Tiina Rajala

Fungal community dynamics in relation to substrate quality of decaying Norway spruce (Picea abies [L.] Karst.) logs in boreal forests

Decaying wood plays an important role in forest biodiversity, nutrient cycling and carbon balance. Community structure of wood-inhabiting fungi changes with mass loss of wood, but the relationship between substrate quality and decomposers is poorly understood. This limits the extent to which these ecosystem services can be effectively managed. We studied the fungal community and physico-chemical quality (stage of decay, dimensions, density, moisture, C : N ratio, lignin and water or ethanol extractives) of 543 Norway spruce logs in five unmanaged boreal forest sites of southern Finland. Fungi were identified using denaturing gradient gel electrophoresis and sequencing of DNA extracted directly from wood samples. Macroscopic fruiting bodies were also recorded. Results showed a fungal community succession with decreasing wood density and C : N ratio, and increasing moisture and lignin content. Fungal diversity peaked in the most decayed substrates. Ascomycetes typically colonized recently fallen wood. Brown-rot fungi preferred the intermediate decay stages. White-rot fungi represented approximately one-fifth of sequenced species in all decay phases excluding the final phase, where ectomycorrhizal (ECM) fungi became dominant. Lignin content of logs with white-rot fungi was low, and ECM fungi were associated with substrates containing abundant nitrogen. Macroscopic fruiting bodies were observed for only a small number of species detected with molecular techniques.

Genetic host-tree effects on the ectomycorrhizal community and root characteristics of Norway spruce

A greenhouse experiment was used to study the effects of host genotype on short root formation and ectomycorrhizal (ECM) fungal community structure in Norway spruce (Picea abies (L.) Karst.). Rooted cuttings representing 55 clones were inoculated with a mix of vegetative hyphae of five ECM fungal species (Laccaria sp., Amphinema byssoides, Piloderma sp., Cadophora finlandia, Paxillus involutus). After one growing season, the ECM fungal community structure was determined by amplifying the fungal internal transcribed spacer (ITS) of ribosomal DNA directly from ECM root tips. Restriction profiles of obtained amplicons were then compared to those of the inoculated strains. Spruce clones differed in their ECM fungal community composition; we found a statistically significant clone-specific effect on ECM fungal diversity and dominating fungal species. Nevertheless, the broad sense heritabilities of the levels of Laccaria sp., Piloderma sp. and A. byssoides colonisations as well as the ECM fungal community structure were low (H2 = 0.04−0.11), owing to the high within-clone variation. As nitrogen concentration of needles correlated negatively with ECM fungal richness, our results imply that in the experimental conditions nutrient acquisition of young trees may benefit from colonisation with only one or two ECM fungal species. The heritability of short root density was moderate (H2 = 0.41) and highest among all the measured shoot and root growth characteristics of Norway spruce cuttings. We suggest that the genetic component determining root growth and short root formation is significant for the performance of young trees in natural environments as these traits drive the formation of the below-ground symbiotic interactions.

Endophyte communities vary in the needles of Norway spruce clones

Endophytic fungi show no symptoms of their presence but can influence the performance and vitality of host trees. The potential use of endophytes to indicate vitality has been previously realized, but a standard protocol has yet to be developed due to an incomplete understanding of the factors that regulate endophyte communities. Using a culture-free molecular approach, we examined the extent to which host genotype influences the abundance, species richness, and community composition of endophytic fungi in Norway spruce needles. Briefly, total DNA was extracted from the surface-sterilized needles of 30 clones grown in a nursery field and the copy number of the fungal internal transcribed spacer (ITS) region of ribosomal DNA was estimated by quantitative PCR. Fungal species richness and community composition were determined by denaturing gradient gel electrophoresis and DNA sequencing. We found that community structure and ITS copy number varied among spruce clones, whereas species richness did not. Host traits interacting with endophyte communities included needle surface area and the location of cuttings in the experimental area. Although Lophodermium piceae is considered the dominant needle endophyte of Norway spruce, we detected this species in only 33% of samples. The most frequently observed fungus (66%) was the potentially pathogenic Phoma herbarum. Interestingly, ITS copy number of endophytic fungi correlated negatively with the richness of ectomycorrhizal fungi and thus potential interactions between fungal communities and their influence on the host tree are discussed. Our results suggest that in addition to environmental factors, endophyte communities of spruce needles are determined by host tree identity and needle surface area.

Profiling functions of ectomycorrhizal diversity and root structuring in seedlings of Norway spruce (Picea abies) with fast‐ and slow‐growing phenotypes

We studied the role of taxonomical and functional ectomycorrhizal (ECM) fungal diversity in root formation and nutrient uptake by Norway spruce (Picea abies) seedlings with fast- and slow-growing phenotypes. Seedlings were grown with an increasing ECM fungal diversity gradient from one to four species and sampled before aboveground growth differences between the two phenotypes were apparent. ECM fungal colonization patterns were determined and functional diversity was assayed via measurements of potential enzyme activities of eight exoenzymes probably involved in nutrient mobilization. Phenotypes did not vary in their receptiveness to different ECM fungal species. However, seedlings of slow-growing phenotypes had higher fine-root density and thus more condensed root systems than fast-growing seedlings, but the potential enzyme activities of ectomycorrhizas did not differ qualitatively or quantitatively. ECM species richness increased host nutrient acquisition potential by diversifying the exoenzyme palette. Needle nitrogen content correlated positively with high chitinase activity of ectomycorrhizas. Rather than fast- and slow-growing phenotypes exhibiting differing receptiveness to ECM fungi, our results suggest that distinctions in fine-root structuring and in the belowground growth strategy already apparent at early stages of seedling development may explain later growth differences between fast- and slow-growing families.

Loss of diversity in wood-inhabiting fungal communities affects decomposition activity in Norway spruce wood.

Hundreds of wood-inhabiting fungal species are now threatened, principally due to a lack of dead wood in intensively managed forests, but the consequences of reduced fungal diversity on ecosystem functioning are not known. Several experiments have shown that primary productivity is negatively affected by a loss of species, but the effects of microbial diversity on decomposition are less studied. We studied the relationship between fungal diversity and the in vitro decomposition rate of slightly, moderately and heavily decayed Picea abies wood with indigenous fungal communities that were diluted to examine the influence of diversity. Respiration rate, wood-degrading hydrolytic enzymes and fungal community structure were assessed during a 16-week incubation. The number of observed OTUs in DGGE was used as a measure of fungal diversity. Respiration rate increased between early- and late-decay stages. Reduced fungal diversity was associated with lower respiration rates during intermediate stages of decay, but no effects were detected at later stages. The activity of hydrolytic enzymes varied among decay stages and fungal dilutions. Our results suggest that functioning of highly diverse communities of the late-decay stage were more resistant to the loss of diversity than less diverse communities of early decomposers. This indicates the accumulation of functional redundancy during the succession of the fungal community in decomposing substrates.

Interactions between soil- and dead wood-inhabiting fungal communities during the decay of Norway spruce logs

We investigated the interaction between fungal communities of soil and dead wood substrates. For this, we applied molecular species identification and stable isotope tracking to both soil and decaying wood in an unmanaged boreal Norway spruce-dominated stand. Altogether, we recorded 1990 operational taxonomic units, out of which more than 600 were shared by both substrates and 589 were found to exclusively inhabit wood. On average the soil was more species-rich than the decaying wood, but the species richness in dead wood increased monotonically along the decay gradient, reaching the same species richness and community composition as soil in the late stages. Decaying logs at all decay stages locally influenced the fungal communities from soil, some fungal species occurring in soil only under decaying wood. Stable isotope analyses suggest that mycorrhizal species colonising dead wood in the late decay stages actively transfer nitrogen and carbon between soil and host plants. Most importantly, Piloderma sphaerosporum and Tylospora sp. mycorrhizal species were highly abundant in decayed wood. Soil- and wood-inhabiting fungal communities interact at all decay phases of wood that has important implications in fungal community dynamics and thus nutrient transportation.

Accumulation rates and sources of external nitrogen in decaying wood in a Norway spruce dominated forest

Summary 1.Microbial respiration in dead wood contributes substantially to the long-lived forest carbon (C) pool and has a significant role in the forest nitrogen (N) cycle. Wood N content has been found to increase during the decay process; however, temporal dynamics and the sources of this external N remain unclear. 2.To examine N dynamics at various stages of decomposition, we combined high variety of analytical methods on Norway spruce logs, including wood δ15N, N%, 14C-dating, fungal composition and N2 fixation rate. For N2 fixation rate we also determined its dependency on ambient temperature and decay class, when estimating annual N2 fixation rates for our study site. 3.N2 fixation was observed to have a major role in increasing wood N content during decay. For the most decayed wood it accounted for 60% of the total N accumulation. Compared to other reports, where the annual temperature was similar to our site, the calculated annual fixation rate of 85 g N/ha/a is a low estimate. However, previous studies have not taken appropriately into account the dependency of N2 fixation rate on ambient temperature and decay class. Our δ15N model describing the sources of external N, statistical analysis and the fungal DNA composition of decayed wood suggest that other sources of external N accumulating in wood were soil foraging wood-decay fungi and mycorrhizal fungi. 4.Our study improves knowledge of the temporal dynamics of N accumulation in wood with advancing wood decay, the potential sources of external N and their relative significance. All of these factors are important for nitrogen as well as carbon models dealing with ecosystem responses to climate change. This article is protected by copyright. All rights reserved.

The community of needle endophytes reflects the current physiological state of Norway spruce

This study investigated fungal endophytes in the needles of Norway spruce (Picea abies) cuttings in relation to host tree growth. We also determined the prevalence of endophytes in needles incubated for six months. The cuttings originated from clonal origins showing slow- and fast-growth in long-term field trials but the heritable differences in growth rate were not yet detected among the studied cutting. Endophytes were isolated from surface-sterilized needles with culture-free DNA techniques. No significant differences were observed between endophyte communities of slow- and fast-growing clonal origins. However, the endophyte community correlated with the current growth rate of cuttings suggesting that endophytes reflect short- rather than long-term performance of a host. The concentration of condensed tannins was similar in slow- and fast-growing clonal origins but it showed a negative relationship with endophyte species richness, implying that these secondary compounds may play an important role in spruce tolerance against fungal infections. More than a third of endophyte species were detected in both fresh and decomposing needles, indicating that many needle endophytes are facultative saprotrophs. Several potentially pathogenic fungal species were also found within the community of saprotrophic endophytes.

Occurrence of similar mycoviruses in pathogenic, saprotrophic and mycorrhizal fungi inhabiting the same forest stand

Fungal viruses (mycoviruses) are considered to be highly host specific, but phylogenetic analysis supports the occasional occurrence of horizontal transmission between species. We used an extensive sampling strategy to investigate whether similar viruses occur in more than one fungal species of the same forest habitat. Mycelial samples were collected from in-growth mesh bags (N = 259), fruiting bodies (N = 173) and cultured isolates (N = 68) at a forest site where the spatial distribution of viral infections in clonal individuals of the wood decay fungus Heterobasidion parviporum was mapped in detail earlier. The investigation revealed previously known Heterobasidion viruses in ∼2% of the single or pooled mycorrhizal samples from mesh bags, ∼3% of the fruiting body samples and none of the fungal cultures analyzed. Novel virus strains distinct from known Heterobasidion viruses were detected in cultures of ectomycorrhizal fungi (Lactarius tabidus, L. rufus) and saprotrophic fungi (Megacollybia platyphylla, Mucoraceae spp.). Overall, our results support the view that mycoviruses do not readily cross species borders. Regarding potential virocontrol applications, the introduction of Heterobasidion viruses into natural habitats is not expected to cause a major infection pressure towards the indigenous fungal community. However, the ecological consequences of the putative interspecies virus transmission events detected require further investigation.

Infection with foliar pathogenic fungi does not alter the receptivity of Norway spruce seedlings to ectomycorrhizal fungi

AimsWe studied whether the induction of defence against foliar pathogens affects the interaction of Norway spruce (Picea abies) with ectomycorrhizal fungi (EMF) and whether the response differs between seedlings originating from families showing variable growth performance in long-term trials.MethodsThe shoots were inoculated with Botrytis cinerea and Gibberella avenacea. The roots were simultaneously inoculated with sieved humus to provide the EMF inoculum. The severity of the pathogenic infection was based on the amount of damage and induced production of condensed tannins in the needles.ResultsEMF richness and colonisation were not affected by the pathogens and were also identical between the fast- and slow-growing seedlings. The fast-growing seedlings were more vulnerable to the pathogens; however, the constitutive level of condensed tannins in the needles did not correlate with their susceptibility to either the pathogenic or symbiotic fungi. G. avenacea induced a marginally greater production of condensed tannins in the slow-growing seedlings, which was linked to a slight reduction in EMF richness and less needle damage after wintering.ConclusionsOur results suggest that there are differences in resource allocation strategies between the fast- and slow-growing spruce families, which may indicate the presence of underlying host effects that regulate interactions with associated fungi.