Oregon Coast Range Research Articles

Rainfall triggers more deep-seated landslides than Cascadia earthquakes in the Oregon Coast Range, USA.

The coastal Pacific Northwest USA hosts thousands of deep-seated landslides. Historic landslides have primarily been triggered by rainfall, but the region is also prone to large earthquakes on the 1100-km-long Cascadia Subduction Zone megathrust. Little is known about the number of landslides triggered by these earthquakes because the last magnitude 9 rupture occurred in 1700 CE. Here, we map 9938 deep-seated bedrock landslides in the Oregon Coast Range and use surface roughness dating to estimate that past earthquakes triggered fewer than half of the landslides in the past 1000 years. We find landslide frequency increases with mean annual precipitation but not with modeled peak ground acceleration or proximity to the megathrust. Our results agree with findings about other recent subduction zone earthquakes where relatively few deep-seated landslides were mapped and suggest that despite proximity to the megathrust, most deep-seated landslides in the Oregon Coast Range were triggered by rainfall.

Squeezed by a habitat split: Warm ocean conditions and old‐forest loss interact to reduce long‐term occupancy of a threatened seabird

AbstractTheory predicts that species requiring multiple habitat types simultaneously should have heightened sensitivity to anthropogenic pressures, yet tests of this prediction are especially rare. We tested whether breeding site occupancy of the threatened marbled murrelet (Brachyramphus marmoratus) was driven by the synergistic effects of nesting habitat loss in forests, and changing ocean conditions. We paired 70,700 murrelet surveys at 19,837 sites across 20 years from the Oregon Coast Range with annual data on the extent of old forest and biophysical ocean conditions. Dynamic occupancy models indicated that local murrelet colonization rates were strongly reduced during warm ocean conditions with low prey availability. Landscapes that contained more old forest and were closer to the ocean showed reduced rates of local extinction. Given predictions of accelerated ocean warming and increased global timber demand, our results suggest murrelets may continue to be imperiled by deterioration of the two habitats upon which they depend.

Top‐down and bottom‐up controls limit woody encroachment into persistent temperate rainforest meadows

AbstractQuestionsMeadows in montane forests have emerged as a global study system for the accelerating phenomenon of woody encroachment, particularly in the highly productive temperate rainforests of the Pacific Northwest. Study of woody encroachment into meadows in western Oregon, however, has been conducted almost entirely in the western and High Cascades, with only two extant observational studies of grassy balds in the Coast Range. This study evaluates evidence for factors limiting woody encroachment into meadows in the Oregon Coast Range, including bottom‐up control by soil properties, plant–plant interactions and top‐down control by large herbivores.LocationCoast Range, western Oregon, United States of America.MethodsThe chemical and physical properties of soils to a depth of 50 cm in meadow and forest measured, and the community, density and browse pressure for shrubs, conifers, and deciduous trees ≤2 m tall from meadow into forest were recorded. A total of 20Pseudotsuga menziesii(Douglas‐fir) seedlings were experimentally planted in each of five meadows and neighbouring plant presence and herbivore access manipulated.ResultsMeadow soils were lower in C and C:N, slightly lower in N and similar in plant‐available water and pH relative to forest soils. Shrubs were most dense but experienced the lowest browse pressure near the meadow edge, while trees were sparse and varied by site (although at one site, browse pressure was heavier in meadow than forest). Seedling survival and growth varied by site and herbivory‐reduced growth; total soil N best explained residual variation in seedling growth among sites.ConclusionsThese findings indicate that ungulate herbivores exert top‐down control on woody encroachment into temperate montane meadows, perhaps in concert with local N‐limitation.

The influence of lithology on channel geometry and bed sediment organization in mountainous hillslope‐coupled streams

AbstractSediment transport and channel morphology in mountainous hillslope‐coupled streams reflect a mixture of hillslope and channel processes. However, the influence of lithology on channel form and adjustment and sediment transport remains poorly understood. Patterns of channel form, grain size, and transport capacity were investigated in two gravel‐bed streams with contrasting lithology (basalt and sandstone) in the Oregon Coast Range, USA, in a region in which widespread landslides and debris flows occurred in 1996. This information was used to evaluate threshold channel conditions and channel bed adjustment since 1996. Channel geometry, slope, and valley width were measured or extracted from LiDAR and sediment textures were measured in the surface and subsurface. Similar coarsening patterns in the first few kilometres of both streams indicated strong hillslope influences, but subsequent downstream fining was lithology‐dependent. Despite these differences, surface grain size was strongly related to shear stress, such that the ratio of available to critical shear stress for motion of the median surface grain size at bankfull stage was around one over most of the surveyed lengths. This indicated hydraulic sorting of supplied sediment, independent of lithology. We infer a cycle of adjustment to sediment delivered during the 1996 flooding, from threshold conditions, to non‐alluvial characteristics, to threshold conditions in both basins. The sandstone basin can also experience complete depletion of the gravel‐size alluvium to sand size, leading to bedrock exposure because of high diminution rates. Although debris flows being more frequent in a basalt basin, this system will likely display threshold‐like characteristics over a longer period, indicating that the lithologic control on channel adjustment is driven by differences in rock competence that control grain size and available gravel for bed load transport. © 2020 John Wiley & Sons, Ltd.

An Analysis of the Reactivation Potential of a Deep-seated Landslide in the Oregon Coast Range under varying Hydrologic conditions with Seismic Triggering

Landslides can occur in many locations across the world and have the potential to be extremely destructive if failures occur near populated areas. Failures are most likely to occur on slopes that have already experienced numerous failures. This means they are a considerable hazard, and the risk involved with building in areas that have previously experienced landslides should be adequately understood. This study examines the reactivation potential of a deep-seated landslide located in the Oregon Coast Range. The analysis of this landslide included creating a map of the surface morphology and computing the factor of safety for the deposit using the Swedish method of slices. The surface morphology map determined that the landslide is relatively young and currently inactive. The factor of safety was computed for two conditions—the base conditions in the field and an estimated fully saturated condition. The factor of safety determined for both conditions was used to calculate the critical acceleration needed to induce failure. This calculation was completed using Newmark’s analysis of a sliding block. The slope is stable in base conditions with a factor of safety of 2.22 and it would take a critical acceleration of 22% of g to cause slope failure (Table 4). This ground acceleration can be related to a Modified Mercalli Intensity of seven or greater. For saturated conditions, the slope is also stable, but a smaller critical acceleration will produce failure, namely 7.7% of g, which is an earthquake of Modified Mercalli intensity of five or above (Table 4).

Quantifying effects of forest harvesting on sources of suspended sediment to an Oregon Coast Range headwater stream

Elevated fine sediment transport to streams can negatively affect aquatic ecosystem health, downstream infrastructure, and community water supply. Forest harvesting activities can increase the delivery of fine sediment to streams due to intensified erosion or mass wasting from hillslopes, roads, and stream channels. However, quantifying the effects of forest harvesting on sediment inputs to streams and the effectiveness of current best management practices (BMPs) at mitigating these effects remains a challenge. In this study, we used sediment source fingerprinting techniques to quantify and compare the sources of suspended sediment to a stream draining a recently harvested catchment and a nearby, unharvested reference catchment in the Oregon Coast Range of the U.S. Pacific Northwest. In each stream, we quantified the proportional contributions of suspended sediment from three potential source areas: hillslopes, roads, and streambanks. The primary source of suspended sediment in the harvested catchment was streambank sediment (90.2 ± 3.4%) with lesser amounts of hillslope (7.1 ± 3.1%) and road (3.6 ± 3.6%) sediment. Interestingly, the proportional contributions of suspended sediment in the reference catchment were similar, with the majority from streambanks (93.1 ± 1.8%) followed by hillslopes (6.9 ± 1.8%). There were no contributions from roads in the reference catchment, despite a similar road network as the harvested catchment. We also quantified monthly sediment mass eroded from 36 × 1 m2 hillslope plots. The sediment mass eroded from the general harvest area (96.5 ± 57.0 (SE) g) was ~10.6-times greater than the sediment collected in the riparian buffer (9.1 ± 1.9 g) and ~4.6-times greater than the sediment collected on the unharvested, reference hillslope (21.0 ± 3.3 g). While this study provides evidence of effectiveness of contemporary BMPs (e.g., riparian management areas, limits to cutblock size, reduced impact forest harvesting techniques, road building and maintenance) at mitigating sediment delivery to streams, additional research is needed as existing studies do not adequately reflect the broad range of climate, geology, topography, and vegetation in the Pacific Northwest, which drive highly variable hydrologic and geomorphic processes in the region.

Long-term effects of forest harvesting on summer low flow deficits in the Coast Range of Oregon

We examined long-term changes in daily streamflow associated with forestry practices over a 60-year period (1959–2017) in the Alsea Watershed Study, Oregon Coast Range, Pacific Northwest, USA. We quantified the response of daily streamflow to (1) harvest of mature/old forest in 1966, (2) 43- to 53-yr and 48- to 58-yr-old industrial plantation forests in 2006–2009, and (3) logging of the plantations using contemporary forest practices, including retention of a riparian buffer, in 2010 and 2014. Daily streamflow from a 40- to 53-yr-old Douglas-fir plantation was 25% lower on average, and 50% lower during the summer (June 15 to Sept 15 of 2006 to 2009), relative to the reference watershed containing mature/old forest. Low flow deficits persisted over six or more months of each year. Surprisingly, contemporary forest practices (i.e., clearcutting of the plantation with riparian buffers in 2009 and 2014) had only a minor effect on streamflow deficits. Two years after logging in 2014, summer streamflow deficits were similar to those observed prior to harvest (under 40- to 53-yr-old plantations). High evapotranspiration from rapidly regenerating vegetation, including planted Douglas-fir, and from the residual plantation forest in the riparian buffer appeared to explain the persistence of streamflow deficits after logging of nearly 100% of the forest plantation. Results of this study indicated that 40- to 50-yr rotations of Douglas-fir plantations can produce persistent, large summer low flow deficits. While the clearcutting of these plantations, with retention of riparian buffers, increased daily streamflow slightly, flows did not return to pre-first entry conditions. Further work is needed to examine how intensively managed plantation forests along with expected warmer, drier conditions in the future may influence summer low streamflow and aquatic ecosystems.

Northward migration of the Oregon forearc on the Gales Creek fault

Abstract The Gales Creek fault (GCF) is a 60-km-long, northwest-striking dextral fault system (west of Portland, Oregon) that accommodates northward motion and uplift of the Oregon Coast Range. New geologic mapping and geophysical models confirm inferred offsets from earlier geophysical surveys and document ∼12 km of right-lateral offset of a basement high in Eocene Siletz River Volcanics since ca. 35 Ma and ∼8.8 km of right-lateral separation of Miocene Columbia River Basalt at Newberg, Oregon, since 15 Ma (∼0.62 ± 0.12 mm/yr, average long-term rate). Relative uplift of Eocene Coast Range basalt basement west of the fault zone is at least 5 km based on depth to basement under the Tualatin Basin from a recent inversion of gravity data. West of the city of Forest Grove, the fault consists of two subparallel strands ∼7 km apart. The westernmost, Parsons Creek strand, forms a linear valley southward to Henry Hagg Lake, where it continues southward to Newberg as a series of en echelon strands forming both extensional and compressive step-overs. Compressive step-overs in the GCF occur at intersections with ESE-striking sinistral faults crossing the Coast Range, suggesting the GCF is the eastern boundary of an R′ Riedel shear domain that could accommodate up to half of the ∼45° of post–40 Ma clockwise rotation of the Coast Range documented by paleomagnetic studies. Gravity and magnetic anomalies suggest the western strands of the GCF extend southward beneath Newberg into the Northern Willamette Valley, where colinear magnetic anomalies have been correlated with the Mount Angel fault, the proposed source of the 1993 M 5.7 Scotts Mills earthquake. The potential-field data and water-well data also indicate the eastern, Gales Creek strand of the fault may link to the NNW-striking Canby fault through the E-W Beaverton fault to form a 30-km-wide compressive step-over along the south side of the Tualatin Basin. LiDAR data reveal right-lateral stream offsets of as much as 1.5 km, shutter ridges, and other youthful geomorphic features for 60 km along the geophysical and geologic trace of the GCF north of Newberg, Oregon. Paleoseismic trenches document Eocene bedrock thrust over 250 ka surficial deposits along a reverse splay of the fault system near Yamhill, Oregon, and Holocene motion has been recently documented on the GCF along Scoggins Creek and Parsons Creek. The GCF could produce earthquakes in excess of Mw 7, if the entire 60 km segment ruptured in one earthquake. The apparent subsurface links of the GCF to other faults in the Northern Willamette Valley suggest that other faults in the system may also be active.

Summer stream temperature changes following forest harvest in the headwaters of the Trask River watershed, Oregon Coast Range

AbstractThe Trask River Watershed Study in the northern Oregon Coast Range was designed to examine physical, chemical, and biological effects of contemporary forest management practices on aquatic ecosystems. We measured stream temperature for 11 summers in 15 small watersheds, eight of which were harvested in 2012. Three riparian buffer treatments, which varied by landowner, were implemented. Using half‐hourly data, we characterized summer water temperature distributions with five percentiles: 5th, 25th, 50th, 75th, and 95th. Each percentile was analysed as a separate response variable using a linear mixed model. After harvest, streams without overstory buffer requirements showed shifts in all the percentiles of the temperature distribution; the largest increase (3.6°C) occurred at the 95th percentile. Sites with narrow riparian buffers showed little to no change. We also calculated changes in duration of thermal exposure above 15°C and 16°C for two species of native stream amphibians; these temperatures occurred 4.7% and 1.3% of the time postharvest in the sites clearcut with no buffer. Analysis of distributions of summer temperatures preharvest and postharvest enabled us to more fully characterize site‐to‐site variability and responses to forest management.

Deciphering ecology from statistical artefacts: Competing influence of sample size, prevalence and habitat specialization on species distribution models and how small evaluation datasets can inflate metrics of performance

AbstractAimSample size and species characteristics, including prevalence and habitat specialization, can influence the predictive performance of species distribution models (SDMs). There is little agreement, however, on which metric of model performance to use. Here, we directly compare AUC and partial ROC as metrics of SDM performance through analyses on the effects of species traits and sample size on SDM performance.LocationThree counties dominated by agricultural lands and coniferous forest in Oregon's Willamette Valley and Coast Range ecoregions.MethodsWe systematically reduced a large avian point count dataset to alter sample sizes of 22 species of songbird. We used boosted regression trees to run SDMs for each species, quantified habitat specialization, and used mixed effects models to compare the influence of sample size, prevalence, and habitat specialization on SDM performance, calculated as AUC and partial ROC, across species. We calculated AUC and partial ROC with subset and independent evaluation data separately to more comprehensively investigate differences in metrics.ResultsWe found a positive quadratic effect of sample size and a strongly positive effect of habitat specialization on both metrics of model performance. We found a weak effect of prevalence on partial ROC and no effect in AUC. Contrary to expectations, when evaluated with a subset evaluation data, partial ROC was consistently highest in models with the smallest sample sizes. These small sample size models had correspondingly small sample sizes in subset evaluation datasets. Partial ROC evaluated with independent data and AUC evaluated with subset or independent data showed the expected positive correlation between sample size and model performance.Main ConclusionsWe found that small evaluation datasets can artificially inflate partial ROC. With literature recommended minimum SDM sample sizes as low as three, attention must be given to the effects of correspondingly low sample sizes in evaluation datasets.

Dendrochronological dating of landslides in western Oregon: Searching for signals of the Cascadia A.D. 1700 earthquake

AbstractLarge-magnitude earthquakes and hydrologic events in mountainous settings commonly trigger thousands of landslides, and slope failures typically constitute a significant proportion of the damage associated with these events. Large, dormant deep-seated landslides are ubiquitous in the Oregon Coast Range, western United States, yet a method for calculating landslide ages with the precision required to diagnose a specific triggering event, including the A.D. 1700 Cascadia earthquake, has remained elusive. Establishing a compelling connection between prehistoric slope instability and specific triggers requires landslide ages with precision greater than that provided by 14C dating of detrital materials. Tree-ring analysis is the only known method capable of determining landslide age with this precision. Dozens of landslide-dammed lakes in western Oregon present an opportunity to use tree rings from drowned snags, or “ghost forests,” to establish the year of death, and thus landsliding. We cross-dated tree-ring indices from drowned Douglas fir trees with live tree-ring records from the Oregon Coast Range that exhibit synchronous, time-specific patterns due to regional climate variations. Our analyses determined that the landslides responsible for creating Wasson and Klickitat Lakes occurred in A.D. 1819 and 1751, respectively. The 14C dates from selected tree rings and landslide deposit detritus are consistent with our tree-ring analysis, although the ages exhibit high variability, revealing the limitations of using 14C dating alone. Because dendrochronology provides annual precision for landsliding, sampling of tree rings at additional landslide-dammed lakes throughout the Oregon Coast Range can be used to constrain the potential effects of ground motion and major storms on Cascadia landscapes.

Deuterium excess and 17O-excess variability in meteoric water across the Pacific Northwest, USA

High-precision triple oxygen isotope analysis of water has given rise to a novel second-order parameter, 17O-excess (often denoted as Δ17O), which describes the deviation from a reference relationship between δ18O and δ17O. This tracer, like deuterium excess (d-excess), is affected by kinetic fractionation (diffusion) during phase changes within the hydrologic cycle. However, unlike d-excess, 17O-excess is present in paleowater proxy minerals and is not thought to vary significantly with temperature. This makes it a promising tool in paleoclimate research, particularly in relatively arid continental regions where traditional approaches have produced equivocal results. We present new δ18O, δ17O, and δ2H data from stream waters along two east–west transects in the Pacific Northwest to explore the sensitivity of 17O-excess to topography, climate, and moisture source. We find that discrepancies in d-excess and 17O-excess between the Olympic Mountains and Coast Range are consistent with distinct moisture source meteorology, inferred from air-mass back trajectory analysis. We suggest that vapor d-excess is affected by relative humidity and temperature at its oceanic source, whereas 17O-excess vapor is controlled by relative humidity at its oceanic source. Like d-excess, 17O-excess is significantly affected by evaporation in the rain shadow of the Cascade Mountains, supporting its utility as an aridity indicator in paleoclimate studies where δ2H data are unavailable. We use a raindrop evaporation model and local meteorology to investigate the effects of subcloud evaporation on d-excess and 17O-excess along altitudinal transects. We find that subcloud evaporation explains much, but not all of observed increases in d-excess with elevation and a minor amount of 17O-excess variation in the Olympic Mountains and Coast Range of Oregon. Key Points 17O-excess correlates spatially with relative humidity across the Pacific Northwest, supporting its use as an aridity indicator in paleoclimate studies. Discrepancies in d-excess and 17O-excess between the Olympic Mountains and Oregon Coast Range suggest that their moisture source is different. Subcloud evaporation explains most of observed increases in d-excess with elevation, and a minor amount of 17O-excess variation in the Olympic Mountains and Oregon Coast Range.

Plant Water Stress and Soil Depletion in Variable-Density, Red Alder/Western Hemlock Coastal Oregon Plantations

Abstract Riparian ecosystems provide critical habitat and functions while being some of the most productive areas in forests. Both conifers and hardwoods contribute to maintenance of habitat and function. To determine the impact of water stress on growth of red alder (Alnus rubra Bong.) and western hemlock (Tsuga heterophylla [Raf.] Sarg.), we installed Nelder type 1a combined with replacement series plots on three Oregon Coast Range sites. Densities ranged from 988 to 85,400 trees/hectare, with ratios (hemlock:alder) of 100:0, 75:25, 50:50, 25:75, and 0:100. In the first 4 years after planting, alder used water in the growing season at greater depths earlier than western hemlock. Higher densities resulted in greater water stress later in the growing season in weeded areas (maintained by herbicide applications), but stress was similar across densities in unweeded areas. Water stress at early ages was correlated with decreased size 14 or 24 years after planting for both species, but these correlations were confounded with other effects of density. Increasing water availability in areas with low summer precipitation could enhance growth of red alder and western hemlock, even in highly productive riparian areas.

Idiosyncratic changes in spring arrival dates of Pacific Northwest migratory birds.

Shifts in the timing of bird migration have been associated with climatic change and species traits. However, climatic change does not affect all species or geographic locations equally. Climate in the Pacific Northwest has shifted during the last century with mean temperatures increasing by 1 °C but little change in total annual precipitation. Few long-term data on migration phenology of birds are available in the Pacific Northwest. We analyzed trends in spring arrival dates from a site in the Oregon Coast Range where nearly daily inventories of birds were conducted in 24 of 29 years. Several species showed statistically significant shifts in timing of first spring arrivals. Six of 18 species occur significantly earlier now than during the initial phase of the study. One species arrives significantly later. Eleven show no significant shifts in timing. We associated trends in spring migration phenology with regional climatic variables, weather (precipitation and temperature), traits of species such as migration strategy, foraging behavior, diet, and habitat use, and regional trends in abundance as indexed by Breeding Bird Survey data. We found no set of variables consistently correlated with avian phenological changes. Post hoc analyses of additional climate variables revealed an association of migratory arrival dates across the 18 species with rainfall totals in northern California, presumably indicating that songbird arrival dates in Oregon are slowed by spring storm systems in California. When only the six species with the most strongly advancing arrival dates were analyzed, winter maximum temperatures in the preceding three winters appeared consistently in top models, suggesting a possible role for food availability early in spring to promote the survival and successful reproduction of the earliest-arriving birds. However, additional data on food availability and avian survival and reproductive success are required to test that hypothesis. Despite the appearance of some climate variables in top models, there remains a mismatch between strongly advancing arrival dates in some songbirds and a lack of clear directional change in those climate variables. We conclude that either some previously unrecognized variable or combination of variables has affected the timing of migration in some species but not others, or the appearance of statistically significant directional changes over time can occur without being driven by consistent environmental or species-specific factors.

Quantifying the restoration success of wood introductions to increase coho salmon winter habitat

Abstract. Large wood (LW) addition is often part of fish habitat restoration projects. However, there is limited information about the spatial–temporal variability in hydraulic changes after LW additions. We investigated reach-scale hydraulic changes triggered after the addition of LW that are relevant to juvenile coho salmon survival. We used Nays2DH, an unsteady two-dimensional flow model, to quantify the patterns and magnitudes of changes of stream velocity and shear stress in three alluvial gravel reaches. The study sites are located in low-gradient reaches draining 5 to 16 km2 in the Oregon Coast Range. Survivable habitat was characterized in terms of critical swim speed for juvenile coho and bed stability considering the critical shear stress required to mobilize the median bed particle size. Model predictions indicated that survivable habitat during bankfull conditions, measured as the area with velocity below the critical swim speed for juvenile coho, increased by 95 %–113 % after the LW restoration. Bed stability also increased between 86 % and 128 % considering the shear stress required to mobilize the median bed particle size. Model predictions indicated more habitat created in the larger site; however, considering that wood would move more frequently in this site there appears to be a trade-off between the timing and the resilience of restoration benefits. Overall, this study quantifies how the addition of LW potentially changes stream hydraulics to provide a net benefit to juvenile salmonid habitat. Our findings are applicable to stream restoration efforts throughout the Pacific Northwest.

DRAINAGE EVOLUTION AND FRESHWATER FISH ZOOGEOGRAPHY IN COASTAL OREGON AND WASHINGTON

In Oregon and Washington coastal streams, there are relatively diverse faunas on the Olympic Peninsula of Washington (Chehalis fauna), in the lower Columbia River, and on the south-central Oregon coast (Tyee fauna). Adjacent to the Tyee fauna are groups of streams with depauperate faunas (0–2 primary freshwater fishes). To the north, from the Nehalem River to the Alsea River, streams have 0–1 species (Oregon Coast Range Isolates), and to the south, from the Rogue River to the Klamath River, streams have 0–2 species (Klamath Isolates). Throughout the entire area there was no relationship between drainage area and freshwater fish diversity. Multiple processes are responsible for these patterns. Two endemic genera, Oregonichthys and Novumbra, originated by the Miocene (23–5.3 Mya) and current distributions are largely bordered by the continental flood-basalt flows of the Columbia River Basalt Group (CRBG). It is hypothesized that both genera were isolated by early Miocene Grande Ronde basalt flows, Oregonichthys to the south of the flows in an ancestral Umpqua River, and Novumbra to the north of the flows in an ancestral Chehalis River. The depauperate streams of the Oregon Coast Range Isolates are hypothesized to be the result of bisection of ancient coastal streams by uplift of the Oregon Coast Range and subsequent extinction of the ancestral fauna on west-slope streams either from changing habitats or stochastic events in smaller streams. The Klamath Isolate streams drain the southern Oregon Coast Range and Klamath Mountains, an ancient region otherwise known for its faunal and floral diversity. It is hypothesized that these streams, some of which are relatively large and would be expected to have relatively diverse faunas, have been isolated from diverse primary freshwater fish faunas to the north (Columbia) and south (Sacramento), and most taxa have simply never had access to these streams.

Increased streamflow in catchments affected by a forest disease epidemic.

Natural disturbances help maintain healthy forested and aquatic ecosystems. However, biotic and abiotic disturbance regimes are changing rapidly. For example, the Swiss needle cast (SNC) epidemic in the Coast Range of Oregon in the U.S. Pacific Northwest has increased in area from 53,050 to 238,705ha over the 1996-2015 period. We investigated whether the hydrologic regime (i.e., annual streamflow, runoff ratio, and magnitude and timing of peak flows and low flows) was affected by SNC in 12 catchments in western Oregon. The catchments ranged in size from 183 to 1834km2 and area affected by SNC from 0 to 90.5%. To maximize the number of catchments included in the study, we analyzed 20years of SNC aerial survey data and 15-26years of stream discharge (Q) and PRISM precipitation (P) and air temperature (Tair) data to test for trends in hydrologic variables for each catchment. As expected, we found that runoff ratios (Q/P) increased in five catchments, all with an area impacted by SNC >10%. This was likely due to the effects of SNC on the hydraulic architecture (i.e., needle retention, sapwood area, sapwood permeability) of affected trees, leading to decreased canopy interception and transpiration losses. Interestingly, two catchments with the greatest area affected by SNC showed no changes in hydrologic regime. The lack of hydrologic response could either be due to compensatory transpiration by vegetation unaffected by the disease or sub-canopy abiotic evaporation, which counteracted reductions in transpiration. This study is the first to illustrate that chronic canopy disturbance from a foliage pathogen can influence catchment scale hydrology.

Biotic factors influencing the unexpected distribution of a Humboldt marten (Martes caurina humboldtensis) population in a young coastal forest.

Pacific martens (Martes caurina) are often associated with mature forests with complex structure for denning, resting, and efficient hunting. Nonetheless, a small isolated population of the Humboldt subspecies of Pacific martens (Martes caurina humboldtensis) occupies a narrow strip of young, coastal forest (< 70 years old) but not inland mature forest in the central Oregon Coast Range. We examined factors contributing to this unexpected distribution of martens by 1) analyzing marten diets using DNA metabarcoding to assess 90 scats, 2) using camera traps to assess differences in the relative abundances of prey, competitors, and predators across a coastal to inland gradient of vegetation types, and 3) quantifying differences in extent of fruit-producing shrubs and vegetation structure within vegetation types. Diets of martens were diverse (12, 10, and 3 species of birds, mammals, and amphibians respectively), and most fall and winter scats contained fruit. Voles, mice, and varied thrushes (Ixoreus naevius) were dominant prey items. Voles, mice, and most birds, but not varied thrushes, were more commonly observed in the coastal shrub-dominated forest than in inland forest. The coastal shrub-dominated forest had the highest diversity of vertebrates and potential prey overall. Bobcats (Lynx rufus), a key potential predator, were more commonly detected in inland forest. Of potential competitors, western spotted skunks (Spilogale gracilis) were more commonly detected in inland forest, with gray foxes (Urocyon cinereoargenteus) and raccoons (Procyon lotor) detected almost exclusively in coastal forests. Vegetation in coastal forests appears to provide, at least seasonally, more prey and fruit, and more overhead shrub cover compared with inland forest. Remaining plausible hypotheses for the restricted distribution of marten to coastal forests include increased prey, fruit, and overhead cover, and reduced predation risk from bobcats.

Severity of Swiss needle cast in young and mature Douglas-fir forests in western Oregon, USA

Swiss needle cast (SNC), caused by Nothophaeocryptopus gaeumannii, is an important foliage disease of Douglas-fir (Pseudotsuga menziesii) forests of the Pacific Northwest. The fungus lives endophytically within the foliage, until forming reproductive structures (pseudothecia) that plug stomates and cause carbon starvation. When pseudothecia appear on one- and two-year-old foliage, significant needle abscission can occur, which reduces productivity of the tree. While there is considerable evidence of SNC disease in coastal Douglas-fir plantations, the severity of SNC in mature and old-growth forests is poorly understood. We compared tree crowns of mature and old-growth conifer forests and nearby young forests at three locations in the Oregon Coast Range and four locations in the western Cascade Range of Oregon. We assessed disease severity for N. gaeumannii on two-year-old foliage, incidence by presence of N. gaeumannii on all foliage, foliage retention for the first four years, and foliar nitrogen of one-year-old foliage. We also compared leaf wetness at three heights in one mature and one young tree at five of the seven sites. Disease severity was greater in young forests than mature forests at all sites except for high elevation Cascade Range areas. Incidence of disease was highest for two-year-old needles in young trees and 3–5 year-old needles in mature trees, except for one coastal site. Retention of 1–4 year-old needle cohorts differed between young and mature trees, and mature trees had much larger complements of > four-year-old needles. Total foliar nitrogen (TN) concentration did not differ in needles of young and mature trees, but at some locations total N differed between canopy positions. Leaf wetness differences were not consistent between young and mature tree crowns. However, at one study site in the core epidemic area, the younger stand had longer periods of wetness in the upper crowns than a nearby old stand. Leaf wetness and foliar N were hypothesized to play a role in SNC disease severity, but they do not explain differences in adjacent young and mature trees. Although the fungus is present in old and young trees, the likelihood of disease expression and lower foliage retention appears to be greater in younger plantation trees than mature and older trees in western Oregon Douglas-fir forests.

Landscape connectivity and conservation prioritization for an old forest species with limited vagility

AbstractOld forests are often patchily distributed in contemporary forested landscapes leading to conservation concerns for species dependent on these forest types. In the absence of broad‐scale recovery of old forests, conservation initiatives have emphasized connecting forested landscapes to facilitate movement of genes or individuals. We assessed forest connectivity for a low‐vagile arboreal rodent (red tree vole; Arborimus longicaudus), an obligate of old forests within a distinct population segment that is a candidate for listing under the US Endangered Species Act in the northern Oregon Coast Range (USA). Within the distinct population segment, old forests (&gt;80 years old) have contracted by &gt;80% in the previous century, comprise 10.9% of the contemporary landscape and reside in a matrix of young forests (20–80 years old). We combined complimentary graph and circuit theory metrics into spatial conservation prioritization to provide insights into the composition and function of the contemporary forested landscape and to identify potential conservation and research priorities for this area. Given average distances from patches of old forests (3.4 km), we predicted that connectivity can only be achieved if red tree voles move and settle iteratively through young forest matrix, which is suboptimal habitat. We found that the top 1% conservation priorities were a series of small, spatially central patches and several large patches located where old forest cover was most extensive. Most of these patches were in protected reserves. Up to 30% of the top 10% prioritization were forests outside of reserves, highlighting the potential for these forests to contribute to landscape connectivity for this low‐vagile species that is dependent on old forest.