Old-growth Douglas-fir Research Articles

A Place for Inquiry, A Place for Wonder: The Andrews Forest. By William G. Robbins

William Robbins’s latest study makes an important contribution to the environmental history of the Pacific Northwest through its thorough, careful scholarship that documents a significant, as-yet little recognized site in our nation’s forest science history. In A Place for Inquiry, A Place for Wonder: The Andrews Forest, Robbins argues that the 15,800-acre experimental forest—“a cloistered place, unknown to the vast majority of Oregon citizens” (p. 192)—is the site of significant contributions to ecosystem science, as well as the environmental humanities, at regional, national, and international scales. The H.J. Andrews (named posthumously for Horace J. Andrews, Regional Forester of the U.S. Forest Service’s Pacific Northwest Region (1943–1951)), located west of the Cascade Mountains within the Willamette National Forest, was established in 1948. Over its seven-decade history, this Experimental Forest of the U.S. Forest Service has generated insights about old-growth Douglas fir forests and plantations and the drainage basin’s watershed processes....

Assessing local impacts of the 1700 CE Cascadia earthquake and tsunami using tree-ring growth histories: a case study in South Beach, Oregon, USA

Abstract. We present an investigation of the disturbance history of an old-growth Douglas-fir (Pseudotsuga menziesii) stand in South Beach, Oregon, for possible growth changes due to tsunami inundation caused by the 1700 CE Cascadia Subduction Zone (CSZ) earthquake. A high-resolution model of the 1700 tsunami run-up heights at South Beach, assuming an “L”-sized earthquake, is also presented to better estimate the inundation levels several kilometers inland at the old-growth site. This tsunami model indicates the South Beach fir stand would have been subjected to local inundation depths from 0 to 10 m. Growth chronologies collected from the Douglas-fir stand shows that trees experienced a significant growth reductions in the year 1700 relative to nearby Douglas-fir stands, consistent with the tsunami inundation estimates. The ±1–3-year timing of the South Beach disturbances are also consistent with disturbances previously observed at a Washington state coastal forest ∼220 km to the north. Moreover, the 1700 South Beach growth reductions were not the largest over the >321-year tree chronology at this location, with other disturbances likely caused by climate drivers (e.g., drought or windstorms). Our study represents a first step in using tree growth history to ground truth tsunami inundation models by providing site-specific physical evidence.

Quantifying the vertical diversification development stage of old-growth Douglas-fir to derive stage-specific targets for restoration silviculture

Structural variability in natural Douglas-fir (Pseudotsuga menziesii (Mirbel) Franco) forests hinders restoration targets for the full old-growth developmental sequence. To guide stage-specific restoration, we present a new approach toward developing a simple index of the vertical diversification (VD) stage. VD-associated sub-stand structures were objectively identified by quantifying the size structures of live trees across a developmental sequence of 10 pristine stands in the Oregon Cascades, USA. Floating neighborhoods were used to delineate natural tree neighborhoods based on triangulated irregular networks in five concentric rings of ever-greater spatial extent (averaging 60–2060 m2). Diameter distributions summarized the most frequently encountered tree size structures among neighborhoods, with increasing deviation from the neutral multi-variate core. Of 18 observed diameter distribution types (DDTs), the core DDT characterized all-sized tree neighborhoods reflective of vertical diversification, which was most abundant in early old-growth (VD) stands and least abundant at the extremes of the sequence. VD declined in older stands, whose more distinct DDTs had peaks in larger trees and multiple size classes, likely reflecting horizontal diversification (HD). This new approach illustrates that structural restoration of VD stands could be facilitated by the single-tree selection method, while that of HD stands may be promoted with both single-tree and group selection as well as targeted release.

Dissolved organic carbon production and flux under long-term litter manipulations in a Pacific Northwest old-growth forest

Dissolved organic carbon (DOC) flux is an important mechanism to convey soil carbon (C) from aboveground organic debris (litter) to deeper soil horizons and can influence the formation of stable soil organic C compounds. The magnitude of this flux depends on both infiltration and DOC production rates which are functions of the climatic, soil, topographic and ecological characteristics of a region. Aboveground litter quantity and quality was manipulated for 20 years in an old-growth Douglas fir forest under six treatments to study relationships among litter inputs, DOC production and flux, and soil C dynamics. DOC concentrations were measured at two depths using tension lysimeters, and a hydrologic model was created to quantify water and DOC flux through the soil profile. DOC concentrations ranged from 3.0–8.0 and 1.5–2.5 mg C/L among treatments at 30 and 100 cm below the soil surface, respectively. Aboveground detrital inputs did not have a consistent positive effect on soil solution DOC; the addition of coarse woody debris increased soil solution DOC by 58% 30 cm belowground, while doubling the mass of aboveground leaf litter decreased DOC concentrations by 30%. We suggest that high-quality leaf litter accelerated microbial processing, resulting in a “priming” effect that led to the lower concentrations. Annual DOC flux into groundwater was small (2.7–3.7 g C/m2/year) and accounts for < 0.1% of estimated litter C at the site. Therefore, direct DOC loss from surface litter to groundwater is relatively negligible to the soil C budget. However, DOC flux into the soil surface was much greater (73–210 g C/m2/year), equivalent to 1.4–2.4% of aboveground litter C. Therefore, DOC transport is an important source of C to shallow soil horizons.

Seasonal patterns of bole water content in old growth Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco)

Large conifer trees in the North American Pacific Northwest (PNW) use stored water to extend photosynthesis, both diurnally and seasonally. This is particularly important during the summer drought, which is characteristic of the region. In the PNW, climate change is predicted to result in hotter, drier summers and warmer, wetter winters with decreased snowpack by mid-century. Understanding seasonal bole water dynamics in relation to climate factors will enhance our ability to determine the vulnerability of forests to climate change. Seasonal patterns of bole water content in old-growth Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) trees were studied in the Cascade Mountains of western Oregon, USA. Relative water content (RWC) was monitored hourly in three 400+ and three ∼150 years-old trees using permanently mounted dielectric devices for 10 years. RWC increased during the late spring and early summer to maximum levels in August then decreased into fall and remained low over winter. The difference between minimum RWC in the winter and maximum in mid-summer averaged 4.5 and 2.3% for the older and younger trees, respectively, across all years. RWC closely followed growth and was positively correlated with air and soil temperature, vapor pressure deficit and photosynthetically active radiation, but lagged plant available soil water. The progressive decrease in RWC seen each year from mid-summer through fall was attributed to net daily loss of water during the summer drought. The marked increase in RWC observed from spring to mid-summer each year was hypothesized to be the period of embolism repair and water recharge in elastic tissues. We conclude that bole water content is an integral part of tree water dynamics enabling trees to extend carbon assimilation into drought periods and during periods when cold soil inhibits water uptake by roots, an adaptation that could benefit the survival of large PNW trees under climate change.

Combustion and Spectral Methods for Quantifying Carbon and Nitrogen Concentrations in Pacific Northwest Douglas-Fir Forest Soils

&lt;p&gt;Traditional combustion methods for assessing soil carbon (C) and nitrogen (N) stocks are time consuming and expensive; visible and near-infrared (VNIR) methods offer a quick and inexpensive alternative for establishing soil C and N concentrations. We compared combustion and spectral methods for quantifying soil carbon and nitrogen concentrations. We sampled organic and mineral soil horizons in managed and old-growth Douglas-fir (&lt;em&gt;Pseudotsuga menziesii&lt;/em&gt;) forests in western Oregon. We applied combustion methods to determine total soil carbon and nitrogen concentrations of these samples. We then applied VNIR methods to derive a reference spectral library for analyzing Pacific Northwest (PNW) andesitic forest soils. Our spectral analysis confirmed that visible range spectra (especially in the 650-750 nm range) are the most useful for distinguishing differences in sample soil organic matter content. Our results provide a robust calibration model for applying spectral analysis combined with Partial Least Square Regression (PLSR) to quantify carbon and nitrogen stocks in PNW Douglas-fir forest soils. Model validation resulted in R&lt;sup&gt;2&lt;/sup&gt; values ranging from 0.92 to 0.95 for C and from 0.73 to 0.84 for N.&lt;/p&gt;

Spatial Relationships between Canopy Structure and Understory Vegetation of an Old-Growth Douglas-Fir Forest

Canopy structure and spatial distributions of understory vegetation are known to be closely correlated in forest community. The primary objective of this study is to examine the spatial relationships between the canopy heterogeneity and understory vegetation at multiple scales in an old-growth Douglas fir forest. A 12 ha plot located in the T.T. Munger Experimental Forest of Washington was stem-mapped, and the understory vegetation was surveyed along two 400 m transects by species and life form groups. Canopy structure of the plot was modeled utilizing a stem map and a crown geometry model. The three-dimensional canopy characteristics derived from the models were then used to assess the effects of canopy structure on understory vegetation at multiple resolutions using correlation and wavelet analysis techniques. Correlation results showed significant associations of understory species with the three-dimensional canopy structure. The majority of dominant herb species were highly associated with canopy opening, and they were more severely affected by lower canopy layers (heights of 10-35 m) than by dominant-codominant layers (heights of 40-50 m). The saplings of western hemlock had a significant negative association with lower canopy layers, while vine maples and the saplings of Pacific silver fir were positively correlated with dominant-codominant canopy layers. Wavelet analysis indicated that the relationship between canopy structure and understory vegetation was highly scale-dependent, i.e., understory variables responded differently to the heterogeneity of canopy patterns at different scales.

Fate of Decomposed Fungal Cell Wall Material in Organic Horizons of Old-Growth Douglas-fir Forest Soils

Organic horizons of old-growth Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco] forests are colonized by ectomycorrhizal fungi, which can form large patches or mats. Respiration, N mineralization, chitinase activity, and amino sugar C and N turnover are higher in soils colonized by ectomycorrhizal mats. To test if microbial activity in mat soil is driven by decomposition of fungal biomass, we incubated mat and non-mat soil with ¹³C¹⁵N-labeled, chitin-enriched fungal cell wall material. Cell wall material was readily decomposed during 22-d incubations in both soils, and cell-wall-derived ¹³C and ¹⁵N were traced into mineralized pools, extractable organic pools, microbial biomass, and microbial phospholipid fatty acids (PLFAs). Basal microbial activity was higher in mat soils. Although total respiration was higher in mat soils with cell walls added, more cell wall C was mineralized in non-mat than mat soils; this was inferred to reflect greater preferential respiration of cell wall C over soil organic matter (SOM). Mineralization of N from cell wall material was low, with no difference between soils, and positive priming of N mineralization from SOM occurred after 3 wk. Both the proportion of cell wall N assimilated and N growth yield efficiency (GYE) were higher than for cell wall C. The relative abundance of fungi was higher in cell-wall-amended soils, and cell-wall-derived ¹³C was found in fungal and Gram-negative bacterial PLFAs. Cell wall C assimilation and C GYE were higher in mat soils, which could be linked to greater mineralization of SOM or to differential activity of select microbial taxonomic groups. Chitin-rich fungal cell wall material was utilized as both a microbial N and C source; N was predominantly assimilated and C was predominantly metabolized. It is possible that fungal necromass may contribute to N stabilization in these soils.

Light on the Devils: Coming of Age on the Klamath by Louise Wagenknecht

 Reviews Richard Louv writes in the introduction, “A growing body of scientific evidence strongly suggests that all of us are starved for nature time” and that spending time outdoors is a central part of healthy living (as proof of that, look no further than North Portland’s new Randall Children’s Hospital on the Emanuel campus, which provides outdoors spaces and plants and trees for patients precisely for their therapeutic benefits).The new edition also contains several walks and essays focused on green infrastructure,which has become a much moreprominentpartof thecity’sbuiltenvironment .A stroll down North Mississippi Avenue can be a lesson in stormwater management, nicely leavened by the thriving food-drinkand -shopping scene. It should be noted that the new edition of the book was supported financially by seemingly every government agency in town with an interest in this subject,from Portland’s Bureau of Environmental Services to the U.S. Fish & Wildlife Service. Many critics of the region’s governmental establishment will no doubt look with suspicion at the values trumpeted in this book. But for readers who simply want a fuller appreciation of what they are seeing as they amble along the Willamette or march determinedly to the top of Mt. Tabor, Wild in the City will be a valuable guide for years to come — until the third edition, no doubt. Jeff Mapes Portland, Oregon Light on the Devils: Coming of Age on the Klamath by Louise Wagenknecht Oregon State University Press, Corvallis, 2011. Illustrations, bibliography. 232 pages. $19.95 paper. Light on the Devils is a memoir set in the remote timber town of Happy Camp, California, in the early 1960s. The book will appeal to readers interested in the history of the Northwest timber industry and particularly in the role of the U.S.Forest Service.The author’s stepfather was a Forest Service forester who vigorously practiced the ethic of scientific “sustained yield”forestry, and who spent his career liquidating the last big tracts of old-growth Douglas fir forest in the Pacific Northwest (pp. 180–81). As Wagenknecht recounts, even her stepfather had his doubts about sustained-yield forestry in the end (pp. 210–11). Her tale is animated by the tension between her early,naïve belief in the rightness and goodness of her way of life,which was dependent on the timber industry,and the dawning realization that she and her close-knit community inhabited a vanishing reality. She begins the book with an essay called “Where it Ended,” an account of the suicide of a man she knew in high school, an outof -work millhand whose wife had left him. This dramatic scene sets up what I took to be the book’s premise: that the implosion of the timber economy was responsible for the man’s death. So I was expecting some sort of hardhitting exposé. To my relief, what emerged instead was a tender, funny autobiography of a bookish, unconsciously charming tomboy, a family-challenged eldest girl perfectly at home in the woods and less at ease in the befuddling thickets of domestic tension and the high school caste system. The first chapter, then, strikes a major false note. It is a cliché — private-tragedy-asmetaphor -for-social-decline is an overworked trope,especiallyinthiscontext.Moreimportant, Wagenknecht does not need it.Her own private perplexities, observations, and awakenings distill the larger story she is trying to tell. There are other rough spots, mostly occurring when Wagenknecht turns from her own experience and begins to explain things. She tells, for example, of a glorious solitary ramble through woods near her house. “My parents didn’t know exactly where I was,”she confides. “I hugged my knees, delighted.”She sits on the ground, “picking deerbrush twigs out of my  OHQ vol. 113, no. 4 hair.”Then she writes:“Deerbrush (Ceanothus integerrimus) is a fast-growing,nitrogen-fixing deciduous shrub that can reach over six feet in height” (p. 57). The shift of voice is jarring. I pictured the adult author sitting at her computer , reaching for her field guide to check the spelling of the binomial. Fortunately, Wagenknecht does not abandon her storytelling voice for long.She lyrically depicts the astonishing...

Soil Properties in Old-Growth Douglas-Fir Forest Gaps in the Western Cascade Mountains of Oregon

Abstract This was a study of vegetation and soil properties in tree-fall gaps in a coniferous forest of the Pacific Northwest. It had three objectives: (1) to determine if there are correlations between above-ground vegetation and below-ground soil properties within large 50 m diameter gaps, (2) to determine how large gaps influence forest soils compared with non-gap soils, and (3) to measure the effects of differently sized gaps on gap soils. To address these objectives, circular canopy gaps were created in old-growth Douglas-fir forests of the H. J. Andrews Experimental Forest in the western Oregon Cascade Mountains. To address the first objective, within-gap soil spatial patterns were compared with above ground distributions of both vegetation and large woody debris in two large gaps. Spatial and Pearson correlation analyses showed no consistent correlations between soil characteristics and above ground vegetation and coarse woody debris. With reference to the second objective, statistically significan...

Flux partitioning in an old-growth forest: seasonal and interannual dynamics

Turbulent fluxes of carbon, water and energy were measured at the Wind River Canopy Crane, Washington, USA from 1999 to 2004 with eddy-covariance instrumentation above (67 m) and below (2.5 m) the forest canopy. Here we present the decomposition of net ecosystem exchange of carbon (NEE) into gross primary productivity (GPP), ecosystem respiration (R(eco)) and tree canopy net CO(2) exchange (DeltaC) for an old-growth Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco)-western hemlock (Tsuga heterophylla (Raf.) Sarg.) forest. Significant amounts of carbon were recycled within the canopy because carbon flux measured at the below-canopy level was always upward. Maximum fluxes reached 4-6 micromol m(-2) s(-1) of CO(2) into the canopy air space during the summer months, often equaling the net downward fluxes measured at the above-canopy level. Ecosystem respiration rates deviated from the expected exponential relationship with temperature during the summer months. An empirical ecosystem stress term was derived from soil water content and understory flux data and was added to the R(eco) model to account for attenuated respiration during the summer drought. This attenuation term was not needed in 1999, a wet La Niña year. Years in which climate approximated the historical mean, were within the normal range in both NEE and R(eco), but enhanced or suppressed R(eco) had a significant influence on the carbon balance of the entire stand. In years with low respiration the forest acts as a strong carbon sink (-217 g C m(-2) year(-1)), whereas years in which respiration is high can turn the ecosystem into a weak to moderate carbon source (+100 g C m(-2) year(-1)).

Biophysical constraints on leaf expansion in a tall conifer

The physiological mechanisms responsible for reduced extension growth as trees increase in height remain elusive. We evaluated biophysical constraints on leaf expansion in old-growth Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) trees. Needle elongation rates, plastic and elastic extensibility, bulk leaf water (Psi(L)) and osmotic (Psi(pi)) potential, bulk tissue yield threshold and final needle length were characterized along a height gradient in crowns of > 50-m-tall trees during the period between bud break and full expansion (May to June). Although needle length decreased with increasing height, there was no height-related trend in leaf plastic extensibility, which was highest immediately after bud break (2.9%) and declined rapidly to a stable minimum value (0.3%) over a 3-week period during which leaf expansion was completed. There was a significant positive linear relationship between needle elongation rates and plastic extensibility. Yield thresholds were consistently lower at the upper and middle crown sampling heights. The mean yield threshold across all sampling heights was 0.12 +/- 0.03 MPa on June 8, rising to 0.34 +/- 0.03 MPa on June 15 and 0.45 +/- 0.05 MPa on June 24. Bulk leaf Psi(pi) decreased linearly with increasing height at a rate of 0.004 MPa m(-1) during the period of most rapid needle elongation, but the vertical osmotic gradient was not sufficient to fully compensate for the 0.015 MPa m(-1) vertical gradient in Psi(L), implying that bulk leaf turgor declined at a rate of about 0.011 MPa m(-1) increase in height. Although height-dependent reductions in turgor appeared to constrain leaf expansion, it is possible that the impact of reduced turgor was mitigated by delayed phenological development with increasing height, which resulted in an increase with height in the temperature during leaf expansion.

Canopy Oribatida: Tree specific or microhabitat specific?

A diverse assemblage of oribatid mites inhabits the canopy of coniferous trees in western North America. We tested the hypothesis that oribatid mites are microhabitat specific in old-growth Douglas fir, Western hemlock and western redcedar at the Wind River Crane Canopy Research Facility, Washington, USA. The upper 3 m of canopy of the three tree species were accessed using the canopy crane. Oribatida were extracted from 4 to 12 g dwt samples of alecterioid and foliose lichens using the twig-washing technique. Overall species richness was low, 16 species representing 11 families, with no species unique to this site. Species were absent from samples taken contemporaneously from the forest floor. All oribatid species were found in foliose lichens, whereas only nine species, in seven families, were recovered from alecterioid lichens. Oribatid species richness was lichen specific depending on the tree species. On Western hemlock both lichens supported similarly rich communities, but on Douglas fir and western redcedar foliose lichens supported the richer community.

The role of epiphytes in rainfall interception by forests in the Pacific Northwest. I. Laboratory measurements of water storage

Old-growth Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) forests frequently contain large populations of epiphytic lichens and bryophytes. To determine the effect these epiphytes have on canopy hydrology we measured the maximum water fraction (f(x)max; maximum mass of internal and external water stored by an epiphyte divided by its tissue dry mass) of common lichens, bryophytes, and dead branches in the laboratory and the water storage and interception efficiency (pi) (the rainfall stored on a branch divided by the rainfall intercepted by the branch) of whole epiphyte-laden branches under a rainfall simulator at three intensities (11.3, 16.1, and 39.8 mm·h–1). The f(x)max values for epiphytic fruticose lichens, foliose lichens, and bryophytes were 2.2 ± 0.4, 3.4 ± 0.6, and 10.0 ± 0.5, respectively. The water stored by an epiphyte-laden branch during and after exposure to simulated rainfall could be predicted if the biomass of epiphytic lichens and bryophytes on the branch was known (R2 = 0.8, p value &lt; 0.0001). For all three rainfall intensities, the branches required &gt;6 mm of rainfall to saturate. Values of pi averaged between 0.5 and 0.7 after 2 mm of rainfall and did not differ among the three intensities (all p values &gt; 0.05). We conclude that epiphytes increase the canopy water storage of a typical old growth Douglas-fir forest by &gt;1.3 mm.

The role of epiphytes in rainfall interception by forests in the Pacific Northwest. II. Field measurements at the branch and canopy scale

To determine how epiphytes affect the canopy hydrology of old-growth Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) forests, we measured rainfall interception by individual branches and an entire stand from March 2003 to May 2004. Epiphyte-laden branches at heights of 3.1, 24.8 and 46.5 m remained partially saturated for most of the wet season and required more than 30 mm of rainfall to become saturated. We used the mean, minimum, and individual storm methods to estimate canopy water storage capacity. Canopy water storage capacity averaged 3.1–5.0 mm, but these are probably underestimates of the maximum canopy water storage capacity, because the canopy was partially saturated prior to most storm events and the saturation of the canopy was delayed by preferential flow through the epiphyte-laden branches. Contrary to expectation, the water stored on epiphyte-laden branches after exposure to natural rainfall increased with rainfall intensity because the rough three-dimensional structure of the lichen and bryophyte mats limits water loss from raindrop splash and impedes the drainage of water from the branch. We conclude that epiphytic lichens and bryophytes increase canopy water storage capacity, prolong the time required for the canopy to saturate and dry, and alter the transfer of water through the canopy.

Microbial community utilization of added carbon substrates in response to long-term carbon input manipulation

The chemical composition and quantity of plant inputs to soil are primary factors controlling the size and structure of the soil microbial community. Little is known about how changes in the composition of the soil microbial community affect decomposition rates and other ecosystem functions. This study examined the degradation of universally 13C-labeled glucose, glutamate, oxalate, and phenol in soil from an old-growth Douglas-fir ( Pseudotsuga menziesii)—western hemlock ( Tsuga heterophylla) forest in the Oregon Cascades that has experienced 7 y of chronic C input manipulation. The soils used in this experiment were part of a larger Detritus Input and Removal Treatment experiment and have received normal C inputs (control), doubled wood inputs, or root and litter input exclusion (no inputs). Soil from the doubled wood treatment had a higher fungal:bacterial ratio, and soil from the no inputs treatment had a lower fungal:bacterial ratio, than the control soil. Differences in the utilization of the compounds added to the field-manipulated soils were assessed by following the 13C tracer into microbial biomass and respiration. In addition, 13C-phospholipid fatty acids (PLFA) analysis was used to examine differential microbial utilization of the added substrates. Glucose and glutamate were metabolized similarly in soils of all three litter treatments. In contrast, the microbial community in the double wood soil respired more added phenol and oxalate, whereas microbes in the no inputs soil respired less added phenol and oxalate, than the control soil. Phenol was incorporated primarily into fungal PLFA, especially in soil of the double wood treatment. The addition of all four substrates led to enhanced degradation of soil organic matter (priming) in soils of all three litter treatments, and was greater following the addition of phenol and oxalate as compared to glucose and glutamate. Priming was greater in the no inputs soil as compared to the control or doubled wood soils. These results demonstrate that altering plant inputs to soil can lead to changes in microbial utilization of C compounds. It appears that many of these changes are the result of alteration in the size and composition of the microbial community.

Spatial and population characteristics of dwarf mistletoe infected trees in an old-growth Douglas-fir – western hemlock forest

We investigated the distribution and severity of trees infected with western hemlock dwarf mistletoe (Arceuthobium tsugense (Rosendahl) G.N. Jones subsp. tsugense) in an old-growth Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) – western hemlock (Tsuga heterophylla (Raf.) Sarg.) forest. With the use of Hawksworth six-class dwarf mistletoe rating system, infection status was assessed for 3516 hemlock and true firs ≥5 cm diameter on a 12-ha stem-mapped plot located in the Cascade Mountains of southwest Washington State. Within the plot, 33% of the area had some level of infection and 25% (719) of western hemlocks, 2.2% (12) of Pacific silver fir (Abies amabilis (Dougl.) Forbes), and 29% (2) of noble fir (Abies procera Rehd.) trees were infected. Infected trees are larger than uninfected trees, on average, and within the infected tree population, the severely infected trees averaged larger than lightly infected trees. Abundant dwarf mistletoe in larger trees definitely positions the dwarf mistletoe population for future spread. Ripley's K analysis indicates a negative association between infected and uninfected hemlock trees, confirming that the infected trees form distinct dwarf mistletoe infection centers. The infection centers are actively spreading at their margins, which was confirmed by nearest neighbor analysis. Heavily infected trees had a negative association with uninfected trees, while lightly infected trees had a positive association with uninfected trees.

Vertical stratification of soil water storage and release dynamics in Pacific Northwest coniferous forests

We characterized vertical variation in the seasonal release of stored soil moisture in old-growth ponderosa pine (OG-PP, xeric), and young and old-growth Douglas-fir (Y-DF, OG-DF, mesic) forests to evaluate changes in water availability for root uptake. Soil water potential ( ψ) and volumetric water content ( θ) were measured concurrently at 10 cm intervals to 1 m depth to create in situ soil water retention curves (SWRC) under drying conditions. Non-linear regression was used to fit SWRC specific to each depth and site. We also quantified root biomass, soil texture, and hydraulic redistribution (HR) of soil water by roots to identify factors affecting the seasonal dynamics of root water uptake and depletion from the soil profile. Soil θ measured at a particular ψ increased with soil depth, and was strongly dependent upon soil texture. For example, when ψ was −0.1 MPa, θ ranged from 13% at 20 cm to 35% at 100 cm for the OG-DF forest. Soil texture and bulk density accounted for 60–90% of the variation in the SWRC. As the summer drought progressed, water extraction shifted to the deeper layers, and recharge from HR approached 0.15 mm day −1 in the upper 60 cm for all sites. Total water use from the upper 2 m at all sites peaked between 1.5–2.5 mm day −1 in mid-July and then declined to 0.5–1.0 mm day −1 by the end of the dry season. Total fine root biomass in the upper 1 m was 0.77 kg m −2 (OG-PP), 1.08 kg m −2 (OG-DF) and 1.15 kg m −2 (Y-DF), with 40% (PP) to 60% (DF) of fine roots located in the upper 20 cm. However, the upper 20 cm only accounted for 20% of total water depletion from the upper 2 m at peak water uptake, declining to 4–6% later in the season, illustrating the contribution of deeper roots to water uptake. Nevertheless, daily water uptake from the entire 2 m profile was strongly dependent on water potential at 20 cm, indicating that fine roots in the upper soil may play an important role in regulating water uptake through hydraulic effects on stomatal conductance.

Investigating scale-dependent stand heterogeneity with structure-area-curves

The creation of stands that exhibit characteristics similar to old-growth forests has become one of the most important objectives in forest management and structure has become the tool of choice for evaluating this condition. Attributes of forest structure, however, are extremely heterogeneous and differ considerably in spatial pattern and extent due to the patchy and variable nature of disturbances. Silvicultural research that aims to better understand structural outcomes of natural disturbances must be conducted at spatial scales that are appropriate to capture the variation in both the disturbance regimes and in these structures. Scale-dependent variation in structural attributes, including the index of old-growth ( I og) and the structural complexity index (SCI), were evaluated in unmanaged mature, transitional, and old-growth Douglas-fir ( Pseudotsuga menziesii (Mirbel) Franco) forests in the south central Cascade Range of Oregon. Moving windows of 100, 500, 1000, 2500, 5000 m 2 were overlain onto stem maps of each of ten 1-ha stands and structural attributes evaluated at each spatial scale. Multiple iterations revealed that structural attributes and indices varied with subplot size, with variation stabilizing with subplots 2500 m 2 and larger. Structural heterogeneity across subplot sizes was consistently highest in old-growth stands. These findings have implications for minimum acceptable plot sizes for research projects that aim to characterize the effect of silvicultural treatments on these structural attributes. For instance, a quarter hectare may be a minimum acceptable plot size to evaluate these structural attributes and potentially even larger plot sizes may be necessary in more complex old-growth forests.

Converging patterns of uptake and hydraulic redistribution of soil water in contrasting woody vegetation types.

We used concurrent measurements of soil water content and soil water potential (Psi(soil)) to assess the effects of Psi(soil) on uptake and hydraulic redistribution (HR) of soil water by roots during seasonal drought cycles at six sites characterized by differences in the types and amounts of woody vegetation and in climate. The six sites included a semi-arid old-growth ponderosa pine (Pinus ponderosa Dougl. ex P. Laws & C. Laws) forest, a moist old-growth Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) forest, a 24-year-old Douglas-fir forest and three Brazilian savanna sites differing in tree density. At all of the sites, HR was confined largely to the upper 60 cm of soil. There was a common threshold relationship between the relative magnitude of HR and Psi(soil) among the six study sites. Below a threshold Psi(soil) of about -0.4 MPa, overnight recharge of soil water storage increased sharply, and reached a maximum value of 80-90% over a range of Psi(soil) from ~ -1.2 to -1.5 MPa. Although amounts of water hydraulically redistributed to the upper 60 cm of soil were relatively small (0 to 0.4 mm day(-1)), they greatly reduced the rates of seasonal decline in Psi(soil). The effectiveness of HR in delaying soil drying diminished with increasing sapwood area per ground area. The relationship between soil water utilization and Psi(soil) in the 20-60-cm layer was nearly identical for all six sites. Soil water utilization varied with a surrogate measure of rhizosphere conductance in a similar manner at all six sites. The similarities in relationships between Psi(soil) and HR, soil water utilization and relative rhizosphere conductance among the six sites, suggests that, despite probable differences in maximum rooting depth and density, there was a convergence in biophysical controls on soil water utilization and redistribution in the upper soil layers where the density of finer roots is greatest.