As the rainy season comes to an end it might be more challenging to spot a mushroom tucked in the duff while on a walk through the oak woodlands at Elkhorn Slough. Above you in the oaks, though, if you look carefully you’ll see many species of lichens which are visible year round on the trunks, branches, and twigs. Lichens are like miniature ecosystems, composed of an association of a fungus, usually an ascomycete (a group of fungi that includes morels) and certain species of single-celled plants (green algae) or blue green algae (cyanobacteria) and support a menagerie of tiny invertebrates barely visible to the naked eye.
The overall shape of the lichen is dictated by the fungal partner, while the photobiont (green algae or cyanobacteria) usually lives in discrete layers within the fungus. In fact, there is debate whether the symbiotic association between algae and fungi forming the lichens is equally beneficial to the partners and might instead be a form of controlled parasitism of algae by fungi. The captive algae convert atmospheric carbon dioxide to carbohydrates via photosynthesis which are then used by the fungus, but no nutrients seem to pass from fungi to algae. Algae may gain physical protection within the fungal body, such as shelter from desiccation and UV radiation. Both partners benefit from being able to colonize habitats otherwise off limits to them individually. For example, the fungus can live in places where there is little available organic matter, while algae can thrive in environments of high light intensity, extreme temperature, and desiccation.
Lichens are broadly divided into three growth forms. Foliose lichens are leaf like and grow somewhat loosely attached to bark or rocks. Fruticose lichens are hair-like, strap-like, or shrubby and form tufts on rocks and trees or dangle from branches. Crustose lichens form crust-like structures that adhere tightly to tree bark, rocks, or other nonliving structures. Lichens are found in nearly every terrestrial habitat from the tropics to the extremes of deserts and arctic tundra, colonizing areas inhospitable to many other organisms. During dry conditions lichens remain dormant, essentially in suspended animation. When moisture becomes available via rain, dew, or fog, the algae resume photosynthesis and lichens can continue growing, though often extremely slowly, from several millimeters to as little as <1mm per year. Lichens are thought to be some of the oldest organisms; some arctic species are estimated to be at least 1000 year old and perhaps over 4500 years old, rivaling the ancient bristlecone pines in the White Mountains east of the Sierra.
In the last 30 years scientists have used lichens as bioindicator species to assess air quality. Lichens vary in their ability to survive in areas exposed to pollutants such as sulfur dioxide, fluoride, and ammonia, and since they tend to concentrate heavy metals lichens also serve as recorders of these other contaminants.
A variety of yellowish to grayish green lichens thrive at Elkhorn Slough, particularly noticeable on the oak trees. Wrinkled evernia, (Evernia prunastri) a fruticose lichen bears a green upper and white lower surface, while another common lichen with strap-like “branches” but entirely green and more stiff and brittle is the mealy ramalina (Ramalina farinacea). Lace lichen (Ramalina menziesii) drapes branches of several species of coastal trees of California, sometimes in great abundance; look for a modest stand in the oaks on the right side of the South Marsh loop trail in front of the small barn.
Common foliose lichens on oaks at Elkhorn Slough include the common greenshield lichen (Flavoparmelia caperata), found on trunk and branches. Another conspicuous foliose lichen is the powdered ruffle lichen Parmotrema chinense, which has hair-like projections along its frilly edges. In the parking lot look for two other species growing on the base of the wooden lampposts, Ramalina subleptocarpha with its wide strap-like branches as well as the beautiful bright orange foliose sunburst lichen Xanthoria sp. So the next time you are out on the trails, be sure to stop among the oaks and enjoy some of the more subtle inhabitants of the Slough.