Plant diversity and the diversity of most other taxa increases as you travel closer to the equator. At these low latitudes, there is more sunlight, more water, and higher temperatures. All in all, there is a much longer, and more suitable growing season for plants. Higher plant diversity radiates up the food chain, as those species rich assemblages support more insects, amphibians, reptiles, birds, and mammals. If you ever are able to see a tropical forest with your own eyes, you will be stunned by its chaos. Everywhere you look you’ll find something new and exciting. The level of competition is intense here, which leads to evolutionary specializations over time. The diversity of fauna and most other biota increases with a decreasing latitudinal gradient. Though, one recent study discloses a different pattern for our fungal friends.
In 2013, Ling-Ling Shi and her team of scientists wanted to understand how fungal diversity changed with a latitudinal gradient. To do this, they selected 17 undisturbed forest ecosystems running from northern boreal forests, through temperate and sub-tropical ecosystems, all the way to the most southern tropical forests in China. To quantify the fungal community present at the forest floor in each latitudinal position, they isolated the genetic material from each soil sample and used a technique called 454 pyrosequencing. Combined, all of their 17 soil samples yielded 62,839 DNA sequences!
Through careful analyzation, they found some striking patterns in fungal diversity across this latitudinal gradient. This study suggests that boreal and temperate ecosystems are dominated by ectomycorrhizal (ECM) fungi. Subtropical forests are dominated by saprobic (decomposing) fungi, and tropical regions had arbuscular mycorrhizae (AMF) dominate. They found that at the most northern (boreal) ecosystems supported a low fungal diversity. I wasn’t surprised at a low fungal diversity associated with boreal forests, because these places have the narrowest growing season, as cold temperatures, minimal sunlight, frozen ground and snow diminish photosynthetic output. With less sugars synthesized through photosynthesis, fewer carbohydrates become sent to underground assemblages of mycorrhizal fungi. Also, the cold temperatures don’t promote the enzymatic activities of saprobic fungi.
Surprisingly contrary to other taxa near the equator, fungal diversity at both sub-tropical and tropical latitudes had the lowest fungal diversity. How could you explain this pattern, given the most species rich places on Earth support fewer fungal species than a frozen boreal wasteland? Well, Greg Gilbert proposes that because these regions support so many different plants, no one species dominates. Without any plant dominating, there no true fitness reward for specializing in breaking down or symbiotically pairing up with a small subset of plants. Most fungi existing in these regions are generalists, AMF generalists to be exact. With a composition of fungal generalists, most species compete with each other. Since competition reduces the fitness of both species (-,- interaction) fewer species can coexist.
Temperate forests support the highest fungal diversity. Could it really be? These places don’t have the longest growing season, but they also don’t have the shortest. This paper showed that ECM fungal diversity has an inverse relationship with plant diversity. That is, when plant diversity decreases in areas dominated by ECM, ECM richness increases. The authors don’t really explain this pattern, but I have my own conclusions. With less plant diversity in temperate ecosystems, there is not only less competition, but more sharing of soil nutrients. Plants will actually send photosynthates to the soil, through mycorrhizal networks to closely related individuals. This mycorrhizal network has been coined the wood wide web, because of how easily it connects trees, similarly to how the internet connects us. With fewer plant species, immense networks develop between stands of trees in temperate ecosystems. Much of the photosynthate being sent through these networks remains in the soil.
Analogous to why adequate sunlight, temperature and water supports plant diversity near the equator, the huge volumes of sugars being pumped into the ground by ECM trees supports the most diverse fungal communities. To me, this is crazy. For some reason, many people I meet in this widespread mycological scene are from temperate places. Could this be because we get to see the most divers assemblages of fungi on the planet? Before I read these papers, I thought you had to head down to the equator to see the most diverse fungal communities. Boy was I wrong. I’m not talking tropical forests down by any means-you truly have to experience them for yourself. I am however proudly exclaiming that if you live in a region like myself, you don’t have to travel far to witness an insane array of fungal species living from the temperate forest floor.