Class from the Field: Lichens & Ecological Succession
View from Huguenot Head on Champlain Mountain.
A typical mountaintop ecosystem on MDI: blueberries thriving under the open jack pine canopies.
For this Class from the Field, we spent the morning on Champlain mountain in Acadia National Park, exploring the scrubby mountaintop ecosystem that exemplifies the pioneer stage of ecological succession.
What is succession? From bare rock to towering forest, ecologists have defined predictable stages of inhabitation as an ecosystem grows more complex over time. Let’s start at the beginning.
The first organisms to make a landing on bare rock and cultivate new soil are lichens, fungi and mosses. We spent some time getting to the bottom of exactly what lichen is. Many people mistake lichens for plants, because of their branching or leafy structures, and because they photosynthesize. However, lichen are actually symbiotic micro-ecosystems of algae, fungi, and sometimes yeast! Algal cells photosynthesize and provide the fungi with sugars, while the fungal threads serve as a protective structure for the algae. This is called a mutual exchange!
Branching lichen intergrowing with blueberries.
Baby jack pine, another pioneer species we looked at today! Unlike most pines, jack pines love sunlight and can grow in barren soils, making them perfect pioneers for desolate habitats.
A glimpse into the history of the ‘lichen controversy.’
During the late 19th century, the true nature of lichens was a subject of controversy. Some scientists were sure that lichens were distinct organisms, while others argued that lichens were in fact symbiotic composites. One of the first people to strongly take a side, after observing lichens growing out of fungal spores, was children's author and amateur botanist, Beatrix Potter! Read more about this story and check out Potter's early botanical watercolors here!
So, back to succession - After lichens and mosses establish a layer of organic material on a rocky substrate, small plants, perennial grasses, and shrubs are able to establish themselves. An ecosystem always starts out with organisms that are physically small, with short life cycles, and high rates of reproduction. As the ecosystem develops, larger, more complex organisms with greater longevity and lower reproduction rates have the resources to grow. So, after perennial shrubs have made a home, deciduous trees like beech, birch, aspen, maple and oak will start to colonize the new habitat. Later on, under the deciduous canopy, shade-tolerant communities of pines and spruces will begin to grow. As the forest ages, these evergreens will eventually grow tall and out-compete the deciduous trees. This is why old-growth forests tend to consist mostly of towering evergreens.
Enter catastrophic events, such as wildfires, that return the ecosystem back to the beginning of the succession cycle. A local example of such an event was the great fire of 1947 that burned down half of Mount Desert Island. (The historical accounts of this fire make for a harrowing tale - read more about it here.) Wherever the fire burned 70 years ago we now see deciduous forests of beech and birch trees. The rest of the island, protected for so long as a national park, consists of an older evergreen-dominant forest.
Smoke covering Mount Desert Island during the 1947 fire.
Fires such as the 1947 fire used to be framed as ecological tragedies. However, ecologists now know that many ecosystems have evolved with fire as an essential contributor to habitat vitality and renewal. Fires clear out dead material and create spaces for new species. In fact, many plant species (such as the Jack Pine!) require fire to germinate, establish, or to reproduce.
Our participants local to Blue Hill then asked - why is Blue Hill mountain covered in a mix of deciduous and evergreen trees? No great fire burned there! The answer: the entire mountain (like most of New England) was completely clear cut in the mid-19th century for timber and mining purposes. Go to Blue Hill Heritage Trust's website to read a brief history of Blue Hill Mountain, here.
Next time you're walking in the woods, or in a meadow, or on a mountaintop, look at the communities around you. How old are they? Given the current conditions of that ecosystem, what may grow next? These are important considerations for restoration ecologists! Each ecosystem has its own succession cycle, and it's essential that we understand them in order to protect them and restore their healthy cycles.
