Of Bluebells and Fungi

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Whether in your garden or in the woods, common bluebells (Hyacinthoides non-scripta) are a delightful respite from the dreary months of winter. It should come as no surprise that these spring geophytes are a staple in temperate gardens the world over. And, as amazing as they are in the garden, bluebells are downright fascinating in the wild.

Bluebells can be found growing naturally from the northwestern corner of Spain north into the British Isles. They are largely a woodland species, though finding them in meadows isn't uncommon. They are especially common in sites that have not experienced much soil disturbance. In fact, large bluebell populations are used as indicators of ancient wood lots.

Being geophytes, bluebells cram growth and reproduction into a few short weeks in spring. We tend to think of plants like this as denizens of shade, however, most geophytes get going long before the canopy trees have leafed out. As such, these plants are more accurately sun bathers. On warm days, various bees can be seen visiting the pendulous flowers, with the champion pollinator being the humble bumble bees.

The above ground beauty of bluebells tends to distract us from learning much about their ecology. That hasn't stopped determined scientists though. Plenty of work has been done looking at how bluebells make their living and get on with their botanical neighbors. In fact, research is turning up some incredible data regarding bluebells and mycorrhizal fungi.

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Bluebell seeds tend not to travel very far, most often germinating near the base of the parent. Germination occurs in the fall when temperatures begin to drop and the rains pick up. Interestingly, bluebell seeds actually germinate within the leaf litter and begin putting down their initial root before the first frosts. Often this root is contractile, pulling the tiny seedling down into the soil where it is less likely to freeze. During their first year, phosphorus levels are high. Not only does the nutrient-rich endosperm supply the seedling with much of its initial needs, abundant phosphorus near the soil surface supplies more than enough for young plants. This changes as the plants age and change their position within the soil.

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Over the next 4 to 5 years, the bluebell's contractile roots pull it deeper down into the soil, taking it out of the reach of predators and frost. This also takes them farther away from the nutrient-rich surface layers. What's more, the roots of older bluebells are rather simple structures. They do not branch much, if at all, and they certainly do not have enough surface area for proper nutrient uptake. This is where mycorrhizae come in.

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Bluebells partner with a group of fungi called arbuscular mycorrhiza, which penetrate the root cells, thus greatly expanding the effective rooting zone of the plant. Plants pay these fungi in carbohydrates produced during photosynthesis and in return, the fungi provide the plants with access to far more nutrients than they would be able to get without them. One of the main nutrients plants gain from these symbiotic fungi is phosphorus.

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For bluebells, with age comes new habitat, and with new habitat comes an increased need for nutrients. This is why bluebells become more dependent on arbuscular mycorrhiza as they age. In fact, plants grown without these fungi do not come close to breaking even on the nutrients needed for growth and maintenance and thus live a shortened life of diminishing returns. This is an opposite pattern from what we tend to expect out of mycorrhizal-dependent plants. Normally its the seedlings that cannot live without mycorrhizal symbionts. It just goes to show you that even familiar species like the bluebell can offer us novel insights into the myriad ways in which plants eke out a living.

Photo Credits: [1] [2] [3] [4] [5]

Further Reading: [1] [2]

 

On Fungi and Forest Diversity

One simply can't talk about plants without eventually talking about fungi. The fact of the matter is the vast majority of plant species rely on fungal interactions for survival. This mutualistic relationship is referred to as mycorrhizal. Fungi in the soil colonize the root system of plants and assist in the acquisition of nutrients such as nitrogen and phosphorus. In return, most photosynthetic plants pay their mycorrhizal symbionts with carbohydrates. 

There are two major categories of mycorrhizal fungi - ectomycorrhizae (EMF) and arbuscular mycorrhizae (AMF). Though there are a variety of different species of fungi that fall into either of these groups, their strategies are pretty much the same. EMF make up roughly 10% of all the known mycorrhizal symbionts. The prefix "ecto" hints at the fact that these fungi form on the outside of root cells. They form a sort of sheath that encases the outside of the root as well as a "hartig net" around the outside of individual cells within the root cortex. AMF, on the other hand, literally penetrate the root cells and form two different kinds of structures once inside. One of these structures looks like the crown of a tree, hence the term "arbuscular." What's more, they are considered the oldest mycorrhizal group to have evolved. 

The type of mycorrhizal fungi a plant partners with has greater implications that simple nutrient uptake. Evidence is now showing that the dominant fungi of a region can actually influence the overall health and diversity forest ecosystems. The mechanism behind this has a lot to do with the two different categories discussed above. 

Researchers have discovered that trees partnering with AMF experience negative feedbacks in biomass whereas those that partner with EMF experience positive feedbacks in biomass. When grown in soils that previously harbored similar tree species, trees that partnered with AMF grew poorly whereas trees that partnered with EMF grew much better. Additionally, by repeating the experiments with seedlings, researchers found that seedling survival was reduced for AMF trees whereas seedling survival increased in EMF trees. 

What is going on here? If mycorrhizae are symbionts, why would there be any detrimental effects? The answer to this may have something to do with soil pathogens. Thinking back to the major differences between EMF and AMF, you will remember that it comes down to the way in which they form their root associations. EMF form a protective sheath around the roots whereas AMF penetrate the cells.  As it turns out, this has major implications for pathogen resistance. Because they form a sheath around the entire root, EMF perform much better at keeping pathogens away from sensitive root tissues. The same can't be said for AMF. Researchers found that AMF trees experienced significantly more root damage when grown in soils that previously contained AMF trees. 

The differences in the type of feedback experienced by EMF and AMF trees can have serious consequences for tree diversity. Because EMF trees are healthier and experience increased seedling establishment in soils containing other EMF species, it stands to reason that this would lead to a dominance of EMF species, thus reducing the variety of species capable of establishing in that area. Conversely, areas dominated by AMF trees may actually be more diverse due to the reduction in fitness that would arise if AMF trees started to dominate. Though they are detrimental, the negative feedbacks experienced by AMF trees may lead to healthier and more diverse forests in the grand scheme of things. 

Infographic by [1]

Further Reading: [1]