While many eagerly await the trees’ new leaves, I’m still enjoying the lichens.
One of the most remarkable things in nature, lichens really grab attention in winter. The ample moisture revives the lichens after the dry season, and the die-back of summer vegetation exposes them.
Many winter-bare branches are flocked with tufts of faded green. Lichens take many other forms as well: patches of crust, tangles of string, broad or narrow ruffles, bony clumps, crowds of miniature match sticks, clusters of golf tees.
The shapes of lichens sometimes appear so bizarre that we may forget they are living organisms -- well, two organisms, actually.
No, wait… At least some lichens are the result of three partners. In 2011 Toby Spribille, a lichen researcher in Montana, discovered a second type of fungus in a common lichen while trying to uncover why that lichen came in two quite different forms when the DNA of the [primary] fungus was identical in both forms.
A lichen is the result of a relationship between one or two fungi and a photosynthetic organism, usually one-celled green algae or cyanobacteria (“blue-green algae”). (Cyanobacteria are a group of bacteria that can photosynthesize—use sunlight to make food from water, carbon, and other nutrients.)
In fact, the word “symbiosis,” to mean a mutually beneficial partnership of two organisms of different species, was coined about 140 years ago to describe lichens.
Under today’s organizational grouping of life, none of the partners in a lichen would be considered a “plant.” The scientific name for a particular lichen is the scientific name of the primary fungal partner.
The primary fungus, a many-celled non-green organism, incorporates many one-celled algae/cyanobacteria within its body. The primary fungus provides structure, nutrients, and some protection for the photosynthetic organisms; the photosynthetic organisms provide food for the fungus. It isn’t yet known exactly how the second fungus participates.
Lichenologist Trevor Goward described lichen as "fungi that have discovered agriculture." It’s not an equal partnership, however, as the fungi occasionally kill some of their partner cells.
Looking under the microscope, the two partners of some lichen species look fairly well organized, with the fungal threads (mycelium) forming two mats that support and contain an inner layer of the photosynthetic partners. In other lichens, the partners seem more randomly placed, with the green partners caught in a snarl of white fungal threads.
While any of the fungal or photosynthetic partners in a lichen can -- and often do -- survive separately, the partnership allows the combination to prosper under conditions too harsh for either alone. From sunbaked desert rocks to frozen tundra, lichens often thrive were other plants cannot. Lichens even colonize barren sand in wind-protected sites in the Oregon dunes. This is a good adaptive strategy: about a fifth of the world's fungi may form such partnerships.
The lichen bond may form at the fungus' earliest stages of development when the outgoing spores of the [primary] fungus are set free already entwined with a photosynthetic partner. Or, a fungus may start out solo and bond later with partners. The process of that combination, called "lichenization," is not well understood but apparently occurs in a variety of ways.
It’s likely that a great deal of distribution of the bushy lichen occurs by simple breakage, with chunks blown off a high branch to become lodged on a low branch or the ground, for example.
The thousand or so Pacific Northwest lichens fill important roles: as habitat for invertebrates and microorganisms; as food for mice and elk and others; and as nesting materials. Lichens also fix large amounts of nitrogen from the air and help moderate the humidity around them.
Further, lichens' tolerance for severe conditions allows them to pioneer environments. Lichens that thrive -- and then die -- on a bare surface, such as rock or stabilized sand, add a bit of organic material that can be the start of a growing progression of other life.
And, no, the masses of lichens adorning twigs, branches, and bark do not hurt the trees. Their extraordinary relationship makes them self-sufficient.