Hello, my dear readers. Many of you have emailed me asking when the next blog post will be posted, what topics we'll cover next, if that cliffhanger from last season will be resolved, and why I decided to partner with a new publishing company. All of those questions will not be addressed in this post because they are made up. Instead, we will pick up the shovel where we dropped it last — the black box of fine-root ecology — and traverse into an even darker territory, fungal guilds and functional traits. 

Guild shifting: a fungal identity crisis

Before moving on to the topic of functional traits, it's important to let you know that everything you just read about fungal guild categorization is a lie. Just kidding, I would consider it more of a ... not-full truth? The thing is, scientists love to categorize. Categorizing is a pragmatic way to understand the world. In an ecological context, it allows us to understand why organisms and communities respond to certain environmental conditions, disturbances, and other organisms. However, because an unconceivable number of environmental conditions + disturbances + other organisms occur in nature, and because kingdom fungi is so diverse,  it's hard to place fungal species into definitive guilds. 

For all of the guilds covered above, there are certain environmental contexts and host-related interactions that may encourage a shift in a fungi's occupation. For example, the tranquil co-existence between an endophytic fungi and their host may turn parasitic if the host begins to decline in health because of some other factor. In some cases, something as small as the host no longer laughing at the endophyte's jokes can turn a relationship sour. In line with this heartbreaking tale of scorned roommates, mycorrhizal fungi may also shift towards parasitic relations with their hosts if nutrient conditions change and it's more beneficial to store nutrients or use it for their own growth. If you are interested in learning more about guild flexibility (also referred to as “dual niche” and “multifunctional” fungi) check out Selosse et al.’s 2018 commentary on the subject. The authors do a great job in encouraging mycologists and ecologists to think beyond their specialty fields when researching fungi, and also provide a helpful review of experiments showing the multifunctionality of many fungal taxa. 

When narrowing the focus to ectomycorrhizal fungal species, guild assignment can get even trickier. Comparatively, it's much easier to rock a rhyme that's right on time (song reference for all of you people who listen to songs out there). The trickiness arises from the diversity in the ectomycorrhizal fungal group and its evolutionary origins from various saprotrophic lineages (80+). Because of this diversity in species and phylogeny, ectomycorrhizal fungi seem to exist on a biotrophy-saprotrophy continuum where some species maintain more saprotrophic traits from their ancestors and are able to obtain carbohydrates both with- and without a plant host (Koide et al. 2008). The existence of this spectrum doesn't discount the fact that some fungal species, to our knowledge, are purely ectomycorrhizal or purely saprotrophic. However, it opens the door for more nuance, and emphasizes the importance in considering the flexibility in some fungal species' trophic habits. 

Hurdles and opportunities in constructing a mycorrhizal trait-based framework

Now that we’re all convinced that traits are cool and can tell us stuff, let me catch you up on the exciting challenges faced by the modern mycologist. First, the lessons we can learn from plant and animal trait studies aren’t always translatable to mycorrhizal fungi and other microbial organisms. This discrepancy comes from the cryptic nature of many mycorrhizal fungal species, and traits that are difficult to measure and observe. Next, the fact that mycorrhizal traits are an emergent property arising from two organisms causes variation in trait values and context dependent results. Finally, similar to what was discussed by Selosse and co, bias of researchers coming from different backgrounds creates plant- or fungal-centric views of mycorrhizal systems and can lead to inconsistent definitions of mycorrhizal traits. 

Moving forward, one way to address these issues is to describe and study mycorrhizal traits based on location. For example, because mycorrhizal traits emerge from both the fungal and plant species engaged in the symbiosis, fungal mycorrhizal traits, plant mycorrhizal traits, and symbiotic mycorrhizal traits can all be defined and studied. Many important fungal traits were already mentioned above, but plant- and symbiotic mycorrhizal traits can be found in Chaudhary et al. 2022 (see Table 2). Until the time comes when I bury you with another blog post, check out the below figure from Chaudhary and co to get a sense of the different categories of mycorrhizal traits. 

What about our experience as fleshy, thumb-bearing, humans, can be shared with a resilient patch of daisies or a sturdy, hardened, oak? Quite a lot, when you consider that we are all formed of the same materials and shaped by time and natural selection to perform the interrelated tasks of growth, survival, and reproduction. For our green friends, various adaptations coined 'plant functional traits' have been accrued, modified, and lost throughout their consistent and biosphere-shaping reign on our planet to address these primary objectives. After a brief introduction on the background of functional trait research, we'll delve below the surface to focus more closely on root functional traits, why they are important, and how they shape ecosystems. It's not a far stretch to see how our own specie's adaptations, powered by our thumbs and tools, allow us to adjust and simultaneously redesign environments across the globe, just like those belonging to our autotrophic planet-mates. 

By now, I'm sure I've thoroughly convinced you that roots are a beautiful, labrinythian, disaster of a research focus that everyone should dedicate their lives to, but to satisfy the rest of the human population who seems to be so obsessed with things they can see during their afternoon walk, we can aim to address how root traits impact the aboveground world. Just like the broader category of plant functional traits covered at the start of this trait tale, root functional traits influence not only individual plants, but whole-ecosystem community structure and nutrient cycling. For us curious folk who enjoy learning for the sake of blogging or annoying their friends with information they didn't ask for, root traits can give us insight into fun topics like species biogeography, evolutionary history, and ecosystem interactions. For the more applied-minded tribe, root traits can assist us in identifying plants that can tolerate certain environments and stressors such as water and nutrient abundance or limitation. Last but not least, for all of you perplexing peeps who enjoy modeling as a past-time, root traits have significant impacts on ecosystem net primary productivity and carbon cycling. 

Evolutionary Ties

Congratulations, you've thoroughly networked with all the traits at this made-up root gala, listening to them talk on and on about their root children, philanthropic ventures, and how much phosphorous they've absorbed this year. Let's now transport ourselves back in time to witness just how these traits have evolved. If you can recall from an earlier post that overviewed the adaptations plants refined while making their transition to land, the Cretaceous (145-65 mya) was a prolific period for plant diversification and geographical expansion into different habitats. It was like the Renaissance for trees, and you should take that simile very seriously coming from someone who knows extremely little about the historical relevance of the Renaissance and slightly more, but still not that much, about plant evolution. Anyway,  poets nor trees can escape the influence of the geologic time of their existence, and both end up being shaped by their environment while simultaneously impacting it. This 2012 publication by Comas et al. does a fantastic job explaining just that - the connection between the evolutionary forces of the Cretaceous environment, the resulting tree adaptations, and the subsequent shift in Earth's biogeochemical cycles.

The above paper, as well as the later 2018 research conducted by Ma et al., emphasizes the importance of RD, SRL, and mycorrhizal fungi association types as three traits that show distinct changes over evolutionary timescales, high phylogenetic signals, and significant influences over nutrient uptake strategies and other interrelated root traits. As atmospheric CO2  decreased during the Cretaceous, and leaves increased their stomatal conductance and leaf vein density to maintain photosynthetic rates, roots had to increase their efficiency belowground to support these aboveground adaptations. Simultaneously, decreased CO2  caused slower decomposition rates in soils, limiting the pool of mobile soil nutrients, and we begin to see the origins of the first ectomycorrhizal fungi evolving separately in different lineages of trees. It is because of these circumstances that tree species across the globe have generally evolved lower RD, higher SRL, and less dependence on arbuscular mycorrhizal fungi over time.

These trends towards lower RD and higher SRL reduced the energy costs of root production, increased water conductance and nutrient uptake rates, and extended the available root surface area for ectomycorrhizal colonization. Species descending from more ancient lineages tell a different story. This can be seen in roots within the Magnoliaceae family, which exhibit thicker RD compared to more recently evolved tree species. Though magnolias are primitive flowering plants that have many unique characteristics, I'm mentioning their root-based differences here to provide support for the strong phylogenetic influence on some morphological root traits. For those of you who missed the definition of 'phylogenetically conserved' amongst the extremely important and fun to read vocabulary words defined above, this means that a specific region of DNA has remained mostly unchanged in many groups of related species. These segments of DNA are highly heritable, consistent, and code for genes that serve vital functions. This is all to say that the persistent evolutionary direction of fine roots seems to be leaning towards thin and nimble, but, a tree's ancestral history still has some say in the manner.

Trait Interactions with Water and Nutrients

Now that we've reviewed a brief background check on the evolutionary and phylogenetic histories of these three root traits, I'll quickly address how RD, SRL, and mycorrhizal fungi association impact water absorption and nutrient uptake. The enhancement in soil exploration strategies that a low RD and high SRL offer directly impact the access to available water and nutrients. Thin and soft roots are able to weave their way through small soil pores like nubby contortionists, and being bendy really comes in handy when in competition with other organisms or in seasonal/infertile environments where water and nutrients are sporadic. The ability of a plant's root system to quickly proliferate new growth in response to abiotic (e.g., rain and temperature) and biotic cues (e.g., herbivory and soil microbe interactions) are also linked to RD and SRL, again with lower RD and higher SRL species performing better in this area. Researches would label a plant wielding these root traits and the accompanying soil exploration strategies as a fast/acquisitive species, but we know from our tree tale at the beginning of this post that the speedy willow doesn't always win the race.

Associations with mycorhizzal fungi also influence access and uptake rates of water and nutrients. Increases in EM fungal colonization in particular greatly expand a tree's ability to explore the soil via hyphal networks, and such networks have longer life-spans than the more ephemeral AM built structures. In addition, mycorrhizal fungi participate in the formation of different 'nutrient economies' because of their impact on soil nutrient availability through decomposition. Fungi within the AM group are primarily assisting trees with phosphorus uptake. They also pass along nitrogen, which they uptake from pools of inorganic N. Their more versatile counterparts, EM fungi, are able to access organic N pools, as well as weather minerals, to offer a more diverse nutrient payment to their tree hosts. These differences are driven by each group's inventory of enzymes. The later-diverged EM fungi have evolved stronger saprotrophic capabilities (or rather, never lost them) which allows them to access organic forms of nutrients through decomposition in a manner that is not available to the AM group. Both fungal groups play a pivotal role in not only individual plant nutrient uptake, but also nutrient cycling in forests and plant and soil community structure. Despite my urge to dive deeper into this subject matter, I will stop right now, thank you very much (Spice Girls lyrics), and encourage you to take a look at the paper I linked earlier in this paragraph, which is co-authored by Morton's very own Dr. Meghan Midgley, and is also the source of the informative graphic below. 

Exploring the Connections Between Traits

There are innumerable examples of how these traits of interest inevitably end up influencing other root traits, and if I really wanted to show you my quirky hyper-fixation side, I would take over a wall in the lab and connect all 300+ root traits using newspaper clippings and strings of red yarn. For now we'll limit our focus to the impacts on root lifespan, root branching intensity, and root branching ratio. Root lifespan is positively linked to root diameter, and therefore negatively to SRL. Roots with higher diameters are investing more in tissue construction, leading to higher root longevity. This has been linked to other plant traits such as root calcium content and plant growth rate. McCormack et al. found that temperate trees with thicker roots had up to a three-fold increase in root lifespan. The exact mechanism driving this increase has yet to be fully uncovered, and it is my suspicion that thicker roots have access to an expensive anti-aging cream which they apply nightly. In actuality, explanations involving root anatomy likely influence this trait (e.g., a thick exodermis could support higher root lifespans because it prevents desiccation, herbivory, and offers a stronger barrier to plant pathogens). Kong and co. explored this further, but more research is needed to elucidate the relationships between root lifespan and other root traits. 

The complicated story involving mycorrhizal colonization and root lifespan is a little more fuzzy and requires additional exploration before making any definitive statements on how these two traits interact. In AM species, mycorrhizal colonization has been shown to increase root lifespan by enhancing tolerance to drying soils and protecting against root pathogens, but these results are dependent on the tree and fungal species involved, as well as the available resources (e.g., water, nitrogen, phosphorous, etc.). For instance, Hooker and colleagues found Populus roots colonized by AM fungi to have shorter lifespans. Studies on EM fungal colonization and root lifespan are even more sparse, but I did find an interesting study by Liese et al. reporting on a dried soil experiment. The results showed fine roots colonized by EM fungi to have shorter lifespans than those with AM fungal partners by ~50 days when in drought conditions.

Root architectural traits such as root branching intensity and branching ratio show strong relationships to mycorrhizal colonization type and weaker associations to the phylogenetic signals that influence RD and SRL. We can turn to another publication by Liese and colleagues for evidence supporting this claim. Architectural trait relations with mycorrhizal fungi show observable trends in angiosperm AM and EM species, with EM species generally showing higher root branching intensity that allows them to produce highly clustered root growth and exploit nutrient rich patches in the soils. Contrastingly, AM species show low root branching intensity, and rely more on changes in root morphological traits (specifically RD) to coordinate with their fungal partners for nutrient acquisition. These strategies can look a little different for gymnosperm EM species, who show low branching intensity due to the even distribution of sparse nutrients in the soils they are typically found growing in. Multiple studies have shown architectural traits to be less constrained by species lineage and more plastic than morphological traits such as RD, SRL, and RTD. This suggests that root branching intensity and branching ratio allows plants to adapt to a variety of environmental and nutrient conditions. 

 The Race to Uncover a Multidimensional Root Trait Spectrum ... that works

The second Liese paper linked above provides us with a smooth segue to the end of  this section, as the group also found correlations between root architectural traits and aboveground traits, which implies that some root traits may serve as increasingly accurate indicators of fast and slow species. This is not only important for figuring out a root economics spectrum (RES) that makes sense, but also for tying these traits and processes to the aboveground economy. To offer a rudimentary summarization of Liese's findings, intensive root branching leads to low C:N ratios and high SRL in fine roots (i.e., fast traits), which indirectly causes high SLA and short leaf longevity (also fast traits). Below is a diagram from the Liese et al. paper that may further confuse you or make this more digestible. Let me know in my non-existent suggestions box, or better yet, allow your virtual voices to be heard in the comment section on Luke's youtube lecture.

If my explanations of the frequently measured root traits above have painted a clear picture for you regarding a universally agreed upon RES, then you should start questioning that picture now. In the last decade alone, there has been a lot of research showing the inconsistencies of linking root traits to strategies, functions, and to each other. This is especially true when looking at morphological traits such as RD, SRL, and RTD. To return to the Weemstra paper linked earlier in this post, issues pinning down an RES that is analogous to the aboveground LES arise because of the complex functions root must perform, the multitude of environmental factors roots they face, as well as the diversity in type and function of their symbioses with soil microbiota. This has led to continuous tweaking of the axes on the root economic spectrum, as well as many unanswered questions. 

So, where are we at now? Should we bury all hopes for a cohesive RES and call it a day? Recent research by Bergmann (2020) and Yi (2023) suggests not, and through summarizing their findings maybe I can glue all of this together into somewhat of a cohesive narrative and finally set you free from this virtual entrapment. By pulling from global trait measurements of close to 2,000 species (woody and non-woody), Bergmann and colleagues were able to find evidence supporting the addition of a mycorrhizal collaboration axis to the classic conservation axis. This collaboration gradient was driven by SRL, RD, and cortex fraction, with thin rooted species (i.e., high SRL) relying on their own roots for water and nutrient acquisition, and thick rooted species (i.e., high RD and cortical space) investing in larger roots that can be colonized by more mycorrhizal fungi, therefore outsourcing their uptake needs. The diagram below depicts Bergmann's framework in addition to the classic fast-slow spectrum driven by root nitrogen and root tissue density. 

Yi and colleagues added to the complexity of the above model by emphasizing the role of root function in the RES. Researchers have been stressing the importance of measuring functional/physiological traits in addition to the frequently measured morphological ones, but as I've mentioned many times in this blog, studying roots is challenge. This is especially true when trying to measure ongoing processes like root respiration, exudation, and so on. This paper focused on measuring root nitrogen uptake rates, and found that high SRL was not associated with higher nitrogen uptake, which differs from the conventional view that a fast trait like SRL will have higher nutrient uptake rates. This finding led the authors to incorporate an 'amount-efficiency' spectrum driven by trait morphology and physiology. In addition, they found higher branching ratios caused higher mycorrhizal fungi colonization rates, therefore a 'self-symbiosis' axis was driven by root architectural traits. These discoveries aren't replacing the former model that rests on the relationship between RD and AM mycorrhizal fungi colonization rates, but rather adding to it. Imagine a model where axes of low to high branching ratio and RD run parallel alongside low to high mycorrhizal fungi colonization rates. If you can't imagine it, I don't have an image for you, and I would suggest that you work on your child-like wonder.  

Although we've reached the end of this trait tour, much like the Spice Girls, I'm not finished performing! Now that you all have been introduced to plant and root functional traits and their importance, when returning to this subject we'll take a look at how climate, soil, season, and plant functional types influence global root trait variation. There's also a lot of ground to cover regarding mycorrhizal fungi's influence in the belowground space, and I'll be sure to dig into that as well.