Old-Growth Forests and Fungi
by John Cook, Seminole County Coordinator (Florida)
In our ever-evolving world, old-growth forest habitats stand as irreplaceable and complex ecosystems where intricate relationships among flora, fauna, and fungi have thrived over millennia. Hiking through these environments reveals towering ancient trees, diverse flora, and a myriad of fauna that name these places as their homes. Old-growth forest ecosystems create diverse environmental conditions and habitats that sustain abundant life, particularly within the fungal realm.
Ancient forests offer unparalleled structural diversity that nurtures thriving habitats. With trees at varying stages of life—from young saplings to centuries-old giants, including those in decay—they collectively contribute to the overall health of the ecosystem, especially benefiting fungi. The fungal kingdom, which includes molds, yeasts, and mushrooms, plays a pivotal role in sustaining and structuring life within old-growth forests. This article will primarily focus on the roles of mushrooms and lichens with these uniquely complex ecosystems.
Photo courtesy of John Cook.
Mushrooms, which are the fruiting bodies of mycelium, encompass a vast array of species, each one performing specialized ecological functions. These roles include decomposing organic matter, forming symbiotic relationships with trees, and regulating insect and tree populations. Fully mature and undisturbed forests showcase complex ecosystems where these organisms - and many others - thrive.
Mycorrhizal mushrooms, including Arbuscular and Ectomycorrhizal fungi, establish crucial symbiotic relationships with trees and plants. In this way they have supported plant life for millions of years. Arbuscular fungi form associations with plant roots, while Ectomycorrhizal fungi form symbiotic relationships with tree roots. These fungi create intricate networks that efficiently harvest nutrients like phosphorus and nitrogen from the soil, exchanging them for carbon sources from plants and trees - thus nourishing both themselves and their plant hosts. These mycorrhizal networks span vast areas, facilitating communication and nutrient exchange among numerous plants and trees in a way we are only just beginning to comprehend.
Photo courtesy of John Cook.
“ These mycorrhizal networks span vast areas, facilitating communication and nutrient exchange among numerous plants and trees in a way we are only just beginning to comprehend.”
Specific examples of mycorrhizal mushrooms include Russula sp., Amanita sp., Lactarius sp., Suillus sp., Hebeloma sp., among many others. Mature forests provide these mushrooms with ideal conditions to establish extensive networks, which then promote plant growth and safeguard other sensitive species that thrive in undisturbed habitats. However, a significant challenge lies in the lack of dedicated research focused on these environments and their vulnerability to external impacts, particularly in regions like my home state of Florida. There is especially minimal research on mushroom species taxonomy and their ecological roles within old-growth forests. A dedicated effort by individuals and professional organizations is crucial to documenting the diverse species of fungi and their ecological contributions, especially in light of the multifaceted threat of habitat loss.
Since so little taxonomic and historical data regarding fungi in Florida forests is available, it is hard to give species-specific examples of mycorrhizal fungi associated with old-growth forests in this state. There are some provisionally named species within the Amanita section that have only been found in mature habitats, where they form symbiotic relationships with either large oak trees or large old-growth sweetgums (Liquidambar styraciflua). There are plenty of other fungi within these forests and, although these other species are not necessarily rare, they will produce fruiting bodies over much larger areas compared to those in disturbed or immature forests. Lactarius indigo is one such example. I have witnessed large fruitings of these mushrooms, numbering into the hundreds, in areas few people have gone. Truly a sight to see.
Photo courtesy of John Cook.
In addition to supporting plant life, mushrooms play vital roles in the forest ecosystem by decomposing dead trees, plants, and other organic matter. Saprotrophic fungi, including the genera Pluteus, Mycena, Gymnopilus, Favolus, Gymnopus, and Entoloma, among others, fulfill this role crucial to the overall health of the environment. These fungi break down organic waste, convert it into nutrients, and stabilize soil. They thrive in mature forests, forming extensive networks across the forest floor and other organic substrates. Interestingly, it is believed that most all species of mushrooms start life as a saprotrophic fungi until they can locate and form a relationship with a host.
Pathogenic and parasitic fungi, like Armillaria and Ganoderma, play critical roles in regulating forest dynamics by parasitizing trees and controlling insect populations. These fungi contribute to maintaining the balance in mature forest ecosystems, whereas younger or less healthy forests may experience dominance by certain of their species, disrupting ecosystem stability. Ganoderma are great examples of a mushroom genus which infects living trees and ultimately causes their slow demise. Once the tree has perished from the infection, the mushroom continues to thrive by acting as a saprotroph, gradually consuming and thus breaking down the tree. This particular group of mushrooms can persist for several months to several years on their hosts.
Photo courtesy of Robert Llewellyn.
Laricifomes officinalis, known as agarikon, is the sole fungi in the Laricifomes genus and is classified as endangered. It faces significant threats due to deforestation, particularly in the Pacific Northwest of the United States where it is primarily found. Agarikon inhabits natural forest areas, exclusively on large, old-growth trees. While it mainly associates with species in the Larix (Larch) genus, it can occasionally be found on other trees such as Pinus (Pine) and Abies (Fir). Similar to Ganoderma species, agarikon initially parasitizes its host tree and later acts as a saprotroph after the tree has died. This mushroom serves as a critical example of a fungal species that requires protection through the preservation of undisturbed old-growth forests to ensure its survival.
Mushrooms also parasitize organisms other than trees. The Cordyceps and Ophiocordyceps genera target specific insect species, including ants, beetles, and moths. In my local environment in central Florida, Ophiocordyceps camponoti-floridani infects Florida carpenter ants, turning them into “zombie ants.” This fungus manipulates the infected ant’s behavior, causing it to climb trees and secure itself to the tips of Bartram’s air plants before perishing. After the ant’s death, mushroom fruiting bodies emerge from its head and body, releasing spores to infect other unsuspecting hosts. This species thrives in mature and old-growth forests and is rare in urban areas or younger forests.
Photo courtesy of John Cook.
In other roles, mushrooms serve as vital food sources for numerous forest inhabitants, such as deer, squirrels, and insects. An interesting example of this is how some mushrooms excrete substances to kill certain insects, in order to attract other, more predatory insects. The insects, attracted to a free meal, get their exoskeletons covered with the mushroom’s spores, and their further travels help the mushroom reproduce in other locations. Predators such as lizards and birds will also come to the mushroom to consume these spore-covered, mushroom-slain insects, thus expanding the potential range of the spores even further.
In another example of ecological interconnection, fungi also form complex symbiotic relationships with cyanobacteria and algae, creating lichens which greatly enrich forest ecosystems. Lichens play multifaceted roles - they provide food and nesting materials for mammals and birds, convert carbon dioxide into oxygen, regulate canopy humidity, and fix gaseous nitrogen into a more stable form for plant use. Lichens specific to old-growth forests serve as crucial environmental indicators, as they are sensitive to changes caused by pollution, climate fluctuations, and other disturbances. Monitoring lichen density and diversity can provide scientists with valuable insights into air quality and the impacts of climate change on a given forest.
Lichens also absorb various atmospheric chemicals, such as sulfuric and nitric acids, ammonia, sulfur dioxide, and fluorine. This absorption then influences their growth and health. Tissue samples of lichens can be collected and analyzed for air contaminants they have absorbed, reflecting environmental conditions. High nitrogen levels adversely affect lichens, similar to other pollutants mentioned earlier. Lichens tend to react to these environmental impacts rather quickly, causing them to perish or show visible signs of distress. These organisms are very sensitive to temperature increases and can thus be used to map and track temperature-based environmental impacts, such as gradual climate change or the urban heat island effect.
Photo courtesy of Robert Llewellyn.
Other species of lichens are unique in their abilities to endure harsh environments, colonizing bare rock and tree bark. They are thus essential in measuring ecosystem succession and stability. These lichens can endure extreme temperatures and lack of water, and they contribute to soil formation by breaking down rocks and releasing nutrients back into the soil. In old-growth forests, lichens cover tree trunks and branches, providing microhabitats for small invertebrates and thereby boosting forest biodiversity.
Understanding the intricate roles that fungi play and the relationships they form underscores the importance of preserving and protecting old-growth forests. These ecosystems provide sanctuary for rare and endangered species across kingdoms, highlighting their enormous conservation significance. The next time you explore these magnificent forests, take a moment to appreciate the mushrooms and lichens you encounter there, and reflect on their essential roles as small but mighty supporters of old-growth forests as we know them.
Photo courtesy of John Cook.