Western Hemlock Research Articles

The Lari-Leuco Container: A Novel Collection Arena for Separating Insects Ascending or Descending From a Plant Foliage Sample.

Efficient separation of insects from plant material for quantification and collection is an important component of entomological research. This paper reports on a novel, easily replicable container designed to efficiently collect two different biological control agents dispersing from hemlock (Tsuga spp.) foliage infested with the invasive hemlock woolly adelgid (HWA), Adelges tsugae Annand (Hemiptera: Adelgidae). The container utilizes a simplified Berlese-style funnel design to collect Laricobius spp. (Coleoptera: Derodontidae) larvae dropping from the foliage into a removable bottom jar, a central jar to house the foliage sample, and a removable top jar to collect adult silver flies (Leucopis spp., Diptera: Chamaemyiidae) emerging from puparia on the twigs. The efficacy of two designs (with and without a funnel leading to the top collection jar) was evaluated using western hemlock [Tsuga heterophylla (Raf.) Sarg.] foliage naturally colonized with HWA and the two predator genera. All Laricobius larvae were effectively collected in the bottom jar, and the addition of an inverted funnel leading to the top collection jar increased the proportion of Leucopis flies reaching the target jar from 60% to 94%. This 'Lari-Leuco' container is presented as a research and motoring tool to benefit the integrated pest management program for HWA in eastern North America and for potential use in simultaneously separating ascending and descending life stages in other insect-plant or predator-prey systems.

Thermo-hydro-mechanical densification of hemlock

ABSTRACT The effect of Thermo-Hydro-Mechanical densification on selected properties of western hemlock was investigated. The drive was to explore the possibility of using hemlock as a replacement for expensive hardwoods for flooring applications. Kiln-dried boards at about 12% moisture content were subjected to densification in a hot press. As a special treatment, half of the samples were surface-sprayed with water before densification and three pressing times (5, 10, and 15 min), as well as temperature levels (120°C, 160°C, and 200°C) were used. Sample density, hardness, colour change, and abrasion resistance were evaluated after densification. Data analysis revealed that the temperature level, densification time, and type of pre-treatment were statistically significant factors. Compared to controls, densification significantly improved density, hardness, and abrasion resistance by 197%, 386%, and 437%, respectively. As a possible desired aesthetic feature, this process darkened hemlock by about 200%.

Mulch versus brash: a case study of in situ harvesting residue treatment and its effects on C and nutrients in soil and plant uptake during natural rewilding

Western hemlock brash decay was investigated over seven years on a UK lowland clay soil site, clear-felled in 2002 for broadleaf restoration. Two brash loadings (9.3 (Heavy) and 5.6 (Light) kg m − 2), a chipped mulch (9.4 kg .m − 2), a seeded grass and a bare soil treatment were incorporated in a fenced split block design. In situ decay of Mulch and Brash was followed by field sampling, and a three year litterbag study of needles, twigs and small branch decay. Natural plant recolonisation was followed in repeated ground surveys within each treatment. Heavy Brash residue lost weight through rain leaching, whilst Light Brash and Mulch gained through the action of decay fungi (Basiodiomycete spp) . The N resource in the Mulch residue 2006 (1660 kg ha−1 of N) was twice that of the Heavy Brash in 2005 (748 kg ha−1of N) from similar initial loadings, with subsequent N and C release from the Mulch resulting in significantly higher soil TN and DOC levels. Above ground uptake was determined using allometric relations of birch height: biomass and ground cover: biomass in 2007. N uptake was initially rapid in the seeded grass plots, where soil TN values were significantly the lowest and soil pH the highest of all treatments in 2005 - an indication that sown grass can be an effective sink for leached N after harvesting. Birch (Betula pendula L) subsequently sequestered >75 kg ha−1 of N in the Light Brash treatments with a further 30 kg ha−1 in the shrub/grass under storey by 2007, representing ~28% of the Light Brash residue N resource. N uptake on the Mulch, with < 50% plant cover, represented <3% of the Mulch resource.Conclusions: Mulching brash in situ produced a tidy site, but the deep layer (~14 cm) resulted in an increased resource of N and C in the residue through fungal action, locking up nutrients for three years. Subsequently (yr 4–7) significantly higher levels of DOC and TN were released to the soil, compared to all other treatments. Total mineral soil C and N stock in year 7 was high but not significantly different under Mulch and Heavy Brash, indicating that this excess N was not retained in the topsoil. A deep layer of mulched material cannot be recommended as a treatment for harvesting residue on N sensitive sites, or for rewilding projects where natural plant colonisation is expected.

A species-specific, site-sensitive maximum stand density index model for Pacific Northwest conifer forests

Maximum stand density index (SDIMAX) models were developed for important Pacific Northwest conifers of western Oregon and Washington, USA, based on site and species influences and interactions. Inventory and monitoring data from numerous federal, state, and private forest management groups were obtained throughout the region to ensure a wide coverage of site characteristics. These observations include information on tree size, number, and species composition. The effects and influence on the self-thinning frontier of plot-specific factors such as climate, topography, soils, and geology, as well as species composition, were evaluated based on geographic location using a multistep approach to analysis involving linear quantile mixed models, random forest, and stochastic frontier functions. The self-thinning slope of forest stands dominated by Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) was found to be –1.517 and that of stands dominated by western hemlock (Tsuga heterophylla (Raf.) Sarg.) was found to be –1.461, leading to regionwide modelled SDIMAX values at the 95th percentile of 1728 and 1952 trees per hectare, respectively. The regional model of site-specific SDIMAX will support forest managers in decision-making regarding density management and species selection to more efficiently utilize site resources toward healthy, productive forests.

Using X-ray CT Scanned Reconstructed Logs to Predict Knot Characteristics and Tree Value

Research Highlights: Stand density was connected with wood quality and lumber production to develop a predictive model to better estimate tree value. Background and Objectives: The available standing wood volume in British Columbia (BC), Canada has consistently decreased since 1990. Better understanding the link between stand growth conditions, knot characteristics, the sawmilling process and product quality is essential in making informed forest management decisions and efficiently utilizing wood. The overall objective was to investigate and predict the impact of tree growth as affected by stand density on knot characteristics, lumber volume and value recoveries for two conifer species, two types of sawmills and three economic scenarios. Materials and Methods: Seventy-two amabilis fir and western hemlock trees were harvested from three stands located on Vancouver Island, BC. Sawlogs were scanned using an X-ray computed tomography (CT) scanner and images were processed to extract knot characteristics and reconstruct three-dimensional (3D) log models. The effects of three diameter at breast height (DBH) classes (30, 40 and 50 cm) and three stand densities on knot characteristics, including knot volume, number of knots, average knot area and knot/tree volume ratio, as well as the simulated lumber volume and value recoveries from two types of sawmills (i.e., Coastal and Interior) under three economic scenarios (i.e., baseline, optimistic, and pessimistic) were investigated. Results: As expected, the knot characteristics of both species increased with the DBH. The difference of knot distribution between amabilis fir and western hemlock suggests that the latter is more sensitive to growth site conditions. The sawmilling simulations revealed that the Coastal mill produced a lower lumber volume due to the type of products manufactured and the primary breakdown patterns being used. Conclusions: The developed linear mixed effects models based on the knot characteristics and tree features could predict the value of a standing tree and can be used for estimating preharvest stand value of similar Coastal Hem-Fir forests.

Modelling specific gravity and diameter inside bark of western hemlock and Sitka spruce growing in southeast Alaska

Western hemlock (Tsuga heterophylla (Raf.) Sarg.) and Sitka spruce (Picea sitchensis (Bong.)) are two commercially important species in Alaska, with harvests beginning to focus on naturally regenerated young-growth. We developed within-tree models of ring specific gravity (SG) and diameter inside bark (DIB) for young-growth western hemlock and Sitka spruce. Eight even-aged stands (age &lt;75 years) in southeast Alaska were felled and disks collected from multiple height levels; 128 trees and 451 disks were collected for western hemlock, and 217 trees and 952 disks were collected for Sitka spruce. Radial strips were prepared and scanned using X-ray densitometry. We fitted nonlinear mixed-effects models to the data, with cambial age, height within tree, and dominance class used as explanatory variables. The R2 values (fixed effects only) for the SG models were 0.48 and 0.42 for western hemlock and Sitka spruce, respectively. The corresponding fit indices (R2) for the DIB models were 0.86 and 0.85 for western hemlock and Sitka spruce, respectively. Tree maps depicting the within-tree variation in SG showed more variability in Sitka spruce than in western hemlock. The wood and growth properties of young-growth trees in Alaska will continue to become more important as the US Forest Service transitions away from harvesting old-growth trees.

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

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

The importance of large-diameter trees to the creation of snag and deadwood biomass

BackgroundBaseline levels of tree mortality can, over time, contribute to high snag densities and high levels of deadwood (down woody debris) if fire is infrequent and decomposition is slow. Deadwood can be important for tree recruitment, and it plays a major role in terrestrial carbon cycling, but deadwood is rarely examined in a spatially explicit context.MethodsBetween 2011 and 2019, we annually tracked all trees and snags ≥1 cm in diameter and mapped all pieces of deadwood ≥10 cm diameter and ≥1 m in length in 25.6 ha of Tsuga heterophylla / Pseudotsuga menziesii forest. We analyzed the amount, biomass, and spatial distribution of deadwood, and we assessed how various causes of mortality that contributed uniquely to deadwood creation.ResultsCompared to aboveground woody live biomass of 481 Mg ha−1 (from trees ≥10 cm diameter), snag biomass was 74 Mg ha−1 and deadwood biomass was 109 Mg ha−1 (from boles ≥10 cm diameter). Biomass from large-diameter trees (≥60 cm) accounted for 85%, 88%, and 58%, of trees, snags, and deadwood, respectively. Total aboveground woody live and dead biomass was 668 Mg ha−1. The annual production of downed wood (≥10 cm diameter) from tree boles averaged 4 Mg ha−1 yr−1. Woody debris was spatially heterogeneous, varying more than two orders of magnitude from 4 to 587 Mg ha−1 at the scale of 20 m × 20 m quadrats. Almost all causes of deadwood creation varied in importance between large-diameter trees and small-diameter trees. Biomass of standing stems and deadwood had weak inverse distributions, reflecting the long period of time required for trees to reach large diameters following antecedent tree mortalities and the centennial scale time required for deadwood decomposition.ConclusionOld-growth forests contain large stores of biomass in living trees, as well as in snag and deadwood biomass pools that are stable long after tree death. Ignoring biomass (or carbon) in deadwood pools can lead to substantial underestimations of sequestration and stability.

Pathogenicity of Sirococcus tsugae on major coniferous tree species of Belgian forest

AbstractTo evaluate the potential impact of Sirococcus tsugae on Belgian coniferous forest, a pathogenicity test was conducted through wound inoculation of two‐year‐old seedlings of different tree species. Picea abies, Pseudotsuga menziesii, Pinus sylvestris, Larix x eurolepsis as well as Tsuga heterophylla and Cedrus atlantica were inoculated with an isolate of S. tsugae collected from the only known infected site in Belgium. The tested strain proved to be highly pathogenic towards seedlings not only of two known host species of the disease (T. heterophylla and C. atlantica) but also of L. x eurolepsis. This result highlights the risk that a possible emergence of S. tsugae might represent for some important forest tree species, including species currently not known as hosts of the disease. However, as the two main coniferous tree species in Belgium (P. abies and P. sylvestris) did not appear sensitive to S. tsugae, the potential impact of the pathogen on the Belgian forest is limited.

Symptom development and mortality rates caused by Armillaria ostoyae in juvenile mixed conifer stands in British Columbia’s southern interior region

ABSTRACTPlots were established in 20 juvenile mixed conifer stands in the Interior Cedar Hemlock biogeoclimatic zone of British Columbia's southern interior region in western Canada to assess mortality and host response to infection in the form of basal lesions caused by Armillaria ostoyae. Western redcedar showed significantly lower mortality and a higher proportion of callused lesions at the root collar than Douglas‐fir and western hemlock. Mortality decreased with increasing tree diameter for all species; however, the rate of decrease was markedly greater for western redcedar than Douglas‐fir or western hemlock. Results from logistic regression analysis showed that the probability of Douglas‐fir being killed is 14.6 times greater than western redcedar in these juvenile stands. Because western redcedar displayed significant resistance to A. ostoyae at both the root and stand level, this species represents an excellent choice as a significant stand component with more susceptible conifers. Should susceptible conifers succumb to Armillaria root disease, western redcedar will likely fill in the stand.

Arboreta reveal the invasive potential of several conifer species in the temperate forests of western Europe

Identifying emerging invasive species is a priority to implement early preventive and control actions. In terms of the number of invasive tree species, forestry represents the second largest pathway of introduction, with an invasive debt likely existing for alien conifers in Europe. In the early 1900s, a network of arboreta was established in southern Belgium to assess the wood production potential of prospective conifer and broadleaved species. Here, we use eight arboreta as natural experiments to identify alien conifers presenting invasive behavior. Through systematic sampling, we quantified the natural regeneration of alien conifers and recorded local environmental variables. For each species, regeneration density, dispersal distances, and age structure were analyzed. Generalized mixed effects models were fitted to test the effect of planted area and tree-stand type on regeneration. The environmental space occupied by regenerating alien conifers was evaluated using principal component analysis. Out of 31 planted alien species, 15 (48%) were identified in natural regeneration, of which eight (26%) exhibited important regeneration density and dispersal distances. The most invasive species were Tsuga heterophylla and Abies grandis, confirming earlier field observations. Both large planted areas and areas planted with alien conifer species increased the density of regeneration. Species that had the highest regeneration density tolerated a wide range of environmental conditions, including shaded understory, which could lead to the invasion of mature, undisturbed forests. This study showed that 17% of the studied alien conifers are potentially invasive because they show important regeneration, long-distance dispersal, and, of importance, have already produced offspring that have matured and are capable of creating new satellite populations. In conclusion, our results provide a guideline for future planting operations, recommending extreme caution when planting these species in the temperate forests of Western Europe.

Partial Retention of Legacy Trees Protect Mycorrhizal Inoculum Potential, Biodiversity, and Soil Resources While Promoting Natural Regeneration of Interior Douglas-Fir

Clearcutting reduces proximity to seed sources and mycorrhizal inoculum potential for regenerating seedlings. Partial retention of legacy trees and protection of refuge plants, as well as preservation of the forest floor, can maintain mycorrhizal networks that colonize germinants and improve nutrient supply. However, little is known of overstory retention levels that best protect mycorrhizal inoculum while also providing sufficient light and soil resources for seedling establishment. To quantify the effect of tree retention on seedling regeneration, refuge plants, and resource availability, we compared five harvesting methods with increasing retention of overstory trees (clearcutting (0% retention), seed tree (10% retention), 30% patch retention, 60% patch retention, and 100% retention in uncut controls) in an interior Douglas-fir-dominated forest in British Columbia. Regeneration increased with proximity to legacy trees in partially cut forests, with increasing densities of interior Douglas-fir, western redcedar, grand fir, and western hemlock seedlings with overstory tree retention. Clearcutting reduced cover of ectomycorrhizal refuge plants (from 80 to 5%) while promoting arbuscular mycorrhizal plants the year after harvest. Richness of shrubs, herbs, and mosses declined with increasing harvesting intensity, but tree richness remained at control levels. The presence of legacy trees in all partially cut treatments mitigated these losses. Light availability declined with increasing overstory cover and proximity to leave trees, but it still exceeded 1,000 W m−2 in the clearcut, seed tree and 30% retention treatments. Increasing harvesting intensity reduced aboveground and belowground C stocks, particularly in live trees and the forest floor, although forest floor losses were also substantial where thinning took place in the 60% retention treatment. The loss of forest floor carbon, along with understory plant richness with intense harvesting was likely associated with a loss of ectomycorrhizal inoculum potential. This study suggests that dispersed retention of overstory trees where seed trees are spaced ~10–20 m apart, and aggregated retention where openings are &amp;lt;60 m (2 tree-lengths) in width, will result in an optimal balance of seed source proximity, inoculum potential, and resource availability where seedling regeneration, plant biodiversity, and carbon stocks are protected.

Transformation of western hemlock (Tsuga heterophylla) tree crowns by dwarf mistletoe (Arceuthobium tsugense, Viscaceae)

AbstractDwarf mistletoes (Arceuthobium species) are arboreal, hemiparasitic plants of conifers that can change the structure and function of the tree crown. Hemlock dwarf mistletoe (Arceuthobium tsugense subsp. tsugense) principally parasitizes western hemlock (Tsuga heterophylla) and effects 10.8% of all western hemlock trees in Oregon, USA. In this study, we climbed 16 western hemlock trees (age 97–321 years, height 33–54.7 m) across a gradient of infection (0%–100% of branches infected) and measured occurrence of all dwarf mistletoe infections, dwarf mistletoe caused deformities, foliage, branch and crown metrics, and sapwood area. We then modelled over 25 different response variables using linear and generalized linear models with three metrics of severity as explanatory variables: total infection incidence, proportion of all live branches infected, and proportion of all live, infected branches with 33 per cent or more foliage distal to infection. A strong effect of dwarf mistletoe intensification was the reduction of branch foliage and an increase in the proportional amount of foliage distal to infections, with severely infected trees having the majority of foliage distal to infections. Increasing severity led to an apparent crown compaction as crown volumes decreased and became increasingly comprised of deformities. Sapwood area was unrelated to infection severity. Branch length and diameters were unrelated to increasing infection severity despite severely infected branches supporting 1–70 infections. The most severely infected tree had 3,615 individual plants in the crown. Our results suggested that shifts in crown structure and branch deformation, foliage amount, and foliage distal to infection, reflected a likely reduction of capacity for tree growth that coincided with a hypothesized increase in resource demand by dwarf mistletoe plants as infection severity intensified.

Effect of wood surface roughness on prediction of structural timber properties by infrared spectroscopy using ANFIS, ANN and PLS regression

Predicting the properties of structural timber using a rapid and reliable non-destructive method is a critical quality control task in production. This study investigates using infrared spectroscopy for predicting the density, moduli of elasticity (MOE) and rupture (MOR) of two structural softwoods. Because the produced timber is sometimes planed during manufacturing resulting in a smooth surface finish, the effect of wood surface roughness on prediction accuracy of timber properties was also investigated. Accordingly, infrared spectroscopy experiments were carried out on Douglas-fir and Western hemlock specimens having a rough and smooth surface. In addition, the effect of the infrared spectroscopy range on the predictive models was studied. Data in the visible infrared (VIS), near-infrared (NIR), and the combined VIS and NIR range were used for properties prediction. The acquired infrared data were processed using principal component analysis (PCA) for data reduction and feature selection. The output of PCA was then fed into an adaptive neuro-fuzzy inference system (ANFIS), multilayer perceptron (MLP) neural network (NN) and partial least square (PLS) regression model. The results showed that the wood surface finish, range of infrared data, and the type of machine learning model impact the prediction accuracy. ANFIS showed superior performance to MLP NN and PLS model for properties prediction. In addition, NIR data acquired from rough surface resulted in better prediction accuracy, which suggests that an infrared spectroscopy test should be performed prior to surface planing. The proposed models yield better accuracy for predicting the MOE and MOR than wood density.

Effect of the specimen configuration on the accuracy in measuring the shear modulus of western hemlock by torsional vibration test

The shear moduli of solid wood in longitudinal–tangential and longitudinal–radial planes were obtained by conducting a torsional vibration test and subsequent numerical analyses on western hemlock specimens by fixing the thickness as 10 mm and varying the width and length in the ranges of 20–80 mm and 100–400 mm, respectively. In the numerical analyses, the shear modulus in the tangential–radial planes was varied, and its effect was also examined as well as those of the width and length of the model. The results obtained indicated that the effect of the shear modulus in the tangential–radial plane was enhanced as the width increased and the length decreased. However, the shear modulus in the wider plane of the specimen could be obtained accurately by reducing the effect of the shear modulus in the tangential–radial plane when appropriate ranges of the width and length were determined. From the numerical and experimental results, the shear moduli in the longitudinal–tangential and longitudinal–tangential planes were accurately obtained when the width was in the range of 20–40 mm and the length was in the range of 250–400 mm.

Effect of specimen configuration and orthotropy on the Young’s modulus of solid wood obtained from a longitudinal vibration test

Abstract Young’s modulus in the longitudinal direction was measured by a longitudinal vibration (LV) test on western hemlock specimens with various length/width ratios. The effects of the configuration and orthotropy of the specimen on the measurement of Young’s modulus was investigated through subsequent finite element (FE) calculations. The experimental results suggested that Young’s modulus could not be obtained accurately when the length/width ratio of the specimen was in a certain range. The FE calculations revealed that Young’s modulus in the transverse direction and Poisson’s ratio in the length-width plane, as well as the length/width ratio, affected the Young’s modulus value. However, the results showed that when the length/width ratio of the specimen was larger than 10, Young’s modulus could be measured accurately with the LV test with a reduction in the effect of orthotropy.

Soil Phosphorus Dynamics Across a Holocene Chronosequence of Aeolian Sand Dunes in a Hypermaritime Environment on Calvert Island, BC, Canada

Phosphorus (P) is an essential nutrient for plant growth, but soil P concentrations decline with increasing soil age. Phosphorus often limits tree growth within the hypermaritime Coastal Western Hemlock zone in British Columbia, Canada, particularly where parent material with low P concentrations have experienced rapid weathering. To sustainably manage forests in this region, more information is needed about changes in soil P concentrations and dynamics that occur with time. This study characterized the forms and abundance of soil and foliar P compounds using a soil chronosequence developed on aeolian sand dunes on Calvert Island and compared results to chronosequences in other locations. Eight time points were examined, from a modern foredune to a relict, stabilized dune (~10,760 years old). Soil horizons were analyzed for bulk density, pH, and concentrations of total carbon (C), nitrogen (N) and total P (TP), iron (Fe) and aluminum (Al), total organic P (Po), and Mehlich-extractable P and cations. For each site, P forms in L, H and organically-enriched mineral (M) horizons were characterized with solution 31P nuclear magnetic resonance spectroscopy (P-NMR), as were foliar samples from tree species spanning all age classes except the youngest dune. This chronosequence followed the Walker and Syers (1976) model, with an exponential decline in TP mass and a humped-shape curve in Po mass with increasing age. The L horizon had lower TP concentrations than foliage samples, but similar P forms. The H horizons had a greater proportion of DNA, phosphonates and nucleotides than the L horizon and increased proportions of myo- and scyllo-inositol hexakisphosphate (IHP) with increasing age. The mineral horizons had much lower TP concentrations than other horizons and increased proportions of IHP and DNA with increasing age, which were correlated to increased exchangeable and amorphous Al concentrations. In all sample types, the proportion of orthophosphate declined with increasing age. These results enhance knowledge of P cycling within hypermaritime soils, particularly the P decline that will occur with age. This will aid in the sustainable management of the low-productivity forests typical of these ecosystems.

Variable-density thinning promotes differential recruitment and development of shade tolerant conifer species after 17 years

Promoting patchy recruitment of shade tolerant tree species into the midstory is an important step in developing structural diversity in second-growth stands. Variable-density thinning (VDT) has been proposed as a strategy for accelerating structural diversity, as its combination of within-stand treatments (harvest gaps, thinning, and non-harvested skips) should create variable overstory and understory conditions. Here we report on western hemlock (Tsuga heterophylla (Raf.) Sarg.) seedling and sapling densities in five mixed-conifer stands and Sitka spruce (Picea sitchensis (Bong.) Carr.) seedling and sapling densities in two stands in western Washington at 3,7, 10, and 16–17 years after VDT. Additionally, we report on western hemlock advance regeneration growth and survival in two stands over 14 years. Western hemlock seedling density was highest in the thinned treatment but only significantly so in Year 10. In contrast, the gaps contained significantly more western hemlock saplings in Years 7 and 10 and significantly greater growth of western hemlock advance regeneration through Year 10. Skips embedded within the VDT did not differ significantly from unharvested reserves in terms of seedling or sapling densities of either species. Sitka spruce seedling density was highest in the gap and thinned treatments, but saplings were uncommon in all treatments. Collectively, these results indicate that our variant of VDT promoted patchy, midstory recruitment of western hemlock but failed to recruit Sitka spruce saplings in either stand where it established. Consequently, more intensive variants of VDT may be required to promote midstory recruitment of species less tolerant of shade than western hemlock.

Climatic factors controlling stem growth of alien tree species at a mesic forest site: a multispecies approach

The introduction of non-native species with various ecological and functional traits to European forests may be a potential tool for mitigating climate risks. We analyzed the growth sensitivity to climate of seven alien (Acer rubrum, Betula maximowicziana, Castanea sativa, Cryptomeria japonica, Metasequoia glyptostroboides, Thuja plicata and Tsuga heterophylla) and two native (Fagus sylvatica and Picea abies) tree species on a productive forest site in western Germany in order to assess their potential suitability in light of climate change. Retrospective basal area increment at breast height (BAI) and total stem volume increment (VI) were statistically related to a total of 11 multiscale climatic variables showing minor discrepancies between BAI and VI in climatic sensitivity. Across the tree species, the strongest climate signals were found for the standardized precipitation evapotranspiration index (spei), vapor pressure deficit (vpd) and the number of frost days (nfr) representing water availability, heat and coldness, respectively. Growth of most species was positively affected by mild winters and growing season spei. Importance of the investigated variables differed among tree species with, e.g., C. sativa and T. plicata being mainly driven by winter vpd and A. rubrum, C. japonica and P. abies by water availability in the growing seasons. We conclude that particularly more anisohydric species (e.g., C. sativa) may profit from their low sensitivity to drier conditions. Highly sensitive and isohydric species (e.g., A. rubrum) will undergo significant growth reductions when climate becomes more variable.

Temporal Asynchrony of Adult Emergence Between Leucopis argenticollis and Leucopis piniperda (Diptera: Chamaemyiidae), Predators of the Hemlock Woolly Adelgid (Hemiptera: Adelgidae), with Implications for Biological Control.

Two species of silver fly, Leucopis argenticollis (Zetterstedt) and Leucopis piniperda (Malloch) (Diptera: Chamaemyiidae), from the Pacific Northwest region of North America have been identified as potential biological control agents of hemlock woolly adelgid (Hemiptera: Adelgidae: Adelges tsugae Annand) in eastern North America. The two predators are collectively synchronized with A. tsugae development. To determine whether adult emergence of the two species of silver fly are also synchronized with one another, we collected adult Leucopis which emerged from A. tsugae-infested western hemlock [Pinaceae: Tsuga heterophylla (Raf.) Sarg.] from four sites in the Pacific Northwest over a 29-d period. Specimens were collected twice daily in the laboratory and identified to species using DNA barcoding. The study found that more adult Leucopis were collected in the evening than the morning. Additionally, the daily emergences of adults over the 29-d sampling period exhibited sinusoidal-like fluctuations of peak abundance of each species, lending evidence to a pattern of temporal partitioning. This pattern could have logistical implications for their use as biological control agents in eastern North America, namely the need to release both species for maximum efficacy in decreasing A. tsugae populations.