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A compelling story of place, Steward’s Fork explores northwest California’s magnificent Klamath Mountains—a region that boasts a remarkable biodiversity, a terrain so rugged that significant landscape features are still being discovered there, and a wealth of natural resources that have been used, and more recently abused, by humans for millennia. James K. Agee, a forest ecologist with more than fifty years experience in the Klamaths, provides a multidimensional perspective on this region and asks: how can we most effectively steward this spectacular landscape toward a sustainable future? In an engaging narrative laced with personal anecdotes, he introduces the dynamics of the Klamath’s ecosystems, including its geology and diverse flora and fauna, and then discusses its native cultures and more recent inhabitants, laying out the effects of industries such as logging, mining, water development, and fishing. Assuming that people will continue to have a close tie to the Klamaths, Agee introduces the principles of restoration ecology to offer a vision of how we can responsibly meet the needs of both people and natural organisms, including plants, fish, and wildlife. This debate over the future of the Klamath’s rich landscape widens into a provocative meditation on nature, culture, and our relationship with the earth itself.

Topography, weather, and fuels are known factors driving fire behavior, but the degree to which each contributes to the spatial pattern of fire severity under different conditions remains poorly understood. The variability in severity within the boundaries of the 2006 wildfires that burned in the Klamath Mountains, northern California, along with data on burn conditions and new analytical tools, presented an opportunity to evaluate factors influencing fire severity under burning conditions representative of those where management of wildfire for resource benefit is most likely. Fire severity was estimated as the percent change in canopy cover (0–100%) classified from the Relativized differenced Normalized Burn Ratio (RdNBR), and spatial data layers were compiled to determine strength of associations with topography, weather, and variables directly or indirectly linked to fuels, such as vegetation type, number of previous fires, and time since last fire. Detailed fire progressions were used to estimate weather (e.g., temperature, relative humidity, temperature inversions, and solar radiation) at the time of burning. A generalized additive regression model with random effects and an additional spatial term to account for autocorrelation between adjacent locations was fitted to fire severity. In this fire year characterized by the relative absence of extreme fire weather, topographical complexity most strongly influenced severity. Upper‐ and mid‐slopes tended to burn at higher fire severity than lower‐slopes. East‐ and southeast‐facing aspects tended to burn at higher severity than other aspects. Vegetation type and fire history were also important predictors of fire severity. Shrub vegetation was more likely to burn at higher severity than mixed hardwood/conifer or hardwood vegetation. As expected, fire severity was positively associated with time since previous fire, but the relationship was non‐linear. Of the weather variables analyzed, temperature inversions, common in the complex topography of the Klamath Mountains, showed the strongest association with fire severity. Inversions trapped smoke and had a dampening effect on severity within the landscape underneath the inversion. Understanding the spatial controls on mixed‐severity fires allows managers to better plan for future wildfires and aide in the decision making when managing lightning ignitions for resource benefit might be appropriate.

Research in the last several years has indicated that fire size and frequency are on the rise in western U.S. forests. Although fire size and frequency are important, they do not necessarily scale with ecosystem effects of fire, as different ecosystems have different ecological and evolutionary relationships with fire. Our study assessed trends and patterns in fire size and frequency from 1910 to 2008 (all fires > 40 ha), and the percentage of high-severity in fires from 1987 to 2008 (all fires > 400 ha) on the four national forests of northwestern California. During 1910-2008, mean and maximum fire size and total annual area burned increased, but we found no temporal trend in the percentage of high-severity fire during 1987-2008. The time series of severity data was strongly influenced by four years with region-wide lightning events that burned huge areas at primarily low-moderate severity. Regional fire rotation reached a high of 974 years in 1984 and fell to 95 years by 2008. The percentage of high-severity fire in conifer-dominated forests was generally higher in areas dominated by smaller-diameter trees than in areas with larger-diameter trees. For Douglas-fir forests, the percentage of high-severity fire did not differ significantly between areas that re-burned and areas that only burned once (10% vs. 9%) when re-burned within 30 years. Percentage of high-severity fire decreased to 5% when intervals between first and second fires were >30 years. In contrast, in both mixed-conifer and fir/high-elevation conifer forests, the percentage of high-severity fire was less when re-burned within 30 years compared to first-time burned (12% vs. 16% for mixed conifer; 11% vs. 19% for fir/high-elevation conifer). Additionally, the percentage of high-severity fire did not differ whether the re-burn interval was less than or greater than 30 years. Years with larger fires and greatest area burned were produced by region-wide lightning events, and characterized by less winter and spring precipitation than years dominated by smaller human-ignited fires. Overall percentage of high-severity fire was generally less in years characterized by these region-wide lightning events. Our results suggest that, under certain conditions, wildfires could be more extensively used to achieve ecological and management objectives in northwestern California.

Climate warming and altered disturbance regimes are changing forest composition and structure worldwide. Given that species often exhibit individualistic responses to change, making predictions about the cumulative effects of multiple stressors across environmental gradients is challenging, especially in diverse communities. For example, warming temperatures are predicted to drive species upslope, whereas fire exclusion promotes the expansion of species at lower elevations where fire was historically frequent. We resampled 148 vegetation plots to assess 46 years (1969–2015) of species and community-level response to warming and fire exclusion in a topographically complex landscape in the Klamath Mountains, California, USA, a diverse region that served as a climate refugia throughout the Holocene. We compared cover and assessed change in the elevational distributions of 12 conifer species at different life stages (i.e., seedlings, saplings, canopy). We observed consistent but non-significant shifts upward in elevation for eight species, and a significant shift upward for one species, all of which were far less than expectations based on recent warming. Six species declined in total cover and another five declined in at least one life stage, whereas the drought- and fire-intolerant Abies concolor increased by 30.7%. The largest declines were at lower elevations in drought-tolerant, early-seral species (Pinus lambertiana and Pinus ponderosa) and at higher elevations for the shade-tolerant Abies magnifica var. shastensis and the regionally rare Abies lasiocarpa. Regionally rare (Picea engelmannii) and endemic (Picea breweriana) species had reductions in early life stages, portending future declines. Multivariate analyses revealed a high degree of inertia with a minor, but significant, shift in composition and a slight decrease in species turnover along the elevation gradient driven by the expansion of A. concolor. Our results indicate that most species are declining, especially at lower and mid-elevations where fire exclusion has increased the cover of shade-tolerant species and reduced the recruitment for fire-adapted species. Collectively, declines in most species, insufficient upward movement to track warming, reductions in drought- and fire-tolerant early-seral species, and an increase in a single, shade-tolerant species will leave these communities maladapted to projected climate scenarios and questions the potential for future climate refugia in this region.

We report the first low-temperature thermochronologic data from the Oregon Coast Range. This includes apatite and zircon fission-track data from fifteen samples of the Umpqua Group and Dothan Formation collected across the Wildlife Safari Fault near Roseburg, Oregon. This structure marks the boundary between the Siletzia terrane and the Klamath Mountains Province, where collision and accretion at 51–49 Ma produced a fold-and-thrust belt. Our sampling was designed to test whether fission track thermochronometry records structurally controlled exhumation across this fault during Siletzia accretion. The data fail to support this hypothesis. Instead, apatite fission track ages define a single thermally reset population at 45.3 ± 1.1 Ma that is uniform across all sampled stratigraphic and structural positions. Unimodal, moderately shortened track lengths (12.5–14.1 μm) record protracted cooling through the apatite partial annealing zone. Zircon fission track data show a time-continuous partial annealing pattern with youngest grain ages of 49–43 Ma, indicating temperatures reached at least the lower zircon partial annealing zone. These data record a regionally pervasive mid-Eocene thermal event that we interpret as syn-collisional heating followed by protracted cooling.

Background Severe fire weather is becoming more common throughout the western United States. Changing conditions demand a better understanding of how prescribed fire treatments perform under extreme burning conditions, including the interactive influence of the age of treatments, vegetation, and fire weather. The Carr Fire of July 2018 burned nearly the entire land area of Whiskeytown National Recreation Area (NRA) under extreme fuel moisture and temperature conditions. Prior to the Carr Fire and since 1997, staff at Whiskeytown NRA treated 23% of the 15,756-ha NRA using large-scale prescribed fire (underburn) treatments ranging in size from 40 to 400 hectares. Methods We used simultaneous autoregressive (SAR) models to describe the effects of landscape-scale fuel treatments on wildfire severity under extreme burning conditions and across diverse biophysical settings at Whiskeytown NRA. Because vegetation type and structure are known drivers of fire severity in diverse ecosystems such as at Whiskeytown NRA, we also considered three different sources of vegetation structure data, including a 2006 physiognomic-floristic classification, a 2011 lidar-based forest structure classification, and a 2016 Landfire map of existing vegetation physiognomy-subclass. Results The greatest effect on 2018 Carr Fire severity was time since treatment of underburn treatments, but treatment effectiveness on fire severity dissipated rapidly—showing notable effectiveness within 5 years of underburning but virtually no effectiveness beyond 10 years post-treatment. Additional factors related to severity included vegetation structure type, topographic position index, aspect, slope, temperature, and wind gust speed. Model variance explained and model parameters, including the effect of underburn treatments, were similar regardless of the source of vegetation structure data. Conclusions Our results show that large-scale underburning treatments can reduce wildfire severity even under extreme fire weather conditions but suggest that frequent maintenance intervals are required to maintain treatment effectiveness ahead of severe wildfire events.

The basement of the Rattlesnake Creek terrane (RCT) in the Klamath Mountains is a mélange of metamorphosed sedimentary and igneous blocks. Recent work shows that the overlying RCT cover sequence has a North American provenance but formed after accretion to the continental margin, so it is unclear if the basement mélange formed exotic or endemic to North America. This study presents petrography and zircon geochronology from RCT metasedimentary blocks and crosscutting intrusions. The southernmost RCT preserves both Early Jurassic and Middle-Late Jurassic cover sequence deposits and records continental clasts and 33% pre-Mesozoic zircons at ~201 Ma, effectively none at ~191 Ma, and 79–90% from 168 to 163 Ma. During active magmatism 207–193 Ma, the RCT was receiving continental sediment, inconsistent with a distant intraoceanic arc. We interpret that the RCT subduction zone formed proximal to North America in the Late Triassic and that there was a sediment pathway to the RCT at ~201 Ma. During Middle to Late Jurassic rifting and subsequent Nevadan compression, the cover sequences were dismembered and incorporated into the mélange by tectonic and sedimentary processes. The age and provenance of metasedimentary deposits in the RCT is inconsistent with west-dipping subduction models in the Klamath Mountains region.

Background: Pre-fire fuels, topography, and weather influence wildfire behaviour and fire-driven ecosystem carbon loss. However, the pre-fire characteristics that contribute to fire behaviour and effects are often understudied for wildfires because measurements are difficult to obtain.Aims: This study aimed to investigate the relative contribution of pre-fire conditions to fire energy and the role of fire advancement direction in fuel consumption.Methods: Over 15 years, we measured vegetation and fuels in California mixed-conifer forests within days before and after wildfires, with co-located measurements of active fire behaviour.Key results: Pre-fire litter and duff fuels were the most important factors in explaining fire energy and contributed similarly across severity categories. Consumption was greatest for the forest floor (litter and duff; 56.8 Mg ha−1) and 1000-h fuels (36.0 Mg ha−1). Heading fires consumed 13.2 Mg ha−1 more litter (232%) and 24.3 Mg ha−1 more duff (202%) than backing fires. Remotely sensed fire severity was weakly correlated (R2 = 0.14) with fuel consumption.Conclusions: 1000-h fuels, litter, and duff were primary drivers of fire energy, and heading fires consumed more fuel than backing fires.Implications: Knowledge of how consumption and fire energy differ among contrasting types of fire behaviours may inform wildfire management and fuels treatments.

The age, provenance, and evolution of some terranes in the Klamath Mountains are poorly constrained because of low detrital zircon yields. We present petrography and 947 new detrital and igneous zircon U-Pb ages from the North Fork (NFT), Eastern Hayfork (EHT), and Western Hayfork (WHT) terranes in the central and southern Klamath Mountains. Chert and argillite are abundant in the NFT and EHT, but matrix sandstones with abundant Proterozoic-to-Archean zircon ages indicate that the EHT received more sediment from North America. Detrital zircon ages from the WHT are ~171 Ma with scattered pre-Mesozoic ages, consistent with previous ages and continental input. A younger population of three grains at 145 Ma is interpreted as Pb loss during metamorphism. In the southernmost EHT, a 143 Ma dike correlates with plutons in the northern Sierra Nevada, which were offset from the Klamath Mountains 140–130 Ma. A 158 Ma metavolcanic/metavolcaniclastic rock in the EHT is a possible extrusive equivalent of the Wooley Creek intrusive suite. The metamorphosed EHT matrix has a young population of six ages at 69 Ma, which we tentatively interpret may represent Pb loss during metamorphism. This study documents an exposure of Late Jurassic arc cover sequence and suggests there may be previously unrecognized local metamorphism/magmatism ≤69 Ma.

Recent studies of arc volcanic systems have shown that major and trace element zoning in calcic amphibole yields information about magmatic processes such as fractional crystallization and mixing. Similar studies of plutonic amphibole are scant, yet hold the potential to yield comparable information. To that end, calcic amphibole from late-stage rocks of the English Peak plutonic complex (EPC; Klamath Mountains, northern California) was analyzed in situ, in textural context. The pluton's late stage consists of three nested intrusive units inwardly zoned from tonalite to granite. Bulk-rock compositions and U-Pb (zircon) ages are consistent either with internal fractional crystallization of a single magma batch or with episodic emplacement of successively evolved magmas, ± magma mixing. Major and trace element abundances and zoning patterns in hornblende (s.l.) are used to test these two interpretations, identify specific magmatic units, determine the temperature range of hornblende stability, and model magma crystallization. In each mapped unit, euhedral to subhedral hornblende displays prominent olive-brown core zones that crystallized at 880-775 °C. Cores are embayed and rimmed by green hornblende crystallized from 775-690 °C. These distinctions are preserved even in samples with moderate deuteric alteration. Some trace elements (Zr, Hf, Sr, Ti, V) decrease monotonically from core to rim, suggesting co-precipitation of hornblende with plagioclase, ilmenite, and zircon. Others (Ba, Rb) are approximately constant in highest-T core zones, then decrease, consistent with onset of biotite crystallization. In contrast, initial rim-ward decreases in Sc, Y, and REE change to near-constant values within olive-brown cores, a change modeled by a decrease in bulk partition coefficients (D) due to onset of biotite crystallization. These elements then increase in abundance in green rims, with as much as a fourfold enrichment. Such enrichments can result from resorption/ re-precipitation attending changing P and T during final emplacement, whereby trace elements in core zones were redistributed to the rims. Although hornblende compositions from the three zones are similar, outer-zone hornblende has higher Ti, Ba, Sc, and REE, whereas interior-zone hornblende has higher Mn. These differences are consistent with episodic ascent of compositionally similar but not identical magmas from a mid-crustal reservoir. Evidence for in situ magma mixing is lacking in hornblende. Core-to-rim decrease in Zr indicates hornblende and zircon crystallized together, at T as high as 880 °C. Because zircon saturation thermometry yields T estimates <720 °C for all EPC samples, many of the analyzed rocks are inferred to be cumulates. This study illustrates the utility of detailed major and trace element analysis of hornblende as a means to identify magmatic units and model petrogenetic processes in calc-alkaline granitic rocks.