Banner art by Sun-Hyuk Kim
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.
Viewing Plants Through Trait-tinted Glasses
Morphological: anything involving form and structure. This not only includes an organism's external structures, but also internal shapes and configurations.
Phenotype: the observable characteristics of an organism that are driven by both genetic makeup and the environment. Genes are providing organisms with a set of traits, and the environment(s) the organism develops in will tweak some of those traits. Basically, "genotype + environment = phenotype." The below diagram provides an example of how a phenotypic trait like plant height can be impacted by elevation.
Specific leaf area (SLA): the area of a single leaf, or all leaves in a tree's canopy, divided by the dry mass of said leaf/leaves. The inverse of SLA, leaf mass per area (LMA) is also used by scientists, depending on what questions they're asking. If focusing on growth rates, SLA is the usual go-to. When focusing on leaf longevity and the investments plants make in proteins per unit of leaf area, LMA is preferred.
Leaf economics spectrum (LES): A theory describing the aboveground trade-off in plants between resource acquisition and leaf construction costs. This trade-off can be observed in the relationships between traits such as LMA, leaf respiration rates, and leaf nitrogen and phosphorous content.
Fine-root Functional Traits
Order based system of fine-root classification: borrowed from scientists who've studied the natural branching patterns of rivers and streams, this method of classification labels the most distal roots within a system as 1st orders.
Absorptive fine roots:
the lowest order of fine roots within a root system, responsible for acquiring soil nutrients and water.
Transport fine roots: higher order roots that result from the merging of multiple absorptive roots, responsible for transporting nutrients and water through the root system.
Specific root length (SRL): I may have defined SRL multiple times on this blog by now, so for those of you rolling your eyes at my redundancy, I apologize ... for being thorough and thoughtful! Okay, so SRL is the belowground analogue to SLA that compares acquisition potential to tissue investment. One of the first scientists to use SRL when looking at root functions and ecosystem processes was Alastair H. Fitter, who also happened to be the scientist responsible for applying the order-based system to roots. Dr. Fitter, if you see this, visit the root lab sometime. We would all enjoy picking your ear about the underground, and also, I'm eager to meet someone with the name 'Alastair.'
Phylogenetically conserved: genes, and therefore traits, that are retained through a specie's evolutionary history with few to minor changes. Examples include the proteins cytochrome c and hemoglobin that can be found in mammals and all other organisms that depend on oxygen for respiration. These proteins serve such vital functions that any mutation acquired is typically selected against and not reproduced. This is evolution's way of communicating "if it ain't broke, don't fix it!"
Root diameter (RD): the measurement of length across the body of a root.
Root tissue density (RTD): the ratio of root dry mass to root volume. The story of this trait starts to get pretty complicated when considering the different tissue layers within a root. For example, the cortical tissue layers in the middle of the root are less dense than the inner stele tissue.
Root nitrogen content: the mass of nitrogen per root mass. I think it is fair to say that this particular trait has caused quite the headache for some researchers, as it doesn't "behave" the same way, or tell us the same things, as leaf nitrogen content. What we do know is that it is largely driven by soil traits and nutrient availability, and impacts root litter decomposition rates as well. Fine roots with higher nitrogen content and lower carbon content decompose at faster rates.
Root respiration: the amount of CO2 released from a given mass of roots. Roots with higher respiration rates are "breathing" or "working" harder, just like you may find yourself doing after powering your way through this long blog post.
Root branching intensity: number of lower-order roots per centimeter length of higher-order root.
Root branching ratio: number of roots in a given order divided by the number of roots in the higher order.
All of the above images are showcasing the most ancient woody roots from lycopods that existed during the Carboniferous, which occurred between 360-300 mya (much earlier than the Cretaceous, but still a great example of how roots have evolved over time). These fossils are called stigmaria, which is a type of 'form genera,' meaning that they are grouped based on a part of the organism rather than by species. Geologists use this classification system because organisms such as large trees tend to fossilize specific parts (e.g., leaves, cones, and bark) that are found spatially separate from one another. Shout out to the geology department at Kentucky University who provided me with these pictures, and make sure to check out that link if you're interested in learning more about stigmaria - UK does a great job at explaining the specifics of the stigmaria fossilization process.
A diagram I'm borrowing from the previously mentioned Comas paper that's showing root diameter (in millimeters) of 20 tropical and subtropical angiosperm trees. I found this extremely helpful when trying to visualize how recently evolved species trend towards lower RD.
Trait Interactions with Water and Nutrients
Exploring the Connections Between Traits
The Race to Uncover a Multidimensional Root Trait Spectrum ... that works