Sugar Maple Symbioses
I've been really diving deep into lichens and learning as much as I can about them; sugar maples seem to to support both a great number and diversity of lichens. I think it has something to do with their textured bark providing a lot of cracks and crevices and lots of different microhabitats. They also provide pretty dense shade, maybe that is another factor. I'm not sure, but I wouldn't be surprised if there was also some interesting chemistry going on that is conducive to lichen growth in some way. So, I've also been really focusing in on sugar maples as well, my last two posts have revolved around sugar maples. Today is no different. While I've been looking for lichens, I've also been amazed that these trees support such a diversity of other life. Many of the species that I have spent the most time observing form symbiotic relationships.
Before we discuss the organisms, lets first talk about symbiosis. Symbioses are close ecological relationships that are sustained for a long preiod of time between two different species. It can be translated as "living together." There are three main categories of symbiosis; mutualisms are mutually beneficial to both organisms, commensalisms are beneficial to one organism and inconsequential to the other, parasitisms are beneficial to one organism and detrimental to the other. Symbioses between organisms are often quite complex relationships, the nature of which is in constant flux based on environmental conditions at any given time. For example, legumes will often form a mutualism with nitrogen-fixing bacteria. They will give carbohydrates in exchange for nitrogen, sometimes this trade is more one-sided and sometimes it is more equally distributed, depending on if there are ideal growing conditions, or droughts, floods, and other stressors. If the soil is fertile enough the plants won't even form the relationship because it becomes unnecessary. Some scientists refer to mutualisms like this as mutually exploitative, as each species is trying to get the most and give away the least. Others, like Martin Nowak, are a little less cynical and instead view mutualism as a "snuggle for survival." Whatever your perspective, all of these relationships whether mutualism, commensalism, or parasitism are usually more complex than they first might appear.
One of the most abundant species I was noticing today was mossy maze polypore (Cerrena unicolor). At first glance, the relationship seems very simple. When growing on dead maples the fungus is a saprotroph, helping decompose the wood and recycle the nutrients back into other parts of the ecosystem. On living maples it is a parasite, essentially doing the same thing but on live wood. But, maples and mossy maze polypores are not the only species involved in this relationship. The fungi is mutualistic with pigeon horntails (Tremex columba). The horntail larva eat wood of maple trees, but by themselves have no specialized enzymes or other mechanisms to break down the hard wood. Instead, they outsource the breakdown of wood to mossy maze polypore, after all breaking down wood is what they do best. So, trees infected with polypores provide the perfect diet for the horntail larva. The adult horntails have a specialized gland called a mycangium which they use to transport fungal spores to infect new trees. It's a good bet that if you see mossy maze polypores on a maple tree that it has pigeon horntail larva eating away in the wood, or at least they were there at some point.
Pigeon horntails are in turn attacked by the long-tailed giant ichneumonid wasp (Megahyssa macrurus macrurus) which is a parasitoid (a parasite that kills its host). Many parasitic wasps have very sensitive receptors on their antenna that they use to locate their prey. I have read that some researchers theorize that the ichneumonid wasp locates the horntails by zeroing in on chemicals produced by the fungus, others claim that it is pheromones from the horntail, or even vibrations as the larva move. Who knows the exact mechanism, or maybe a combination of all these theories, that allow the wasp to find the horntail which is buried deep inside the protection of the wood and bark. Once it finds its prey the wasp sticks its long ovipositor into the tree and lays its egg in the horntail larva with a concoction of chemicals to paralize the larva. In a while the eggs will hatch, consume the larva, pupate, and emerge as adults. Although they don't have the specialized glands, I would guess they transport some of the fungal spores to the next tree they visit. It looks like this wasp is either mutualistic or commensalistic with the fungi.
I only saw the fungus today, but it made me reflect on all of these other relationships that allowed for that fungus to grow. Those other species were likely present in at least some of the trees I saw today as well. To summarize we have a fungus that is parasitic on a tree, but also mutualistic with a horntail, that horntail is parasitized by a wasp, and that wasp is likely mutualistic or commensalistic with the fungus. These interaction really get me thinking about the vast complexities present in nature.
The story of the mossy maze polypore took up most of my headspace on my hike; I also saw some other cool examples of symbiosis on sugar maples. A similar fungus to the mossy maze polypore that I saw is the violet toothed polypore (Trichaptum biforme). Just like the other fungus it can act as a saprotroph or parasite on maple and other species of trees. Growing on some of these fungi was fairy pins (Phaeocalicium polyporaeum), a lichen that forms a commensalism with the fungi. The lichen gets a substrate to grow on and the fungi is largely unaffected. The mossy maze and violet toothed polypores also often have green algae growing on them, maybe they hold more moisture than the surrounding areas and form another commensalism.
Perhaps the most noticeable, yet still commonly overlooked example of symbiosis I saw was the maple canker fungus (Eutypella parasitica). It is extremely common in our woods at Wolf Ridge. It can affect a lot of maple species, but is most common on sugar maples. The canker starts with a very small necrotic spot, it grows larger each time the trunk grows a new layer of vascular tissue. It eventually will form a significant, large callus that is usually a few feet off the ground. If you look closely, you will often see the mycelia growing in the crevices of the canker. Trees affected by this parasite seem to be more susceptible to other diseases and attacks from insects, the canker provides an entry point where there is no bark protecting the tree.
Like always, I also saw a bunch of lichen species, which are in themselves a mutualism between a fungi and and algae or cyanobacteria. On one of the shield lichens I saw Homostegia piggotii, a parasitic fungi that grows on that species. Lichens also form commensalistic relationships with trees. The trees provide structure and support to grow and the lichen does not give much in return, or that's what most people would say. I would argue that this relationship is more of a mutualism. Lichens with cyanobacteria as the photobiont can fix nitrogen from the atmosphere, they often catch debris that nutrients leech out of when it rains, they hold extra moisture in the ecosystem, and eventually die and fall to the ground to decompose, returning their nutrients to the soil. All of these ecosystem services are beneficial to the trees that they grow on.
Today I was just focused on a handful of symbiotic relationships that sugar maples play some role in. I'm sure there were plenty more examples of these relationships on the sugar maples I didn't notice, and sugar maples are just one species. There are countless other examples of symbiosis that I saw today, on other plants which I didn't focus much on. There are probably exponentially more that I walked by without even noticing. Who knows what symbiotic relationships I will slow down enough to notice tomorrow.
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