Hemlock Dwarf Mistletoe Research Articles

Susceptibility of sugar pine, Shasta red fir and Sierra lodgepole pine to mountain hemlock dwarf mistletoe (Arceuthobiumtsugense subsp. mertensianae, Viscaceae) in south central Oregon

AbstractIn southern Oregon and northern California, mountain hemlock dwarf mistletoe (MHDM; Arceuthobium tsugense subsp. mertensianae) commonly parasitizes mountain hemlock (Tsuga mertensiana) in multiple mountain ranges such as the Cascades, Sierra Nevada and Klamath Mountains. However, limited information is available on the susceptibility of sugar pine (Pinus lambertiana), Shasta red fir (Abies magnifica var. shastensis) and lodgepole pine (Pinus contorta) to MHDM. From 2015–2019, I found MHDM on sugar pine at multiple sites in northwestern Klamath County, Oregon. To evaluate the susceptibilities of sugar pine, Shasta red fir and Sierra lodgepole pine to MHDM infection, 41 temporary, fixed‐radius plots (area 182.4 m2) were established at five sites in 2018 and 2019 around an overstory tree (western white, sugar and whitebark pines along with one mountain hemlock) severely infected with MHDM in each plot centre. Live trees ≥2.6 cm in diameter at breast height (1.37 m; dbh) were assigned a dwarf mistletoe severity rating (6‐class system) and incidence (% infection) of MHDM on live trees at the tree level were used to estimate host susceptibilities. To confirm the identity of the dwarf mistletoe infecting live trees in each plot, dwarf mistletoe specimens were collected from the severely infected tree at the centre of each plot and from all tree species bearing dwarf mistletoe in each plot. Morphological characters were used to identify the dwarf mistletoe plants on each tree species and included plant heights, length of the third internode and plant colour. Dwarf mistletoe plants were collected from each affected tree species, and morphological characteristics of aerial shoots of dwarf mistletoes were examined in the laboratory. Morphological measurements confirmed the dwarf mistletoe infesting each plot was MHDM. Mountain hemlock dwarf mistletoe was found on sugar pine in all 41 plots. At the tree level after pooling larger sugar pine trees from all plots (total of 164 live trees ≥2.6 cm dbh), incidence was 76%. Morphological characteristics of MHDM aerial shoots, such as colour, shoot height and third internode length, were consistent with previous descriptions of MHDM plants. These data indicate that sugar pine should be classified as a secondary host of MHDM in the Oregon East Cascades, and not as a rare host as previously reported. No dwarf mistletoe plants were found on 98 living Shasta red fir trees ≥2.6 cm dbh in 26 plots. Shasta red fir should be classified as immune to MHDM. No MHDM was found on 573 living lodgepole pines of this size in 41 plots, and the tree species should be classified as immune to MHDM. Western white (Pinus monticola) and whitebark (Pinus albicaulis) pines were severely parasitized by MHDM with 92% and 100% incidence, respectively. However, because the sample sizes for these white pines were relatively small, additional work is needed to evaluate the susceptibility of these pines to MHDM.

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.

Tree growth declines and mortality were associated with a parasitic plant during warm and dry climatic conditions in a temperate coniferous forest ecosystem.

Insects and pathogens are widely recognized as contributing to increased tree vulnerability to the projected future increasing frequency of hot and dry conditions, but the role of parasitic plants is poorly understood even though they are common throughout temperate coniferous forests in the western United States. We investigated the influence of western hemlock dwarf mistletoe (Arceuthobium tsugense) on large (≥45.7cm diameter) western hemlock (Tsuga heterophylla) growth and mortality in a 500year old coniferous forest at the Wind River Experimental Forest, Washington State, United States. We used five repeated measurements from a long-term tree record for 1,395 T. heterophylla individuals. Data were collected across a time gradient (1991-2014) capturing temperature increases and precipitation decreases. The dwarf mistletoe rating (DMR), a measure of infection intensity, varied among individuals. Our results indicated that warmer and drier conditions amplified dwarf mistletoe effects on T. heterophylla tree growth and mortality. We found that heavy infection (i.e., high DMR) resulted in reduced growth during all four measurement intervals, but during warm and dry intervals (a) growth declined across the entire population regardless of DMR level, and (b) both moderate and heavy infections resulted in greater growth declines compared to light infection levels. Mortality rates increased from cooler-wetter to warmer-drier measurement intervals, in part reflecting increasing mortality with decreasing tree growth. Mortality rates were positively related to DMR, but only during the warm and dry measurement intervals. These results imply that parasitic plants like dwarf mistletoe can amplify the impact of climatic stressors of trees, contributing to the vulnerability of forest landscapes to climate-induced productivity losses and mortality events.

Arceuthobium tsugense (Viscaceae): four subspecies with contrasting morphologies and host distributions

There have been four subspecies of hemlock dwarf mistletoe (Arceuthobium tsugense: Viscaceae) described by various investigators of this complex group of parasitic flowering plants: subsp. tsugense, subsp. amabilae, subsp. contortae, and subsp. mertensianae. As suggested by their subspecific epithets, these taxa differ in their host affinities; parasitizing different and the same hosts to varying degrees. Although these taxa also have morphological differences, their classifica-tion has been under debate for many years. Therefore, we compared the morphological characteristics of each subspecies using both univariate and multivariate statistical analyses in order to better assess their differences. Because some investigators have grouped hemlock dwarf mistletoe with western dwarf mistletoe (A. campylopodum), we also compared the subspecies of hemlock dwarf mistletoe with western dwarf mistletoe. Our morphometric analyses demonstrated that all of the subspecies are morphologically distinct from western dwarf mistletoe and that subsp. contortae is the most morphologically differentiated of the subspecies. Overlap in the morphological characters across two of the other three subspecies was evident; yet, subsp. amabilae and subsp. mertensianae were also consistently delimited using female and male plant morphologies. Statistical comparisons of female or male plants via standard and stepwise discriminant function analyses demonstrated that without consideration of host plant, female and male A. tsugense subsp. tsugense are morphologically similar to corresponding plants of subsp. amabilae and mertensianae but not subsp. contortae or A. campylopodum.

Fire and dwarf mistletoe (Viscaceae: Arceuthobium species) in western North America: contrasting Arceuthobium tsugense and Arceuthobium americanum

Dwarf mistletoes (Viscaceae: Arceuthobium spp.) and fire interact in important ways in the coniferous forests of western North America. Fire directly affects dwarf mistletoes by killing the host, host branch, or heating/smoking the aerial shoots and fruits. Fire is a primary determinant of dwarf mistletoe distribution on the landscape, and time since fire controls many aspects of dwarf mistletoe epidemiology. Conversely, dwarf mistletoes can influence fire by causing changes in forest composition, structure, and fuels. Prescribed fire is important for management of dwarf mistletoes, while fire suppression is thought to have increased dwarf mistletoe abundance in western forests. Two dwarf mistletoes are compared in order to illustrate fire interactions in Oregon and Washington, USA: Arceuthobium americanum Nutt. ex Engelm. (lodgepole pine dwarf mistletoe) and Arceuthobium tsugense (Rosendahl)(western hemlock dwarf mistletoe). Arceuthobium persists on the landscape where the host is not killed by fire. Arceuthobium americanum spreads directly into the regenerating Pinus contorta Dougl. ex Loud., while A. tsugense persists in refugia for 200 years or more following fire. Host successional status is a driver of fire – dwarf mistletoe interactions, but forest disturbance agents also play a role. Given the importance of these interactions to the ecology of fire-prone forests, dwarf mistletoes warrant inclusion in disturbance ecology research.

Incorporating climate change into pest risk models for forest pathogens: a role for cold stress in an era of global warming?

Climate change may alter the distribution and activity of native and alien pathogens that infect trees and, in severe cases, cause tree death. In this study, potential future changes in climate suitability are investigated for three forest pathogens that occur in western North America: the native Arceuthobium tsugense subsp tsugense, hemlock dwarf mistletoe, and two alien invasive species, Dothistroma septosporum, the cause of red band needle blight or Dothistroma needle blight, and Phytophthora ramorum, the cause of sudden oak death or ramorum blight. Specifically, the software CLIMEX is used to calculate Cold-Stress, Heat-Stress, and Dry-Stress indices for each pathogen in 98,224 grid cells in North America. Downscaled climate projections from the general circulation models CGCM1, CSIROMk2, and HadCM3 drive forecasts for 2020, 2050 and 2080. These climate projections are then analyzed to forecast shifts in the geographic extent of abiotic stresses that are severe enough to directly kill pathogen propagules and prevent year-round establishment of these pathogens. Cold stress currently has a major impact on climate suitability for all three pathogens; heat stress is likely to become more significant in the future. I forecast that the geographic extent of cold stress will decline from its current levels by a constant 5% (± 1%) of all grid cells in each 30-yr projection horizon for all three pathogens. Forecasts suggest the extent of heat stress will increase concurrently by 4% (± 1%) in each 30-yr projection horizon. Drought stress shows no consistent trend over time. No disproportionate effect of climate change on the two alien invasive pathogens over the native is forecasted. These results suggest that forecasts of future climate suitability for pathogens based on historical climate normals are accurate for less than 30 yrs. Adaptive management strategies in forestry will be needed to respond as these changes unfold.

Host–parasite distributions under changing climate:Tsuga heterophyllaandArceuthobium tsugensein Alaska

Dwarf mistletoes ( Arceuthobium species) influence many processes within forested ecosystems, but few studies have examined their distribution in relation to climate. An analysis of 1549 forested plots within a 14.5 million ha region of southeast Alaska provided strong indications that climate currently limits hemlock dwarf mistletoe ( Arceuthobium tsugense (Rosendahl) G.N. Jones) to a subset of the range of its primary tree host, western hemlock ( Tsuga heterophylla (Raf.) Sarg.), with infection varying from a high of 20% of trees at sea level to only 5% by 200 m elevation. Three types of modeling approaches (logistic, most similar neighbors, and random forests) were tested for the ability to simultaneously predict abundance and distribution of host and pathogen as a function of climate variables. Current distribution was explained well by logistic models using growing degree-days, indirect and direct solar radiation, rainfall, snowfall, slope, and minimum temperatures, although accuracy for predicting A. tsugense presence at a particular location was only 38%. For future climate scenarios (A1B, A2, and B1), projected increases for A. tsugense habitat over a century ranged from a low of 374% to a high of 757%, with differences between modeling approaches contributing more to uncertainty than differences between climate scenarios.

Assessment of Colletotrichum gloeosporioides as a biological control agent for management of hemlock dwarf mistletoe (Arceuthobium tsugense)

The ascomycete Colletotrichum gloeosporioides was tested for biological control of Arceuthobium tsugense on western hemlock (Tsuga heterophylla) in British Columbia (BC), Canada. A field trial was conducted near Nanaimo, BC, using C. gloeosporioides isolate PFC 2415 in three treatments applied in late August 2002. The treatments consisted of C. gloeosporioides formulated with ‘Stabileze’ (incorporation of fungi in a water-absorbent starch matrix with oil and sucrose, then granulating the matrix with hydrated silica) which were sprayed on (1) intact mistletoe swellings, (2) swellings on which all mistletoe shoots had been cut at 0.5 cm from the base of the shoot and (3) C. gloeosporioides formulated in a sucrose and gelatin preparation sprayed on intact mistletoe swellings. The ultimate goal of this research was to substantially reduce the ability of the treated dwarf mistletoe plants to produce seed inoculum. The ‘Stabileze’ and sucrose–gelatin treatments formulated with C. gloeosporioides reduced the current berry crop by 36.8 and 40.5%, respectively (p < 0.05). While the results for shoots appeared promising, heavy background infection and/or secondary infection, especially on controls, limited the ability to detect clear treatment effects. Careful culturing from various live and dead host tissues showed that C. gloeosporioides was unable to invade and kill the mistletoe endophytic system within the living xylem and phloem of the host.

Susceptibility of Conifers to Three Dwarf Mistletoes in the Klamath-Siskiyou Mountains

Abstract Dwarf mistletoes (Arceuthobium spp., Viscaceae) are parasitic flowering plants that infect members of the Pinaceae family in the western United States. This article reports additional host susceptibility data for three dwarf mistletoes found in the Klamath-Siskiyou Mountains of southwestern Oregon and northwestern California. Three mixed conifer stands, each infested with either mountain hemlock dwarf mistletoe, western white pine dwarf mistletoe, or Wiens' dwarf mistletoe (nine stands total) were sampled to evaluate the susceptibility of conifers to these parasites. At each of the study sites, 10–20 temporary circular plots with a 6-m radius (0.012 ha) were established around large, severely infected trees. Within plots, species, dbh, and dwarf mistletoe rating (six-class system) were determined for each live tree. On the basis of the incidence of infection, conifers were assigned to host susceptibility classes. Western white pine and mountain hemlock were principal hosts of western white pine and mountain hemlock dwarf mistletoes, respectively. Brewer spruce and red fir were principal hosts of Wiens' dwarf mistletoe. Other conifers sampled were less susceptible to these mistletoes. This information can be used by forest managers to mitigate the damage associated with infestations of these dwarf mistletoes in mixed conifer forests of the Klamath-Siskiyou Mountains.

First Report of Mountain Hemlock Dwarf Mistletoe (Arceuthobium tsugense subsp. mertensianae) on Sugar Pine (Pinus lambertiana) from Oregon.

Mountain hemlock dwarf mistletoe (Arceuthobium tsugense (Rosendahl) G.N. Jones subsp. mertensianae Hawksw. & Nickrent, Viscaceae) commonly parasitizes mountain hemlock (Tsuga mertensiana (Bong.) Carr.) from the central Sierra Nevada Mountains to the southern Cascades in Oregon (1,3). It has also been reported to commonly parasitize whitebark pine (Pinus albicaulis Engelm.) and occasionally western white pine (P. monticola Dougl. ex D. Don) (1,4). In September of 2008, we found mountain hemlock dwarf mistletoe infecting two sugar pines (P. lambertiana Dougl.) 4.5 km north of Windigo Pass, Oregon (42°24'40″N, 123°35'26″W, elevation 2,710 m). One of the sugar pines was 63.5 cm (25 inches) in diameter and had three infections. The other tree was 22.4 cm (9 inches) in diameter, but had 17 infections, many with mistletoe plants that allowed identification of the mistletoe using published descriptions (1,2). Mountain hemlock dwarf mistletoe can be distinguished from sugar pine dwarf mistletoe (A. californicum Hawksw. & Wiens) by its smaller plant size (mean height 5 cm versus 8 cm) and plant color (green-brown versus green to yellow) (1,2). An area (site) of approximately 1 ha around the infected sugar pines was examined and none of the other sugar pines we observed (33 trees) were found to be infected. Because mountain hemlock dwarf mistletoe also occurs in the principal range of sugar pine in the Sierra Nevada Mountains, it may also parasitize this tree there. However, our observations in several mountain hemlock stands infested with mountain hemlock dwarf mistletoe in California have failed to uncover infection of sugar pine by this mistletoe. Therefore, we would tentatively classify sugar pine as a rare host of mountain hemlock dwarf mistletoe (1). Specimens of mountain hemlock dwarf mistletoe on sugar pine were collected and deposited at the Deaver Herbarium (ASC), Northern Arizona University, Flagstaff (Accession No. 87122). To our knowledge, this is the first report of mountain hemlock dwarf mistletoe parasitizing sugar pine.

Distribution of Red Fir and Noble Fir in Oregon Based on Dwarf Mistletoe Host Specificity

The distribution of California red fir (Abies magnifica) and noble fir (Abies procera) in Oregon has long been debated. The objective of this study was to determine if selective parasitism of these true firs by subspecies of hemlock dwarf mistletoe (Arceuthobium tsugense) could be used to provide additional data supporting one of the geographic distributions proposed for these conifers. A total of ten mixed-conifer stands in California and Oregon infested with mountain hemlock dwarf mistletoe (Arceuthobium tsugense subsp. mertensianae) and 11 stands in Oregon infested with Pacific silver fir dwarf mistletoe (Arceuthobium tsugense subsp. amabilae) were sampled to compare host susceptibility to these parasitic plants. Populations of true firs morphologically resembling noble fir south of latitude 44° N in Oregon were immune to infection by both dwarf mistletoes. Populations of noble fir north of latitude 44° N in Oregon were principal hosts of Pacific silver fir dwarf mistletoe. The complete immunity of California red fir and trees morphologically resembling noble fir to Pacific silver fir dwarf mistletoe south of approximately latitude 44° N in Oregon supports the classification of these true fir populations as California red fir, and not noble fir. The severe parasitism of trees resembling noble fir by Pacific silver fir dwarf mistletoe north of latitude 44° N in central Oregon supports the classification of these populations as noble fir. These results provide the best example of using dwarf mistletoe-host specificity as evidence to distinguish between different interpretations of the geographic distribution of their coniferous hosts.

Basal area growth impacts of dwarf mistletoe on western hemlock in an old-growth forest

We investigated the effect of western hemlock dwarf mistletoe ( Arceuthobium tsugense (Rosend.) G.N. Jones ssp. tsugense ) on the 13 year basal area growth of large (&gt;45.7 cm diameter at breast height) western hemlock ( Tsuga heterophylla (Raf.) Sarg.) trees in an old-growth forest in southwestern Washington state. We controlled for spatial effects on tree growth by utilizing twenty-seven 0.4 ha plots that had uninfected, lightly, moderately, and severely infected classes of trees present on each plot. Basal area growth was analyzed using analysis of covariance of a randomized block design that was a balanced design with four treatments (infection classes) and 27 replicates (blocks or 0.4 ha plots), with initial diameter as the covariate. Basal area growth was found to be significantly different among the infection classes (F[3,77] = 10.09 and p &lt; 0.0001). Growth of the light and moderate infection classes was not detectably different from growth of uninfected trees. However, severely infected trees grew from 16% to 46% (mean = 36%) less than uninfected trees of the same initial diameter over the period of study (1991–2004). The large trees in this stand did not show growth impacts until they were severely infected.

Arceuthobium tsugense subsp. Amabilae, a New Subspecies of Hemlock Dwarf Mistletoe (Viscaceae) from Oregon

The dwarf mistletoe severely parasitizing Pacific silver fir in Oregon is described as a new subspecies of hemlock dwarf mistletoe. This classification is based on morphological and host range differences between hemlock dwarf mistletoe, Arceuthobium tsugense (Rosendahl) G. N. Jones subsp. tsugense, and the new subspecies, Pacific silver fir dwarf mistletoe, Arceuthobium tsugense subsp. amabilae Mathiasen & C. Daugherty.

Assessment of Neonectria neomacrospora (anamorph Cylindrocarpon cylindroides) as an inundative biocontrol agent against hemlock dwarf mistletoe

A field trial was conducted on Vancouver Island to evaluate the efficacy of the native fungus Neonectria neomacrospora as a biocontrol agent of hemlock dwarf mistletoe (Arceuthobium tsugense), a conifer parasite. Treatments consisted in formulated inoculum of N. neomacrospora applied to unwounded and wounded A. tsugense swellings. After 10 months, the amount of bark necrosis for the wounded, inoculated treatment was significantly greater than for the other treatments. Similarly, sporodochia were more common (present on 50% of swellings) for the wounded, inoculated treatment than the other treatments (present on less than 10% of swellings). Neonectria neomacrospora was isolated from 55.0% of unwounded, inoculated swellings and from 73.7% of those wounded and inoculated, while isolated from 20.0% of unwounded, uninoculated swellings and from 35.0% of those wounded and uninoculated. These results suggest that, although some infection does occur when inoculum is applied to unwounded swellings of hemlock dwarf mistletoe, wounding significantly enhances infection. Hemlock dwarf mistletoe swellings with confirmed N. neomacrospora infection (isolation of the fungus and (or) presence of sporodochia) had their numbers of healthy mistletoe shoots significantly reduced (by 1.6, or about 36%) when compared with mistletoe swellings with unconfirmed infection (P = 0.014).

Comparative Susceptibility of Conifers to Western Hemlock Dwarf Mistletoe in the Cascade Mountains of Washington and Oregon

Abstract A total of 24 mixed conifer stands in Washington and Oregon infested with western hemlock dwarf mistletoe (Arceuthobium tsugense subsp. tsugense) were sampled to compare host susceptibility to this parasitic plant. Temporary circular plots were established around dominant, severely infected western hemlocks (Tsuga heterophylla). Within plots, species, diameter at breast height, and dwarf mistletoe rating (6-class system) were determined for each live tree. More than 5,700 trees were sampled in 275 plots. Based on the incidence of infection, tree species were assigned to host susceptibility classes. Western hemlock is classified as the only principal host of western hemlock dwarf mistletoe. Pacific silver fir (Abies amabilis), noble fir (Abies procera), and mountain hemlock (Tsuga mertensiana) are classified as occasional hosts. Resource managers can use the host susceptibility classifications based on this study for decisions regarding which trees to retain when selectively harvesting mixed conifer stands infested with western hemlock dwarf mistletoe in the Pacific Northwest. West. J. Appl. For. 20(2):94–100.

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.

British Columbia Regional Meeting, 2004

British Columbia Regional Meeting, 2004

Hemlock Dwarf Mistletoe Stand Establishment Decision Aid

Hemlock dwarf mistletoe (Arceuthobium tsugense), a parasitic seed plant, is endemic and common in coastal western hemlock forests from northern California to southern Alaska. The mistletoe spreads by seeds dispersed from residual to nearby trees. Over time, infestations can significantly reduce tree growth. Annual growth losses attributed to mistletoe infection are estimated at over 1 million m3 in coastal British Columbia. Variable retention silviculture regimes recently implemented in hemlock-dominated coastal forests create conditions that could exacerbate the spread, intensification, and effects of mistletoe.The Stand Establishment Decision Aid (SEDA) format has been used to extend information on various vegetation and forest health concerns in British Columbia. This is the second SEDA produced for the Coast Forest Region. It summarizes information that relates current management regimes to the spread and effects of hemlock dwarf mistletoe. The first page provides general information, hazard ratings for the biogeoclimatic zones and subzones of the Coast Forest Region, and silvicultural considerations for hemlock dwarf mistletoe. The second page outlines the growth and yield implications and other effects and associations of the pathogen. This page also includes a valuable resource and reference list to provide readers with more detailed information. Reference material that is not available on-line can be ordered through libraries or the Queen�s Printer at: www.qp.gov.bc.ca.

Hemlock Dwarf Mistletoe Stand Establishment Decision Aid

Hemlock dwarf mistletoe (Arceuthobium tsugense), a parasitic seed plant, is endemic and common in coastal western hemlock forests from northern California to southern Alaska. The mistletoe spreads by seeds dispersed from residual to nearby trees. Over time, infestations can significantly reduce tree growth. Annual growth losses attributed to mistletoe infection are estimated at over 1 million m3 in coastal British Columbia. Variable retention silviculture regimes recently implemented in hemlock-dominated coastal forests create conditions that could exacerbate the spread, intensification, and effects of mistletoe.The Stand Establishment Decision Aid (SEDA) format has been used to extend information on various vegetation and forest health concerns in British Columbia. This is the second SEDA produced for the Coast Forest Region. It summarizes information that relates current management regimes to the spread and effects of hemlock dwarf mistletoe. The first page provides general information, hazard ratings for the biogeoclimatic zones and subzones of the Coast Forest Region, and silvicultural considerations for hemlock dwarf mistletoe. The second page outlines the growth and yield implications and other effects and associations of the pathogen. This page also includes a valuable resource and reference list to provide readers with more detailed information. Reference material that is not available on-line can be ordered through libraries or the Queen�s Printer at: www.qp.gov.bc.ca.

Integrated responses of hydraulic architecture, water and carbon relations of western hemlock to dwarf mistletoe infection

ABSTRACTDwarf mistletoe (Arceuthobium spp.) is a hemiparasite that is said to be the single‐most destructive pathogen of commercially valuable coniferous trees in many regions of the world. Although its destructive nature is well documented in many respects, its effects on the physiology of its host are poorly understood. In the present study, water and carbon relations were characterized over a range of scale from leaf to whole tree in large (40‐ to 50‐m‐tall) individuals of western hemlock (Tsuga heterophylla (Raf.) Sarg.) that were either heavily infected, or uninfected with hemlock dwarf mistletoe (Arceuthobium tsugense). Specific hydraulic conductivity (ks) of infected branches was approximately half that of uninfected branches, yet leaf‐specific conductivity (kL) was similar because leaf area : sapwood area ratios (AL : AS) of infected branches were lower. Pre‐dawn and minimum leaf water potential and stomatal conductance (gs) were similar among infected and uninfected trees because adjustments in hydraulic architecture of infected trees maintained kL despite reduced ks. Maximum whole‐tree water use was substantially lower in infected trees (approximately 55 kg d−1) than in uninfected trees (approximately 90 kg d−1) because reduced numbers of live branches in infected trees reduced whole‐tree AL : AS in a manner consistent with that observed in infected branches. Maximum photosynthetic rates of heavily infected trees were approximately half those of uninfected trees. Correspondingly, leaf nitrogen content was 35% lower in infected trees. Foliar δ13C values were 2.8‰ more negative in infected than in uninfected individuals, consistent with the absence of stomatal adjustment to diminished photosynthetic capacity. Adjustments in hydraulic architecture of infected trees thus contributed to homeostasis of water transport efficiency and transpiration on a leaf area basis, whereas both carbon accumulation and photosynthetic water use efficiency were sharply reduced at both the leaf and whole‐tree scale.