Morphological traits predict host-tree specialization in wood-inhabiting fungal communities

Symbiotic status alters fungal eco-evolutionary offspring trajectories

Despite host-fungal symbiotic interactions being ubiquitous in all ecosystems, understanding how symbiosis has shaped the ecology and evolution of fungal spores that are involved in dispersal and colonization of their hosts has been ignored in life-history studies. We assembled a spore morphology database covering over 26,000 species of free-living to symbiotic fungi of plants, insects and humans and found more than eight orders of variation in spore size. Evolutionary transitions in symbiotic status correlated with shifts in spore size, but the strength of this effect varied widely among phyla. Symbiotic status explained more variation than climatic variables in the current distribution of spore sizes of plant-associated fungi at a global scale while the dispersal potential of their spores is more restricted compared to free-living fungi. Our work advances life-history theory by highlighting how the interaction between symbiosis and offspring morphology shapes the reproductive and dispersal strategies among living forms.

Analyzing trophic ecosystem functions with the interaction functional space

Quantifying the vulnerability of ecosystems to global change requires a better understanding of how trophic ecosystem functions emerge. So far, trophic ecosystem functions have been studied from the perspective of either functional diversity or network ecology. To integrate these two perspectives, we propose the interaction functional space (IFS) a conceptual framework to simultaneously analyze the effects of traits and interactions on trophic functions. We exemplify the added value of our framework for seed dispersal and wood decomposition and show how species interactions influence the relationship between functional trait diversity and trophic functions. We propose future applications for a range of functions where the IFS can help to elucidate mechanisms underpinning trophic functions and facilitate understanding of functional changes in ecosystems amidst global change.

Habitat and tree species identity shape aboveground and belowground fungal communities in central European forests

Introduction

Trees interact with fungi in mutualistic, saprotrophic, and pathogenic relationships. With their extensive aboveground and belowground structures, trees provide diverse habitats for fungi. Thus, tree species identity is an important driver of fungal community composition in forests.

Methods

Here we investigate how forest habitat (bark surface vs. soil) and tree species identity (deciduous vs. coniferous) affect fungal communities in two Central European forests. We assess differences and interactions between fungal communities associated with bark surfaces and soil, in forest plots dominated either by Fagus sylvatica, Picea abies, or Pinus sylvestris in two study regions in southwestern and northeastern Germany.

Results

ITS metabarcoding yielded 3,357 fungal amplicon sequence variants (ASVs) in the northern and 6,088 in the southern region. Overall, soil communities were 4.7 times more diverse than bark communities. Habitat type explained 48-69% of the variation in alpha diversity, while tree species identity explained >1-3%. NMDS ordinations showed that habitat type and host tree species structured the fungal communities. Overall, few fungal taxa were shared between habitats, or between tree species, but the shared taxa were highly abundant. Network analyses, based on co-occurrence patterns, indicate that aboveground and belowground communities form distinct subnetworks.

Discussion

Our study suggests that habitat (bark versus soil) and tree species identity are important factors structuring fungal communities in temperate European forests. The aboveground (bark-associated) fungal community is currently poorly known, including a high proportion of reads assigned to "unknown Ascomycota" or "unknown Dothideomycetes." The role of bark as a habitat and reservoir of unique fungal diversity in forests has been underestimated.

Fungal Community Development in Decomposing Fine Deadwood Is Largely Affected by Microclimate

Fine woody debris (FWD) represents the majority of the deadwood stock in managed forests and serves as an important biodiversity hotspot and refuge for many organisms, including deadwood fungi. Wood decomposition in forests, representing an important input of nutrients into forest soils, is mainly driven by fungal communities that undergo continuous changes during deadwood decomposition. However, while the assembly processes of fungal communities in long-lasting coarse woody debris have been repeatedly explored, similar information for the more ephemeral habitat of fine deadwood is missing. Here, we followed the fate of FWD of Fagus sylvatica and Abies alba in a Central European forest to describe the assembly and diversity patterns of fungal communities over 6 years. Importantly, the effect of microclimate on deadwood properties and fungal communities was addressed by comparing FWD decomposition in closed forests and under open canopies because the large surface-to-volume ratio of FWD makes it highly sensitive to temperature and moisture fluctuations. Indeed, fungal biomass increases and pH decreases were significantly higher in FWD under closed canopy in the initial stages of decomposition indicating higher fungal activity and hence decay processes. The assembly patterns of the fungal community were strongly affected by both tree species and microclimatic conditions. The communities in the open/closed canopies and in each tree species were different throughout the whole succession with only limited convergence in time in terms of both species and ecological guild composition. Decomposition under the open canopy was characterized by high sample-to-sample variability, showing the diversification of fungal resources. Tree species-specific fungi were detected among the abundant species mostly during the initial decomposition, whereas fungi associated with certain canopy cover treatments were present evenly during decomposition. The species diversity of forest stands and the variability in microclimatic conditions both promote the diversity of fine woody debris fungi in a forest.

Response of Fruit Body Assemblage Color Lightness to Macroclimate and Vegetation Cover

Understanding how species relate mechanistically to their environment via traits is a central goal in ecology. Many macroecological rules were found for macroorganisms, however, whether they can explain microorganismal macroecological patterns still requires investigation. Further, whether macroecological rules are also applicable in microclimates is largely unexplored. Here we use fruit body-forming fungi to understand both aspects better. A recent study showed first evidence for the thermal-melanism hypothesis (Bogert’s rule) in fruit body-forming fungi and relied on a continental spatial scale with large grid size. At large spatial extent and grid sizes, other factors like dispersal limitation or local microclimatic variability might influence observed patterns besides the rule of interest. Therefore, we test fungal assemblage fruit body color lightness along a local elevational gradient (mean annual temperature gradient of 7°C) while considering the vegetation cover as a proxy for local variability in microclimate. Using multivariate linear modeling, we found that fungal fruiting assemblages are significantly darker at lower mean annual temperatures supporting the thermal-melanism hypothesis. Further, we found a non-significant trend of assemblage color lightness with vegetation cover. Our results support Bogert’s rule for microorganisms with macroclimate, which was also found for macroorganisms.

Fungal fruit body assemblages are tougher in harsh microclimates

Forest species are affected by macroclimate, however, the microclimatic variability can be more extreme and change through climate change. Fungal fruiting community composition was affected by microclimatic differences. Here we ask whether differences in the fruiting community can be explained by morphological traits of the fruit body, which may help endure harsh conditions. We used a dead wood experiment and macrofungal fruit body size, color, and toughness. We exposed logs of two host tree species under closed and experimentally opened forest canopies in a random-block design for four years and identified all visible fruit bodies of two fungal lineages (Basidio- and Ascomycota). We found a consistently higher proportion of tough-fleshed species in harsher microclimates under open canopies. Although significant, responses of community fruit body size and color lightness were inconsistent across lineages. We suggest the toughness-protection hypothesis, stating that tough-fleshed fruit bodies protect from microclimatic extremes by reducing dehydration. Our study suggests that the predicted increase of microclimatic harshness with climate change will likely decrease the presence of soft-fleshed fruit bodies. Whether harsh microclimates also affect the mycelium of macrofungi with different fruit body morphology would complement our findings and increase predictability under climate change.

Wood-inhabiting fungal responses to forest naturalness vary among morpho-groups

The general negative impact of forestry on wood-inhabiting fungal diversity is well recognized, yet the effect of forest naturalness is poorly disentangled among different fungal groups inhabiting dead wood of different tree species. We studied the relationship between forest naturalness, log characteristics and diversity of different fungal morpho-groups inhabiting large decaying logs of similar quality in spruce dominated boreal forests. We sampled all non-lichenized fruitbodies from birch, spruce, pine and aspen in 12 semi-natural forest sites of varying level of naturalness. The overall fungal community composition was mostly determined by host tree species. However, when assessing the relevance of the environmental variables separately for each tree species, the most important variable varied, naturalness being the most important explanatory variable for fungi inhabiting pine and aspen. More strikingly, the overall species richness increased as the forest naturalness increased, both at the site and log levels. At the site scale, the pattern was mostly driven by the discoid and pyrenoid morpho-groups inhabiting pine, whereas at the log scale, it was driven by pileate and resupinate morpho-groups inhabiting spruce. Although our study demonstrates that formerly managed protected forests serve as effective conservation areas for most wood-inhabiting fungal groups, it also shows that conservation planning and management should account for group- or host tree -specific responses.

What can intraspecific trait variability tell us about fungal communities and adaptations?

Analyses of species functional traits are suitable to better understand the coexistence of species in a given environment. Trait information can be applied to investigate diversity patterns along environmental gradients and subsequently to predict and mitigate threats associated with climate change and land use. Species traits are used to calculate community trait means, which can be related to environmental gradients. However, while species traits can provide insights into the mechanisms underlying community assembly, they can lead to erroneous inferences if mean trait values are used. An alternative is to incorporate intraspecific trait variability (ITV) into calculating the community trait means. This approach gains increasing acceptance in plant studies. For macrofungi, functional traits have recently been applied to examine their community ecology but, to our knowledge, ITV has yet to be incorporated within the framework of community trait means. Here, we present a conceptual summary of the use of ITV to investigate the community ecology of macrofungi, including the underlying ecological theory. Inferences regarding community trait means with or without the inclusion of ITV along environmental gradients are compared. Finally, an existing study is reconsidered to highlight the variety of possible outcomes when ITV is considered. We hope this Opinion will increase awareness of the potential for within-species trait variability and its importance for statistical inferences, interpretations, and predictions of the mechanisms structuring communities of macro- and other fungi.

Revealing hidden drivers of macrofungal species richness by analyzing fungal guilds in temperate forests, West Hungary

We explored the most influential stand-scaled drivers of ectomycorrhizal, terricolous saprotrophic, and wood-inhabiting (main functional groups) macrofungal species richness in mixed forests by applying regression models. We tested 67 potential explanatory variables representing tree species composition, stand structure, soil and litter conditions, microclimate, landscape structure, and management history. Within the main functional groups, we formed and modeled guilds and used their drivers to more objectively interpret the drivers of the main functional groups. Terricolous saprotrophic fungi were supported by air humidity and litter mass. Ectomycorrhizal fungi were suppressed by high soil nitrogen content and high air temperature. Wood saprotrophs were enhanced by litter pH (deciduous habitats), deadwood cover, and beech proportion. Wood saprotrophic guilds were determined often by drivers with hidden effects on all wood saprotrophs: non-parasites: total deadwood cover; parasites: beech proportion; white rotters: litter pH; brown rotters: air temperature (negatively); endophytes: beech proportion; early ruderals: deciduous stands that were formerly meadows; combative invaders: deciduous tree taxa; heart rotters: coarse woody debris; late stage specialists: deciduous deadwood. Terricolous saprotrophic cord formers positively responded to litter mass. Studying the drivers of guilds simultaneously, beech was a keystone species to maintain fungal diversity in the region, and coniferous stands would be more diverse by introducing deciduous tree species. Guilds were determined by drivers different from each other underlining their different functional roles and segregated substrate preferences. Modeling guilds of fungal species with concordant response to the environment would be powerful to explore and understand the functioning of fungal communities.