Wood Decomposers Research Articles

The wood-decaying fungus Hygrophoropsis aurantiaca increases P availability in acid forest humus soil, while N addition hampers this effect

We evaluated the influence of the brown rot fungus Hygrophoropsis aurantiaca on P solubility in the humus layer of a podzolic forest soil. This fungus is known to exude large amounts of oxalic acid that may stimulate weathering of minerals and increase dissolution of humus, which in turn may increase P availability in the soil surrounding the fungus. Humus was inoculated using small wooden pieces colonised by the fungus. The presence of the fungus resulted in elevated concentration of PO4− in the humus solution. In a second experiment birch seedlings grown in the same humus were able to utilise the PO4− mobilised by the fungus to increase their internal P content. The factor determining this increased P uptake and the increased available P might be oxalate produced by fungus. The acid may directly dissolve P or change organic forms of P making it more susceptible to reaction with phosphatases. This fungal effect on P solubility diminished when N was added to the soil in the form of a slow release N fertilizer (methyl urea), or when a soil with a higher soil N concentration was used. We found a strong correlation between NH4+ concentration and total organic carbon in the soil solution at high NH4+ concentrations, suggesting the dissolution of humus as a result of the high NH4+ content in the solution. This study demonstrates that the wood-decaying fungus H. aurantiaca influences nutrient turnover in forest soil, and thereby nutrient uptake by forest trees. An intensified harvest of forest products such as whole tree harvesting may decrease the active biomass of the wood decomposers and may thereby change the availability of P and the leaching of N.

Effects of resource availability on mycelial interactions and 32P transfer between a saprotrophic and an ectomycorrhizal fungus in soil microcosms

Interactions between mycelia of the ectomycorrhizal fungus Suillus variegatus and the wood decomposing fungus Hypholoma fasciculare were studied in soil microcosms. The ectomycorrhizal mycelium extended from Pinus sylvestris seedlings and the saprotrophic fungus grew out into the soil from birch wood blocks of two alternative sizes. Transfer of 32P between the interacting mycelia was measured non-destructively using electronic autoradiography. The outcome of the interactions was clearly affected by the size of the wood blocks. In systems with large wood blocks (1.6 cm3) the wood decomposer fungus overgrew the mycelium of the mycorrhizal fungus whereas in systems with small wood blocks (0.44 cm3) the mycorrhizal mycelium overgrew the wood decomposer fungus. The saprotrophic fungus was able to capture significantly more 32P from labelled mycorrhizal mycelium when growing out from larger wood blocks. The mycorrhizal fungus was able to capture significantly more 32P from labelled saprotrophic mycelium when the wood blocks were smaller.

Effects of resource availability on mycelial interactions and 32P transfer between a saprotrophic and an ectomycorrhizal fungus in soil microcosms

Interactions between mycelia of the ectomycorrhizal fungus Suillus variegatus and the wood decomposing fungus Hypholoma fasciculare were studied in soil microcosms. The ectomycorrhizal mycelium extended from Pinus sylvestris seedlings and the saprotrophic fungus grew out into the soil from birch wood blocks of two alternative sizes. Transfer of 32P between the interacting mycelia was measured non-destructively using electronic autoradiography. The outcome of the interactions was clearly affected by the size of the wood blocks. In systems with large wood blocks (1.6 cm 3) the wood decomposer fungus overgrew the mycelium of the mycorrhizal fungus whereas in systems with small wood blocks (0.44 cm 3) the mycorrhizal mycelium overgrew the wood decomposer fungus. The saprotrophic fungus was able to capture significantly more 32P from labelled mycorrhizal mycelium when growing out from larger wood blocks. The mycorrhizal fungus was able to capture significantly more 32P from labelled saprotrophic mycelium when the wood blocks were smaller.

Mycelial dynamics during interactions between Stropharia caerulea and other cord-forming, saprotrophic basidiomycetes.

• Macroscopic mycelial interactions between Stropharia caerulea and four other cord-forming wood decomposer basidiomycetes (Phanerochaete velutina, Phallus impudicus, Hypholoma fasciculare and Agrocybe gibberosa) on nonsterile soil were quantified using nondestructive image analysis. • Interaction development and outcome was species dependent. Once S. caerulea mycelium became fully confronted by nonself mycelia, extension usually ceased and mycelia formed (defensive) aerial ridges and/or (invasive) cords. P. velutina rapidly replaced S. caerulea, but with significantly (P<0.05) reduced biomass and mass fractal dimension (DBM ). S. caerulea regressed, biomass, DBM and surface fractal dimension (DBS ) became significantly (P<0.05) reduced. • S. caerulea produced temporary defensive ridges against P. impudicus, the latter extending through and replacing S. caerulea mycelium, but with significantly (P≤0.05) reduced biomass. S. caerulea and H. fasciculare deadlocked, the latter producing dense (high DBM ) noninvasive lateral mycelial fans resulting in persistent mycelial fronts. S. caerulea and A. gibberosa initially deadlocked, the latter completely encircling S. caerulea. In response, S. caerulea produced defensive mycelial ridges at the interaction margin, and fans extended over A. gibberosa. • These results are discussed in relation to mycelial foraging strategies, mycelial morphogenesis and determinants of interaction outcome.

Degradation of lipophilic wood extractive constituents in Pinus sylvestris by the white-rot fungi Bjerkandera sp. and Trametes versicolor

The white-rot fungi Trametes versicolor and Bjerkandera spp. are among the most frequent decomposers of angiosperm wood in forest ecosystems and in wood products in service. Wood extractives have a major impact on wood properties and wood utilization. This work evaluated the ability of two white-rot fungal strains (Bjerkandera sp. strain BOS55 and T. versicolor strain LaVec94-6) to degrade the main lipophilic extractive constituents in Scots pine (Pinus sylvestris L.). The time course of wood decay and wood extractive degradation was monitored in stationary batch assays incubated for eight weeks. The strains tested eliminated high levels of total resin, 34 to 51% in two weeks. Wood triglycerides were the most readily degraded extractive components (over 93% elimination in only two weeks). Free fatty acids and resin acids, which are potential fungal inhibitors, were also rapidly decomposed by the fungal strains. Sterols were used more slowly, nonetheless, the fungal degradation of this extractive fraction ranged from 50 to 88% after four weeks.

A review of selected methods for the detection of degradative fungi

The primary decomposers of wood in non-aquatic environments are the brown and white rot fungi. These are basidiomycetous fungi which possess the appropriate enzymatic and non-enzymatic systems to allow them to degrade the wood lignocellulose. Many current methods for the detection of these organisms within the wood are unreliable or necessitate extensive destructive sampling. Our laboratory has focused on serological and molecular assays for the detection of degradative fungi. Polyclonal and monoclonal antibodies to specific fungal metabolites have been used with TEM immunolabelling and ELISA for the detection of decay agents. More recently we have developed a polymerase chain reaction assay which can allow us to reliably detect basidiomycete DNA in small wood samples. When used to examine fungi in wood, these primers are specific for basidiomycetes and do not react with non-degradative wood-inhabiting ascomycetes. Fungal colonization of wood can be detected early in the degradation process. Restriction digests can be used to confirm the identity of the amplified DNA. These assays are of potential utility for the detection and identification of wood degrading fungi.

Factors related to diversity of decomposer fungi in tropical forests

Recent studies suggest that host-preferences are common among certain groups of tropical fungal decomposers but rare in others, and sometimes occur where we least expect them. Host preferences among microfungi and ascomycetes that decompose leaf litter are common but usually involve differences in relative frequencies more than presence/absence, so their diversity may be loosely correlated with species richness of host trees. Strong host-specificity appears to be rare among wood decomposer fungi, whereas characteristics of their substrata and habitat are very important for this group. Anthropogenic disturbance predisposed a tropical forest to subsequent hurricane damage, and the resulting direct and indirect effects on host diversity and habitat heterogeneity were reflected in the decomposer fungal community more than sixty years after the original disturbance. While species richness of dictyostelid slime molds and functional diversity of their bacterial prey increased with disturbance, the more diverse microfungi and ascomycetes were apparently negatively affected by disturbance.

A new species of Hypholoma (Agaricales) from Western Australia

Hypholoma australe sp. nov., a primary wood decomposer, is described and illustrated from Western Australia. It appears to be closely related to Hypholoma sublateritium (Fr.) Quél, which is widespread in the Northern Hemisphere. Self crosses of both species reveal a bipolar mating system. Mating tests carried out between single-spore isolates from populations from Western Australia and North America were completely intersterile.

Fungal communities in wet tropical forests: variation in time and space

Understanding variation in tropical forest fungal populations and communities is important for assessing fungal biodiversity, as well as for understanding the regulatory roles fungi play in tropical forests. In wet tropical forests, the canopy is typically occupied by certain wood decomposers, endophytes, epiphylls, and pathogens. Aphyllophoraceous canopy fungi are a subset of species found in the understory. Marasmioid agarics in the understory often form extensive networks of rhizomorphs that trap litter; these and other aerial species are rare on the forest floor. Decomposers are stratified within the forest floor, with some species colonizing only fresh litter, others preferring decomposed litter, and others restricted to soil organic matter. Specificity to particular host substrates is frequent among tropical forest litter decomposers and contributes to spatial heterogeneity in fungal communities over the landscape. Litter basidiomycete and microfungal communities in patches of 1 m2 or less do not significantly resemble communities in similar patches located at distances greater than 100 m. Disturbances induce changes in the environment and the abundance of different substrates, resulting in changes in fungal communities through time, and variation over the landscape. Severe disturbances, as well as the slight daily variations in rainfall, profoundly affect populations of fungal decomposers and their influence on plant nutrient availability. Key words: fungi, tropical forests, diversity, stratification, spatial variation, temporal variation.

Saprotrophic cord-forming fungi: warfare strategies and other ecological aspects

A number of saprotrophic fungi, particularly wood-decaying basidiomycetes, from mycelial cords, which are aggregations of predominantly parallel, longitudinally aligned hyphae. These linear organs often form extensive, long-lived systems which connect between discrete nutrient resources on the floor of temperate and tropical forests, and can form nets in tropical canopies. They are a very successful group of decomposers which exhibit many strategies analogous to those adopted in warfare, including: securing territory by aggressive/combative action; economic and efficient discovery of potential resources; establishing lines of communication; economic and efficient deployment of resources; and adoption of different strategies according to the home economic situation. Thus, nutrients are sequestered by foraging mycelial fronts and translocated to different parts of the mycelial system. On encountering new resources mycelial biomass and nutrients may be reallocated depending on a number of factors including: the relative size of inoculum base and new resources, state of decay of resources, whether the new resource is already colonized, the relative positioning of one new resource compared with another, and the fungal species involved. Cord-forming saprotrophic fungi probably play a major role in ecosystems, since they are major wood decomposers and can translocate large quantities of essential nutrients over several metres. They may, however, immobilize nutrients for considerable periods, perhaps months or longer, but by so doing may act as a buffer against nutrient loss from the system. The ease with which they can be inoculated into the forest floor could possibly be used as a management tool by virtue of their antagonism to tree root pathogens and their abilities as agents of nutrient cycling and translocation.

Wood Degrading Ability of Basidiomycetes That Are Wood Decomposers, Litter Decomposers, or Mycorrhizal Symbionts

The wood degrading ability of basidiomycetes, 41 wood decomposers, 22 litter decomposers, and 5 mycorrhizal symbionts, was measured in aseptic culture on an agar medium. The wood decomposers had greater variation in this ability than the litter decomposers. Ratios of percent lignin loss to percent weight loss of the wood specimens (the lignin loss ratio) were somewhat higher in the white-rot fungi than in the brown-rot fungi. The species collected from the L and F layers of litter deposits, such as Collybia confluens, Collybia dryophila, and Hygrocybe sp., and Marasmius maximus, decomposed wood more than the species from the H layer. These species also produced lignin loss ratios two to three times that of the white-rot species of wood decomposers. Ten litter decomposers bleached wood specimens to yellowish white, as typical white-rot species do. None of the mycorrhizal species degraded wood during 80 days of culture. The wood and litter decomposers showed highly significant variation among species within a genus for wood degrading ability.

Mercury and methyl mercury in higher fungi and their relation with the substrata in a cinnabar mining area

195 specimens of higher fungi and their substrata collected in the Hg mining area of M.Amiata and around Siena (Central Italy), were analyzed for their total Hg content. Whereas wood decomposers and many species of mycorrhizal fungi accumulated the metal at a very low rate, some mycorrhizal species and all the humus decomposers may accumulate up to 100 μg.g−1d.w. of Hg and in the least contaminated sites, up to 63 times as much Hg as the substratum. Instead in mineralized areas the concentration factor rarely exceeded 1. The methyl mercury content of 35 species (almost all edible), ranged between 0.01 and 3.7 μg.g−1 d.w..The possible significance of these findings and the toxicologic implications for local con sumers are considered.

The role of termites in the decomposition of wood and leaf litter in the Southern Guinea savanna of Nigeria.

This is part of a study on the litter dynamics of Southern Guinea savanna, with special reference to the role of termites (Isoptera) as decomposers of wood and leaves.The biomass of accumulated wood litter on the 6 ha study area varied from 2.299 to 3.488 t ha-1, with a mean of 2.821 t ha-1 Leaf litter biomass varied from 0.290 to 1.643 t ha-1, with a mean of 0.903 t ha-1. The measured decomposition constant for wood of 51% a-1 agreed well with a calculated value of 49% a-1. Leaf decomposition was calculated as 264% a-1. Termites removed an estimated 835.5 kg ha-1a-1 of wood litter (60% of annual wood-fall), and 68.4 kg ha-1a-1 of leaf litter (3% of annual leaf-fall), i.e. 24% of total annual litter production. Conversely, the annual bush-fire removed 1.173 t ha-1a-1 (49%) of annual leaf-fall but only 3 kg ha-1a-1 (0.2%) of annual wood-fall, i.e. 31% of total annual litter production. Baiting studies showed that the fungus-growing Macrotermitinae took 95% of the litter removed by termites (23% of annual litter production).The ability of the Macrotermitinae to utilise fresh litter is a diagnostic feature of decomposition processes in Southern Guinea and other savanna ecosystems. Their fungus combs produce a processed diet which allows the Macrotermitinae to feed in a seasonally arid environment where the curtailment of free fungal and microbial activity inhibits feeding by other decomposer invertebrates.