Central Coast Range Research Articles

Nutrient content of vegetation and soil of four conifer species growing under different site and competing vegetation management conditions

In this study we examined the long-term effects of competing vegetation control on the total nutrient content of different ecosystem components of 19-year-old Douglas-fir (DF), western hemlock (WH), western redcedar (WRC), and grand fir (GF) stands growing in Oregon’s central Coast Range (CR) and of DF and WRC growing in Oregon’s Cascade mountain foothills (CF). Nutrient content responses were evaluated under two contrasting vegetation control treatments, including the Control (no herbicide application post-planting), and VM (5 consecutive years of herbicide application post-planting). Both treatments involved a pre-planting herbicide application. The ecosystem components were divided into overstory (planted crop trees), midstory, understory, forest floor, fine roots, and mineral soil. All samples were analyzed for content (mass per unit ground area) of total carbon, nitrogen, phosphorus, potassium, calcium, magnesium, sulfur, boron, copper, iron, manganese, sodium, and zinc. VM effects on total plant derived nutrient masses were more prominent than differences in concentrations. Ca was the only nutrient for which all species showed higher plant derived masses in the VM condition. Plant derived tissue content of C, Cu, P, and B all tended to be higher in VM plots, with the exception of WRC plots at the CR site. This case was an outlier due to the fact that Control plots developed significantly more biomass as a result of high midstory biomass, whereas the VM plots developed relatively little midstory and crop tree biomass. There were few differences in soil nutrients content between species and treatments, and those that were significant were unable to be explained by differences in uptake by plant species. Notably, total soil N of WRC at the CR site was significantly lower for VM plots. This may indicate the potential for VM applied to a slow growing species, such as WRC, to reduce ecosystem retention of N. With the exception of C and N, total soil nutrient reserves were orders of magnitude greater than total plant derived masses. This indicated that there is low probability of an adverse effect of VM on soil nutrient stores.

Deformation of the Juan de Fuca Plate Beneath the Central Cascadia Continental Margin (44°-45°N) in Response to an Upper Plate Load

A 3D crustal model for the central Cascadia continental shelf and Coast Range between 44°N and 45°N shows that the crystalline crust of the forearc wedge beneath the coastline is characterized by a NW-trending, vertical slab of high-velocity rock interpreted to represent the dike complex that fed the Yachats Basalt, which was intruded into the forearc approximately 37 million years ago. A spatial correlation is observed between downward deflection of the crust of the subducting Juan de Fuca plate, inferred from inversion of PmP arrivals to image the Moho surface, and the high velocity (and consequently high density) anomaly underlying the Yachats Basalt. Apparent subsequent rebound of the subducting plate at greater depth suggests a primarily elastic response of the subducting plate to this load. Calculations for a range of plausible values for the magnitude of the load and the width and depth of the depression indicate that the effective elastic thickness of the subducted Juan de Fuca plate is < 6 km. Although our simple analytical models do not include partial support of the load of the slab by the adjacent upper plate crust or time dependence to account for the motion of the slab beneath the load, incorporation of those effects should decrease rather than increase the apparent strength of the subducted plate. We conclude that the subducted Juan de Fuca plate beneath the central Oregon margin is elastically thin and has the potential to store elastic strain energy before rupturing. Our model of a well-defined, focused and static upper plate load that locally deforms the subducted plate within the nominally seismogenic or transitional part of the Cascadia plate boundary may be unique in providing a relatively straightforward scenario for estimating the mechanical properties of the subducted Juan de Fuca plate. We extrapolate from these results to speculate that elastic deformation of the subducting plate may contribute to the low level of seismicity throughout much of the Cascadia forearc in the inter-seismic period between great earthquakes but note that our local results do not preclude faulting or elasto-plastic deformation of a thin and weak plate as it subducts. These results also suggest that the subducting plate should deform in response to larger scale variations in upper plate thickness and density.

Evaluating the influence of seasonal stratification on mercury methylation rates in the water column and sediment in a contaminated section of a western U.S.A. reservoir

Mercury methylation frequently occurs at the active oxic/anoxic boundary between the sediment bed and water column of lakes and reservoirs. Previous studies suggest that the predominant mercury methylation zone moves to the water column during periods of stratification and that high potential methylation rates (Km) in sediment require oxygenated overlying water. However, simultaneous measurements of methylmercury (MeHg) production in both the sediment and water column remain limited. Understanding the relative importance of sediment versus water column methylation and the impact of seasonal stratification on these processes has important implications for managing MeHg production. This study measured Km and potential demethylation rates (Kdm) using stable isotope tracers of unfiltered inorganic mercury and MeHg in sediments and water of the littoral and profundal zones of a shallow branch of the Nacimiento Reservoir in California's central coastal range. Field sampling was conducted once during winter (well-mixed/oxygenated conditions) and once during late summer (thermally stratified/anoxic conditions). The results showed very high ambient MeHg concentrations in hypolimnetic waters (up to 7.5 ng L−1; 79% MeHg/total Hg). During late summer, littoral sediments had higher Km (0.024 day−1) compared to profundal sediments (0.013 day−1). Anoxic water column Km were of similar magnitude to Km in the sediment (0.03 day−1). Following turnover, profundal sediment Km did not change significantly, but water column Km became insignificant. Summer and winter sediment Kdm were higher in profundal (2.35, 3.54 day−1, respectively) compared to the littoral sediments (0.52, 2.56 day−1, respectively). When modelled, Km in the water column could account for approximately 40% of the hypolimnetic MeHg. Our modelling results show that the remaining MeHg in the hypolimnion could originate from the profundal sediment. While further study is needed, these results suggest that addressing methylation in the water column and profundal sediment are of equal importance to any remediation strategy.

A Late Cenozoic Kinematic Model for Fault Motion Within Greater Cascadia

AbstractWidely accepted tectonic reconstructions indicate at least 100 km of coast‐parallel northwestward translation of the Sierra Nevada block of California and 15–20° clockwise rotation of most of Oregon since the current phase of Basin and Range extension began ∼17 Ma. These reconstructions require at least 100 km of convergence between the central Coast Range of Oregon and rigid North America in mainland British Columbia, yet there is little discussion of how such convergence might be distributed. This study offers a kinematic model of the distribution of such deformation, constrained by geodesy, paleomagnetism, and fault offsets in Nevada, California and Oregon. The model includes differential rotation across the thrust faults of the Yakima fold and thrust belt (YFTB), compressive right‐lateral faulting in the Washington Cascade Range, substantial thrust faulting within the Puget Lowland, and oroclinal bending and doming in the Olympic Mountains. Shortening across YTFB along 120°W longitude is modeled as 47 km, across Puget Lowland at 123°W (Olympia‐Bellingham) is 94 km, and total shortening between the central Oregon Coast Range and northern Washington (Corvallis‐Bellingham) is 125 km. Current motion of the coastal regions above the Cascadia subduction zone results from both permanent deformation of the continent and elastic coupling to the subducting plate. Permanent deformation in the model is based on extrapolating geodesy from east of 120°W or south of 40°N, indicating a very uniform convergence velocity with the Juan de Fuca plate for northernmost California and Oregon near 31 mm/yr at N61°E.

Breeding Swainson’s Hawks in the Central Coast Range of California

During the 20th century the Swainson’s Hawk (Buteo swainsoni) was an infrequent migrant through the central Coast Range of California with no breeding documented. Since 2005, however, several breeding pairs have established nest sites in the region. We studied Swainson’s Hawks during the breeding seasons of 2019 and 2020 in a portion of the central Coast Range to estimate their abundance and distribution there. We observed individual hawks systematically and monitored their nesting activity and breeding outcomes in suitable habitat in Alameda, Santa Clara, and San Benito counties within the Coast Range and sought to characterize the habitat of this small outlying population. From 2019 to 2020 the numbers of nesting pairs increased from 3 to 5, of young fledged from 1 to 6, and of individual adults from 15 to 22. Swainson’s Hawks were more closely associated with farmland than with grassland, and nests were located in areas that were closely associated with both grassland and farmland. The increase of this species in the central Coast Range and its use of agricultural areas is of conservation significance for land managers and wildlife and conservation agencies, given that the California Department of Fish and Wildlife has designated it as threatened.

A BRYOPHYTE INVENTORY OF MOUNT DIABLO, CONTRA COSTA COUNTY, CALIFORNIA

Mount Diablo in Contra Costa County is a notable peak in California's central Coast Ranges, well known for its biodiversity. To date, no accounts of the bryophytes on this mountain have been published. We present a catalogue of the mosses, liverworts, and hornworts known to occur on Mount Diablo. We base this on collections made by us and augmented by specimens from other collectors that are housed in North American herbaria. We collected extensively on the mountain from 2014 to 2020. Online databases were searched to locate specimens from Mount Diablo. These were located predominantly at the University of California Berkeley (UC) or California Academy of Sciences (CAS). Specimens were examined at these herbaria to confirm their presence and to confirm identifications as needed. We list 160 bryophyte taxa on the mountain, including 137 mosses, 20 liverworts, and three hornworts. Forty families and 75 genera are represented, with Pottiaceae having the most species (39). We list separately 22 taxa that we consider unconfirmed on Mount Diablo because they are represented only by herbarium specimens that we could not locate, or for other reasons as given. Our own collections added 19 bryophyte species (17 mosses and two liverworts) since 2014. We did not find 41 of the confirmed taxa reported on Mount Diablo by others. Thirty-three taxa have so far been collected only once on the mountain. These numbers imply that additional taxa occur there and have not yet been found. We discuss the distribution of taxa within different parts of the mountain, and the Mount Diablo bryophyte flora is briefly compared with that of neighboring regions and of California as a whole.

Fish response to successive clearcuts in a second-growth forest from the central Coast range of Oregon

Research dating back to the 1950 s has documented negative effects from harvesting of primeval forests on stream ecosystems of the Pacific Northwest. By the early 1990 s, state and federal forest practice rules governing timber harvest were modified throughout North America to better protect aquatic habitats and biotic resources, principally salmonids. These rules inspired a generation of studies using a before-after-control-impact (BACI) design to document the capacity of contemporary timber harvest rules to protect salmonids in headwater streams of second-growth forests. One important unanswered question concerns the potential effects of successive clearcuts in second growth forests. Consequently, we used a paired watershed approach to evaluate the effects of two successive clearcut harvests in the Alsea Watershed, site of the seminal Alsea Watershed Study that was conducted from 1958 to 1973, on relative biomass, movement, survival, and distribution of coastal cutthroat trout (Oncorhynchus clarkii clarkii) and three physical habitat characteristics (pool area and depth, and water temperature). Although the total clearcut harvest encompassed 87% of the treatment catchment in six years, no negative effects of logging were detected for either age-1 + coastal cutthroat trout or habitat variables. Comparisons between the harvested and reference catchments suggested the survival of coastal cutthroat trout (>94 mm fork length) and total catchment relative biomass of age-1+ (i.e., > 80 mm) exhibited similar patterns, increasing from the pre-logging period (2006–2009) through the Phase I post-logging period (2009–2014), and decreasing to levels observed in the pre-logging period during the Phase II post-logging period (2014–2017). Additionally, there was no evidence for differences in movement of coastal cutthroat trout related to the harvesting treatment. In terms of habitat variables, there was a relative increase in annual total pool area in the harvested catchment during the Phase II post-logging period, but there was no evidence the 7-day moving mean maximum stream temperature changed after the Phase I and Phase II harvests. Moreover, stream water temperatures never exceeded the criterion designed to protect core coldwater habitat for salmonids (16 °C). As such, it is unlikely that cutthroat trout experienced thermal stress following either harvest. More generally, results from this and other recent studies suggest that forest practice rules developed in conjunction with current best management practices for logging in headwater catchments have substantially improved outcomes for stream biota relative to unregulated forest harvest, at least for short periods of time after logging (i.e., ≤ 8 years).

Plant derived tissue and soil nutrient concentration for plantations of four conifer species growing under different site and vegetation management conditions

This study investigates the long-term effects of vegetation management on nutrient concentration of various tissues and ecosystem components of 16 to 18 year-old Douglas-fir (DF), western hemlock (WH), western redcedar (WRC), and grand fir (GF) stands growing in Oregon’s central Coast Range (CR) and DF and WRC growing in Oregon’s Cascade mountain foothills (CF) under two contrasting vegetation management (VM) treatments. The treatments consist of: Control, which received no herbicide application post planting, and VM, which received five years of spring release herbicide application. Both treatments include a fall site preparation herbicide application. The ecosystem was broken down into crop trees (separated into foliage, live branches, bark, and stemwood), midstory species (separated into foliage and stem), understory, forest floor, fine roots, and mineral soil (with depth increments 0.0–0.2 m, 0.2–0.4 m, 0.4–0.6 m, and 0.6–1.0 m). All samples were analyzed for concentration of total carbon, nitrogen, phosphorus, potassium, calcium, magnesium, sulfur, boron, copper, iron, manganese, sodium, and zinc. This study design resulted in 1,740 unique nutrient concentration results being reported. The effect of VM (treatment) on tissue concentration varied by nutrient, overstory crop species (species), ecosystem component, and site. Forest floor and crop tree bark, followed by fine roots, were the ecosystem component nutrient concentrations that showed the greatest number of treatment effects across all species. Soil concentrations showed large variation across sites but were generally unaffected by treatment and species. At the CR site, magnesium and calcium soil concentrations were higher in VM plots across species, while zinc concentrations were lower. There were no other effects of treatment on soil nutrient concentrations, but there were some significant treatment × crop species interactions. Most notably, at the CF site, the concentration of C and N were higher in VM plots than control plots of DF, while the opposite was true for WRC. While total soil concentrations were generally unaffected by treatment and are unlikely to be adversely affected in the long term, it is possible that VM can reduce soil nitrogen for slow growing species like WRC.

Terrain attributes and forage productivity predict catchment-scale soil organic carbon stocks

Accurate assessments of soil organic carbon (SOC) stocks are needed at multiple scales given their importance to both local soil health and global C cycles. Rangelands cover 54% of California, representing a large stock of SOC, but existing SOC estimates are uncertain. To improve understanding of fine-resolution SOC stocks in complex terrain and provide guidance to rangeland SOC inventories, we grid-sampled 105 locations (21-m grid cells) at two depths (0–10 and 10–30 cm) in a 10-ha annual grassland catchment in California’s Central Coast Range. Soils were analyzed for bulk density, coarse fragments, SOC and texture. Monthly aerial imagery was acquired by an unmanned aerial vehicle to compare surface reflectance during two contrasting years (wet vs. dry) to SOC stocks. We found that the 0–30 cm soil thickness held 3.64 ± 0.71 kg SOC m−2 (mean ± SD) with a range of 1.97–5.49 kg SOC m−2. The 0–10 cm soil thickness stored 47% of the 0–30 cm SOC stock with SOC concentrations twice as high in the 0–10 cm layer (1.40 ± 0.38%) as in the 10–30 cm layer (0.71 ± 0.15% SOC). Multiple linear regression (MLR) models explained 50–57% of SOC variability at 0–30 and 10–30 cm, but only 25% of variability at 0–10 cm. Based on cross-validation tests, MLR outperformed spatial interpolation methods and Random Forest models, best explaining SOC stocks with five environmental covariates: wet-year greenness, mean curvature, elevation, insolation, and slope. Lower hillslope positions, concave landforms, and enhanced wet-year greenness were associated with more SOC, and explained 11%, 24%, and 31% of variability in 0–30 cm SOC stocks, respectively. This study demonstrates that the accuracy of regional-scale SOC mapping of California rangelands benefits from considering microclimatic and topographic controls at the catchment-scale, in addition to broader scale mineralogical and macroclimatic controls identified in previous SOC studies.

Microclimate–forage growth linkages across two strongly contrasting precipitation years in a Mediterranean catchment

AbstractGiven the complex topography of California rangelands, contrasting microclimates affect forage growth at catchment scales. However, documentation of microclimate–forage growth associations is limited, especially in Mediterranean regions experiencing pronounced climate change impacts. To better understand microclimate–forage growth linkages, we monitored forage productivity and root‐zone soil temperature and moisture (0–15 and 15–30 cm) in 16 topographic positions in a 10‐ha annual grassland catchment in California's Central Coast Range. Data were collected through two strongly contrasting growing seasons, a wet year (2016–17) with 287‐mm precipitation and a dry year (2017–18) with 123‐mm precipitation. Plant‐available soil water storage (0–30 cm) was more than half full for most of the wet year; mean peak standing forage was 2790 kg ha−1 (range: 1597–4570 kg ha−1). The dry year had restricted plant‐available water and mean peak standing forage was reduced to 970 kg ha−1 (range: 462–1496 kg ha−1). In the wet year, forage growth appeared energy limited (light and temperature): warmer sites produced more forage across a 3–4°C soil temperature gradient but late season growth was associated with moister sites spanning this energy gradient. In the dry year, the warmest topographic positions produced limited forage across a 10°C soil temperature gradient until late season rainfall in March. Linear models accounting for interactions between soil moisture and temperature explained about half of rapid, springtime forage growth variance. These findings reveal dynamic but clear microclimate–forage growth linkages in complex terrain, and thus, have implications for rangeland drought monitoring and dryland ecosystems modeling under climate change.

Long-term effects of vegetation management on biomass stock of four coniferous species in the Pacific Northwest United States

Silvicultural treatments applied during the early stages of stand development can have long-lasting impacts on forest ecosystem structure. Forest vegetation management (VM) is an important component of many reforestation programs and although several studies have demonstrated the benefits of VM on planted conifer growth and survival, few reports have been published on the long-term effects of VM treatments on total ecosystem biomass accumulation. In this study we assessed the effects of two contrasting VM treatments on total tree and ecosystem biomass stock for Douglas-fir, western hemlock, western redcedar, and grand fir growing in Oregon’s central Coast Range (CR) and Douglas-fir and western redcedar growing in Oregon’s Cascade foothills (CF). The assessments were made at age 16 years, 11 years after treatment application ended. The study contained two vegetation management treatments: control (C) and vegetation management (VM). Both the C and VM plots received a pre-planting herbicide site preparation treatment. The VM plots had, additionally, sustained vegetation control using herbicides during the first 5 years after planting. At age 16 years, at the CR site, the VM treatment increased the biomass stock of crop trees by 26.5, 91.2, 44.7, and 96.1 Mg ha−1 for Douglas-fir, western hemlock, western redcedar, and grand fir, respectively. At the same age, at the CF site, the VM treatment increased crop tree biomass stock by 48.1 Mg ha−1 for Douglas-fir and 42.2 Mg ha−1 for western redcedar. When other ecosystem components were considered, however, total ecosystem biomass did not differ between C and VM treated plots for western hemlock, western redcedar and grand fir at the CR site largely due to the development of an abundant hardwood midstory. On the other hand, VM treatments increased the ecosystem biomass stock of Douglas-fir and western redcedar at the site with a low abundance of hardwood midstory (CF site). Midstory biomass of C plots at the CR site averaged 52.9, 64.7, and 36.0 Mg ha−1, for western hemlock, western redcedar, and grand fir, respectively. At the CF site, midstory biomass of C plots averaged 1.2 and 5.9 Mg ha−1, for Douglas-fir and western redcedar, respectively. The results of this study demonstrate that sustained VM treatments during the first 5 years of stand establishment increases the biomass stock of crop trees, directing site resources towards planted crop trees.

Effect of Vegetation Management and Site Conditions on Volume, Biomass and Leaf Area Allometry of Four Coniferous Species in the Pacific Northwest United States

Allometric equations are useful tools for calculating tree and stand-level attributes, such as above-ground biomass or stem volume, using simple measurements that can be obtained from stand inventory data. These equations tend to be species-specific and can be affected by site conditions and silvicultural treatments. Forest vegetation management treatments (VM) are an important component of reforestation programs in the Pacific Northwest of the United States; however, no study has investigated the impact of these treatments on crop tree allometry. In this study we assessed the long-term effects of two contrasting VM treatments on the allometry of sixteen-year-old Douglas-fir, western hemlock, western redcedar, and grand fir trees growing in Oregon’s central Coast Range (CR) and fifteen-year-old Douglas-fir and western redcedar trees growing in Oregon’s Cascade foothills (CF). The VM treatments included a control which received only a pre-planting herbicide application and a VM treatment consisting of five consecutive years of vegetation control after planting. The equations developed in this study were species-specific and were not affected by VM with the exception of western redcedar foliage biomass. For western redcedar, tress of similar diameter had more foliage biomass when growing on plots without VM after planting. The allometry of Douglas-fir and western redcedar was also found to be affected by site, such that trees of similar diameter and height had larger stem volume when growing at the CR site than the CF site. This difference in stem volume was found to be the result of differences in stem tapering. There was a strong relationship between stand basal area and leaf area index that was the same for all species tested and was unaffected by site. The equations presented in this study are useful for calculating stem volume, leaf area and individual tree and component biomass for stands of the studied species that are of similar age.

Effects of Vegetation Management on Wood Properties and Plant Water Relations of Four Conifer Species in the Pacific Northwest of the USA

In plantation forests, competition from unwanted vegetation may reduce survival and negatively impact tree growth. The goal of this study was to examine the influence of vegetation management treatments on plant water relations and wood properties. Control trees (no treatment) were compared to trees subjected to post-planting competing vegetation control for five consecutive years after planting. Four conifer species (Douglas-fir, western hemlock, western redcedar, and grand fir) were studied on two different sites in western Oregon, USA. Carbon isotope (13C) analysis was used to study intrinsic water use efficiency (iWUE) and X-ray densitometry was used to measure specific gravity, ring width, and latewood percent. We found a significant interaction between vegetation management treatment and wood ring (growing season) in iWUE for Douglas-fir. There was little effect of vegetation management treatment on ring specific gravity for all species. Only western redcedar growing at a central Coast Range site showed increased ring specific gravity under sustained competing vegetation control. When growing under conditions of sustained control of competing vegetation, western redcedar at a central Coast Range site had a significant increase in earlywood specific gravity, while Douglas-fir at a Cascade Foothills site had a significant decrease in latewood specific gravity. Western redcedar and grand fir had a significant interaction-effect on its latewood percentage, with treatment trees having a higher latewood percentage than control trees after ring 8. Further, Douglas-fir and western hemlock had a significant increase in ring, earlywood, and latewood area with treatment, and grand fir had a significant interaction-effect of treatment × ring for ring, earlywood, and latewood area. This study indicates that, for conifer trees growing under sustained vegetation control, growth gains could be achieved without compromising wood properties. However, if harvested at a target diameter, these trees will have a larger proportion of low quality corewood compared to trees from conventionally managed stands.

Fish response to contemporary timber harvest practices in a second-growth forest from the central Coast Range of Oregon

We used a paired-watershed approach to investigate the effects of contemporary logging practices on headwater populations of coastal cutthroat trout (Oncorhynchus clarkii clarkii) and juvenile coho salmon (Oncorhynchus kisutch) in a second-growth Douglas-fir forested catchment in Oregon. Stream habitat and fish population characteristics, including biomass, abundance, growth, size, and movement, were assessed over a 9-year period (4 years pre- and 5 years postlogging). The logged catchment was located on private industrial forestland and had been previously logged in 1966. The reference catchment was covered by an unharvested, fire-regenerated forest approximately 150–160 years old, which was unroaded and managed as a Research Natural Area by the USDA Forest Service. A single clearcut harvest unit of the upper 40% of the treatment catchment was implemented following current forest practice regulations, including the retention of riparian buffer of standing trees adjacent to fish bearing channels. No statistically significant negative effects on coastal cutthroat trout or coho salmon occurred following logging, and in fact, both late-summer density and total biomass of age-1+ coastal cutthroat trout increased in the logged catchment following logging. Increases in age-1+ coastal cutthroat were greatest closest to the harvest area and declined downstream as distance from the logged area increased. In contrast to the previous timber harvest in the catchment when few logging regulations existed, current forest practice regulations and logging techniques appear to have reduced acute negative effects on coastal cutthroat trout.

A revision of Bittacomorphinae with a review of the monophyly of extant subfamilies of Ptychopteridae (Diptera)

The phylogenetic relationship between the two extant subfamilies of Ptychopteridae is examined using morphological characters. Bittacomorphinae is supported as a monophyletic group with several synapomorphies. Monophyly of the Ptychopterinae is less certain. Descriptions are provided for the family and subfamilies. The genera and species of Bittacomorphinae are reviewed, and phylogenetic relationships within the subfamily are discussed. While Bittacomorpha is a monophyletic group, Bittacomorphella is probably paraphyletic. Redescriptions of the species of Bittacomorpha and most of Bittacomorphella are presented. Bittacomorphella furcata sp. nov, is described from the central Coast Range of Oregon and Olympic Peninsula of Washington, USA. The range of Bittacomorphinae is found to extend into the Neotropical region with a record of Bittacomorpha clavipes from Costa Rica. A key to the world species of Bittacomorphinae is provided. Printed copy of this book is available from US$25.8 plus postage, Click here to order.

Soil development over mud-rich rocks produces landscape-scale erosional instabilities in the northern Gabilan Mesa, California

In soil-mantled landscapes, the length of hillslopes is set by the relative efficiency of hillslope transport processes and incision by channelized flows. In cases where rapid channel incision exposes less-weathered, more-resistant rocks, incision should slow, creating a damping feedback that prevents runaway channel expansion. We present observations from the northern Gabilan Mesa (central Coast Ranges, California), which is underlain by mud-rich rocks that provide less resistance to erosion than the soils produced from them. This strength inversion arises because unconfined, mud-rich units of the Pancho Rico Formation undergo a process called slaking, in which pervasive fractures, spaced centimeters apart, develop when subjected to wet-dry cycles. In contrast, consecutive wetting and drying cycles have produced a cohesive regolith with higher tensile strength than the slaked bedrock. Exposure of the relatively weak bedrock may create a positive feedback, where the acceleration of erosion within the bedrock undermines the cohesive regolith and allows channels to expand headward at the expense of hillslopes. Geomorphic mapping reveals numerous channelized features that have incised deeply into bedrock, and crosscutting relationships and topographic analyses indicate that these features have consumed regions of the landscape previously occupied by hillslopes. However, these features are only present in south-flowing drainages, where soil is thinnest and therefore bedrock is most likely to be exposed by increases in erosion rate. Preferential headward expansion of these south-flowing channels has established a profound topographic asymmetry, where south-flowing tributaries occupy ∼75% of drainage basin width. These results suggest that processes operating at the scale of the critical zone, which may be unique to certain rock types, conspire to produce an instability that shapes the morphology of drainage basins.

Geologic evolution of a Cretaceous tectonometamorphic unit in the Franciscan Complex, western California

ABSTRACTThe Franciscan Yolla Bolly terrane of the NE California Coast Ranges consists mainly of quartzose metagreywackes containing sparse high-pressure/low-temperature (HP/LT) neoblastic minerals, including ubiquitous lawsonite. Some Yolla Bolly rocks also contain one or more of the newly grown phases, pumpellyite, aragonite, glaucophane, and/or jadeitic pyroxene. These blueschist-facies metasandstones recrystallized under physical conditions of ~200–300°C and ~8 kbar at subduction-zone depths approaching 30 km. Petrologically similar Franciscan metaclastic-rich map units – Yolla Bolly terrane-like rocks, here designated the ‘YB’ unit – crop out in the central and southern California Coast Ranges. Recently published detrital zircon U‒Pb SIMS and LA-ICPMS data for 19 ‘YB’ metagreywackes indicate maximum ages of formation as follows: ~110–115 Ma (8) in the NE California Coast Ranges; ~95–107 Ma (7) in the San Francisco Bay area + Diablo Range; and ~85–92 Ma (4) in the dextrally offset Nacimiento Block. These fault-bounded ‘YB’ strata do not constitute coeval parts of a single tectonostratigraphic unit. Instead the term tectonometamorphic is proposed for such time-transgressive map units. Based on the current and likely Cretaceous 30° angular divergence between NS-palaeomagnetic stripes of the Farallon oceanic plate and the NNW-trending California convergent margin, I infer that arrival at the arc margin and underflow of a relatively thick segment of oceanic crust and its largely clastic sedimentary blanket may have resulted in progressive southeastward migration of an accreted, subducted, then exhumed HP/LT metagreywacke section. During the ~30 million year interval, ~115–85 Ma, the locus of ‘YB’ accretion, underflow, and tectonic regurgitation evidently moved SE along an ~1000 km stretch of the accretionary margin of western California.

The Point Sal–Point Piedras Blancas correlation and the problem of slip on the San Gregorio–Hosgri fault, central California Coast Ranges

Existing models for large-magnitude, right-lateral slip on the San Gregorio–Hosgri fault system imply much more deformation of the onshore block in the Santa Maria basin than is supported by geologic data. This problem is resolved by a model in which dextral slip on this fault system increases gradually from 0–10 km near Point Arguello to ∼150 km at Cape San Martin, but such a model requires abandoning the cross-fault tie between Point Sal and Point Piedras Blancas, which requires 90–100 km of right-lateral slip on the southern Hosgri fault. We collected stratigraphic and detrital zircon data from Miocene clastic rocks overlying Jurassic basement at both localities to determine if either section contained unique characteristics that could establish how far apart they were in the early Miocene. Our data indicate that these basins formed in the early Miocene during a period of widespread transtensional basin formation in the central Coast Ranges, and they filled with sediment derived from nearby pre-Cenozoic basement rocks. Although detrital zircon data do not indicate a unique source component in either section, they establish the maximum depositional age of the previously undated Point Piedras Blancas section to be 18 Ma. We also show that detrital zircon trace-element data can be used to discriminate between zircons of oceanic crust and arc affinity of the same age, a potentially useful tool in future studies of the California Coast Ranges. Overall, we find no characteristics in the stratigraphy and provenance of the Point Sal and Point Piedras Blancas sections that are sufficiently unique to prove whether they were far apart or close together in the early Miocene, making them of questionable utility as piercing points.

Landscape genetics and population structure in Valley Oak (Quercus lobata Née).

Although long-distance pollen movement is common in wind-pollinated trees, barriers to gene flow may occur in species that have discontinuous ranges or are confined to certain habitat types. We investigated the genetic structure of Quercus lobata Née populations throughout much of their range in California. We assessed the connectivity of populations and determined if barriers to gene flow occurred, and if so, if they corresponded to landscape features. We collected leaf samples from 270 trees from 12 stands of Quercus lobata and genotyped these trees using eight polymorphic microsatellite loci. Genetic structure and clustering was evaluated using genetic distance methods, Bayesian clustering approaches, and network analysis of spatial genetic structure. The southernmost population of Quercus lobata sampled from the Santa Monica area comprised a separate genetic cluster from the rest of the species, suggesting that Transverse Ranges such as the San Gabriel Mountains limit gene flow. Population differentiation among the other sites was small but significant. Network analysis reflected higher connectivity among populations along the Central Coast range, with few connections spanning the dry, low Central Valley. While long distance pollen movement has been shown to be common in oaks, on larger spatial scales, topographic features such as mountain ranges and the large, flat Central Valley of California limit gene flow. Such landscape features explain gene flow patterns much better than geographic distance alone.

Identifying the location and population served by domestic wells in California

Abstract Study region California, USA. Study focus Identification of groundwater use is an important step in the regional-scale assessment of groundwater quality. In California, 1990 US Census data indicate that domestic wells provide drinking-water to about 1.2 million people. However, the location of these domestic well users of groundwater is poorly identified because the census tracts can be quite large (up to 20,000 km 2 ). The purposes of this paper are to present methods used for (1) estimating the location of domestic wells, (2) estimating the location of households using domestic well water; and (3) identifying where in California groundwater is an important source of domestic drinking supply. New hydrological insights for the region Aggregating the results indicates that three hydrogeologic provinces contain nearly 80% of all domestic wells and also have the highest density of domestic well users: Central Valley (31.6%), Sierra Nevada (31.5%), and Northern Coast Ranges (16.6%). Results were also aggregated into groundwater basins and highland areas, collectively called Groundwater Units (GUs). Twenty-eight of the 938 GUs contain more than 50% of the total population served by domestic wells, 70 GUs contain more than 75%, and 150 GUs contain 90%. The 28 GUs are mostly located in the eastern and southern San Joaquin Valley (11), the Sacramento Valley (7), and the western foothills of the Sierra Nevada province (5). Using the information presented in this research along with other information about domestic-well use, the US Geological Survey has begun sampling high-use GUs for the Shallow Aquifer Assessment component of the Groundwater Ambient Assessment (GAMA) program.