Washington Cascades Research Articles

Chromian spinel–olivine phase chemistry and the origin of primitive basalts of the southern Washington Cascades

Geochemically diverse basalts occur in an ∼160-km-wide transect that stretches across the Quaternary southern Washington Cascades (SWC) volcanic arc. Two fundamental groups of SWC basalts can be identified on the basis of major and trace element chemistry. On primitive mantle-normalized “spidergrams”, Group I lavas resemble within-plate basalts, whereas Group II lavas exhibit chemical characteristics (e.g., Nb, Ta, and Ti depletions) typical of subduction-related magmas. The primitive nature of many SWC basalts is indicated by their high MgO (>6.5 wt.%, up to 9.5 wt.%), Ni (>85 ppm) and Cr (>180 ppm) contents. Electron microprobe analyses of olivine–spinel pairs in a diverse suite of SWC basalts are used to further evaluate the primitive nature of these magmas. Some of the observed variations in olivine–spinel compositions can be attributed to oxidation, fractionation, mixing, and/or pressure variations during magma evolution. To minimize these effects, we focus on olivine-hosted spinels in samples for which olivine-host rock equilibrium can be demonstrated. Spinels in such rocks have Fe 2O 3<16.5 wt.%, 100 Fe 3+/(Fe 3++Cr+Al)<22, and exhibit high Mg# [=100 Mg/(Mg+Fe 2+)>44] and low Cr# [=100 Cr/(Cr+Al) <58]. For the most primitive spinel–olivine pairs: (1) spinel Cr# ranges from 20 to 57; (2) spinel Cr# for Group I lavas are distinctly lower (< 41) than those (> 45) for Group II lavas; (3) spinel Mg# ranges from 45 to 71, and overlaps for Groups I and II. However, each group defines distinct trends on Mg# vs. Cr# and other variation diagrams, with Group II having more refractory characteristics. This observation is consistent with higher whole-rock Mg# and Fo contents of olivines for Group II lavas, and suggests that sources for those magmas are more refractory (i.e., more melt-depleted) than sources for Group I lavas. The “within-plate”-like chemistry of Group I lavas precludes significant slab-derived contributions and is consistent with their derivation by decompression melting of little modified and relatively fertile mantle wedge material upwelling beneath the arc. The fluid-mobile element-enriched nature of Group II lavas is consistent with melting of a mantle source(s) that has been modified by slab-derived fluids. However, the standard “flux melting” process invoked to explain formation of many arc magmas is problematic for the Cascades because (a) high temperatures inferred for the Cascadia subduction zone imply that the slab may be extensively dehydrated, in which case the inventory of volatiles and fluid-mobile elements is likely to be strongly depleted; and (b) Group I lavas occur at the volcanic front and appear to originate at depths proximal to the subducting slab. These geochemical and geometric constraints seem inconsistent with formation of Group II magmas by flux melting of the mantle wedge as commonly proposed for arcs. An alternative source for Group II lavas is chemically refractory lithospheric mantle that was previously metasomatized by earlier (40 Ma to present) Cascadia subduction and associated magmatism. Reheating and melting of such fluid-enriched domains could result from influx of hot decompression melts (Group I magmas). In either case, chemical and mineralogical evidence implicates two (or more) distinct processes of melt production and at least two distinct types of mantle source.

Boron and lithium isotopic variations in a hot subduction zone—the southern Washington Cascades

New B and Li isotopic data are presented for well-characterized basalts from a transect across the Cascades volcanic arc, S. Washington, for a sediment core on the incoming Juan de Fuca plate, and for dacites from Mt. St. Helens. The basalts define two compositional groups, one (I) having affinities to those from intraplate settings and the other (II) resembling typical volcanic arc calcalkalic lavas. These groups are inferred to have distinct sources. Group I lavas have low Li/Y (0.25–0.6) and B/Nb (∼0.1–0.45), near-constant Ba/Nb (7–18), and show little cross-arc variation in fluid-mobile/immobile element ratios. In contrast, Group II lavas have slightly elevated Li/Y (∼0.5) and higher Ba/Nb (21–100) regardless of location; B/Nb is enriched (to ∼2.2) in the frontal arc, but toward the backarc drops to lower values (<0.4) overlapping with intraplate basalts. δ 11B values are among the lowest reported for volcanic arcs. In detail, δ 11B exhibits spatial variation, being highest (∼−3‰ to −0.4‰) in frontal arc lavas then decreasing sharply to OIB-like values (−8‰ to −9‰) in the backarc region; no systematic variation with magma types is apparent. δ 7Li values (2.5‰ to 5‰) are MORB-like and show no systematic spatial variation. Combined with Sr–Nd–Pb isotopic and other trace element data, these results are consistent with weak slab contributions beneath the frontal arc ranging to negligible inputs below the backarc. This pattern is attributed to progressive dehydration of the subducting slab in an unusually warm subduction zone (SZ); initial inventories of fluid-mobile elements (FMEs) apparently were reduced extensively before the slab reached subarc depths, and essentially depleted as it moved beneath the backarc. Slab contributions appear to be superimposed on a compositionally layered mantle structure consisting of two domains sampled by the distinct basalt groups. Group I lavas appear to form by decompression melting of convectively upwelling deeper mantle that is little modified from an OIB–MORB source composition, except for the relatively small effect on B composition beneath the frontal arc. Group II lavas may form by melting of enriched domains within shallower lithospheric mantle. Because both groups have similar δ 11B, production of Group II magmas need not involve modern inputs of slab fluid. This interpretation differs from the standard paradigm for arc magmatism in that flux melting is not invoked as the dominant cause for partial melting; efficient dehydration of the slab may remove the impetus to drive this mechanism.

Truffle abundance and mycophagy by northern flying squirrels in eastern Washington forests

Although much is known about truffle abundance and rodent mycophagy in mesic Douglas-fir forests in the Pacific Northwest, few data are available for dry interior montane forests dominated by ponderosa pine ( Pinus ponderosa), Douglas-fir ( Pseudotsuga menziesii), and grand fir ( Abies grandis). Our objective was to quantify the relationship between the abundance and diversity of ectomycorrhizal fungal sporocarps in the soil and in the diets of northern flying squirrels ( Glaucomys sabrinus) in low-elevation forests of the eastern Washington Cascades. We randomly sampled four stands each of three cover types: dry open ponderosa pine, mesic young mixed-conifer forest, and mesic mature mixed-conifer forest. We sampled the soil for hypogeous sporocarps during the spring of 1999 and 2000. We collected fecal pellets from 318 flying squirrels live-trapped during the fall of 1997–2000. We sampled 2400 m 2 of soil surface and found truffles in 40% of 600 plots. Total biomass collected was 609 g. Spring truffle biomass on a kg/ha basis averaged 1.72 in open pine, 3.56 in young, and 4.11 in mature forest. Twenty-two species were collected across all cover types, with all but three species belonging to the Basidiomycotina. Eleven dominant species accounted for 91–94% of truffle biomass in each cover type. Four dominant species accounted for 60–70% of spring truffle biomass: Gautieria monticola, Hysterangium coriaceum, Rhizopogon parksii, and R. vinicolor. Truffle assemblages, richness and total biomass differed among cover types: richness and biomass were highest in young and mature mixed-conifer forest, and lowest in open ponderosa pine forest. Fall squirrel diets were composed of 23 genera or groups of fungi, plus about 22% plant material. Rhizopogon was the most abundant genus in the diet, followed by plant material, then Gautieria, Leucogaster, Alpova, and Hysterangium. Diets in different cover types were similar in the composition, richness, evenness, and the ratio of fungus to plant material. Diet richness varied over the study period. Nineteen truffle genera were detected in fall fecal samples versus 12 in spring soil samples. Management of low-elevation dry forest to maintain or restore stable fire regimes might reduce truffle diversity at stand scales by simplifying stand composition and structure; but, such management might increase long-term beta and landscape truffle diversity and persistence by reducing the occurrence of high-intensity fires and stabilizing inherent fire disturbance regimes.

Development of Canopy Structure in Pseudotsuga menziesii Forests in the Southern Washington Cascades

Abstract We quantified stand-level structural diversity in eight stands of temperate coniferous forests that ranged in age from 50 to 950 years in the southern Washington Cascade Range. Stands were chosen based on the dominance, or former dominance in the case of the oldest stands, of Pseudotsuga menziesii. In addition, to avoid confusing patterns of structural development, only stands that had a single disturbance of origin were used. Within each stand, randomly located transects were laid out that were proportional in size to the dominant tree height. Tree heights and diameters ranged up to 90 m and 309 cm, respectively. A total of 5,737 trees among the 32 transects were mapped and measured for height, diameter, crown height, and four crown radii. The structure of the canopy in the younger stands is a simple, fairly even monolayer. This contrasts with the older stands, which have a very complex, highly diffuse canopy with extreme variability in both horizontal and vertical dimensions. The stands with the highest structural complexity were between 500 and 650 years old and retained from 17 to 76 Pseudotsuga stems/ha. Our oldest site had only one living Pseudotsuga/ha. Mean diameter of just the Pseudotsuga trees was less than 55 cm for stands less than 200 years and over 180 cm for the most complex old-growth stands. One measure of stand structural diversity, the standard deviation of dbh, ranged from less than 25 (stands less than 200 years) to values greater than 50 (most complex old-growth sites). A similar relationship exists when using the standard deviation of height. Large snags and logs important for wildlife were present only in the old-growth stands and were abundant in the most structurally diverse stands. FOR. SCI. 50(3):326–341.

Three-dimensional Structure of an Old-growth Pseudotsuga-Tsuga Canopy and Its Implications for Radiation Balance, Microclimate, and Gas Exchange

We describe the three-dimensional structure of an old-growth Douglas-fir/western hemlock forest in the central Cascades of southern Washington, USA. We concentrate on the vertical distribution of foliage, crowns, external surface area, wood biomass, and several components of canopy volume. In addition, we estimate the spatial variation of some aspects of structure, including the topography of the outer surface, and of microclimate, including the within-canopy transmittance of photosynthetically active radiation (PAR). The crowns of large stems, especially of Douglas-fir, dominate the structure and many aspects of spatial variation. The mean vertical profile of canopy surfaces, estimated by five methods, generally showed a single maximum in the lower to middle third of the canopy, although the height of that maximum varied by method. The stand leaf area index was around 9 m2 m−2, but also varied according to method (from 6.3 to 12.3). Because of the deep narrow crowns and numerous gaps, the outer canopy surface is extremely complex, with a surface area more than 12 times that of the ground below. The large volume included below the outer canopy surface is very porous, with spaces of several qualitatively distinct environments. Our measurements are consistent with emerging concepts about the structure of old-growth forests, where a high degree of complexity is generated by diverse structural features. These structural characteristics have implications for various ecosystem functions. The height and large volume of the stand indicate a large storage component for microclimatic variables. The high biomass influences the dynamics of those variables, retarding rates of change. The complexity of the canopy outer surface influences radiation balance, particularly in reducing short-wave reflectance. The bottom-heaviness of the foliage profile indicates much radiation absorption and gas exchange activity in the lower canopy. The high porosity contributes to flat gradients of most microclimate variables. Most stand respiration occurs within the canopy and is distributed over a broad vertical range.

Relationship Between Xerophyllum tenax and Canopy Density in the Southern Cascades of Washington

Abstract Large-scale commercial harvest of beargrass (Xerophyllum tenax) has been taking place in the Cascades of Washington and Oregon for the past 15 to 20 years. The long, slender leaves are either used fresh or dried and dyed for use in the floral industries in the United States and Europe. Our objectives were to develop a better understanding of beargrass production under different tree canopy (overstory) densities in the Pacific silver fir/big huckleberry/beargrass and the mountain hemlock/big huckleberry/beargrass plant associations in and around the Cispus Adaptive Management Area. We examined differences in beargrass production for different overstory canopy conditions on 10 sites in each association. Results indicated that beargrass quality is not of commercial grade under open canopies (&lt;60% overstory density). For medium and high densities, the interaction between plant association and overstory density was significant for all response variables except harvestable dry mass. Harvestable dry mass of beargrass did not differ between the two associations, but was greater under medium- compared with high-density conditions. For the Pacific silver fir association, the high-overstory-density class had greater basal area of beargrass per site, and plants were larger with longer leaves compared to medium-canopy-density sites. We did not find this relationship for the mountain hemlock association, except for the longest leaf variable. It is unclear why basal area and size of beargrass were more closely related to overstory conditions for the Pacific silver fir association. Evaluation of the sustainability of beargrass as a nontimber forest product will require long-term study of the relationships among environmental variables, beargrass productivity, and beargrass population dynamics. West. J. Appl. For. 19(2):82–87.

Coupled spatial variations in precipitation and long-term erosion rates across the Washington Cascades.

Past studies of tectonically active mountain ranges have suggested strong coupling and feedbacks between climate, tectonics and topography. For example, rock uplift generates topographic relief, thereby enhancing precipitation, which focuses erosion and in turn influences rates and spatial patterns of further rock uplift. Although theoretical links between climate, erosion and uplift have received much attention, few studies have shown convincing correlations between observable indices of these processes on mountain-range scales. Here we show that strongly varying long-term (>10(6)-10(7) yr) erosion rates inferred from apatite (U-Th)/He cooling ages across the Cascades mountains of Washington state closely track modern mean annual precipitation rates. Erosion and precipitation rates vary over an order of magnitude across the range with maxima of 0.33 mm yr(-1) and 3.5 m yr(-1), respectively, with both maxima located 50 km west (windward) of the topographic crest of the range. These data demonstrate a strong coupling between precipitation and long-term erosion rates on the mountain-range scale. If the range is currently in topographic steady state, rock uplift on the west flank is three to ten times faster than elsewhere in the range, possibly in response to climatically focused erosion.

Epiphytic lichen diversity and biomass in low-elevation forests of the eastern Washington Cascade range, USA

I used litterfall sampling to estimate the biomass, diversity, and community structure of epiphytic lichens in three dry forest cover types in the eastern Washington Cascade range. Cover types represented a temperature/moisture and stand structural complexity gradient. Lichen litterfall biomass increased with increasing stand complexity and moisture. Lichen litterfall biomass was 3.42 kg/ha in open pine stands, 7.51 kg/ha in young mixed-species stands, 8.55 kg/ha in mature mixed-species stands. Six species accounted for 63% of the total sample biomass: Bryoria capillaris (20.2%), Hypogymnia imshaugii (15.8%), Letharia vulpina (14.5%), B. fremontii (7.2%), Nodobryoria abbreviata (2.8%), and Alectoria sarmentosa (2.6%). Bryoria tortuosa, considered rare in most of its range, was relatively common in young forest. Epiphytic lichen species richness and dominance did not differ among the three forest cover types. Lichen associations differed, however, among cover types. The dominant lichen in open pine stands was L. vulpina, followed by Bryoria spp., H. imshaugii, and L. columbiana. Vulpicida canadensis and L. columbiana were weak indicator species. Young stands were dominated by H. imshaugii, a strong indicator species of young stands, followed by B. capillaris and L. vulpina. Cetraria platyphylla and C. pallidula also were strong indicator species of young stands. Mature stands were overwhelmingly dominated by Bryoria species, primarily B. capillaris and B. fremontii, both strong indicators of mature stands. A. sarmentosa, H. imshaugii, and L. vulpina were other moderately abundant species. Alectoria imshaugii was also a strong indicator species of mature forest. Impacts on epiphytic lichens of dry forest management options that reduce stand density in fire-suppressed stands might vary. Lichens associated with dry open conditions would presumably benefit. The impact on species associated with old or moist conditions, especially forage lichens, might be lessened by variable-density thinning and other practices that maintain open- and closed-canopy patchiness and large trees. That patchiness might be hard to maintain with a maintenance program of understory burning patterned after natural fire regimes. At landscape scales, however, long-term persistence of forage and other lichens associated with mature moist forest would be enhanced by restoration of stable fire regimes.

Black bear resource selection in the northeast Cascades, Washington

We examined resource selection of black bears in the northeast Cascades of Washington at two spatial scales. Specifically, we compared habitats selected for within home ranges to those available in the study area, and habitats selected for versus those available within home ranges. Compositional analysis showed selection of similar habitats at each spatial scale, with some differences. In the dry climate of the eastern Cascades, black bears appeared to locate home ranges within habitats that would provide abundant food resources, such as riparian and deciduous forests, meadows and shrubfields. Once established in a home range, black bears selected for a mosaic of habitat types that provided security cover in proximity to food resources, such as riparian and deciduous forests, other forest types and meadows.

Influence of step composition on step geometry and flow resistance in step‐pool streams of the Washington Cascades

Step‐pool streams dissipate flow energy primarily through spill resistance. We compared the geometry, step characteristics, and flow hydraulics of 20 step‐pool reaches without large woody debris (LWD) to 20 step‐pool reaches with LWD. Non‐LWD streams exhibited significantly shallower flows, lower steps, shorter step spacings, greater percentages of water‐surface drop created by steps, larger grain sizes, and smaller Darcy‐Weisbach friction factors. Grain resistance was negligible in both stream types. Form resistance created by irregularities in the channel shape associated with steps contributed more to the total flow resistance in LWD reaches. Although both stream types showed poor correlation between step height and flow resistance, the significant positive correlation between flow resistance and step height/length ratio in the non‐LWD reaches demonstrates the increasing effect of spill resistance with increasing step height. The lack of such a trend in the LWD‐loaded reaches suggests that spill resistance was highly influenced by a few large log steps in these reaches. LWD creates deep pools and increases flow resistance along step‐pool streams. It thus stabilizes channels and stores sediment in steep headwater streams recently scoured by debris flows.

Influence of wildfires on apatite and zircon (U-Th)/He ages

Low closure temperatures of the apatite and zircon (U-Th)/He thermochronometers allow valuable constraints on timing and rates of bedrock exhumation through shallow crustal depths, but raise the possibility that shallow-level processes other than exhumation-related cooling may also influence He ages. A simple He diffusion model predicts that wildfires can completely or partially reset apatite He ages as much as 3 cm below rock surfaces and partially reset zircon He ages in the outermost 1 cm. Measured He ages in bedrock and sediments from the Washington Cascades that were exposed to extensive wildfires in 2001 show strong agreement with these model predictions. Apatite He ages decrease from a regionally consistent age of 19.5 ′ 1.2 Ma at a distance >3 cm from the rock surface to as low as 1.9 Ma in the outermost 1 cm, whereas zircon He ages decrease from 65 to 55 Ma over the same distance. Thin (<3 cm) flakes shed from a nearby boulder during or after the most recent fire have apatite He ages ranging from 9.7 ′ 0.6 to 17.2 ′ 1.0 Ma. The partial-resetting profiles are best explained by model thermal histories involving at least one short-duration (∼5-10 min), high-temperature (575-650 °C) event and at least one longer (30-40 min), lower-temperature (350-450 °C) event. Age-depth profile data may be useful in determining wildfire intensities or locations and also suggest that He ages of detrital apatites from some environments may be subject to bias from the thermal effects of wildfires.

Nest-Site Habitat Use by White-Headed Woodpeckers in the Eastern Cascade Mountains, Washington

The white-headed woodpecker (Picoides albolarvatus) is strongly associated with old-growth ponderosa pine (Pinus ponderosa) forest, a degraded and increasingly uncommon habitat in the Pacific Northwest. We investigated the nest-site habitat use of this species by col- lecting vegetation data at 21 known nest sites in the eastern Cascades of Washington, 12 of which we found in 1999. Sixteen of 17 (94%) nests in the ponderosa pine vegetation zone were in the 72% of the zone that occurred below 1219 m, and 15 (88%) nests were on slopes of <20% (which comprised 48% of the zone); the total area characterized by both slopes <20% and elevation below 1219 m was 181,664 ha or 33.6% of the vegetation zone. Most (16 of 21) nests were in ponderosa pines and 81% (17 of 21) were in snags. The nest snags and trees were generally large, with a mean diameter at breast height of 51.5 cm (sx = 5.13). The mean height of nest snags and trees was 12.6 m (sx = 2.33), and the mean height of the nest cavity entrance was 5.8 m (sx = 1.37). Compared to random sites located <1 km from each nest, nest sites were characterized by a greater abundance, size, and basal area of large trees and snags, primarily ponderosa pines. Management of habitat for this species should include retention of 6 to 8 large snags/0.8 ha and 8 to 10 large trees/0.8 ha in the immediate vicinity of nesting areas. Habitat requirements at the home range scale should be investigated.

Patterns of Survival, Damage, and Growth for Western White Pine in a 16-Year-Old Spacing Trial in Western Washington

Abstract Information about effects of planted spacing on growth and development of western white pine is scant because white pine blister rust disease has severely limited the survival of trees in young plantations. In the early 1980s, we established a western white pine spacing trial in the southern Cascades of Washington, using seedlings from a seedlot comprised primarily of open-pollinated seed from trees screened and certified as resistant to white pine blister rust. Initial square spacings ranged from 2 to 6 m; treatment plots were 0.4 ha in size and were replicated three times (six times for the 3 m spacing). Permanent measurement plots were installed at age 8, and the lower portion of all stems was pruned in the 10th growing season to reduce blister rust infections. The plots were remeasured at ages 11 and 16. Sixteen years after planting, survival averaged 80%; most mortality was associated with blister rust, but the rate of new infections and mortality diminished substantially between ages 11 and 16; 71% of the planted trees were free of blister rust at age 16. Trees averaged 8.4 m tall (ranging from 7.6 m in the 2 m spacing to 8.8 m in the 4 and 5 m spacings) and 12.7 cm dbh (10.3 in the 2 m spacing to 14.0 cm in the 5 m spacing). Periodic annual growth from 11 to 16 yr in the three wider spacings averaged 0.7 m in height and 1.0 cm in diameter. Antler rubbing by elk caused substantial damage to stems, but wounds on most trees were overgrown in 2 to 4 yr. Rub damage did not appear directly related to spacing but rather to stem diameter, with damage limited primarily to stems &gt;6 and &lt;15 cm. Early growth rates in this trial were much greater than those attained in older, natural stands (as inferred from site index curves and yield tables) and in progeny tests and other young silvicultural trials planted elsewhere. We believe rust-resistant stock of western white pine merits greater consideration for planting in the Douglas-fir region. West. J. Appl. For. 18(1):35–43&gt;.

Integration of phase equilibria modelling and garnet Sm-Nd chronology for construction of P-T-t paths: examples from the Cordilleran Coast Plutonic Complex, USA

Abstract Integration of petrographic observations, mineral chemistry, garnet Sm-Nd isochrons, and MnNaCaKFMASH pseudosection phase equilibria models constructed with THERMOCALC provides quantitative metamorphic pressure-temperature-time (P-T-t) paths which allow determination of assemblage/reaction history for pelites. Examples are presented for the Cretaceous to Tertiary magmatic arc of the North American Coast Plutonic Complex. Metamorphism in the western Coast Plutonic Complex of southeastern Alaska and in the North Cascades of Washington resulted from at least three widespread events from &gt; 100 Ma to c. 60 Ma, and in both areas partly resulted from crustal thickening, evidenced by local occurrences of kyanite after andalusite. Pressure-temperature pseudosections constructed from bulk rock compositions and the intersection of garnet core composition isopleths provide estimates for the pressure and temperature of garnet core growth. Intersections of these isopleths indicate garnet growth 18 to 85 °C above the predicted garnet-in reaction temperatures. Rim and near-rim garnet compositions and matrix mineral chemistry provide estimates for near-peak metamorphic conditions. Finite pressure-temperature-time paths of garnet zone metamorphism were determined from the combined core growth and pressure-temperature conditions determined from near-rim garnet and matrix mineral compositions. The western Coast Plutonic Complex near the Stikine River, southeastern Alaska, displays a complex pattern of regional metamorphism overprinted by contact metamorphic aureoles. Many of the c. 90 Ma aureoles contain andalusite, andalusite plus sillimanite, or andalusite plus kyanite with complex replacement textures. A pseudosection constructed for a contact metamorphic rock on Kadin Island (95GL11c), predicts that garnet grew c. 555 ± 10 °C and 4.8 ± 0.7 kbar, above the garnet-in line and the aluminium silicate triple-point pressure. These results suggest that andalusite in samples from this aureole likely grew prior to garnet and that the pressure may have increased by ≤ 1 kbar during metamorphism. The southern part of the North Cascades in Washington also contain complex aluminium silicate replacement textures with early andalusite and later kyanite and sillimanite. A sample (96NC67), collected near the andalusite-bearing aureole of the Mt Stuart batholith, contains sillimanite and c. 10 mm garnet crystals containing staurolite inclusions in their cores. Temperatures estimated from the garnet core of this sample are within the pseudosection staurolite stability field, compatible with initial garnet growth significantly above the garnetin line. The garnet rim thermometry estimate of c. 668 ± 59 °C for this sample is c. 85 °C higher than the core growth temperature. The calculated P-T-t path provides important information for interpreting regional and contact metamorphism. An extensive region NE of the Mt Stuart batholith in the North Cascades underwent a significant pressure increase; however, the timing and nature of medium- to high-pressure metamorphism is controversial. Quantitative P-T-t paths constructed for garnet growth along the NE margin of the batholith indicate that 87-85 Ma garnet growth was younger than the nearby Mt Stuart batholith (93.5 ± 1.4 Ma, U-Pb zircon). Garnet core and rim segments are isochronous indicating a short interval for garnet growth. P-T-t paths indicate that garnet growth occurred in the sillimanite stability field during a maximum pressure increase of 1 to 2 kbar, after rocks passed through the andalusite stability field (Mt Stuart contact metamorphism). Careful sampling, hand-picking, acid leaching, and isotopic analysis of garnet provide geologically consistent ages with uncertainties of ≤ 1.0 Ma. Thermodynamic modelling in the MnNaCaKFMASH system provide reasonable P-T predictions for pelite mineral stability that can be integrated with isotope ages to provide quantitative P-T-t paths. The P-T-t paths developed for both regional and contact metamorphic rocks allow critical evaluation of tectonic models and of interpretations for mineral textures.

Cavities in Snags along a Wildfire Chronosequence in Eastern Washington

We sampled occurrence of bird-excavated cavities in snags in a chronosequence of 26 wildfire burns (ages 1-81 years) on the east slope of the Washington Cascade Range, USA. Cavities occurred in 5.5% of the 1,867 recorded snags; most (69%) were in burns 250) of subalpine fir (Abies lasiocarpa; 0.2%) and lodgepole pine (Pinus contorta; 0.0%) snags. Cavities occurred in about 4% of the small samples (n 34 cm diameter at breast height [dbh]) snags >2 m tall and located in middle-age to older burns (>19 years old). Cross-validated accuracy of the classification tree model was 97% for cavity snags and 82% for snags without cavities. Cavity-bearing Douglas-fir snags were best characterized as large-diameter snags (>33 cm dbh), or as smaller soft snags (Class 4, 5) at elevations <1,200 m. Accuracy of the classification tree model was 88% and 73% for snags with and without cavities, respectively. In burns <20 years old, Douglas-fir snags with broken tops had higher cavity excavation rates (6%) than snags with whole tops (0.25%). Aspect, slope, slope position, and elevation had negligible or no value for classification. To ensure good post-fire snag habitat, manage pre-fire green stands for tree species, large size, defect (e.g., broken tops), and spatial pattern that will provide cavity snags in short and long terms. When salvaging burns, retain snags with defects incurred prior to fire, especially broken tops, and large-diameter snags of species known to be most suitable for cavity excavation in that area.

POTENTIAL UPPER BOUNDS OF CARBON STORES IN FORESTS OF THE PACIFIC NORTHWEST

Placing an upper bound to carbon (C) storage in forest ecosystems helps to constrain predictions on the amount of C that forest management strategies could sequester and the degree to which natural and anthropogenic disturbances change C storage. The potential, upper bound to C storage is difficult to approximate in the field because it requires studying old-growth forests, of which few remain. In this paper, we put an upper bound (or limit) on C storage in the Pacific Northwest (PNW) of the United States using field data from old-growth forests, which are near steady-state conditions. Specifically, the goals of this study were: (1) to approximate the upper bounds of C storage in the PNW by estimating total ecosystem carbon (TEC) stores of 43 old-growth forest stands in five distinct biogeoclimatic provinces and (2) to compare these TEC storage estimates with those from other biomes, globally. Finally, we suggest that the upper bounds of C storage in forests of the PNW are higher than current estimates of C stores, presumably due to a combination of natural and anthropogenic disturbances, which indicates a potentially substantial and economically significant role of C sequestration in the region. Results showed that coastal Oregon stands stored, on average, 1127 Mg C/ha, which was the highest for the study area, while stands in eastern Oregon stored the least, 195 Mg C/ha. In general, coastal Oregon stands stored 307 Mg C/ha more than coastal Washington stands. Similarly, the Oregon Cascades stands stored 75 Mg C/ha more, on average, than the Washington Cascades stands. A simple, area-weighted average TEC storage to 1 m soil depth (TEC100) for the PNW was 671 Mg C/ha. When soil was included only to 50 cm (TEC50), the area-weighted average was 640 Mg C/ha. Subtracting estimates of current forest C storage from the potential, upper bound of C storage in this study, a maximum of 338 Mg C/ha (TEC100) could be stored in PNW forests in addition to current stores.

Potential Upper Bounds of Carbon Stores in Forests of the Pacific Northwest

Placing an upper bound to carbon (C) storage in forest ecosystems helps to constrain predictions on the amount of C that forest management strategies could sequester and the degree to which natural and anthropogenic disturbances change C storage. The potential, upper bound to C storage is difficult to approximate in the field because it requires studying old-growth forests, of which few remain. In this paper, we put an upper bound (or limit) on C storage in the Pacific Northwest (PNW) of the United States using field data from old-growth forests, which are near steady-state conditions. Specifically, the goals of this study were: (1) to approximate the upper bounds of C storage in the PNW by estimating total ecosystem carbon (TEC) stores of 43 old-growth forest stands in five distinct biogeoclimatic provinces and (2) to compare these TEC storage estimates with those from other biomes, globally. Finally, we suggest that the upper bounds of C storage in forests of the PNW are higher than current estimates of C stores, presumably due to a combination of natural and anthropogenic disturbances, which indicates a potentially substantial and economically significant role of C sequestration in the region. Results showed that coastal Oregon stands stored, on average, 1127 Mg C/ha, which was the highest for the study area, while stands in eastern Oregon stored the least, 195 Mg C/ha. In general, coastal Oregon stands stored 307 Mg C/ha more than coastal Washington stands. Similarly, the Oregon Cascades stands stored 75 Mg C/ha more, on average, than the Washington Cascades stands. A simple, area-weighted average TEC storage to 1 m soil depth (TEC100) for the PNW was 671 Mg C/ha. When soil was included only to 50 cm (TEC50), the area-weighted average was 640 Mg C/ha. Subtracting estimates of current forest C storage from the potential, upper bound of C storage in this study, a maximum of 338 Mg C/ha (TEC100) could be stored in PNW forests in addition to current stores.

Fringe Conservation: a Call to Action

Fringe Conservation: a Call to Action

Small mammal response to experimental canopy gaps in the southern Washington Cascades

General patterns of small mammal species occurrence across major successional gradients have been studied extensively in western Washington forests, but the effects of smaller-scale disturbances are less clear. Understanding such effects will help to evaluate alternatives to traditional clear-cut logging. We compared relative abundances, population characteristics, and species occurrence of small mammals in experimentally created canopy gaps and adjacent forests in the southern Washington Cascade Range. Contrary to our predictions, species characteristic of closed-canopy conditions dominated captures in all gap sites. No significant declines in abundance between forests and large gap patches were observed for any closed-canopy species. Measures of persistence, reproductive status, sex ratios, and weight distributions did not indicate that gaps were sinks or lower quality habitat for Trowbridge’s shrew ( Sorex trowbridgii), Keen’s mouse ( Peromyscus keeni), or the southern red-backed vole ( Clethrionomys gapperi). The creeping vole ( Microtus oregoni) was the only early-successional species to respond strongly to the openings. It occurred frequently in large (0.1–0.2 ha) gaps but rarely in smaller gaps and forested patches. The large gaps targeted in this study are at the upper end of the range of natural gap sizes reported for the western Cascades, but may be too small to provide habitat for most early-successional species. Our results indicated that the main direct effect of disturbance-created small gaps in these areas was to increase resources for forest species rather than to facilitate recruitment of early-successional small mammals.

Estimating Precipitation in the Central Cascades of Washington

Abstract Precipitation in the central Cascades of Washington correlates well over 1966–96 with wind and moisture in twice-daily upper-air soundings at a radiosonde station near the Pacific coast, 225 km away. A simple model estimates precipitation by using the component of the wind roughly normal to the north–south range of the Cascade Mountains, which it raises to a power and scales by the relative humidity. Values at 850 mb are taken as an index of the total moisture flux. Thresholds are imposed for the wind component and the relative humidity to reduce the likelihood of estimating precipitation from weak onshore flow on dry days. A split-sample analysis indicates that the model parameters are highly robust against sampling error. This moisture flux model estimates precipitation over 5-day periods with the coefficient of determination r2 ≈ 0.55, which increases for precipitation aggregated over 30-day periods to 0.75 and decreases to 0.30 for daily precipitation. Results over a 4-month period in the win...