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Warming in southern California during the 21st century is unprecedented in the instrumental record. To place this warming in a multi-century historical context, we analyzed tree ring data sampled from Jeffrey pine (Pinus jeffreyi) and sugar pine (Pinus lambertiana) collected from minimally disturbed, old-growth high-elevation forests within Mt. San Jacinto State Park California, USA. Based on a calibration/verification period of 1960–2020 between earlywood radial growth and California Climate Division 6 climate data, we reconstructed annual (November–October) minimum temperature (Tmin) from 1658 to 2020. During the 61-year calibration/verification period, instrumental Tmin increased (r = 0.69, p < 0.01) and was positively associated with annual radial growth (r = 0.71, p < 0.01). Using regime shift analysis, we found that the 363-year reconstruction revealed Tmin stability until 1958 and then decreased until 1980, followed by the two warmest regimes (1981–2007, 2008–2020) on record. The last 13-year period was 0.77 °C warmer than the multi-century average with nine of the ten warmest years in the reconstruction recorded. These results suggest that 21st century warming in southern California is unique in the context of the past four centuries, indicating the rarity of exceptional warmth captured in the tree ring record. [ABSTRACT FROM AUTHOR]

Hot droughts, droughts attributed to below-average precipitation and exceptional warmth, are increasingly common in the twenty-first century, yet little is known about their effect on coniferous tree growth because of their historical rarity. In much of the American West, including California, radial tree growth is principally driven by precipitation, and narrow ring widths are typically associated with either drier or drought conditions. However, for species growing at high elevations (e.g., Larix lyalli, Pinus albicaulis), growth can be closely aligned with above-average temperatures with maximum growth coinciding with meteorological drought, suggesting that the growth effects of drought span from adverse to beneficial depending on location. Here, we compare radial growth responses of three high-elevation old-growth pines (Pinus jeffreyi, P. lambertiana, and P. contorta) growing in the San Jacinto Mountains, California, during a twenty-first-century hot drought (2000–2020) largely caused by exceptional warmth and a twentieth-century drought (1959–1966) principally driven by precipitation deficits. Mean radial growth during the hot drought was 12% above average while 18% below average during the mid-century drought illustrating that the consequences of environmental stress exhibit spatiotemporal variability. We conclude that the effects of hot droughts on tree growth in high-elevation forests may produce responses different than what is commonly associated with extended dry periods for much of western North America's forested lands at lower elevational ranges and likely applies to other mountainous regions (e.g., Mediterranean Europe) defined by summer-dry conditions. Thus, the climatological/biological interactions discovered in Southern California may offer clues to the unique nature of high-elevation forested ecosystems globally. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Fire exclusion over the last two centuries has driven a significant fire deficit in the forests of western North America, leading to widespread changes in the composition and structure of these historically fire-adapted ecosystems. Fuel treatments have been increasingly applied over the last few decades to mitigate fire hazard, yet it is unclear whether these fuel-focused treatments restore the fire-adapted conditions and species that will allow forests to persist into the future. A vital prerequisite of restoring fire-adaptedness is ongoing establishment of fire-tolerant tree species, and both the type and reoccurrence of fuel treatments are likely to strongly influence stand trajectories. Here, we leveraged a long-term study of repeated fuel treatments in a Sierra Nevada mixed-conifer forest to examine the regeneration response of six native tree species to the repeated application of common fuel treatments: prescribed fire, mechanical, mechanical plus fire, and untreated controls. Our objectives were to (1) quantify differences in forest structure and composition following the repeated application of alternative fuel treatments that may influence the establishment environment and then (2) identify the stand structure and climate conditions influencing seedling dynamics. We found that both treatment type and intensity are highly influential in shifting forests toward more fire-adapted conditions and determining species-specific regeneration dynamics. Specifically, the conifer species tracked here increased in either colonization or persistence potential following repeated applications of fire, indicating fire may be most effective for restoring regeneration conditions broadly across species. Fire alone, however, was not enough to promote fire-adapted composition, with concurrent mechanical treatments creating more favorable conditions for promoting colonization and increasing abundances of fire-tolerant ponderosa pine. Yet, even with repeated fuel treatment application, establishment of fire-intolerant species far exceeded that of fire-tolerant species over this 20-year study period. Moreover, increasing growing season water stress negatively impacted seedling dynamics across all species regardless of treatment type and intensity, an important consideration for ongoing management under heightened climatic stress. While repeated treatments are waypoints in restoring fire-adapted conditions, more intense treatments via gap-creation or hotter prescribed fires targeting removal of fire-intolerant species will be necessary to sustain recruitment of fire-tolerant species., (© 2024 The Ecological Society of America.)

This paper evaluates multi-factor, non-linear dependencies of main (FC) and normal (trust) (FN) cutting forces, from chip thickness (ap), rake angle (γF) and some strength properties, namely the compression strength parallel to wood fibers (RC∥), tensile strength perpendicular to wood fibers (RT⊥), shearing strength parallel to wood fibers (RS∥) and bending strength (RB), for three North American wood species (analyzed altogether), by opened, flat, longitudinal cutting and very low cutting speed, based on work [1]. In the analyzed relations, several strong interactions were evidenced, graphically illustrated and discussed. Although a wood density (D) and a moisture content (mc), were not taken into account in this study, very good fit was obtained. The lowest influence of the RC∥ on the FC and the FN cutting forces was observed. [ABSTRACT FROM AUTHOR]

Sugar pine, Pinus lambertiana Douglas, is a keystone species of montane forests from Baja California to southern Oregon. Like other North American white pines, populations of sugar pine have been greatly reduced by the disease white pine blister rust (WPBR) caused by a fungal pathogen, Cronartium ribicola , that was introduced into North America early in the twentieth century. Major gene resistance to WPBR segregating in natural populations has been documented in sugar pine. Indeed, the dominant resistance gene in this species, Cr1 , was genetically mapped, although not precisely. Genomic single nucleotide polymorphisms (SNPs) placed in a large scaffold were reported to be associated with the allele for this major gene resistance ( Cr1 R ). Forest restoration efforts often include sugar pine seed derived from the rare resistant individuals (typically Cr1 R / Cr1 r ) identified through an expensive 2-year phenotypic testing program. To validate and geographically characterize the variation in this association and investigate its potential to expedite genetic improvement in forest restoration, we developed a simple PCR-based, diploid genotyping of DNA from needle tissue. By applying this to range-wide samples of susceptible and resistant ( Cr1 R ) trees, we show that the SNPs exhibit a strong, though not complete, association with Cr1 R . Paralleling earlier studies of the geographic distribution of Cr1 R and the inferred demographic history of sugar pine, the resistance-associated SNPs are marginally more common in southern populations, as is the frequency of Cr1 R . Although the strength of the association of the SNPs with Cr1 R and thus, their predictive value, also varies with geography, the potential value of this new tool in quickly and efficiently identifying candidate WPBR-resistant seed trees is clear.