamics. [ABSTRACT FROM AUTHOR]

Trees in more competitive environments appear to respond to climate differently than trees in less competitive environments. In turn, climate patterns may affect inter- or intra-specific competition, favoring certain individuals over others. Using dendrochronological methods, we sampled cores from dominant pine trees and their nearby competitors in 40 stands in the northern Sierra Nevada. Our dendroclimatological analyses showed complex responses to climate for the dominant pine and their competitors. Pinus lambertiana Dougl. and Pinus ponderosa Dougl. Ex P. & C. Laws benefited from cooler temperatures in the past May; all species benefited from warmer winter temperatures; Calocedrus decurrens (Torr.) Florin benefited from cooler temperatures in the past summer, while Abies concolor (Gord. & Glend.) Lindl.) and the two pine species benefited from cooler temperatures in the present summer. All species showed consistently positive effects from increased precipitation and increased moisture availability. A competition ratio calculated as the ratio between the dominant tree’s radial increments to the competitor’s indicated that climate could influence competition through one of two mechanisms. One, negative responses to climate conditions that were negative for one species but neutral for another tended to drive the competition ratios. Two, conditions that were negative for both competing trees tended to hurt competitors more than dominant trees, while conditions that were generally positive tended to help competitors more than dominant trees. These results suggest that the dominant pine trees may be more resilient to climate stress than competitors. Reducing competition via management such as thinning may increase climatic resilience for all species and possibly have positive competitive results for competitors. Increasing temperatures will tend to harm Sierra Nevada conifers in the summer and spring, but may show some benefit over the winter as long as precipitation is not reduced. [ABSTRACT FROM AUTHOR]

• Density reduction from fire or mechanical damage increased post-fire growth rates. • Density reduction from bark beetle mortality did not affect post-fire growth rates. • Pinus had a stronger post-fire growth response to competitive release than Abies. • Mortality and diameter distribution, not growth, control composition of large trees. 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. [ABSTRACT FROM AUTHOR]

• Treatments help multiple species resist drought and bark beetle outbreak. • Thinning and prescribed fire promote tree resistance to multiple disturbances. • Despite unprecedented drought, thinning and prescribed fire mitigate tree mortality. 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. [ABSTRACT FROM AUTHOR]

Field identification of dwarf mistletoes (Arceuthobium; Viscaceae) often is complicated due to their reduced morphology and sexual dimorphism, requiring the integration of not only morphologic measurements, but also, geographic, host, and/or phenological data. Difficulties in species determination have plagued the taxonomic classification of closely-related Arceuthobium, and, hence, their recognition as distinct species has also been questioned. Troubles in the classification of Arceuthobium are typified by five taxas in section Campylopoda, ser. Campylopoda--A. apachecum, A. blumeri, A. californicum, A. cyanocarpum, and A. monticola--that infect white pines (Pinus; subg. Strobus) as principal hosts. Each of the aforementioned taxa recently were circumscribed in synonymy with or reduced to subspecies of A. campylopodum; we contend that they deserve separate species recognition. To support our position, we conducted morphological analysis for these six taxa using univariate and multivariate statistical approaches. Results demonstrated that Arceuthobium apachecum, A. blumeri, A. californicum, A. cynanocarpum, and A. monticola can be determined readily to species using morphological data without consideration of geographic location or host specificity. Moreover, all but two of 20 morphological characters across female and male plants of the white pine dwarf mistletoes were significantly different when compared to A. campylopodum. Female and male plants of the latter species were also easily segregated from each of the white pine dwarf mistletoes in the combined analysis of only three characters. Thus, the white pine dwarf mistletoes are well-differentiated morphologically, and therefore, should be considered distinct species. [ABSTRACT FROM AUTHOR]

Many forest ecosystems with a large pine component in the western United States have experienced environmental stress associated with climate change and increased competition with forest densification in the absence of fire. Information on how changes in climate and competition affect carbon allocation to tree growth and defense is needed to anticipate changes to tree vigor and, ultimately, stand structure. This study retrospectively examined the influence of annual climate and competition measures on the growth and defense of 113 large sugar pines (Pinus lambertiana) in a mixed-conifer forest of the central Sierra Nevada of California. We found that growth in large sugar pine was positively associated with higher January temperatures and lower intraspecific competition. Resin duct size was negatively associated with climatic water deficit and total competition, while resin duct area contrastingly showed a positive relationship with total competition. From 1979 to 2012, the rates of growth increased, while resin duct size decreased. Our results suggest that tree vigor measures can respond differently to climate and competition factors that may lead to separate growth and defense trends over time. Stress associated with warmer temperatures and higher competition may distinctly influence individual tree and stand-level vigor with potential implications for future forest dynamics.

• Radial thinning aims to reduce focal tree mortality by reducing local competition. • 16 years post-treatment, large sugar pine mortality did not differ among treatments. • Radial thinning 12.5 m around surviving focal trees increased tree diameter growth. • Radial thinning 12.5 m around surviving focal trees increased sugar pine seedlings. • No clear, consistent benefits resulted from the radial thinning treatments tested. White pines (Pinus , subgenus Strobus) in North America are highly vulnerable to the native mountain pine beetle (Dendroctonus ponderosae), invasive white pine blister rust (Cronartium ribicola), and consequences of altered disturbance regimes, with sugar pine (Pinus lambertiana) being among the most in peril. Radial thinning treatments reflect an effort to balance the needs of late seral, high forest cover for some wildlife species with the desire to also promote habitat for species requiring higher light and lower competition, as well as to create heterogenous forest structure that can be more resilient to wildfire and drought. We examined 16-year effects of three radial thinning treatments that cleared approximately 0.017 ha to 0.05 ha around focal large sugar pines on reducing tree mortality and increasing sugar pine recruitment relative to untreated, control trees. Thinning treatments by order of increasing intensity included a compressed radius treatment which removed all trees and shrubs to 3 m from the crown edge, an extended radius with large tree retention treatment which removed trees <64 cm DBH and shrubs to 7.6 m from the crown edge, and an extended radius treatment which removed all trees and shrubs to 7.6 m outside of the crown edge. None of the three radial thinning treatments tested reduced mortality relative to the control, and mountain pine beetle caused the most mortality across the four treatments. However, mortality trended lower in the extended radius with retention treatment (19% compared to 30–37%) and was the only treatment where no mortality from wind or insects other than mountain pine beetle occurred. For trees that survived through the 16-year monitoring period, only trees in the extended radius treatment experienced increased growth above pre-treatment values and higher presence of sugar pine seedlings. Our results show no clear benefit was derived from the radial thinning treatments tested. While radial thinning may be a useful tool when used in conjunction with other silvicultural prescriptions that create heterogeneous stands and landscapes with a variety of forest structures, it seems unlikely that employing only radial thinning will meet restoration efforts that aim to decrease large tree mortality. This study highlights the value of long-term monitoring in the continual effort to develop new treatments that are effective at increasing forest resilience. [ABSTRACT FROM AUTHOR]

Fire suppression has resulted in dramatic changes to species composition and structural diversity in the Sierra Nevada mixed-conifer forests. We need a better understanding of how these forests have changed during the fire suppression era, but empirical historical datasets are rare and methodologies for developing new historical reference information are subject to limitations. We sought to develop historical reference information for the Yosemite Forest Dynamics Plot (YFDP), a research plot located in an old-growth mixed-conifer forest in Yosemite National Park. We performed a dendrochronologial fire history analysis to characterize the historical fire regime of the YFDP, resulting in an estimated pre-1900 point fire return interval of 29.5 years. We then developed two different forest reconstruction models to estimate structural and compositional forest changes since 1900, the year the last widespread fire burned the YFDP, to the present. We explored the use of two different tree growth models—a regionally parameterized competition-dependent model and a parsimonious site-specific model—as well as a decay model based on published estimates of wood decay rates. The competition-dependent growth model predicted slightly higher stem densities (175 trees ha −1 vs. 112 trees ha −1 in 1900) and slightly lower basal area (20.9 m 2 ha −1 vs. 24.1 m 2 ha −1 in 1900) than the site-specific growth model. Predictions about dead trees, especially large diameter sugar pines, are potentially inaccurate, both in this study and other reconstruction studies in the Sierra Nevada, due to a lack of size-specific snag and log decay rate data. Our study highlights the need for more detailed decay data for Sierra Nevada mixed-conifer forests. While reconstruction models are constrained by the data used to parameterize them, they can still produce estimates of historical conditions that are useful for understanding the direction and magnitude of forest change, as well as for planning forest management. Our analysis demonstrates dramatic changes in forest conditions since fire-exclusion—a clear ecological basis for restoration. We encourage managers to focus on restoring key ecological processes that maintain ecosystem structure and function, in this case fire, rather than attempting tree-for-tree recreations of historical structure without reintroducing fire. [ABSTRACT FROM AUTHOR]

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

Mosaics of tree clumps and openings are characteristic of forests dominated by frequent, low- and moderate-severity fires. When restoring these fire-suppressed forests, managers often try to reproduce these structures to increase ecosystem resilience. We examined unburned and burned forest structures for 1937 0.81 ha sample areas in Yosemite National Park, USA. We estimated severity for fires from 1984 to 2010 using the Landsat-derived Relativized differenced Normalized Burn Ratio (RdNBR) and measured openings and canopy clumps in five height strata using airborne LiDAR data. Because our study area lacked concurrent field data, we identified methods to allow structural analysis using LiDAR data alone. We found three spatial structures, canopy-gap, clump-open, and open, that differed in spatial arrangement and proportion of canopy and openings. As fire severity increased, the total area in canopy decreased while the number of clumps increased, creating a patchwork of openings and multistory tree clumps. The presence of openings > 0.3 ha, an approximate minimum gap size needed to favor shade-intolerant pine regeneration, increased rapidly with loss of canopy area. The range and variation of structures for a given fire severity were specific to each forest type. Low- to moderate-severity fires best replicated the historic clump-opening patterns that were common in forests with frequent fire regimes. Our results suggest that managers consider the following goals for their forest restoration: 1) reduce total canopy cover by breaking up large contiguous areas into variable-sized tree clumps and scattered large individual trees; 2) create a range of opening sizes and shapes, including ~ 50% of the open area in gaps > 0.3 ha; 3) create multistory clumps in addition to single story clumps; 4) retain historic densities of large trees; and 5) vary treatments to include canopy-gap, clump-open, and open mosaics across project areas to mimic the range of patterns found for each forest type in our study. [ABSTRACT FROM AUTHOR]

Variability of postfire injury and stand and individual tree factors that affected survival responses of eastern Sierra Nevada conifers to wildfire were examined. Prefire measurements served as a basis of comparison for postfire conditions in a mixed conifer stand located in the eastern portion of the Lake Tahoe Basin and provided insight into predisposing influences on survival. Species composition consisted primarily of Jeffrey pine (Pinus jeffreyi Grev. & Balf.) and California white fir (Abies concolor var. lowiana [Gord.] Lemm.) along with a minor component of sugar pine (Pinus lambertiana Dougl.). Postfire survival was higher in Jeffrey pine than white fir but was highest overall in sugar pine. Catastrophic crown loss occurred less frequently in Jeffrey pine than in the fir but was least common in sugar pine. Survival generally increased with tree size, but this relationship did not extend to the largest trees in the stand. Among an array of regression models used to evaluate selected variables for their predictive capacity regarding postfire survival, prefire stand density was found to negatively influence that of Jeffrey and sugar pine, and survival of Jeffrey pine and white fir was negatively correlated with bole char. These results provide natural resource managers guidance in the selection of viable trees for retention during the salvage harvesting operations that often follow wildfire events. [ABSTRACT FROM PUBLISHER]

We investigated the physical and burning characteristics of sugar pine (Pinus lambertiana Douglas) cones and their contribution to woody surface fuel loadings. Field sampling was conducted at the Yosemite Forest Dynamics Plot (YFDP), a 25.6 ha mapped study plot in Yosemite National Park, California, USA. We developed a classification system to describe sugar pine cones of different sizes and decay conditions, and examined differences among cone classes in biomass, bulk density, flame length, burning time, consumption, and relative contribution to surface fuel loads. Sugar pine cones comprised 601 kg ha-1 of surface fuels. Mature cones comprised 54 % of cone biomass, and aborted juvenile cones accounted for 44 %. Cone biomass, diameter, and bulk density differed among cone condition classes, as did burning characteristics (one-way ANOVA, P < 0.001 in all cases). Flame lengths ranged from 5 cm to 94 cm for juvenile cones, and 71 cm to 150 cm for mature cones. Our results showed that the developmental stage at which sugar pine cones become surface fuels determines their potential contribution to surface fire behavior in Sierra Nevada mixed-conifer forests. Sugar pine cones burn with greater flame lengths and flame times than the cones of other North American fire-tolerant pine species studied to date, indicating that cones augment the surface fire regime of sugar pine forests, and likely do so to a greater degree than do cones of other pine species. [ABSTRACT FROM AUTHOR]

Abstract: Historical logging, fire suppression, and an invasive pathogen, Cronartium ribicola, the cause of white pine blister rust (WPBR), are assumed to have dramatically affected sugar pine (Pinus lambertiana) populations in the Lake Tahoe Basin. We examined population- and genetic-level consequences of these disturbances within 10 sugar pine populations by assessing current population structure and trends for 1129 individuals, genetic diversity for 250 individuals, and frequency of WPBR-resistance for 102 families. Logging had occurred in 9 of 10 sites and fire suppression was evident in all stands. High density of white fir (Abies concolor) is often an indicator of fire suppression and we found a negative relationship between sugar pine survivorship and white fir basal area (r 2 =0.31). C. ribicola was present in 90% of stands (incidence range: 0–48%) and we found a significant relationship between mean host survivorship and disease incidence (r 2 =0.46). We estimated population growth rates (λ) from size-based transition matrices. For six of 10 sugar pine populations λ was ⩾1.0, indicating that these populations appear to be stable; for four populations, λ was <1.0, indicating populations that may be in decline. A population specific drift parameter, ci , which is a measure of genetic differentiation in allele frequencies relative to an ancestral population, ranged from 0.009 to 0.048. Higher values of ci indicate greater genetic drift, possibly due to a bottleneck caused by historical logging, other agents of mortality or much older events affecting population sizes. Effects of drift are known to be greater in small populations and we found a negative relationship between sugar pine density and ci (r 2 =0.36). Allele frequency of the Cr1 gene, responsible for WPBR-resistance in sugar pine, averaged 0.068 for all populations sampled; no WPBR infection was found in one population in which the Cr1 frequency was 0.112. Historical disturbances and their interactions have likely influenced the population biology of sugar pine in the Tahoe Basin; for some populations this has meant reduced population size, higher genetic drift, and poor survival of small- and intermediate-sized individuals. Possible management strategies include restoring population numbers, deploying WPBR-resistance, treating stands to promote natural sugar pine regeneration, and enhancing genetic diversity. [Copyright &y& Elsevier]

Abstract: Mature shrubs can provide microhabitats that are beneficial to tree seedling growth and development. Sugar pine trees (Pinus lambertiana) grow in a narrow zone on the eastern slope of the Carson Range in extreme western Nevada, whereas Jeffrey pine (Pinus jeffreyi) is the dominant tree species in the region, an area extensively disturbed by wild fire. This study compares seedling establishment of sugar pine and Jeffrey pine relative to mature shrubs. In the fall of 2002 (cohort 1) and 2003 (cohort 2), 13,600 seeds of both species were planted in wire mesh enclosures, at three sites, under a variety of microhabitat treatments: under shade and in the open, under two species of shrub cover, and with and without plant litter. Seedlings were monitored for survival through two growing seasons. Even though more sugar pine seedlings emerged, more Jeffrey pine seedlings survived, and Jeffrey pine was the more drought tolerant species, better suited for the xeric climate found in the Carson Range. Litter slightly hindered seedling emergence but had no effect on survival and there was no significant species×litter interaction. Supplemental water facilitated survival in all treatments with highest survival in shade treatments. Sugar pine seedlings showed a significant increase in survival over Jeffrey pine seedlings with the addition of water, particularly in open treatments and more of both species survived under manzanita shrubs with water. The highest seedling mortality occurred when shrub canopy was removed, and seedlings experienced the effect of full sun and competition for soil water. For either species, microhabitat is a significant factor in determining success or failure in rehabilitation efforts after disturbance. [Copyright &y& Elsevier]

Abstract: Genetic relationships among Mexican white pines have not been completely resolved by DNA sequencing analyses. The use of random amplified polymorphic DNA (RAPD) markers for the study of interspecific relationships has been questioned because of the possible lack of homology of co-migrating bands between species. However, several RAPD based studies on pines have provided sufficient information to discriminate between closely related taxa. Genetic relationships among four species of Mexican white pines (Pinus ayacahuite, Pinus strobiformis, Pinus lambertiana and Pinus chiapensis) were estimated based on RAPD markers. Sixty-nine primers generated 247 bands in pooled DNA samples from ten populations. In addition, four selected primers generated 27 bands in 176 individual DNA samples. Unweighted Pair Group Method with Arithmetic Average (UPGMA) dendrograms based on Jaccard similarity indices were constructed. The results suggest that the closest pine species analyzed were P. ayacahuite and P. strobiformis, followed by P. lambertiana. The most genetically distant species was P. chiapensis. Cluster analyses did not support P. strobiformis as a distinct species from P. ayacahuite. [Copyright &y& Elsevier]

Selective pressures that influence the form of seed dispersal syndromes are poorly understood. Morphology of plant propagules is often used to infer the means of dispersal, but morphology can be misleading. Several species of pines, for example, have winged seeds adapted for wind dispersal but owe much of their establishment to scatter- hoarding animals. Here the relative importance of wind vs. animal dispersal is assessed for four species of pines of the eastern Sierra Nevada that have winged seeds but differed in seed size: lodgepole pine (Pinus conlorla inurrayana, 8 mg); ponderosa pine (Pinus ponderosa ponderosa, 56 mg); Jeffrey pine (Pinus jeffreyi, 160 mg); and sugar pine (Pintis lambertiana, 231 mg). Pre-dispersal seed mortality eliminated much of the ponderosa pine seed crop (66%), but had much less effect on Jeffrey pine (32% of seeds destroyed), lodgepole pine (29%), and sugar pine (7%). When cones opened most filled seeds were dispersed by wind. Animals removed >99% of wind-dispersed Jeffrey and sugar pine seeds from the ground within 60 days, but animals gathered only 93% of lodgepole pine seeds and 38% of ponderosa pine seeds during the same period. Animals gathered and scatter hoarded radioactively labeled ponderosa, Jeffrey, and sugar pine seeds, making a total of 2103 caches over three years of study. Only three lodgepole pine caches were found. Caches typically contained 1-4 seeds buried 5-20 mm deep, depths suitable for seedling emergence. Although Jeffrey and sugar pine seeds are initially wind dispersed, nearly all seedlings arise from animal caches. Lodgepole pine is almost exclusively wind dispersed, with animals acting as seed predators. Animals treated ponderosa pine in an intermediate fashion. Two-phased dispersal of large, winged pine seeds appears adaptive; initial wind dispersal helps to minimize pre-dispersal seed mortality whereas scatter hoarding by animals places seeds in sites with a higher probability of seedling establishment. [ABSTRACT FROM AUTHOR]

For many species of long-lived organisms, such as trees, survival appears to be the most critical vital rate affecting population persistence. However, methods commonly used to quantify tree death, such as relating tree mortality risk solely to diameter growth, almost certainly do not account for important spatial processes. Our goal in this study was to detect and, if present, to quantify the relevance of such processes. For this purpose, we examined purely spatial aspects of mortality for four species, Abies concolor, Abies magnifica, Calocedrus decurrens, and Pinus lambertiana, in an old-growth conifer forest in the Sierra Nevada of California, USA. The analysis was performed using data from nine fully mapped long-term monitoring plots. In three cases, the results unequivocally supported the inclusion of spatial information in models used to predict mortality. For Abies concolor, our results suggested that growth rate may not always adequately capture increased mortality risk due to competition. We also found evidence of a facilitative effect for this species, with mortality risk decreasing with proximity to conspecific neighbors. For Pinus lambertiana, mortality risk increased with density of conspecific neighbors, in keeping with a mechanism of increased pathogen or insect pressure (i.e., a Janzen-Connell type effect). Finally, we found that models estimating risk of being crushed were strongly improved by the inclusion of a simple index of spatial proximity. Not only did spatial indices improve models, those improvements were relevant for mortality prediction. For P. lambertiana, spatial factors were important for estimation of mortality risk regardless of growth rate. For A. concolor, although most of the population fell within spatial conditions in which mortality risk was well described by growth, trees that died occurred outside those conditions in a disproportionate fashion: Furthermore,.as stands of A. concolor,become increasingly dense, such spatial factors are likely to become increasingly important. In general, models that fail to account for spatial pattern are at risk of failure as conditions change. [ABSTRACT FROM AUTHOR]

We monitored infection and mortality of full-sib families of sugar pine (SP) and western white pine (WWP) selected for different mechanisms of resistance to white pine blister rust for more than 30 years in a field test in northern California. Natural infection was enhanced by interplanting alternate host Ribes spp. among test seedlings. Parents of the families were from three geographic provinces, representing the northern Rocky Mountains of Idaho (WWP), the Cascade Range of Oregon and Washington (WWP), and the Siskiyou Mountains of California and Oregon (SP). Several families of each pine species had major gene resistance (MGR), or genes for partial resistance (PR), or both types. Under the chronic epidemic conditions that prevailed, susceptible controls of both species became infected and died within a few years. Families with MGR segregated in expected Mendelian ratios (susceptibility/resistance), but each R gene—Cr1 in sugar pine and Cr2 in western white pine—was eventually defeated by biotypes of the rust with specific virulence to it (vcr1 and vcr2, respectively). WWP families from Idaho, which lacked Cr2, performed the best overall, ranging from 44 to 64% rust-free; a few WWP families from the Cascades, both with and without Cr2, performed comparably. Families with PR were not specifically vulnerable to vcr1 or vcr2. PR was less frequent in SP than WWP, and SP families had steeper infection rate curves than WWP. Combining PR in pedigrees with MGR may prolong protection of MGR by inhibiting selection and epidemic increase of vcr genes in the rust. [ABSTRACT FROM AUTHOR]

Pinus lambertiana (sugar pine) is an economically and ecologically important conifer with a 1600-km latitudinal range extending from Oregon, USA, to northern Baja California, Mexico. Like all North American white pines (subsect. Strobus), sugar pine is highly susceptible to white pine blister rust, a disease caused by the fungus Cronartium ribicola. We conducted a chloroplast DNA (cpDNA) survey of Pinus subsect. Strobus with comprehensive geographical sampling of P. lambertiana. Sequence analysis of 12 sugar pine individuals revealed strong geographical differentiation for two chloroplast haplotypes. A diagnostic restriction site survey of an additional 72 individuals demarcated a narrow 150-km contact zone in northeastern California. In the contact zone, maternal (megagametophtye) and paternal (embryo) haplotypes were identified in 31 single seeds, demonstrating bidirectional pollen flow extending beyond the range of maternal haplotypes. The frequencies of the Cr1 allele for white pine blister rust major gene resistance, previously determined for 41 seed zones, differ significantly among seed zones that are fixed for the alternate haplotypes, or contain a mixture of both haplotypes. Interspecific phylogenetic analysis reveals that the northern sugar pine haplotype belongs to a clade that includes Pinus albicaulis (whitebark pine) and all of the East Asian white pines. Furthermore, there is little cpDNA divergence between northern sugar pine and whitebark pine ( dS = 0.00058). These results are consistent with a Pleistocene migration of whitebark pine into North America and subsequent chloroplast introgression from whitebark pine to sugar pine. This study demonstrates the importance of placing phylogeographical results in a broader phylogenetic context. [ABSTRACT FROM AUTHOR]

The sexual reproductive process in Pinus lambertiana has not been completely described, and previous attempts to generate hybrids with Pinus monticola and other North American pines have not been successful. The nature of incompatibility barriers between P. lambertiana and P. monticola is unknown. This needs to be understood if strategies are to be developed to overcome the said barriers. In this paper, development on interspecific crosses is compared with that on intraspecific crosses on the same parent trees. Pollen grains of both species germinated on the nucellus of both species within a week after pollination. Seed cone receptivity in P. lambertiana came approximately 2 weeks after receptivity in P. monticola, and this delay was perpetuated throughout ovule development in the first year of the reproductive process. Development of the second-year seed cones proceeded more gradually in P. lambertiana. However, seed cones reached maturity only for P. monticola x P. lambertiana. In both crosses, the barriers to hybridization occurred during the second year of the reproductive process. With the P. lambertiana as the seed parent, it was manifested through the failure of the megaspores at the free-nuclear stage to resume development. When P. monticola was used as the seed parent, the male and female gametes failed to fuse. Our results clearly show that the barriers to hybridization in these species occur before or at fertilization. However, the exact mechanisms behind these are still unknown. Based on the results of this study, we present several strategies to bypass the developmental barriers and possibly produce hybrid progenies. [ABSTRACT FROM AUTHOR]

We assayed the distribution and frequency of two genes of the blister rust pathogen with specific virulence to major resistance genes in sugar pine and western white pine in inoculum from extensive parts of the hosts' ranges. The genes, vcr1 and vcr2, differentially neutralize the cognate resistance alleles Cr1 and Cr2 of the two respective hosts and are clearly marked by their interaction phenotypes. Basidiospores from each inoculum source were cast over Cr1 and Cr2 host genotypes simultaneously, and interaction phenotypes scored when developed, vcr1 was confined to sites with high concentrations of Cr1 (mostly plantations) where frequencies tended toward fixation, vcr2 showed a similar tendency, except high frequencies were occasionally observed from natural and planted stands of western white pine with very low frequencies of Cr2. Otherwise, no pattern was evident for either allele: frequencies were very erratic from site to site within short distances (<1 to 7 km) of each other and oscillated with high amplitudes at the same sites measured in consecutive years. Intense selection for virulence by Cr alleles occurs locally, but spread of vcr alleles over the landscape is mitigated by remarkably low gene flow. Absence of heterozygotes among single telia inoculum on Cr2 genotypes indicated cytoplasmic inheritance of vcr2, similar to vcr1 (previously reported). [ABSTRACT FROM AUTHOR]

The positive growth response of healthy young trees to density reduction is well known. In contrast, large old trees are usually thought to be intrinsically limited in their ability to respond to increased growing space; therefore, density reduction is seldom used in stands of old-growth trees. We tested the null hypothesis that old-growth trees are incapable of responding with increased growth following density reduction. The diameter growth response of 271 Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco), ponderosa pine (Pinus ponderosa Dougl. ex Laws) and sugar pine (Pinus lambertiana Dougl.) trees ranging in age from 158 to 650 years was examined 20 to 50 years after density reduction. Density reduction involved either light thinning with removal of less vigorous trees, or shelterwood treatments in which overstory trees were not removed. Ratios of basal area growth after treatment to basal area growth before treatment, and several other measures of growth, all indicated that the old trees sometimes benefited and were not harmed by density reduction. Growth increased by 10% or more for 68% of the trees in treated stands, and nearly 30% of trees increased growth by over 50%. This growth response persisted for at least 20 years. During this 20-year period, only three trees in treated stands (1.5%) exhibited a rapid decrease in growth, whereas growth decreased in 64% of trees in untreated stands. The length of time before a growth response to density reduction occurred varied from 5 to 25 years, with the greatest growth response often occurring 20 to 25 years after treatment. These results have important implications both for the basic biology of aging in woody plants as well as for silvicultural practices in forests with old-growth trees. [ABSTRACT FROM AUTHOR]

Abstract Aim This study appraises historical fire regimes for Californian mixed-conifer forests of the Sierra San Pedro Mártir (SSPM). The SSPM represents the last remaining mixed-conifer forest along the Pacific coast still subject to uncontrolled, periodic ground fire. Location The SSPM is a north–south trending fault bound range, centred on 31°N latitude, 100 km SE of Ensenada, Baja California. Methods We surveyed forests for composition, population structure, and historical dynamics both spatially and temporally over the past 65 years using repeat aerial photographs and ground sampling. Fire perimeter history was reconstructed based on time-series aerial photographs dating from 1942 to 1991 and interpretable back to 1925. A total of 256 1-ha sites randomly selected from aerial photographs were examined along a chronosequence for density and cover of canopy trees, density of snags and downed logs, and cover of non-conifer trees and shrubs. Twenty-four stands were sampled on-the-ground by a point-centred quarter method which yielded data on tree density, basal area, frequency, importance value, and shrub and herb cover. Results Forests experience moderately intense understory fires that range in size to 6400 ha, as well as numerous smaller, low intensity burns with low cumulative spatial extent. SSPM forests average 25–45% cover and 65–145 trees per ha. Sapling densities were two to three times that of overstory trees. Size-age distributions of trees ≥ 4 cm dbh indicate multi-age stands with steady-state dynamics. Stands are similar to Californian mixed conifer forests prior to the imposition of fire suppression policy. Livestock grazing does not appear to be suppressing conifer regeneration. Main conclusions Our spatially-based reconstruction shows the open forest structure in SSPM to be a product of infrequent, intense surface fires with fire rotation periods of 52 years, rather than frequent, low intensity fires at intervals of 4–20 years proposed from California fire-scar dendrochronology (FSD) studies. Ground fires in SSPM were intense enough to kill pole-size trees and a significant number of overstory trees. We attribute long fire intervals to the gradual build-up of subcontinuous shrub cover, conifer recruitment and litter accumulation. Differences from photo interpretation and FSD estimates are due to assumptions made with respect to site-based (point) sampling of fire, and nonfractal fire intensities along fire size frequency distributions. Fire return intervals determined by FSD give undue importance to local burns which collectively use up little fuel, cover little area, and have little demographic impact on forests. [ABSTRACT FROM AUTHOR]