Intact Bark Research Articles

Endophytic community in juvenile Acer pseudoplatanus and pathogenicity of Cryptostroma corticale and other associated fungi under controlled conditions

The causative agent of sooty bark disease, Cryptostroma corticale, has, for some years, caused increased damage to maples (Acer spp.) in Germany and throughout Europe. It has been stated that this pathogen can occur latently in Acer. In this study, the composition of endophytic fungi of woody stem tissues of sycamore (Acer pseudoplatanus) saplings were analysed in order to investigate whether C. corticale is present in young, visually healthy maple trees from natural regeneration. The most abundant taxa of the 30 isolated endophytes were Diaporthe cf. rudis and Petrakia irregularis. An association of five fungal species (Biscogniauxia mediterranea, Coniochaeta velutina, Gibellulopsis catenata, Neocucurbitaria quercina, Tangerinosporium thalitricola) with A. pseudoplatanus was reported for the first time. Cryptostroma corticale was not present in the juvenile sycamore stems. Furthermore, its pathogenicity was studied in comparison to other pathogens associated with A. pseudoplatanus, namely Diplodia mutila, Dothiorella sp., Nectria cinnabarina, Neonectria coccinea, Neonectria punicea and Stegonsporium pyriforme. The longest necroses were induced by C. corticale followed by N. coccinea. In a trial with a C. corticale spore suspension sprayed directly on stem bark, the fungus could be re-isolated from woody tissue, however necroses were evoked only when the bark was wounded prior to infection. The results from the experiments presented here function as additional evidence for the ability of C. corticale to establish endophytically within the host and, in particular, for the ability of the fungus to establish a successful infection/entrance through intact bark.

Oak Wilt Fungus (Bretziella fagacearum) Survival in Logs Following Fumigation with Ethanedinitrile

Abstract The phaseout of methyl bromide (MB) fumigation creates an urgent need for an alternative phytosanitary treatment to limit the risk of international spread of the oak wilt fungus, Bretziella fagacearum. Fumigation with ethanedinitrile (EDN) is considered a potential alternative to MB fumigation to eradicate wood-inhabiting pests and pathogens. We evaluated the efficacy of EDN fumigation by comparing the rate of B. fagacearum isolation before and after fumigation of red oak (Quercus rubra or Quercus ellipsoidalis) log sections from oak wilt–affected trees. Logs (range 15.2 to 98.0 cm long; diameter 9.1 to 46.1 cm) were obtained from red oak trees that were naturally infected (NI) or artificially inoculated (AI) with B. fagacearum. The logs were fumigated for 24, 48, and/or 72 hours with 120 g/m3 EDN. Frequencies of pathogen isolation from the sapwood before treatment were higher for AI logs than for NI logs. EDN treatments greatly reduced the frequency of viable pathogen recovery, but eradication occurred only in experiments using the smallest log diameters (9 to 14 cm). Our results suggest that EDN may have limited penetration in oak logs with intact bark, similar to fumigants currently used on wood products, such as MB and sulfuryl fluoride. Results of future work may help in the understanding of the limitations for consistent and full efficacy of EDN against B. fagacearum in logs harvested from diseased trees.

Geotechnical investigation of the Naval dry dock at the Mietsu Naval Facility World Heritage Site, Japan

The Mietsu Naval Facility was established as a Western-style military facility by the Saga Clan in 1858. A geotechnical investigation of the Mietsu Naval Dock (MND) at the Mietsu Naval Facility World Heritage Site was performed using portable cone penetration tests, unconfined compression tests with suction measurement, and standard oedometer tests. The pile length of the wooden frame structure was determined using pulse waves. The slope of the western wall consists of four layers, and straw soil bags were arranged for slope stabilization. The slope of the western wall during construction and under use was analyzed based on total and effective stresses, respectively, and was found to be stable. The wooden structure of the eastern wall consists of four stages and has a complex structure of pine wood with intact bark. The second and third-stage piles have anchorage structures with several horizontal elements linked to the first and second-stage piles. The number of the piles in the third-stage indicates that it plays a central structural role. Even though ship were repaired at the bottom of the dock, there is no evidence of ground improvement to prevent damage from ship loads. Because the amount of decrease in effective overburden pressure corresponded to the amount of excavated soil with the load of the Denryu-maru, which was repaired in this dry dock, it is possible that ground improvement for entry and repair of the ship was not needed. The results of this study are useful for the maintenance, management, and restoration of the MND remains.

Emergence levels of pine shoot beetles from roundwood piles of Scots pine and the cascading damage in the surrounding forests

Bark beetles are amongst the most aggressive pest agents of coniferous forests. Due to this, many boreal countries have designated laws aiming to lower the risk of bark beetle epidemics. Finland’s forest legislation has pre-emptive measures targeted against bark beetles, and for Scots pine ( L.), the law concerns pine shoot beetles ( spp.). This study used data collected around 25 piles of Scots pine roundwood that were harvested in the winter but left in the forest until the following November. Thus, the pine shoot beetles were able to use the piles for breeding. We assessed the number of emerged insects from the piles and the cascading damage they caused in the surrounding forests. All roundwood piles, regardless of their volume, were used by the beetles for breeding. Highest densities of beetle exit holes were found from the parts of the log with thick and intact bark. If the bark of the log was damaged by the harvester head, the number of beetles decreased significantly. Depending on the volume of the roundwood pile, the cascading damage (fallen shoots) was noticeable up to ca. 40–60 m from the roundwood pile. Storing of piles smaller than 50 m did not cause excess damage. The number of fallen shoots per tree was generally below the known thresholds for when growth losses can occur. However, the study was conducted in mature forests, and it can be assumed that the recorded damage levels would severely affect the growth of young pines, raising the question of where to store the roundwood. As with other bark beetles, the role of beetles as damage agents may change in the future, but based on this as well as past studies, the species can be viewed as a notable damage agents only around long-term wood storage sites in the current northern conditions.Pinus sylvestrisTomicus3Tomicus

Chemical defence responses of Norway spruce to two fungal pathogens

AbstractConstitutive and inducible terpene production is involved in conifer resistance against insects and fungal infestations. To gain knowledge about local defence responses of Norway spruce bark against pathogens and to find potential chemical markers for resistance breeding, we inoculated the stem of 8‐year‐old Norway spruce (Picea abies) clonal trees with both Endoconidiophora polonica (Ep, a common fungal pathogen associated with the spruce bark beetle Ips typographus) and Heterobasidion parviporum (Hp, a severe pathogen causing root and stem rot disease). Three weeks after inoculation, the fungal‐inoculated and intact bark from each tree was sampled. The terpenes in tree bark were extracted with hexane and characterized by gas chromatography–mass spectrometry (GC‐MS). The two fungi induced varied terpene responses in the four spruce clones used. Three of the clones showed a 2.3‐fold to 5.7‐fold stronger terpene response to Hp relative to Ep inoculation, while one clone responded similarly to inoculation with the two fungal pathogens. The amount of the diterpenes thunbergol and geranyllinalool varied between the clones. The level of thunbergol was higher in both intact and fungal‐inoculated bark from the less susceptible clones compared with the more susceptible clones. Geranyllinalool was present in higher amounts in the susceptible clones and is thus a possible marker for susceptibility. Our observations show that Norway spruce employs a similar chemical mechanism against the two fungal pathogens. Based on the present and earlier published data, we suggest that certain Norway spruce genotypes have a strong defence reaction against these two pathogens. The diterpenes thunbergol and geranyllinalool might be useful markers of susceptibility in tree‐breeding programmes and should be the focus of further detailed investigations.

Fate of Antioxidative Compounds within Bark during Storage: A Case of Norway Spruce Logs.

Softwood bark is an important by-product of forest industry. Currently, bark is under-utilized and mainly directed for energy production, although it can be extracted with hot water to obtain compounds for value-added use. In Norway spruce (Picea abies [L.] Karst.) bark, condensed tannins and stilbene glycosides are among the compounds that comprise majority of the antioxidative extractives. For developing feasible production chain for softwood bark extractives, knowledge on raw material quality is critical. This study examined the fate of spruce bark tannins and stilbenes during storage treatment with two seasonal replications (i.e., during winter and summer). In the experiment, mature logs were harvested and stored outside. During six-month-storage periods, samples were periodically collected for chemical analysis from both inner and outer bark layers. Additionally, bark extractives were analyzed for antioxidative activities by FRAP, ORAC, and H2O2 scavenging assays. According to the results, stilbenes rapidly degraded during storage, whereas tannins were more stable: only 5–7% of the original stilbene amount and ca. 30–50% of the original amount of condensed tannins were found after 24-week-storage. Summer conditions led to the faster modification of bark chemistry than winter conditions. Changes in antioxidative activity were less pronounced than those of analyzed chemical compounds, indicating that the derivatives of the compounds contribute to the antioxidative activity. The results of the assays showed that, on average, ca. 27% of the original antioxidative capacity remained 24 weeks after the onset of the storage treatment, while a large variation (2–95% of the original capacity remaining) was found between assays, seasons, and bark layers. Inner bark preserved its activities longer than outer bark, and intact bark attached to timber is expected to maintain its activities longer than a debarked one. Thus, to ensure prolonged quality, no debarking before storage is suggested: outer bark protects the inner bark, and debarking enhances the degradation.

Efficacy of methyl bromide and alternative fumigants against pinewood nematode in pine wood samples

Alternatives to methyl bromide fumigation are needed for the phytosanitary treatment of log exports. Among the potential alternative, fumigants are phosphine, and sulfuryl fluoride, which are already in use for other commodities and are labeled for use on wood products. Ethanedinitrile is being used for log treatment in Australia and its registration for that purpose is being sought in other countries. These alternative fumigants were tested against the pinewood nematode, in a range of artificially infested wood samples with high moisture content and, in some cases, the bark layer intact on the surface exposed to the fumigant. Phosphine and sulfuryl fluoride successfully eliminated pinewood nematodes on pine wood chips but neither was completely effective on pine blocks with intact bark. Ethanedinitrile was effective on pine chips, blocks and logs at all the doses tested: 40–100 mgL−1 for 24 h at 20 °C. Ethanedinitrile appears to be a promising alternative to methyl bromide for the fumigation of pine logs. Further testing at lower temperatures and doses, and with commercial-scale log loads, is recommended.

Fungal proliferation and calcium accumulation in the orange slime of Cornus controversa

ABSTRACTA giant dogwood (Cornus controversa) was bleeding transparent sap that turned into gummy orange slime with a pungent odor on the bark in early April. The tree exudation originated from a branch wound that occurred during the most recent spring pruning. Scanning electron microscopy revealed the intact bark surface was irregularly undulating and covered with at least three types of epicuticular waxes: polygonal rodlets; polygonal tubules; and threads, whereas the dried orange plaques were covered with fungi, possibly the co-occurrence of yeast and filamentous forms. The yeast forms were ovoid, lemon- or club-shaped, and approximately 10 μm in length. Obviously constricted septa and clamp connections were rarely detected on the filamentous forms. No bacterial proliferation was detected on the orange slime. X-ray microanalysis revealed a steep increase in calcium concentration in the orange plaques compared with that in the intact bark. These results suggest that bark-dwelling dimorphic red yeasts may constitute one of the dominant microbial consortia of the orange slime of giant dogwood. Given the high amount of sugars and calcium in the sap, and tolerance to diurnal temperature fluctuations in early spring, the red yeasts may be specialist species that thrive under a limited range of environmental conditions.

CHEMICAL INDUCTION OF TRAUMATIC GUM DUCTS IN CHINESE SWEETGUM, LIQUIDAMBAR FORMOSANA

The effect of ethephon (Et) and methyl jasmonate (MeJA) on the induction of traumatic gum ducts (TGDs) was studied in Chinese sweetgum, Liquidambar formosana, a broad-leaved tree species. Lanolin pastes with concentrations of 1, 2 and 5% (w/w) of these chemicals were applied to the intact bark of the trees in May, July and September without any wounding. The trees did not show any response to the treatment of MeJA, but TGDs were formed in response to treatment with Et. Trees treated with Et in the active growing season (May) produced more rows of TGDs than those treated in July or September, suggesting a strong relationship between cambial activity and susceptibility to TGD induction. These results are discussed in comparison with responses of conifers and some angiosperms to MeJA and Et treatments with and without associated wounding reported in the literature.

The protective role of bark against fire damage: a comparative study on selected introduced and indigenous tree species in the Western Cape, South Africa

The objective of this study was to compare the protective role of bark against fire for three selected indigenous and five exotic species in the Western Cape, South Africa. Bark thickness, compass direction, stem diameter at breast height, bark moisture content and relative height of the sample in the stem were tested for their effect on heat insulation capacity of bark. Trees were felled and heating experiments were conducted at 400 oC on fresh billets with intact bark. Time to heat the cambium to lethal 60 oC was determined. Statistical analysis based on correlation, multi-model inference and multiple regression revealed no significant influence of compass direction and diameter at breast height. Heat resistance was mainly determined by bark thickness, to a lesser degree by moisture content. In several species relative height at the stem modulated the bark thickness effect. Higher up the stem bark of the same thickness offered less protection against heat. Significant species-specific differences in heat resistance were apparent in the results, which could not be explained by bark thickness thus indicating further need for research in scrutinising these factors, which might help to explain the relative higher fire tolerance of certain species compared to others.

Histological and cytological investigations of the infection and colonization of apple bark by Valsa mali var. mali

The infection process of Valsa mali var. mali (Vmm), causal agent of canker, in apple (Malus domestica cv. Fuji) tissue was investigated by light and electron microscopy. The germination pattern of conidia was similar on wounded or intact twigs, but there was a time delay on intact bark. On intact bark tissue, conidia swelling could be observed at 16 h post inoculation (hpi) and conidia germinated at 20 hpi, however, no infection was observed. On wounded twig surface, conidia swelled at 6 hpi, and germinated at 16 hpi. Direct penetration of germ tubes was observed by 20 hpi and the pathogen caused canker lesions 4 days post inoculation (dpi) on wounded twigs. Electron microscopy observation of the canker region showed that fungal hyphae mainly spread intercellularly causing severe tissue maceration and cell necrosis. By 15 dpi, hyphae colonized all bark tissues and xylem vessels and generated pycnidia on the canker region. Fungal colonization resulted in marked alterations in host tissues including plasmolysis and degeneration of protoplasts and cell walls. Based on enzyme- and immuno-gold labeling of cell wall components, pectinases were involved in pathogenesis of Vmm of apple tissue.

Disease symptoms and fungi on dying ash trees (Fraxinus excelsior L.) in Staszów Forest District stands

Symptomy chorobowe i grzyby na zamierających jesionach (Fraxinus excelsiorL.) w drzewostanach Nadleśnictwa Staszów

Improving the Efficacy of Cytokinin Applications for Stimulation of Lateral Branch Development in Young Sweet Cherry Trees in the Orchard

Improving lateral branch development in young sweet cherry trees without reliance on pruning is a desirable component of tree training programs, especially for high-density systems. Applications of two proprietary formulations of 6-benzyladenine and gibberellins A4 and A7 (Promalin, Valent Biosciences, Walnut Creek, Calif.; and Perlan, Fine Americas, Walnut Creek, Calif.) to individual buds or intact bark of unpruned sweet cherry central leader shoots at green-tip had little effect on lateral shoot growth from buds or on distribution of new shoot growth along the treated leader shoots. Scoring, nicking, or notching cuts alone also had inconsistent effects on shoot development and distribution. In some trials, bud removal (or disbudding, removing every fourth bud on 1-year-old shoots) produced limited improvement of lateral shoot development and vertical distribution. Combining nicking, notching, scoring, or bark scraping with the application of cytokinin–gibberellic acid solution to the cut area greatly improved both number of shoots developed and the numbers originating from the lower portions of treated leader shoots. Removing the physical barrier to bioregulator product contact with active tissues was a primary factor in improving treatment efficacy.

First Report on an Infestation of Phytophthora cinnamomi in Natural Oak Woodlands of California and its Differential Impact on Two Native Oak Species.

During an intense survey of natural woodlands around Lake Hodges (33°N, 117°W) in June 2001, symptoms typical of root and collar rot caused by Phytophthora spp. were observed on 27% of 474 coast live oaks (Quercus agrifolia Nee.) and on none of 86 Engelmann oaks (Q. engelmannii Greene), in spite of complete spatial intermixing of the two species. Symptoms on coast live oaks included viscous exudates emerging through intact bark matched by underbark dark lesions with irregular margins. Lesions were delineated by a dark line and present on the root collar or the buttress of symptomatic trees. Crowns of trees with lesions ranged from completely healthy to declining or dead. All symptomatic trees were in proximity of the lake or streams. Phytophthora cinnamomi Rands was isolated from four trees in three distinct sites by plating tissues from lesion margins on PARP selective medium and from four soil samples by using standard pear baiting and plating lesions from pear tissue onto PARP. Identification of the isolates was obtained from microscopic observations and direct sequencing of the internal transcribed spacer region of the rDNA (Genbank Accession Nos. AY302148, MC2 and AY302149, MC3). P. citricola Sawada was also isolated once. Pathogenicity tests were completed to compare the susceptibility of the two species of oaks growing in the Lake Hodges region with P. cinnamomi. Two P. cinnamomi isolates from Lake Hodges (MC2, ATCC MYA-3711; MC3) and one isolate from an avocado orchard in San Diego County (MC6) were used to inoculate separately 10 5-year-old trees each of Q. agrifolia and Q. engelmannii grown in 5-gallon containers. Inoculations were performed in two lath-house experiments during February and September 2002 by placing an 8-mm diameter V8-agar plug from the margin of a P. cinnamomi colony underbark and sealing the wound with Parafilm and grafting wax. Lesion lengths were measured 2 months after inoculation, and the presence of the pathogen confirmed by reisolation on PARP. Mean average, maximum, and minimum temperatures were 14, 19, and 9°C and 21, 24, and 18°C for the February and September inoculations, respectively. The February inoculation resulted in small lesions only on Q. agrifolia (26 ± 15 mm, SD). The September inoculation resulted in 135 ± 68 mm (SD) lesions on Q. agrifolia and 49 ± 35 mm (SD) lesions on Q. engelmannii. Controls did not show any lesions. The length of lesions was significantly different between the two hosts (P < 0.0001) and significant differences were observed among the three isolates (P = 0.0018). Although Q. agrifolia is a known host for P. cinnamomi in California (2,3), to our knowledge, this is the first report of widespread infestation of P. cinnamomi in natural oak woodlands in the western United States. Survey and inoculation results indicated Q. engelmannii to be less susceptible to infection. Inoculation results confirm previous research that cold temperatures are unfavorable to this pathogen and isolates differed in pathogenicity toward Q. agrifolia. Decline of oaks infected by P. cinnamomi was observed only in conjunction with other factors, in particular with the presence of the oak twig girdler, Agrilus angelicus Horn., an insect favored by stress conditions such as drought. Similar effects have been reported for Mediterranean oaks infected by the same pathogen (1).

Bark-carving behavior of the Japanese horned beetle Trypoxylus dichotomus septentrionalis (Coleoptera: Scarabaeidae)

The Japanese horned beetle, Trypoxylus dichotomus septentrionalis, usually aggregates at the wounds caused by boring insects on Quercus oak trees to feed on exuding sap. Here, a unique behavior of the beetle, which enables it to independently obtain sap without any activities of other insects, is reported. Beetles carved intact bark of Fraxinus griffithii, where no sap was exuding, using clypeus. As a result of carving behavior, bark was stripped, and sap exuded from these scars temporarily, which beetles immediately began to lick. A digital video image relating to the article is available at (http://www.momo-p.com/showdetail-e.php?movieid=momo050525td01a).

The effects of moisture and oxygen availability on rhizomorph generation by Armillaria tabescens in comparison with A. gallica and A. mellea

Compared with other Armillaria species, natural melanized rhizomorphs of A. tabescens are rarely observed. Growing in autoclaved Vitis stem segments, A. tabescens isolates from the Ozark Mountains in the central USA formed fully melanized rhizomorphs, thinner and shorter than those observed for other Armillaria species under field conditions, and only under conditions of both high oxygen availability, ζ2 ûg cm−2 min−1 and moisture near saturation. Conducive conditions were used to compare the rhizomorph generation capacities of A. tabescens, A. gallica, and A. mellea, which have overlapping host and geographic ranges in central North America. While the rhizomorphs of A. tabescens were significantly shorter than those of the other two species, A. gallica and A. tabescens produced similar numbers of rhizomorph initials. Finally, we demonstrated the ability of melanized A. tabescens rhizomorphs to span woody food bases and thereby establish viable infections by penetration of intact bark of Vitis stem segments. We hypothesize that A. tabescens rhizomorphs form under conditions of periodic saturation which promote rapid water movement through naturally occurring lacunae in the soil. Thus, the role of A. tabescens rhizomorphs in the spread of the fungus should be re-evaluated.

Application of methyl jasmonate on Picea abies (Pinaceae) stems induces defense‐related responses in phloem and xylem

Application of 100 mmol/L methyl jasmonate (MJ) to the intact bark of 30-yr-old Norway spruce induced anatomical reactions related to defense. Within 30 d, a single MJ treatment induced swelling of existing polyphenolic parenchyma cells (PP cells) and an increase in their phenolic contents and formation of additional PP cells and of traumatic resin ducts (TDs) at the cambial zone. These changes occurred up to 7 cm away from the application zone. Treatment enhanced resin flow and increased resistance to the blue-stain fungus, Ceratocystis polonica. Methyl jasmonate application to the oldest internode of 2-yr-old saplings also induced TD formation, and, more surprisingly, TDs were formed in the untreated internode. Traumatic ducts were not formed in branches, ruling out an effect of volatile MJ on the upper internode. Methyl jasmonate application never gave rise to a hypersensitive response, cell death, tissue necrosis, or wound periderm, indicating the amount of MJ transported across the periderm was very low relative to the application concentration. This is the first report of a single compound giving rise to major cellular features related to acquired resistance and previously shown to be induced by wounding, fungal infection, and bark beetles in Norway spruce.

Sudden oak death, a new forest disease in California

A forest disease caused by a newly described pathogen, Phytophthora ramorum, is presently affecting a number of woody plant species in central and northern coastal California and southern Oregon, U.S.A. Among the most severely affected tree species are coast live oak, Quercus agrifolia, and tanoak, Lithocarpus densiflorus. In March 2000, we established plots in Marin County, CA, to assess symptom progression and the association of other organisms with diseased trees. Symptomatic trees exhibit 'bleeding' of viscous sap from apparently intact bark, typically within approximately 2 m of the soil. At least three species of bark and ambrosia beetles colonize bleeding oaks. These beetles colonized every bleeding tree that died during the first year of the study (N = 23) while the foliage was still green. By March 2001, numbers of symptomatic and dead trees increased for both coast live oak and tanoak. Symptomatic coast live oaks totaled 26% in March 2000 and 27% in March 2001. During the same period, mortality increased from 6% to 10%. For tanoaks, 41% were symptomatic in 2000, rising to 49% in 2001. Mortality was 11% in 2000 and 13% in 2001. The values for coast live oak are broadly consistent with independently acquired infection and mortality estimates derived from an unbiased transect method.

STATIC LIFE-TABLE ANALYSIS OF FALL RATES OF SUBALPINE FIR SNAGS

A conflict exists in managed forests between the maintenance of snags (standing dead trees) for biological diversity and the requirement to remove snags for worker safety. This conflict could be mitigated by the identification of safe classes of snags, those with low rates of fall, that can be easily identified and retained in forest operations. I outline an approach to estimating rates of fall of identifiable decay classes of snags based on static life-table analysis of the current age distribution and relative abundance of classes of snags in a stand, applying the analysis to subalpine fir (Abies lasiocarpa) snags. The two most recent classes of snags, identified by the presence of fine branches and intact bark, showed fall rates of 0.17–0.20%/yr, similar to or lower than live subalpine fir trees. Fall rates increased to 0.70%/yr for the third class of snags, identified by loss of fine branches and cracking of bark, and to 3.79% and 6.16%/yr for the fourth and fifth classes, respectively, identified by ...

Static Life-Table Analysis of Fall Rates of Subalpine Fir Snags

A conflict exists in managed forests between the maintenance of snags (standing dead trees) for biological diversity and the requirement to remove snags for worker safety. This conflict could be mitigated by the identification of safe classes of snags, those with low rates of fall, that can be easily identified and retained in forest operations. I outline an approach to estimating rates of fall of identifiable decay classes of snags based on static life-table analysis of the current age distribution and relative abundance of classes of snags in a stand, applying the analysis to subalpine fir (Abies lasiocarpa) snags. The two most recent classes of snags, identified by the presence of fine branches and intact bark, showed fall rates of 0.17–0.20%/yr, similar to or lower than live subalpine fir trees. Fall rates increased to 0.70%/yr for the third class of snags, identified by loss of fine branches and cracking of bark, and to 3.79% and 6.16%/yr for the fourth and fifth classes, respectively, identified by increasing loss of bark and incipient softening of outer wood. Snags in the two earliest classes could be retained during forest operations without substantial increase in risk to workers. The static life-table approach to estimating fall rates of snags can complement long-term monitoring of snags in older forests where the assumption of a stationary age distribution is met.