Decay Fungi of Riparian Trees in the Southwestern U.S
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WESTERN A rborist Decay fungi of riparian trees in the Southwestern U.S. Jessie A. Glaeser and Kevin T. Smith Introduction: Most of the tree species cialist needs a working knowledge of a brown residue, composed largely that characterize riparian woodlands the fungi associated with hardwood of lignin, which becomes part of soil are early or facultative seral species decay. We present here some of the humus and resists further degrada- including Fremont cottonwood (Popu- common fungi responsible for decay tion. This brown-rot residue is an lus fremontii), Arizona alder (Alnus of riparian species of the Southwest. important component of the carbon oblongifolia), Arizona sycamore (Plata- Many of these fungi are nonspecial- sequestered in forest soil. White-rot nus wrightii), Modesto ash (Fraxinus ized and will be encountered fre- fungi frequently decay hardwoods, velutina), boxelder (Acer negundo), and quently throughout North America. and brown-rot fungi often colonize narrowleaf poplar (Populus angustifo- Wood decay fungi can be grouped conifers, but many exceptions occur. lia). Arizona walnut (Juglans major) is in different ways. Academic my- The decayed wood within the tree a riparian species that can persist at cologists use evolutionary or genetic can take different forms described by a low density in late seral or climax relationships, largely discerned from its appearance and texture, including forests. DNA sequence analyses, to group “stringy rot,” “spongy rot,” “pocket The Southwest is a harsh environ- fungi. In recent years, improved rot,” “cubical rot,” and “laminated ment for trees. The frequent occur- analytical techniques have greatly in- rot.” Each of these decay types has rence of early-seral tree species in creased our understanding of fungal different physical properties that af- riparian forests reflect the frequency, evolution and upended many tradi- fect the amount of strength remaining severity, and extent of disturbance tional groupings of fungi that shared in the wood. In brown-rot decay, large events. Disturbance from fire, sea- morphological similarities. Ecological amounts of strength loss occur early sonal flooding, and landslides all groupings based on observations in the decay process due to the rapid provide special opportunity for injury and infection by wood decay fungi. Even riparian species can un- dergo periodic drought conditions …the hazard tree specialist needs as water levels drop during the heat a working knowledge of the fungi of summer. Human activity can lead to soil compaction and root damage. associated with hardwood decay. Stressed trees are more susceptible to agents of mortality, including “op- portunistic pathogens” that are only of fungal habitat, spatial position, depolymerization of cellulose (Cowl- able to cause disease in weakened and the appearance of the decayed ing 1961). In white rot, wood strength trees. Infections in a young tree can wood (Tainter and Baker 1996), are declines more gradually with time. produce a cascade of processes that also relevant to the hazard tree spe- An eco-nutritional approach result in long-standing decay and cialist. White-rot fungi degrade the groups some wood decay fungi as cavities in large, mature individuals. lignin, cellulose and hemicellulose saprotrophs that attack wood in ser- As the tree loses structural support to of wood, leaving behind a white or vice or as felled logs, slash, or snags decay, it becomes a potential hazard. off-white residue. Some white-rot (Toupin et al. 2008) or as pathogens The degree of hazard depends on the fungi produce many small pockets that decay wood in living trees. physical condition of the tree, includ- of decay throughout the infected Pathogenic wood decay fungi can be ing the presence of cracks and weak volume of wood, a condition known further subdivided based on the type branch attachments, as well as the size as “pocket rot”. Brown-rot fungi, on of wood degraded and the position and position of decayed wood and the other hand, degrade the cellulose of the fungus within the living tree. cavities. Identification of the fungi and hemicellulose in the wood cell Heartrot fungi can decay heartwood responsible for the decay improves wall, but do not significantly degrade in living trees despite the tree’s abil- prediction of tree performance and the lignin. Brown-rotted wood in ad- ity to produce protective chemicals the quality of management decisions, vanced decay is often seen as more and low oxygen conditions in the including tree pruning or removal. or less cubical fragments. Eventu- central cylinder (Highley and Kirk Consequently, the hazard tree spe- ally, brown-rotted wood becomes 1979). When sapwood is exposed by Fall 2013 40 WESTERN Arborist tree species in this group do not have Glossary: Definitions of mycological terms a strong compartmentalization re- (Gilberston and Ryvarden, 1987) sponse to resist the spread of infection after the wounding of live sapwood. Annulus – a ring found on the stipe of certain mushrooms. Dead wood, including the dead wood Applanate– thin, flattened horizontally. Usually used to describe ses- attached to living trees, is generally of sile fruiting bodies or the pileate portion of effused-reflexed fruiting low durability and therefore decays bodies. readily. The inability of these species Dimidiate – semi-circular in outline when viewed from above. to compartmentalize and limit the development of further decay makes Effused – flat (resupinate) with no pileus or shelf. Adheres entirely them particularly hazardous in highly to the substrate. trafficked areas. Effused –reflexed – a fruiting body that is partially resupinate and The types of decay, morphologi- partially shelving into the pileus. cal features of the fruiting body, and Mycelium – the vegetative stage of the fungus, usually observable as a short discussion about the impact a mass of individual threads, termed “hyphae.” on hazard tree analysis are presented for some of the major decay fungi Ochraceous – a yellowish buff color. associated with riparian trees in the Pileus – the portion of a fruiting body with a sterile upper surface Southwest. A glossary of mycological and a fertile lower surface. terms is included. Resupinate – flat Armillaria mellea Rhizomorph – a macroscopic strand, often resistant to drying, that spreads throughout the soil. Black in Armillaria species. Although the traditional concept of A. mellea has been split into 10 Sessile – without a stipe [or stalk] biological species in North America, Stipe – stalk-like or stem-like structure that supports the pileus. the name is still valid for the root-rot Stipitate – with a stipe. The stipe can be central or attached later- fungus present on hardwoods and ally (eccentric). some conifers in California and the Southwest. Fruiting bodies are gilled Ungulate – hoof-shaped. mushrooms, produced in clusters usually of 8 – 10 but sometimes 30 or more. The cap (pileus) is honey-col- mechanical injury, many saprot fungi of sapwoodd by a succession of fungi. ored, 3-13 cm wide, generally with a can act as primary pathogens and The spread of those fungi throughout smooth surface, but which may show directly kill living cells in advance of the tree is resisted through the bound- a few sparse hairs or scales on the up- infection. These include canker rot aries and barriers of compartmental- per surface. Gills are attached to the and many root rot fungi (Shortle et al. ization (Smith 2006). As the vascular stalk (stipe), which tapers at the base 1996). Fruiting bodies of saprot fungi cambium continues to produce new and usually has a persistent ring, or around the outer circumference of the xylem to the outside of existing wood, “annulus,” at the upper portion. Black stem can indicate structural weak- and healthy sapwood continues to be rhizomorphs form on the surface of ness and increased risk for climbing converted into heartwood, the infec- colonized roots and under the bark arborists. tion becomes more or less centered in of infected trees (Burdsall and Volk Many decay fungi can be catego- the middle of the tree. The early-seral 1993). Mycelial fans may also form rized using the above static criteria. However, the great advance in un- Armillaria mellea cluster Armillaria mellea gills derstanding of the biology of wood decay in living trees involves the compartmentalization process (Shigo 1984), a foundation concept in forest pathology (Manion 2003). Prior to the description of the compartmentaliza- tion process, heartrot was thought to form by the direct infection of dead heartwood exposed by injury. In the compartmentalization concept, heart- rot generally begins with the infection 41 Fall 2013 WESTERN A rborist (M. Fairweather, personal communi- cation). It is usually associated with conifers but will also colonize hard- woods and causes a serious root- and butt-rot. In some areas it seems to be nonpathogenic (Klopfenstein et al. 2008) but may become an opportu- nistic pathogen on trees stressed by drought, insect attack or other factors. Armillaria solidipes can be differenti- ated from other species of Armillaria Armillaria nabsnona trudell by its brown cap and stipe, the fairly Coniophora puteana prominent scales on the cap, and the beneath the bark of diseased roots and well-developed ring (annulus). Mush- trembling aspen (Populus tremuloides), the root crown. Armillaria mellea may rooms form in clusters at the base of Gambel oak (Quercus gambelii), as well be the most virulent species of Armil- affected trees and on the dead wood as southwestern white pine (Pinus laria in California, causing a serious of fire scars and other wounds. The reflexa) and Douglas-fir (Pseudotsuga whiterot of roots, especially in over- wood becomes yellow to brown in menziesii) (Gilbertson et al. 1974). watered urban trees (Baumgartner color and advanced decay appears and Rizzo 2001 a & b). Infected wood water-soaked and stringy. Coniophora species initially appears water-soaked, then Fruiting bodies are thin, brown, and becomes light colored and spongy Bjerkandera adusta resupinate (flattened) with an ir- with more advanced decay (Swiecki “Smoky Polypore.” Fruiting body ef- regular, wrinkled surface.