Southern Cascade Research Articles

Regional and local controls on historical fire regimes of dry forests and woodlands in the Rogue River Basin, Oregon, USA

Fire regimes structure plant communities worldwide with regional and local factors, including anthropogenic fire management, influencing fire frequency and severity. Forests of the Rogue River Basin in Oregon, USA, are both productive and fire-prone due to ample winter precipitation and summer drought; yet management in this region is strongly influenced by forest practices that depend on fire exclusion. Regionally, climate change is increasing fire frequency, elevating the importance of understanding historically frequent-fire regimes.We use cross-dated fire-scars to characterize historical fire return intervals, seasonality, and relationships with climate beginning in 1650 CE for 13 sites representative of southwestern Oregon dry forests. Using systematic literature review, we link our local fire histories to a regional dataset and evaluate our data relative to more intensively studied conifer/hardwood forest types in California.Fire-scars show that fires in the Rogue Basin were frequent and regular until disrupted in the 1850s through 1910s, corresponding with forced displacement of Native Americans and Euro-American settlement. Median historical fire return intervals were 8 years at the stand-scale (<25 ha), with site medians ranging from five to 14 years and no significant differences between sampled vegetation types. Burn seasonality was broadly distributed with 47% of recorded fires in the latewood (midsummer), 30% at the ring boundary (late summer and fall), and 23% in the earlywood (spring and early summer).The number of sites recording fire each year was associated with Palmer Drought Severity Index (PDSI) and El Niño Southern Oscillation Index (ENSO). Fires were detected in the study area every other year, and synchrony among sites was associated with stronger annual drought. The ENSO synchronization of fire suggests an herbaceous fuel signal, with warm winters/wet summers two years prior to widespread fire-years, a pattern observed globally in fuel-limited systems.Stand-scale fire histories in the Klamath, southern Cascades, and northern Sierra Nevada ecoregions resemble Rogue River Basin stand-scale fire histories. Across dry mixed conifer, yellow pine, and mixed evergreen forests, fire return intervals converged on 8 years. Moist mixed conifer and red fir forests exhibited 13-year fire return intervals. Across ecoregions, fire periodicity was weakly correlated with climatic water deficit, but well-modeled by elevation, precipitation, and temperature. These data highlight the need for decadal fire and burning outside of the contemporary fire season for forest restoration and climate adaptation in the dry forests of the Rogue Basin.

Cross-scale occupancy dynamics of a postfire specialist in response to variation across a fire regime.

Fire creates challenges and opportunities for wildlife through rapid destruction, modification and creation of habitat. Fire has spatially variable effects on landscapes; however, for species that benefit from the ephemeral resource patches created by fire, it is critical to understand characteristics of fires that promote postfire colonization and persistence and the spatial scales on which they operate. Using a model postfire specialist, the black-backed woodpecker (Picoides arcticus), we examined how colonization and persistence varied across two spatial scales as a function of four characteristics of fire regimes-fire severity, fire size, fire ignition date and number of years since fire. We modelled black-backed woodpecker colonization and persistence using data from 108 recently burned forests in the Sierra Nevada and southern Cascades ecoregions of California, USA, that we monitored for up to 10years following fire. We employed a novel, spatially hierarchical, dynamic occupancy framework which differentiates colonization and persistence at two spatial scales: across fires and within fires. We found strong effects of fire characteristics on dynamic rates, with colonization and persistence declining across both spatial scales with increasing years since fire. Additionally, at sites within fires, colonization decreased with fire size and increased with fire severity and for fires with later ignition dates. Our results support the notion that different aspects of a species' environment are important for population processes at different spatial scales. As habitat quality is ephemeral for any given postfire area, our results illustrate the importance of time since fire in structuring occupancy at the fire level, with other characteristics of fires playing larger roles in determining abundance within individual fires. Our results contribute to the broader understanding of how variation in fire characteristics influences the colonization and persistence of species using ephemeral habitats, which is necessary for conserving and promoting postfire biodiversity in the context of rapidly shifting fire regimes.

Whitebark pine foliar fungal endophyte communities in the southern Cascade Range, USA: Host mycobiomes and white pine blister rust

Abstract We present an assessment of relationships between correlates, signs, and symptoms of white pine blister rust and fungal endophyte communities of whitebark pine (Pinus albicaulis) foliage in the southern Cascade Range, OR, USA. Parsimonious combinations of historically important predictors of blister rust provided effective logistic models (AUC = 0.83) for predicting blister rust infection in whitebark pines at our sites. Holding ten other predictors constant, multivariate endophyte community structure varied significantly with host tree diameter, mountain aspect, the latitudinal coordinates at which samples were collected, and proximal alternative host plants of the white pine blister rust pathogen. After controlling for multiple comparisons, mountain aspect remained a significant source of variation in endophyte community composition even at the taxonomic rank of class, while the presence/absence of blister rust cankers did not remain significant. We discuss overarching mechanisms that may contribute to novel trends observed in mycobiome variation.

New zircon (U-Th)/He and U/Pb eruption age for the Rockland tephra, western USA

Eruption ages of a number of prominent Quaternary volcanic deposits remain inaccurately and/or imprecisely constrained, despite their importance as regional stratigraphic markers in paleo-environment reconstruction and as evidence of climate-altering eruptions. Accurately dating volcanic deposits presents challenging analytical considerations, including poor radiogenic yield, scarcity of datable minerals, and contamination of crystal populations by magma, eruption, and transport processes. One prominent example is the Rockland tephra, which erupted from the Lassen Volcanic Center in the southern Cascade arc. Despite a range in published eruption ages from 0.40 to 0.63 Ma, the Rockland tephra is extensively used as a marker bed across the western United States. To more accurately and precisely constrain the age of the Rockland tephra-producing eruption, we report U/Pb crystallization dates from the outermost ∼2 μm of zircon crystal faces (surfaces) using secondary ion mass spectrometry (SIMS). Our new weighted mean 238U/206Pb age for Rockland tephra zircon surfaces is 0.598 ± 0.013 Ma (2σ) and MSWD = 1.11 (mean square weighted deviation). As an independent test of the accuracy of this age, we obtained new (U-Th)/He dates from individual zircon grains from the Rockland tephra, which yielded a weighted mean age of 0.599 ± 0.012 Ma (2σ, MSWD = 5.13). We also obtained a (U-Th)/He age of 0.628 ± 0.014 Ma (MSWD = 1.19) for the Lava Creek Tuff member B, which was analyzed as a secondary standard to test the accuracy of the (U-Th)/He technique for Quaternary tephras, and to evaluate assumptions made in the model-age calculation. Concordance of new U/Pb and (U-Th)/He zircon ages reinforces the accuracy of our preferred Rockland tephra eruption age, and confirms that zircon surface dates sample zircon growth up to the time of eruption. We demonstrate the broad applicability of coupled U/Pb zircon-surface and single-grain zircon (U-Th)/He geochronology to accurate dating of Quaternary tephra, and highlight the challenges and opportunities of this technique.

Aboveground biomass responses to organic matter removal, soil compaction, and competing vegetation control on 20-year mixed conifer plantations in California

We measured vegetation growth 5, 10, and 20years following plantation establishment at 12 Long-Term Soil Productivity installations in California’s Sierra Nevada and Southern Cascades. The combined effects of soil compaction (none, moderate, severe), organic matter removal (tree bole only, whole tree, whole tree plus forest floor), and competing vegetation control (complete: VC, none: NVC) on aboveground biomass were tested. Soils ranged from sandy loams to clay loams and were compacted (10–25% bulk density increase) prior to planting of mixed-conifer seedlings. Soil compaction resulted in a 15% increase in planted tree biomass on a plot-scale basis, attributed to improved seedling survival, along with reduced competing vegetation biomass on NVC plots. The unexpected response of tree growth to compaction was consistent across the LTSP sites. After year 5, there were no differences among the diverse organic matter treatments in tree biomass, periodic annual increment, or competing vegetation biomass. In contrast, vegetation control had a strong positive effect on tree biomass, about 68% greater tree growth (129.13Mgha−1) compared to trees grown on NVC plots (76.83Mgha−1). However, total vegetation biomass (trees+competing vegetation) was greater without vegetation control for the initial 10years of the study, prior to canopy closure. The results showed near-complete tolerance by forest biomass to soil compaction and surface organic matter manipulation on LTSP plantations across a wide geographic range in California. Vegetation control was the single most important factor affecting 20-year tree biomass.

Fire History and Forest Structure along an Elevational Gradient in the Southern Cascade Range, Oregon, USA

We examined stand structure, demography, and fire history using tree cores and fire scar data across an approximately 7000-hectare study area over an elevational gradient in the southern Cascade Range, Oregon, USA. Our plots were located in mountain hemlock (Tsuga mertensiana [Bong.] Carr), red fir (Abies magnifica A. Murr.), lodgepole pine (Pinus contorta Loudon), and mixed conifer forest types. Stand demography from high elevation mountain hemlock forests showed continuous regeneration since the early 1600s and no fire scars present. Red fir forests showed both continuous and episodic regeneration over the past several centuries, providing evidence for a mixed-severity fire regime. Lodgepole pine stands were even-aged with no fire scar evidence and likely established following high severity fire events. Mixed conifer forests were uneven-aged. The majority of trees that we sampled established between 1880 and 1920. Interpretation of our data is limited by a small number of fire scars and relatively small sample size. However, our study highlights the spatial complexity of forest types and concomitant fire regimes on this landscape.

Primitive arc magma diversity: New geochemical insights in the Cascade Arc

The origin of the large range of compositional diversity encompassed by primitive arc magmas and the respective roles of mantle heterogeneity versus slab-derived contributions are the center of ongoing debate. The Cascade Arc of western North America is a global endmember hot subduction zone and an ideal setting in which to examine fundamental questions about the origin of arc magmas. Cascade Arc primitive magmas define several distinct compositional endmembers, the most widespread of which are high alumina olivine tholeiite (HAOT), calc-alkaline basalt (CAB), and intraplate-type basalt (IPB). New high precision Sr-Nd-Hf-Pb isotopic and trace element data for 49 of the most primitive magmas from seven major High Cascades volcanic centers are used to assess whether the basalt groups are derived from geochemically distinct mantle sources. Along with recently published data for the Garibaldi Volcanic Belt (GVB), the northern and other major segment of the Cascade Arc, and Mt. Rainier, the new data expand the high-precision isotopic coverage to seventeen volcanic centers spanning the ~1300km length of the arc. In all investigated isotopic systems, the new High Cascades data allow for finer resolution with much less scatter than previous data. HAOT mantle sources are systematically more depleted in incompatible elements than CAB sources, yet High Cascades CABs and HAOTs are isotopically indistinguishable, defining a remarkable linear Pb isotopic array consistent with a single mantle endmember that is similar to the source of Juan de Fuca mid-ocean ridge basalts. This mantle has been variably modified by contributions from a single subducting sediment endmember that is a close isotopic match to average Northern Cascadia basin sediment. Although Astoria Fan sediment was previously used as a proxy for the subducting sediment composition in the Cascadia subduction zone, the isotope ratios of Cascades basalts record no input from Astoria Fan sediment. High Cascades CABs and HAOTs both contain subducting sediment input in the form of a melt, but CABs also contain melts of subducting oceanic crust which is most apparent in the southern Cascades high-Sr/P suite and in the GVB. The least sediment input is recorded by GVB basalts, which also become progressively lower in 208Pb*/206Pb* and Hf isotope ratios to the north, reflecting the influx of an isotopically distinct enriched mantle component that generates IPBs at the northern slab edge. Enriched mantle with a composition trending towards HIMU is also recorded by a distinct isotopic array that is defined by IPBs from the Mt. Adams-Simcoe back arc region. We propose that slab gaps permit the influx of sub-slab asthenospheric mantle in both the northern GVB and Mt. Adams-Simcoe back-arc. Otherwise, western North America is underlain predominantly by isotopically homogeneous, MORB-type depleted mantle from which both the CAB and HAOT basalt groups are derived. Our results from the Cascades imply that most of the compositional heterogeneity encompassed by primitive arc magmas is not related to the presence of multiple mantle components but to variability in the composition and quantity of slab contributions. This study highlights the importance of high precision Sr-Nd-Hf-Pb isotope and trace element data in obtaining new insights on the petrogenesis of arc magmas.

Tectonic Geomorphology and Volcano-Tectonic Interaction in the Eastern Boundary of the Southern Cascades (Hat Creek Graben Region), California, USA

The eastern boundary of the Southern Cascades (Hat Creek Graben region), California, USA, is an extensively faulted volcanic corridor between the Cascade Range and Modoc Plateau. The east-west extending region is in the transition zone between the convergence and subduction of the Gorda Plate underneath the North American Plate; north-south shortening within the Klamath Mountain region; and transcurrent movement in the Walker Lane. We describe the geomorphological and tectonic features, their alignment and distribution, in order to understand the tectonic geomorphology and volcano-tectonic relationships. One outcome of the work is a more refined morpho-structural description that will affect future hazard assessment in the area. A database of volcanic centers and structures was created from interpretations of topographic models generated from satellite images. Volcanic centers in the region were classified by morphological type into cones, sub-cones, shields and massifs. A second classification by height separated the bigger and smaller edifices and revealed an evolutionary trend. Poisson Nearest Neighbor analysis shows that bigger volcanoes are spatially dispersed while smaller ones are clustered. Using volcano centroid locations, about 90 lineaments consisting of at least three centers within 6km of one another were found, revealing that preferential north-northwest directed pathways control the transport of magma from the source to the surface, consistent with the strikes of the major fault systems. Most of the volcano crater openings are perpendicular to the maximum horizontal stress, expected for extensional environments with dominant normal regional faults. These results imply that the extension of the Hat Creek Graben region and impingement of the Walker Lane is accommodated mostly by extensional faults and partly by the intrusions that formed the volcanoes. Early in the history of a volcano or volcano cluster, melt produced at depth in the region propagates to the surface using the easiest and most efficient pathway, mostly controlled by these pre-existing extensional faults and near-surface stress fields. Continued volcano growth is less closely dependent on the regional structures, suggesting control by the development of the volcanic edifice itself.

Slab melting and magma formation beneath the southern Cascade arc

Abstract The processes that drive magma formation beneath the Cascade arc and other warm-slab subduction zones have been debated because young oceanic crust is predicted to largely dehydrate beneath the forearc during subduction. In addition, geochemical variability along strike in the Cascades has led to contrasting interpretations about the role of volatiles in magma generation. Here, we focus on the Lassen segment of the Cascade arc, where previous work has demonstrated across-arc geochemical variations related to subduction enrichment, and H-isotope data suggest that H2O in basaltic magmas is derived from the final breakdown of chlorite in the mantle portion of the slab. We use naturally glassy, olivine-hosted melt inclusions (MI) from the tephra deposits of eight primitive ( MgO > 7 wt % ) basaltic cinder cones to quantify the pre-eruptive volatile contents of mantle-derived melts in this region. The melt inclusions have B concentrations and isotope ratios that are similar to mid-ocean ridge basalt (MORB), suggesting extensive dehydration of the downgoing plate prior to reaching sub-arc depths and little input of slab-derived B into the mantle wedge. However, correlations of volatile and trace element ratios (H2O/Ce, Cl/Nb, Sr/Nd) in the melt inclusions demonstrate that geochemical variability is the result of variable addition of a hydrous subduction component to the mantle wedge. Furthermore, correlations between subduction component tracers and radiogenic isotope ratios show that the subduction component has less radiogenic Sr and Pb than the Lassen sub-arc mantle, which can be explained by melting of subducted Gorda MORB beneath the arc. Agreement between pMELTS melting models and melt inclusion volatile, major, and trace element data suggests that hydrous slab melt addition to the mantle wedge can produce the range in primitive compositions erupted in the Lassen region. Our results provide further evidence that chlorite-derived fluids from the mantle portion of the slab ( ∼ 7 – 9 km below the slab top) cause flux melting of the subducted oceanic crust, producing hydrous slab melts that migrate into the overlying mantle, where they react with peridotite to induce further melting.

Targeted Capture Sequencing in Whitebark Pine Reveals Range-Wide Demographic and Adaptive Patterns Despite Challenges of a Large, Repetitive Genome.

Whitebark pine (Pinus albicaulis) inhabits an expansive range in western North America, and it is a keystone species of subalpine environments. Whitebark is susceptible to multiple threats – climate change, white pine blister rust, mountain pine beetle, and fire exclusion – and it is suffering significant mortality range-wide, prompting the tree to be listed as ‘globally endangered’ by the International Union for Conservation of Nature and ‘endangered’ by the Canadian government. Conservation collections (in situ and ex situ) are being initiated to preserve the genetic legacy of the species. Reliable, transferrable, and highly variable genetic markers are essential for quantifying the genetic profiles of seed collections relative to natural stands, and ensuring the completeness of conservation collections. We evaluated the use of hybridization-based target capture to enrich specific genomic regions from the 27 GB genome of whitebark pine, and to evaluate genetic variation across loci, trees, and geography. Probes were designed to capture 7,849 distinct genes, and screening was performed on 48 trees. Despite the inclusion of repetitive elements in the probe pool, the resulting dataset provided information on 4,452 genes and 32% of targeted positions (528,873 bp), and we were able to identify 12,390 segregating sites from 47 trees. Variations reveal strong geographic trends in heterozygosity and allelic richness, with trees from the southern Cascade and Sierra Range showing the greatest distinctiveness and differentiation. Our results show that even under non-optimal conditions (low enrichment efficiency; inclusion of repetitive elements in baits), targeted enrichment produces high quality, codominant genotypes from large genomes. The resulting data can be readily integrated into management and gene conservation activities for whitebark pine, and have the potential to be applied to other members of 5-needle pine group (Pinus subsect. Quinquefolia) due to their limited genetic divergence.

Time-scale and mechanism of subsidence at Lassen Volcanic Center, CA, from InSAR

Observations of volcanic subsidence have contributed to our understanding of the eruption cycle, hydrothermal systems and the formation of continental crust. Lassen Volcanic Center is one of two volcanoes in the southern Cascades known to have subsided in recent decades, but the onset, temporal evolution, and cause of subsidence remain unconstrained. Here we use multiple sets of InSAR data, each corrected using the North American Regional Reanalysis atmospheric model, to determine the temporal and spatial characteristics of deformation between 1992 and 2010. Throughout this period all datasets reveal subsidence of a broad, 30–40km wide region at rates of ~10mm/yr. Evaluating past geodetic studies we suggest that subsidence may have been ongoing since the 1980s, before which it is unlikely that significant ground deformation occurred. By combining multiple tracks of InSAR data we find that the ratio of horizontal to vertical displacements is high (up to 3:1), and source inversions favour a point source located at ~8km depth. Time-series analysis suggests that the rate of volume change of this source may have varied over time. The source geometry and the temporal evolution of deformation contrasts to subsidence observed at nearby Medicine Lake Volcano since the 1950s. We evaluate possible causes of subsidence at Lassen Volcanic Center in light of tectonic setting and hydrothermal activity, and suggest that regional GPS measurements will be key to understanding the role of crustal extension plus other hydrothermal/magmatic processes in deformation during recent decades.

High severity fire and mixed conifer forest-chaparral dynamics in the southern Cascade Range, USA

Understanding how alternative vegetation types co-exist in a landscape is important in managing for biodiversity within an ecosystem. In California, mixed conifer forest is often interrupted by stands of shrubs known as montane chaparral. The development of chaparral stands following recent high severity or stand-replacing wildfires in mixed conifer forests has been well documented. Fire has been excluded from mixed conifer forests for over a century, and fuel loads are at historically high levels across much of this landscape. Despite contemporary post-fire research on mixed conifer forest, little is known about montane chaparral fire regimes or forest-chaparral dynamics in an ecosystem with a functioning fire regime. This study quantifies fire regimes in chaparral and adjacent forest and determines how chaparral responded to fire before fire exclusion in Lassen Volcanic National Park, California, a park that was never logged. Chaparral stems regenerated immediately after high severity fires in the 19th and early 20th century, and stem recruitment continued until the present. Fire return intervals in chaparral were longer than in adjacent forest (25years vs. 11years), and chaparral fires occurred during drier, potentially more extreme conditions. The apparent maintenance of stands of chaparral by less frequent, more severe fires suggests chaparral represents a self-reinforcing alternative stable state to forest. Following fire exclusion, chaparral stands gradually converted to forest as trees progressively invaded chaparral from the forest edge. Forest developing in chaparral is denser and more fir-enriched than the adjacent forest, similar to the understory that develops beneath a pine overstory following fire exclusion. Replacement of chaparral by forest reduces mixed conifer forest landscape diversity. However, the mixture of shrubs and trees in long unburned former chaparral is likely to burn with high severity effects in a subsequent fire. Since chaparral is also establishing in recent, very large high severity burn patches, chaparral extent may be expanding in the new configuration. If the decades needed for trees to invade from forest at the edge of severe burns exceed the fire return interval, chaparral may emerge as an alternative stable state to forest. Consequently, developing management strategies to increase resilience of mixed conifer forests to altered fire regimes is a pressing management challenge.

Postglacial vegetation and fire history of the southern Cascade Range, Oregon

The Cascade Range of southwestern Oregon contains some of North America's most diverse forests, but the ecological history of this area is poorly understood. A 7900-yr-long pollen and charcoal record was examined to better understand past changes in vegetation and fire activity in relation to large-scale climate variability. From 7900 to 3500calyr BP, the dominance of xerophytic species and the frequent fires are consistent with a climate that was warmer and drier than at present. The period from 3500calyr BP to present experienced an abundance of mesophytic taxa and reduced fire frequency, suggesting cooler and wetter conditions. The regional history of Abies indicates that it was most widespread during the late-glacial period; its range contracted during the early Holocene thermal maximum, and it steadily expanded during the middle and late Holocene. In contrast, Pseudotsuga was restricted in range during the glacial period, became abundant at low-elevation sites in the Coast and northern Cascade ranges during the early Holocene, and was more prevalent in southern mid-elevation sites as the climate became cooler and wetter in the late Holocene. The sensitivity of these species to past climate change suggests that biogeographic responses to future conditions will be highly variable in this region.

Quaternary tephrochronology and deposition in the subsurface Sacramento–San Joaquin Delta, California, U.S.A.

We document characteristics of tephra, including facies and geochemistry, from 27 subsurface sites in the Sacramento–San Joaquin Delta, California, to obtain stratigraphic constraints in a complex setting. Analyzed tephra deposits correlate with: 1) an unnamed tephra from the Carlotta Formation near Ferndale, California, herein informally named the ash of Wildcat Grade (<~1.450 to >~0.780Ma), 2) the Rockland ash bed (~0.575Ma), 3) the Loleta ash bed (~0.390Ma), and 4) middle Pleistocene volcanic ash deposits at Tulelake, California, and Pringle Falls, Bend, and Summer Lake, Oregon, herein informally named the dacitic ash of Hood (<~0.211 to >~0.180Ma). All four tephra are derived from Cascades volcanic sources. The Rockland ash bed erupted from the southern Cascades and occurs in up to >7-m-thick deposits in cores from ~40m subsurface in the Sacramento–San Joaquin Delta. Tephra facies and tephra age constraints suggest rapid tephra deposition within fluvial channel and overbank settings, likely related to flood events shortly following volcanic eruption. Such rapidly deposited tephra are important chronostratigraphic markers that suggest varying sediment accumulation rates in Quaternary deposits below the modern Sacramento–San Joaquin Delta. This study provides the first steps in a subsurface Quaternary stratigraphic framework necessary for future hazard assessment.

Niche divergence of Abies grandis–Abies concolor hybrids

Homoploid hybridization has increasingly been recognized as an important source of genetic variation to support adaptation to changing environmental conditions, the invasion of novel ecological niches and, ultimately, speciation. Although ecological selection pressures have been theorized to play a key role in the stabilization of homoploid hybrid lineages, surprisingly few detailed quantitative studies compare the realized ecological niches of hybrids and their progenitors. In this study, we compare twenty bioclimatic variables (Worldclim) at representative occurrence sites of Abies grandis, Abies concolor, and two classes of their natural hybrids, which are variably intergradient between their progenitors, to assess the potential role of ecological selection in the evolution of the observed morphological patterns of introgression. We find that both hybrid classes occupy distinct habitats that are variably intermediate between those associated with A. grandis and A. concolor for a majority of environmental variables, but with lower precipitation in the summertime in the southern Cascade, Siskiyou, and Sierra Nevada Mountains than associated with either progenitor. We hypothesize that moisture stress and high levels of solar insolation in the summertime associated with hybrid populations have played an important role in the observed patterns of introgression of key traits: leaf angle, specific leaf area, and abundance of adaxial stomates. Phenotypic plasticity in two of these traits may allow hybrids to adapt to an environment with a higher degree of variability in moisture stress. Moreover, in the context of moisture stress associated with predicted climate change, hybrids may have an adaptive advantage over A. grandis.

Quaking Aspen Regeneration Following Prescribed Fire in Lassen Volcanic National Park, California, USA

AbstractPrescribed fire is commonly used for restoration, but the effects of reintroducing fire following a century of fire exclusion are unknown in many ecosystems. We assessed the effects of three prescribed fires, native ungulate browsing, and conifer competition on quaking aspen (Populus tremuloides Michx.) regeneration in four small groves (0.5 ha to 3.0 ha) in Lassen Volcanic National Park, California, USA, over an 11 yr period. The effects of fire on aspen regeneration density and height were variable within and among sites. Post-fire aspen regeneration density generally decreased with greater conifer basal area (rs = −0.73), but there was a wide range of aspen regeneration densities (4000 to 36 667 stems ha−1) at transects with no live conifers post-fire. The height of aspen regeneration increased as a function of increasing years-since-fire (1 yr to 11 yr), but heavy browsing by mule deer (Odocoileus hemionus Rafinesque) may alter future growth trajectories. Median percent of aspen regeneration browsed was high in burned (91 %) and unburned (81 %) transects. Only 7 % (282 stems ha−1 to 333 stems ha−1) of post-fire aspen regeneration in 11-year old burns exceeded the height necessary to escape mule deer browsing (150 cm). Browsing may also be altering aspen growth form, such that multi-stemmed aspen regeneration was positively associated with proportion of aspen regeneration browsed. These four case studies indicate that the effects of prescribed fires on quaking aspen in the southern Cascade Range of northern California were highly variable and, when coupled with biotic factors (such as deer browsing and competing vegetation) and varying fire severity, fire may either benefit or hasten the decline of small aspen groves.

Straddling the tholeiitic/calc-alkaline transition: the effects of modest amounts of water on magmatic differentiation at Newberry Volcano, Oregon

Melting experiments have been performed at 1 bar (anhydrous) and 1- and 2-kbar H2O-saturated conditions to study the effect of water on the differentiation of a basaltic andesite. The starting material was a mafic pumice from the compositionally zoned tuff deposited during the ~75 ka caldera-forming eruption of Newberry Volcano, a rear-arc volcanic center in the central Oregon Cascades. Pumices in the tuff of Newberry caldera (TNC) span a continuous silica range from 53 to 74 wt% and feature an unusually high-Na2O content of 6.5 wt% at 67 wt% SiO2. This wide range of magmatic compositions erupted in a single event makes the TNC an excellent natural laboratory in which to study the conditions of magmatic differentiation. Our experimental results and mineral–melt hygrometers/thermometers yield similar estimates of pre-eruptive H2O contents and temperatures of the TNC liquids. The most primitive (mafic) basaltic andesites record a pre-eruptive H2O content of 1.5 wt% and a liquidus temperature of 1,060–1,070 °C at upper crustal pressure. This modest H2O content produces a distinctive fractionation trend that is much more enriched in Na, Fe, and Ti than the calc-alkaline trend typical of wetter arc magmas, but slightly less enriched in Fe and Ti than the tholeiitic trend of dry magmas. Modest H2O contents might be expected at Newberry Volcano given its location in the Cascade rear arc, and the same fractionation trend is also observed in the rim andesites of the rear-arc Medicine Lake volcano in the southern Cascades. However, the Na–Fe–Ti enrichment characteristic of modest H2O (1–2 wt%) is also observed to the west of Newberry in magmas erupted from the arc axis, such as the Shevlin Park Tuff and several lava flows from the Three Sisters. This shows that modest-H2O magmas are being generated directly beneath the arc axis as well as in the rear arc. Because liquid lines of descent are particularly sensitive to water content in the range of 0–3 wt% H2O, they provide a quantitative and reliable tool for precisely determining pre-eruptive H2O content using major-element data from pumices or lava flows. Coupled enrichment in Na, Fe, and Ti relative to the calc-alkaline trend is a general feature of fractional crystallization in the presence of modest amounts of H2O, which may be used to look for “damp” fractionation sequences elsewhere.

Long‐term effects of fire severity on oak–conifer dynamics in the southern Cascades

We studied vegetation composition and structure in a mixed conifer-oak ecosystem across a range of fire severity 10 years following wildfire. Sample plots centered on focal California black oaks (Quercus kelloggii) were established to evaluate oak and neighboring tree and shrub recovery across a gradient of fire severity in the southern Cascade Range, USA. Shrub and oak resprouting was strongest around focal oaks where conifer mortality was greatest. Linear modeling revealed negative relationships between California black oak sprout height or basal area and residual overstory tree survival, primarily white fir (Abies concolor). The two dominant competing species, California black oak and white fir, showed opposite responses to fire severity. Sprouting California black oak and associated shrubs dominated in severely burned areas, while surviving, non-sprouting white fir maintained dominance by its height advantage and shading effects in areas that burned with low fire severity. Our results indicate that high-severity fire promotes persistence and restoration of ecosystems containing resprouting species, such as California black oak, that are increasingly rare due to widespread fire exclusion in landscapes that historically experienced more frequent fire. We present a conceptual model based on our results and supported by a synthesis of postfire resprouting dynamics literature. Our results and conceptual model help illuminate long-term postfire vegetation responses and the potential ability of fire to catalyze formation of alternate vegetation community structures that may not be apparent in studies that evaluate postfire effects at shorter time-since-fire intervals or at coarser scales.

Modern departures in fire severity and area vary by forest type, Sierra Nevada and southern Cascades, California, USA

Acute changes in ecological disturbance regimes can have major consequences for ecosystems and biota, including humans, living within them. Human suppression of fire in the western United States over the last century has caused notable changes to many ecosystems, especially in lower elevation, semiarid forest types dominated historically by fire tolerant taxa likePinusandQuercus. Recent increases in fire activity in western US forests have highlighted the need for restoration of ecological structure and function, but management targets for restoration in different forest types remain uncertain. Working in the forests of eastern California, we evaluated the direction and magnitude of change in burned area and fire severity between the period prior to Euro‐American settlement (∼1500–1850) and the “modern” period (1984–2009). We compared total annual area burned; proportional area burned at low‐moderate severity and high severity; and annual area burned at low‐moderate severity and high severity between the two time periods in seven forest types. We also examined modern trends in fire area and severity. We found that modern rates of burning are far below presettlement levels for all forest types. However, there were major differences between low to middle elevation forests and high elevation forests regarding the components of this departure. Low and middle elevation forests are currently burning at much higher severities than during the presettlement period, and the departure in fire area is overwhelmingly expressed in the low to moderate severity categories; in these forest types, mean annual area of high severity fire is not notably different between the modern and presettlement periods. In higher elevation forests on the other hand, the modern departure in fire area is expressed equally across fire severity categories. Our results underline the critical need for forest and fire restoration in the study area, especially in low and middle elevation forests adapted to frequent, low severity fire. Expanded management of naturally ignited fires for resource benefit is clearly needed, but in many parts of our study area, strategic reduction of forest fuels will likely be necessary before large‐scale restoration of fire becomes ecologically, politically, and financially feasible.

Constraints from magnetotelluric measurements on magmatic processes and upper mantle structure in the vicinity of Lassen volcanic center, northern California

A 250-km-long broadband and long-period (0.01–20,000 s periods) magnetotelluric (MT) line along latitude 40½°N spans from the California-Nevada border at longitude 120°W across the buried extension of the northern Sierra Nevada, the southern Cascades arc, and the subducting Gorda plate to longitude 123°W. The resulting resistivity cross section reveals conductors at the locations of dewatering of the subducting metasomatized crust at 100 km depth, partial melting of the mantle wedge at 40–60 km depth, and melting in the crust shallower than 40 km. The conductor at 100 km is too conductive (8 S/m) to result solely from magma; very conductive fluids must be present either separately or incorporated into hydrous melts. A melt fraction of 7% is estimated for the mantle wedge (average conductivity is 0.06 S/m). Sites of Pleistocene and Holocene volcanism at Lassen Peak, Hat Creek, and Poison Lake are closely associated with crustal conductors inferred to be shallow magma. The MT study also reveals presumably basaltic magma at 40–50 km depth in the upper mantle where the eastern end of the profile extends into the Basin and Range province.