Christopher W. Fernandez

Redefining fine roots improves understanding of below-ground contributions to terrestrial biosphere processes.

Fine roots acquire essential soil resources and mediate biogeochemical cycling in terrestrial ecosystems. Estimates of carbon and nutrient allocation to build and maintain these structures remain uncertain because of the challenges of consistently measuring and interpreting fine-root systems. Traditionally, fine roots have been defined as all roots ≤ 2 mm in diameter, yet it is now recognized that this approach fails to capture the diversity of form and function observed among fine-root orders. Here, we demonstrate how order-based and functional classification frameworks improve our understanding of dynamic root processes in ecosystems dominated by perennial plants. In these frameworks, fine roots are either separated into individual root orders or functionally defined into a shorter-lived absorptive pool and a longer-lived transport fine-root pool. Using these frameworks, we estimate that fine-root production and turnover represent 22% of terrestrial net primary production globally - a c. 30% reduction from previous estimates assuming a single fine-root pool. Future work developing tools to rapidly differentiate functional fine-root classes, explicit incorporation of mycorrhizal fungi into fine-root studies, and wider adoption of a two-pool approach to model fine roots provide opportunities to better understand below-ground processes in the terrestrial biosphere.

The role of chitin in the decomposition of ectomycorrhizal fungal litter

Ectomycorrhizal fungal tissues comprise a significant forest-litter pool. Ectomycorrhizal (EM) fungi may also influence the decomposition of other forest-litter components via competitive interactions with decomposer fungi and by ensheathing fine roots. Because of these direct and indirect effects of ectomycorrhizal fungi, the factors that control the decomposition of EM fungi will strongly control forest-litter decomposition as a whole and, thus, ecosystem nutrient and carbon cycling. Some have suggested that chitin, a component of fungal cell walls, reduces fungal tissue decomposition because it is relatively recalcitrant. We therefore examined the change in chitin concentrations of EM fungal tissues during decomposition. Our results show that chitin is not recalcitrant relative to other compounds in fungal tissues and that its concentration is positively related to the decomposition of fungal tissues. Variation existing among EM fungal isolates in chitin concentration suggests that EM fungal community structure influences C and nutrient cycling.

General principles in the community ecology of ectomycorrhizal fungi.

IntroductionKnowledge of the factors that determine ectomycorrhizal fungal community structure is essential in many areas of practical significance including conservation, habitat restoration, prevention or amelioration of species invasions, and the prediction of responses to climate change. With respect to these goals, however, community ecology, in general, and ectomycorrhizal fungal community ecology, in particular, have been rather disappointing.DiscussionIndeed, some feel that community ecology is, at best, an esoteric discipline and, at worst, an inane one. But there is hope. As we apply what has been learned about other organisms concerning the relationships between functional traits and success (abundance), it may be possible to elucidate general principles that govern much of the structuring of ectomycorrhizal fungal communities.ConclusionA hierarchical model of ectomycorrhizal fungal community structure is presented that involves abiotic filtering of immigrant propagules based on functional traits, followed by interspecific competition as ameliorated by disturbance and habitat partitioning, the outcomes of which are dependent on functional traits.

Can ectomycorrhizal colonization of Pinus resinosa roots affect their decomposition

In many forest ecosystems, fine root litter comprises a large pool of organic carbon and nutrients. In temperate climates ectomycorrhizal fungi colonize the roots of many forest plant species. If ectomycorrhizal colonization influenced root decomposition, it could significantly influence carbon sequestration and nutrient cycling. Fungal tissues and fine roots may decompose at different rates and, therefore, ectomycorrhizal colonization may either hasten or retard root decomposition. Unfortunately, no comparisons of the decomposition of roots and ectomycorrhizal fungi have yet been made. Therefore, we compared decomposition of Pinus resinosa fine roots and ectomycorrhizal fungi from a Pinus resinosa plantation. We also compared the decomposition rates of nonmycorrhizal Pinus resinosa fine roots with roots colonized by nine species of ectomycorrhizal fungi. We found that the several tested isolates of ectomycorrhizal fungi decomposed far more rapidly than the fine roots and that ectomycorrhizal colonization either had no significant effect on root decomposition or significantly increased root decomposition depending on the isolate of fungus. We conclude that the composition of an ectomycorrhizal fungal community may affect carbon and nutrient cycling through its influence on root decomposition.

Building a better foundation: improving root-trait measurements to understand and model plant and ecosystem processes

Trait-based approaches provide a useful framework to investigate plant strategies for resource acquisition, growth, and competition, as well as plant impacts on ecosystem processes. Despite significant progress capturing trait variation within and among stems and leaves, identification of trait syndromes within fine-root systems and between fine roots and other plant organs is limited. Here we discuss three underappreciated areas where focused measurements of fine-root traits can make significant contributions to ecosystem science. These include assessment of spatiotemporal variation in fine-root traits, integration of mycorrhizal fungi into fine-root-trait frameworks, and the need for improved scaling of traits measured on individual roots to ecosystem-level processes. Progress in each of these areas is providing opportunities to revisit how below-ground processes are represented in terrestrial biosphere models. Targeted measurements of fine-root traits with clear linkages to ecosystem processes and plant responses to environmental change are strongly needed to reduce empirical and model uncertainties. Further identifying how and when suites of root and whole-plant traits are coordinated or decoupled will ultimately provide a powerful tool for modeling plant form and function at local and global scales.

Melanization of mycorrhizal fungal necromass structures microbial decomposer communities

Handling Editor: Karina Clemmensen Abstract 1. Mycorrhizal fungal necromass is increasingly recognized as an important contributor to soil organic carbon pools, particularly in forest ecosystems. While its decomposition rate is primarily determined by biochemical composition, how traits such as melanin content affect the structure of necromass decomposer communities remains poorly understood. 2. To assess the role of biochemical traits on microbial decomposer community composition and functioning, we incubated melanized and non-melanized necromass of the mycorrhizal fungus Meliniomyces bicolor in Pinusand Quercus-dominated forests in Minnesota, USA and then assessed the associated fungal and bacterial decomposer communities after 1, 2 and 3 months using high-throughput sequencing. 3. Melanized necromass decomposed significantly slower than non-melanized necromass in both forests. The structure of the microbial decomposer communities depended significantly on necromass melanin content, although the effect was stronger for fungi than bacteria. On non-melanized necromass, fungal communities were dominated by r-selected ascomycete and mucoromycete microfungi early and then replaced by basidiomycete ectomycorrhizal fungi, while on melanized necromass these groups were co-dominant throughout the incubation. Bacterial communities were dominated by both specialist mycophageous and generalist taxa. 4. Synthesis. Our results indicate that necromass biochemistry not only strongly affects rates of decomposition but also the structure of the associated decomposer communities. Furthermore, the observed colonization patterns suggest that fungi, and particularly ectomycorrhizal fungi, may play a more important role in necromass decomposition than previously recognized.

Measuring and modeling roots, the rhizosphere, and microbial processes belowground

As the field of ecology has progressed, one would be hard pressed to find an empiricist who has not worked with a model or a modeler who has not spent significant time in the field. Still, clear divisions exist between these communities, and despite efforts within both groups to bridge these divisions we still see a distinct break in communication between the two. Part of this disconnect likely comes from fundamentally different aims, as empiricists often highlight novel detail while modelers necessarily look for broad, consistent patterns. Both angles are appropriate and important, but there must be a common ground where empiricists can relay understanding of ecological processes at appropriate scales and modelers can be open to incorporating these insights into new model applications. The gap between empiricists and modelers is particularly evident for the belowground environment, where many ecosystem and most global terrestrial models have been forced to greatly simplify many critical ecological processes due to a lack of empirical understanding and broad descriptive patterns. However, recent advances in our understanding of belowground processes now enable us to push the boundaries of models at both the ecosystem and global scale. Here, models have the potential to improve our descriptive and predictive capabilities in the long term and, perhaps more importantly, can help us to identify key areas of weakness in our empirical understanding that are ripe for focused research in the short term. In a recent session at the annual meeting of the Ecological Society of America, speakers from both the field-based and modeling communities came together to share cutting-edge belowground research and discuss potential new areas of mutually beneficial research.

The continuing relevance of “older” mycorrhiza literature: insights from the work of John Laker Harley (1911–1990)

To new generations of scientists beginning their careers in research, we strongly recommend the practice of reading older literature. To illustrate the value of doing so, we highlight six insights of one of the most influential mycorrhiza researchers of the twentieth century, Jack Harley. These insights concerning mycotrophy, the new niche, the sheath, C cycling, N cycling, and mutualism were published prior to 1975 and so may have escaped the notice of many, but they laid the groundwork for some of the most important research of today.

Mycorrhizal Interactions With Saprotrophs and Impact on Soil Carbon Storage

In most ecosystems roots and their associated mycorrhizal fungi are a significant component of the soil food web where many bacteria, fungi, and animals directly or indirectly depend on them for much of their nutrition. This dependency is to a large extent mutual: decomposers (saprotrophs) in soil are the primary agents in releasing vital nutrients locked up in organic matter, which in many cases neither plant roots nor mycorrhizal fungi can effectively release. Therefore plants and mycorrhizal fungi engage in an intriguing relationship with saprotrophs in which they act as a main substrate for saprotrophs and simultaneously depend on them and compete with them for soil nutrients. These multiple interactions present us with difficulties in generalizing the effect of mycorrhizal fungi on saprotrophs. In some cases their interests are aligned and saprotrophs and mycorrhizal fungi can act in concert in releasing and taking up essential nutrients; in others competition may be more important. In this chapter we discuss the ways in which different types of mycorrhizal fungi (arbuscular, ectomycorrhizal, and ericoid) can prime soil organic matter decomposition, compete with saprotrophs for substrate, and engage in other ways with members of the soil food web to affect soil organic carbon stocks.

Measuring and modeling roots, the rhizosphere, and microbial processes belowground: Meetings

As the field of ecology has progressed, one would be hard pressed to find an empiricist who has not worked with a model or a modeler who has not spent significant time in the field. Still, clear divisions exist between these communities, and despite efforts within both groups to bridge these divisions we still see a distinct break in communication between the two. Part of this disconnect likely comes from fundamentally different aims, as empiricists often highlight novel detail while modelers necessarily look for broad, consistent patterns. Both angles are appropriate and important, but there must be a common ground where empiricists can relay understanding of ecological processes at appropriate scales and modelers can be open to incorporating these insights into new model applications. The gap between empiricists and modelers is particularly evident for the belowground environment, where many ecosystem and most global terrestrial models have been forced to greatly simplify many critical ecological processes due to a lack of empirical understanding and broad descriptive patterns. However, recent advances in our understanding of belowground processes now enable us to push the boundaries of models at both the ecosystem and global scale. Here, models have the potential to improve our descriptive and predictive capabilities in the long term and, perhaps more importantly, can help us to identify key areas of weakness in our empirical understanding that are ripe for focused research in the short term. In a recent session at the annual meeting of the Ecological Society of America, speakers from both the field-based and modeling communities came together to share cutting-edge belowground research and discuss potential new areas of mutually beneficial research.