Conifer Stem Decays
Caused by several fungi
Host(s) in Alaska: All conifers
Habitat(s): Most decay heartwood, some occupy sapwood & heartwood
- General information about conifer stem decays in Alaska
- Symptoms, biology & impacts of white and brown rots
- Historic activity & survey methods
- Select stem decay fungi of Alaskan conifers
- Recent species observations
- Detection maps & identification
- Links & Resources
Stem decays occur on conifer hosts throughout the state, but have been studied in greatest depth in Southeast Alaska. Stem decay incidence changes little over time without active management. In mature forests of Southeast Alaska, conifer stem decays cause enormous wood volume loss. Approximately one-third of the old-growth timber volume in Southeast Alaska is defective, largely due to stem decay. There is very little decay in young-growth stands unless there is prevalent wounding. Stem decays are key disturbance agents in the coastal rainforest, because they predispose large old trees to bole breakage and windthrow. Stem decays create canopy gaps, influence stand structure and succession, perform essential nutrient-cycling functions, increase biodiversity, and enhance wildlife habitat. Trees with stem decay can be hazardous in managed recreation areas. Visit our hazard tree management webpage. Brown rots are the most significant source of cull for Sitka spruce, while white rots are most significant for western hemlock and western redcedar. Western redcedar is the most defective species, followed by western hemlock and Sitka spruce. A variety of different fungi cause stem decay in Alaskan conifers.
This table displays the most common conifer stem decays in Alaska, with information about the type of decay they cause, their tree hosts, modes of infection and known distributions in Alaska. For some conifer stem decay fungi, we have enough georeferenced ground observations to provide maps of detection locations, included with species-specific information below.
Stem decays rot or deteriorate wood, primarily in tree trunks, rather than roots and butts. They can be identified based on the presence and characteristics of conks, mushrooms, or other fungal structures on tree boles, when present. The characteristics of decayed wood and species of host tree can also be helpful for identification. Wildlife holes, cavities, and hollows indicate the presence of stem decay on live trees, even when conks and mushrooms are absent. Heart rot develops primarily in the heartwood (inner wood) of living trees, whereas sap rot develops in the sapwood (outer wood beneath bark) and is usually extensive only in dead trees. Bole wounds and cracks provide entry points for many stem decay fungi, although some decays enter through natural openings like branch stubs.
Brown rots are particularly detrimental to tree strength. They degrade cellulose fibers leaving behind brownish lignin, which dries in brittle cubes. White rots decompose all wood components (cellulose and lignin); wood remains fibrous until very late stages of decay. The color and texture of white rots is dependent upon the causal fungi.By predisposing large old trees to bole breakage and windthrow, stem decays are key disturbance agents. Individual tree mortality, much of it caused by heart rot fungi, creates small-scale canopy gaps and appears to be the leading form of disturbance in the coastal rainforest (Hennon 1995), where fire and other large-scale disturbances are uncommon. All major tree species in Southeast Alaska have been found killed in this manner. Stem decays influence stand structure and succession, perform essential nutrient cycling functions, increase biodiversity, and enhance wildlife habitat. Heart rot has an obvious and essential role in wood decomposition and has been demonstrated to be a site of nitrogen fixation by other microorganisms. Cavities created by stem decay fungi in standing trees provide crucial habitat for many species (bears, voles, squirrels and birds). Stem decays reduce merchantable timber volume from mature harvest units (especially old-growth) and can be hazardous in managed recreation areas. Many stem decay fungi cause heart rot of living trees, others decay the wood of dead trees, and some grow on dead tissue of both live and dead trees. Most of these decays do not actually interfere with the normal growth and physiological processes of live trees since the vascular system is unaffected. However, some decay pathogens, such as Phellinus hartigii and P. pini may attack the sapwood and cambium of live trees after existing as a heart rot fungus. Many of the fungi that are normally found on dead trees (e.g., Fomitopsis pinicola) can grow on large stem wounds, broken tops and dead tissue of live trees. Root and butt rot fungi can also cause stem decay in the lower bole (e.g., Phaeolus schweinitzii).
Wounds on live trees caused by logging activities are potential sites of infection for decay fungi to cause appreciable timber losses (Wright and Isaac 1956). Generally, larger, deeper wounds and larger diameter breaks in tops result in a faster rate of decay (Hennon 1990). Without logging injury, heart rot in young forests are typically at very low levels until stand age 100 to 150 years. Eventually, heart rot consumes as much or more wood volume annually than is produced by the live trees. There are methods that could be used to promote earlier development of stem decays for wildlife habitat in young-growth stands with non-timber objectives (Filip et al. 2011, Hennon and Mulvey 2014), such as intentional bole wounding and top breakage during stand entries. In some instances, bole breakage can be encouraged to occur in a specific direction (e.g., across steams for coarse woody debris input) by causing wounds to one side of the bole (e.g., the side that faces the stream).
Our knowledge of stem decay impacts on timber volume loss primarily come from two cull studies in Southeast Alaska (Kimmey 1956 and Far et al. 1976). A study by Hennon and McClellan (2003) evaluated modes of tree mortality (e.g., died standing, broken bole, uprooting) in old-growth forests. Permanent monitoring plots throughout Southcentral and Interior Alaska (evaluated 2013-2016) are helping to build information about the distribution and relative importance of stem decays on various tree hosts in other regions and forest types in Alaska.
An important cull study conducted by James Kimmey in Southeast Alaska in the 1950s found that brown rots were the most significant source of cull for Sitka spruce, while white rots were most significant for western redcedar (especially Obba/Ceriporiopsis rivulosa and Phellinus weirii) and western hemlock. Farr et al. (1976) found similar high rates of decay in old-growth forests as Kimmey (1956). These and other studies have shown that stem decay incidence and volume increase with tree size. The amount of defect also depends on tree species: for any given size or age class, redcedar was the most defective species, followed by western hemlock and Sitka spruce. Although redcedar wood products are known for decay resistance, it seems that a few species of decay fungi are specialized to overcome the decay resistance of live redcedar but do not affect wood in service.
Contribution of fungi that cause brown rot (tan wedges) and white rot (light gray wedges) in live Sitka spruce (left) and western hemlock (right). Adapted from Kimmey (1956) for Hennon and Mulvey (2014).
Influence of tree age on percentage of wood volume that is cull (left) and incidence of decay in trees (right). Adapted using data from Table 10 from Kimmey (1956) of mean gross volume cull values for dissected trees grouped by 50-year age intervals. Curves were fit with polynomial equations. Also used in Hennon and Mulvey (2014).
Recent observations, key identification characteristics, and damage information is provided for each species. Click images to view albums of stem decay fungi from the Forest Health Protection, Forest Service, USDA, Alaska Region on Flickr. Detection maps show georeferenced observations of fungi, and many include the modeled range of tree hosts. Host tree distributions were developed by the Forest Health Assessment and Applied Sciences Team in 2011 (240m-resolution, presence based on dominant tree species by tree diameter). View our ground survey data dashboard to view maps and ground survey records of stem decays and other damage agents in Alaska.
In 2015, the paint fungus (Echinodontium tinctorium), thought to be absent in Southeast Alaska south of Skagway, was found to be abundant on western and mountain hemlock in one stand on Mitkof Island south of Petersburg. Continued survey work may allow us to detect this fungus in other locations.
Fomitopsis pinicola species complex
Fomitopsis mounceae J.-E. Haight & Nakasone, sp. nov
Fomitopsis ochracea Ryvarden & Stokland
Of thirteen observations of Fomitopsis pinicola sensu lato (a species complex that has recently been redescribed) recorded in 2022 by FHP staff, Fomitopsis ochracea was the most common species recorded. Recent phylogenetic work has revealed that three species from this complex are present in North America and two occur in Alaska: F. mounceae, which has a red-orange band that inspired the “red belt conk” common name, while F. ochracea does not (Haight et al. 2019, https://doi.org/10.1080/00275514.2018.1564449). F. pinicola sensu stricto was originally described from Europe and is now thought to be restricted to Eurasia. In iNaturalist, there were 45 research grade observations of F. mounceae, 122 of F. ochracea, and four observations that did not have characteristics for identification to species. iNaturalist is improving our ability to capture georeferenced and photo-documented observations of this very common species complex. Members of the Fomitopsis pinicola complex are presumed to occur throughout their spruce and hemlock host ranges in Alaska. In Southcentral Alaska, conks of the F. pinicola complex were associated with white spruce bole snap during the recent spruce beetle activity in the Matanuska-Susitna valley. It is assumed that the trees had been infected long before they snapped because of the extensive advanced decay.
Ganoderma applanatum (Pers.) Pat.
In 2022, Ganoderma applanatum was detected ten times during ground detection surveys and 24 research grade observations were contributed through iNaturalist. The conk was especially abundant on western hemlock along the Carlanna Lake Trail in Ketchikan. Ganoderma applanatum is likely a species complex, found on both hardwoods and conifers in coastal Alaska
Ganoderma tsugae Murrill
While we have been identifying the species of varnish conk, which occurs on hemlock in Alaska, as Ganoderma tsugae, a likely alternative is Ganoderma oregonense. There were 16 observations made through iNaturalist and one during our ground detection surveys. This fungus tends to occur on dead wood and appears to be most common in southern parts of the Panhandle of Southeast Alaska.
Laetiporus conifericola Burds. & Banik
In Alaska, Laetiporus conifericola causes brown cubical rot of conifers, primarily spruce and hemlock in coastal Southeast and Southcentral Alaska. Five closely related species have been identified in North America (Linder and Banik 2008, https://doi.org/10.3852/07-124R2). Eleven observations were recorded from Prince of Wales, Ketchikan, and Juneau, while 62 research grade observations were recorded in iNaturalist spanning coastal Alaska from Ketchikan to Kodiak Island, including Middleton Island in the Gulf of Alaska. The brightly colored, ephemeral fruiting structures, popular among fungal foragers, were exceptionally abundant in coastal Alaska last year (2021), spanning coastal Alaska from Ketchikan to Haines and Gustavus in Southeast, the Chugach National Forest and Chugach State Park from Cordova to Seward and around to Kachemak Bay on the Kenai Peninsula, and Kodiak Island. The iNaturalist application can be particularly helpful in cataloging the occurrence of popular, easily identified fungi, like the sulfur fungus, with ephemeral fruiting structures.
Laricifomes officinalis (Batsch) Kotl. & Pouzar. (=Fomitopsis officinalis)
Phaeolus schweinitzii (Fr.:Fr.) Pat.
Phaeolus schweinitzii is most common in Southeast Alaska on Sitka spruce of the coastal forest but has also been recorded on shore pine and white spruce. In 2022, it was recorded by FHP on Sitka spruce at seven locations near Juneau. Fourteen research grade observations were contributed through iNaturalist in Southeast near Wrangell, Sitka, Juneau, and Gustavus, and in Southcentral Alaska near Girdwood, Primrose, Seward, and Kodiak Island. The fruiting bodies are most noticeable when they emerge from the decayed wood of broken tree boles or from below ground roots in late summer and fall. Root and lower bole damage can promote infection, an important management consideration at developed recreation sites. The brown cubical rot symptom of P. schweinitzii may easily be mistaken for that caused by the much more common Fomitopsis pinicola senus lato. Advanced decay can emit a pleasant sweet-spicy licorice scent!
Phellinus hartigii (Allesch. & Schnabl) Pat.
We recorded Phellinus hartigii on western hemlock at two locations near Juneau in 2022. This fungus can invade through stem wounds, including bole swellings caused by hemlock dwarf mistletoe. Although infrequently encountered in Southeast Alaska, we have repeatedly noticed mortality of infected trees within a decade of initial detection due to disease activity in the sapwood girdling the stem.
Porodaedalea pini (Brot.) Murrill (=Phellinus pini)
Porodaedalea pini was recorded by FHP staff at 15 sites near Juneau and on Prince of Wales Island in Southeast Alaska and at one site in Southcentral Alaska on the Kenai Peninsula near Kenai Lake. Two observations were made on Sitka spruce and the rest on western hemlock. Additionally, thirteen research grade observations were recorded in iNaturalist, with half collected along the length of the Kenai Peninsula and the rest from Southeast Alaska around Skagway, Haines, Juneau, and Kruzof Island near Sitka. Although more common in coastal forests, P. pini can also be found in Interior Alaska. Multiple fruiting bodies along the length of the tree bole indicate extensive internal decay. Although primarily considered a heart rot, P. pini can progress into sapwood and kill trees. Fruiting bodies often occur near branch stubs on live trees and are an indicator of heart rot.
Conifer Stem Decay Detection Maps
Farr, W. A.; LaBau, V. J.; Larent, T.L. 1976. Estimation of decay in old-growth western hemlock and Sitka spruce in southeast Alaska. Research Paper PNW-204. Portland, OR: U.S. Department of Agriculture, Forest Service. 24 p.
Filip, G.; Chadwick, K.; Zambino, P.; and others. 2011. Seven- to 12-year effects of artificially inoculating living conifers to promote stem decay and subsequent wildlife use in Oregon and Washington forests. Portland, OR: USDA Forest Service, Forest Health Protection.
Hennon, P. E. 1990. Wounding on residual Sitka spruce and western hemlock remaining after thinning on Prince of Wales Island, Alaska. USDA Forest Service, State and Private Forestry, Juneau, AK. Forest Pest Management Report R10 90 2. 9p.
Hennon, P. E. 1995. Are heart rot fungi major factors of disturbance in gap-dynamic forests? Northwest Science. 69: 284-293. Available here.
Hennon, P.E.; McClellan, M. H. 2003. Tree mortality and forest structure in temperate rainforests of southeast Alaska. Canadian Journal of Forest Research 33: 1621-1634.
Hennon, P. E.; Mulvey, R. L. 2014. Managing heart rot in live trees for wildlife habitat in young-growth forests of coastal Alaska. Gen. Tech. Rep. PNW-GTR-890. Portland, OR: U.S. Department of Agriculture, Forest Service, Pacific Northwest Research Station. 23 p. Available here.
Kimmey, J. W. 1956. Cull factors for Sitka spruce, western hemlock, and redcedar in southeast Alaska. USDA Forest Service. Alaska Forest Research Center, Juneau, Alaska. Station Paper No. 6. 31p.
Kimmey, J. W. 1964. Heart Rots of Western Hemlock. USDA Forest Pest Leaflet 90. Available here.
Wright, E.; Isaac, L. A. 1956. Decay following logging injury to western hemlock, Sitka spruce, and true firs. USDA Tech. Bull. No. 1148. 34p.
Pocket guide for the identification of common forest diseases and insects in Alaska.
Content prepared by Robin Mulvey, Forest Pathologist, Forest Health Protection, firstname.lastname@example.org