Fungal diversity at fragmented landscapes: synthesis and future perspectives

Different spatial structure of plant-associated fungal communities above- and belowground

The distribution and community assembly of above- and belowground microbial communities associated with individual plants remain poorly understood, despite its consequences for plant-microbe interactions and plant health. Depending on how microbial communities are structured, we can expect different effects of the microbial community on the health of individual plants and on ecosystem processes. Importantly, the relative role of different factors will likely differ with the scale examined. Here, we address the driving factors at a landscape level, where each individual unit (oak trees) is accessible to a joint species pool. This allowed to quantify the relative effect of environmental factors and dispersal on the distribution of two types of fungal communities: those associated with the leaves and those associated with the soil of Quercus robur trees in a landscape in southwestern Finland. Within each community type, we compared the role of microclimatic, phenological, and spatial variables, and across community types, we examined the degree of association between the respective communities. Most of the variation in the foliar fungal community was found within trees, whereas soil fungal community composition showed positive spatial autocorrelation up to 50 m. Microclimate, tree phenology, and tree spatial connectivity explained little variation in the foliar and soil fungal communities. Foliar and soil fungal communities differed strongly in community structure, with no significant concordance detected between them. We provide evidence that foliar and soil fungal communities assemble independent of each other and are structured by different ecological processes.

Diversity and distribution of tropical ectomycorrhizal fungi

The tropics were long considered to have few ectomycorrhizal fungi, presumably due to a paucity of ectomycorrhizal host plants relative to higher-latitude ecosystems. However, an increase in research in tropical regions over the past 30 years has greatly expanded knowledge about the occurrence of tropical ectomycorrhizal fungi. To assess their broad biogeographic and diversity patterns, we conducted a comprehensive review and quantitative data analysis of 49 studies with 80 individual data sets along with additional data from GlobalFungi to elucidate tropical diversity patterns and biogeography of ectomycorrhizal fungi across the four main tropical regions: the Afrotropics, the Neotropics, Southeast Asia, and Oceania. Generalized linear models were used to explore biotic and abiotic influences on the relative abundance of the 10 most frequently occurring lineages. We also reviewed the available literature and synthesized current knowledge about responses of fungi to anthropogenic disturbances, and their conservation status and threats. We found that /russula-lactarius and /tomentella-thelephora were the most abundant lineages in the Afrotropics, the Neotropics, and Southeast Asia, whereas /cortinarius was the most abundant lineage in Oceania, and that /russula-lactarius, /inocybe, and /tomentella-thelephora were the most species-rich lineages across all of the tropical regions. Based on these analyses, we highlight knowledge gaps for each tropical region. Increased sampling of tropical regions, collaborative efforts, and use of molecular methodologies are needed for a more comprehensive view of the ecology and diversity of tropical ectomycorrhizal fungi.

Contrasting responses of above- and belowground diversity to multiple components of land-use intensity

Land-use intensification is a major driver of biodiversity loss. However, understanding how different components of land use drive biodiversity loss requires the investigation of multiple trophic levels across spatial scales. Using data from 150 agricultural grasslands in central Europe, we assess the influence of multiple components of local- and landscape-level land use on more than 4,000 above- and belowground taxa, spanning 20 trophic groups. Plot-level land-use intensity is strongly and negatively associated with aboveground trophic groups, but positively or not associated with belowground trophic groups. Meanwhile, both above- and belowground trophic groups respond to landscape-level land use, but to different drivers: aboveground diversity of grasslands is promoted by diverse surrounding land-cover, while belowground diversity is positively related to a high permanent forest cover in the surrounding landscape. These results highlight a role of landscape-level land use in shaping belowground communities, and suggest that revised agroecosystem management strategies are needed to conserve whole-ecosystem biodiversity.

Microbial Species-Area Relationships in Antarctic Cryoconite Holes Depend on Productivity

The island species-area relationship (ISAR) is a positive association between the number of species and the area of an isolated, island-like habitat. ISARs are ubiquitous across domains of life, yet the processes generating ISARs remain poorly understood, particularly for microbes. Larger and more productive islands are hypothesized to have more species because they support larger populations of each species and thus reduce the probability of stochastic extinctions in small population sizes. Here, we disentangled the effects of "island" size and productivity on the ISAR of Antarctic cryoconite holes. We compared the species richness of bacteria and microbial eukaryotes on two glaciers that differ in their productivity across varying hole sizes. We found that cryoconite holes on the more productive Canada Glacier gained more species with increasing hole area than holes on the less productive Taylor Glacier. Within each glacier, neither productivity nor community evenness explained additional variation in the ISAR. Our results are, therefore, consistent with productivity shaping microbial ISARs at broad scales. More comparisons of microbial ISARs across environments with limited confounding factors, such as cryoconite holes, and experimental manipulations within these systems will further contribute to our understanding of the processes shaping microbial biogeography.

Forest edge effects on the mycorrhizal communities of the dual-mycorrhizal tree species Alnus glutinosa (L.) Gaertn

Forest conversion into agricultural land has resulted in a continuous decline in forest cover and in a reduced size and increased edge-to-core ratio of the remaining fragments. Forest edges are more directly exposed to sunlight, wind and pollutants and the resulting changes in habitat quality might have a large impact on plant and animal communities. Few studies, however, have focused on forest edge effects on mycorrhizal fungus communities. Here, we used high-throughput sequencing to study how communities of arbuscular mycorrhizal (AMF) and ectomycorrhizal fungi (EcMF), present in both the roots of the dual mycorrhizal tree Alnus glutinosa and in the soil, changed with increasing distance from the forest edge within fragmented forests embedded in an intensively managed agricultural matrix. Overall, we found 158 AMF OTUs and 275 EcMF OTUs. Soil moisture content increased with increasing distance from the forest edge, whereas soil nitrate concentration increased with increasing distance in south-facing and decreased in north-facing edges. Distance to the forest edge had a significant effect on EcMF community composition that largely overlapped with the observed changes in soil variables, especially soil moisture content. Apart from this distance effect, there were also clear effects of edge orientation on mycorrhizal diversity and community composition. While AMF OTU richness was higher at south- than at north-facing edges, the opposite pattern was found for EcMF. Community composition of both mycorrhiza types also differed significantly between south- and north-facing edges. We conclude that altered environmental conditions at forest edges cause significant changes in mycorrhizal communities, which could subsequently affect ecosystem functioning.

Local abiotic conditions are more important than landscape context for structuring arbuscular mycorrhizal fungal communities in the roots of a forest herb

Due to human influence, large tracts of natural vegetation have been cleared and replaced by other types of land use, resulting in highly fragmented landscapes consisting of small fragments of well-conserved habitat scattered within a matrix of intensively managed land. Changes in land use and associated fragmentation have important consequences for biodiversity in the remaining fragments. Most studies so far have investigated the impact of land use change on macro-organisms, but little is known about how landscape fragmentation affects microbial communities. Here, we studied how changes in land use and abiotic conditions affected the arbuscular mycorrhizal fungal (AMF) communities in the roots of the forest herb Stachys sylvatica. Root samples were collected from 40 populations occurring in fragmented forest patches of varying age and size embedded within an agricultural landscape. Our results showed that forest age and isolation did not affect AMF diversity or community composition, suggesting that AMF disperse easily throughout the landscape and that AMF communities reassemble fast in recently established forest patches. On the other hand, AMF richness increased with increasing forest area, indicating that small forest sizes limit AMF richness. Additionally, AMF richness increased with increasing soil pH and decreased with soil nitrate content, while AMF community composition was affected plant-available phosphorus. Overall, these results show that landscape context is less important than local abiotic conditions for structuring AMF communities. However, the significant area effect indicates that further reductions in forest area will lead to impoverished AMF communities, potentially affecting long-term plant fitness and community structure.

The effects of rainforest fragment area on the strength of plant-pathogen interactions

Pathogenic interactions between fungi and plants facilitate plant species coexistence and tropical rainforest diversity. Such interactions, however, may be affected by forest fragmentation as fungi are susceptible to anthropogenic disturbance. To examine how fragmentation affects fungus-induced seed and seedling mortality, we sowed seeds of six plant species in soils collected from 21 forest fragments. We compared seedling establishment in unmanipulated soils to soils treated with fungicides. Fungicides increased germination of Toona ciliata seeds and decreased mortality of Syzygium rubicundum and Olea dioica seedlings. The fungus-induced mortality of one of these species, S. rubicundum, decreased with decreasing fragment size, indicating that its interactions with pathogenic fungi may weaken as fragments become smaller. We provide evidence that a potential diversity-maintaining plant-fungus interaction weakens in small forest fragments and suggest that such disruptions may have important long-term consequences for plant diversity. However, we emphasize the need for further research across rainforest plant communities to better understand the future of diversity in fragmented rainforest landscapes.