Macrolichens of the Pacific Northwest
Introduction
What are lichens?
Lichens are enormously successful and evolutionarily ancient composite organisms comprising members of two, and sometimes three, biological kingdoms. Specifically, a lichen consists of a fungus and its photosynthetic partner (green algae, cyanobacteria or both) growing together in a mutually controlled, symbiotic relationship. The composite form is strongly altered in appearance, physiology, reproduction and chemistry, compared to freeliving fungi, algae or bacteria.
Lichens are named after their fungal partner
(mycobiont),
usually an ascomycete. Worldwide, about one fifth of all known
fungi are lichenized (Honneger 1996). In contrast there are only
about 40 genera of photosynthetic partners (Tschermak-Woess 1989,
Büidel 1992). The most common photosynthetic partners
(photobionts) are green algae. In the Pacific Northwest,
Trebouxia and trebouxioid species are the primary photobionts in at least 60% of macrolichen genera. Other green alga photobionts of PNW
macrolichens are
Chlorella, Coccomyxa, Cystococcus, Dictyochloropsis,
Elliptochloris, Myrmecia and
Pleurococcus. Although it is often
possible to determine the genus of the algal photobiont in lichens,
the alga must be isolated and cultured to determine species. This is
because life cycle stages and chloroplast morphology, must be
observed to identify species, but do not occur or are altered in the
lichenized state.
The primary photobionts of about 15% of PNW macrolichen genera are the cyanobacteria,
Nostoc and
Scytonema. Identification of cyanobacterial species, even from cultures, is nearly impossible since species concepts for these bacteria are usually defined by ecological features. Lichens with cyanobacterial photobionts can "fix" atmospheric nitrogen into forms useable by plants and animals. The lichen genera
Collema, Hydrothyria, Koerberia, Lcmpholcmma, Leptochidium, Leptogium, Massalongia, Pannaria, Parmeliella, Polychidium, and
Pseudocyphellaria all have cyanobacterial photobionts. A small number of lichens contain both green algal and cyanobacterial photobionts within the same individuals, e.g.,
Solorina, Psoroma hypnorum and some species of
Lobaria, Nephroma, Peltigera, Pilophorns, and
Stereocaulon. In these cases, green algae are the primary photobionts and special internal or external nitrogen fixing structures
(cephalodia) are formed containing cyanobacteria. Cephalodia are surrounded by a dense, cemented layer of fungal tissue which reduces oxygen diffusion.
The partially anaerobic environment of the cephalodium increases the density of cyanobacterial nitrogen fixing cells (heterocysts) compared to the freeliving state, boosting nitrogen fixation rates (Hitch & Millbank 1976).
Why are lichens important?
Lichens contribute to biological diversity. The lichenized state is thought to have originated several times in unrelated fungal genera (Tehler 1996). Over subsequent eons a magnificent wealth of genetic, chemical and ecological diversity emerged. Today we are the inheritors of a planet rich in lichen colors, forms, and species. And many lichens still await discovery. Between 1987 and 1995 almost 400 lichens were reported in North America for the first time (Esslinger and Egan 1995). About 1,000 different lichens from the Pacific Northwest are now known. Conservation of good air quality and critical habitats will be essential to maintain this diversity.
In addition to their contribution to biological diversity, lichens are ecologically important in the Pacific Northwest as food, shelter and nesting material for wildlife. Deer, elk, moose, caribou, mountain goats, bighorn sheep, pronghorn antelope and various species of squirrels, chipmunks, voles, pikas, mice and bats eat lichens or use them in nest building (Banfield 1974, Conner 1983, Denison 1973, Edwards and Ritcey 1960, Fox and Smith 1988, Gunther et al. 1983, Richardson and Young 1977, Stevenson and Rochelle 1984, Thomas and Rosentreter 1992, Ure and Maser 1982). At least 45 species of North American birds use lichens as nesting material (Richardson and Young 1977). Among invertebrates, bristletails, barklice, katydids, grasshoppers, webspinners, butterflies, moths, lacewing larvae, mites, spiders, snails, slugs and many beetles live on, mimic, or eat lichens (Gerson and Seaward 1977).
Lichens are not just appetizers or supplementary nesting material, but a critical component of the Pacific Northwest forest food web. For example, flying squirrels rely on the brown beard lichen,
Bryoria, as their principal winter food source (Maser et al. 1985 and 1986). Flying squirrels, in turn, are one of the two or three primary prey of the northern spotted owl, an endangered species whose protection needs have redefined forest management on public lands in the Pacific Northwest (USDAFS and USDIBLM 1994a). In summer, the main staple of flying squirrels is underground-fruiting fungi. As they travel through the forest, the squirrels disperse fungal spores in their droppings. The spores germinate and form mycorrhizal associations with tree roots which are critical to the tree's ability to absorb nutrients (Maser et al.1986). The trees provide habitat for lichens, flying squirrels, spotted owls and other organisms.
Lichens play significant roles in mineral and hydrological cycles, notably nitrogen fixation. In certain coastal and western Cascade oldgrowth forests of the Pacific Northwest, nitrogenfixing
Lobaria oregana dominates the lichen biomass. As
Lobaria leaches during rains or decomposes in litterfall, it contributes significant amounts of nitrogen to the forest ecosystem (Pike 1978). When biomass is high, lichens and bryophytes hold moisture, moderating humidity and temperature fluctuations within the canopy. They also intercept and release nutrients from atmospheric sources such as rain, dew, fog, dry deposition and gases, which might otherwise be lost or unavailable (Knops 1994). Desert crusts of lichens, fungi, cyanobacteria and moss reduce soil erosion by intercepting surface runoff and facilitating inflitration of water into hardpan soils (Harper and Marble 1988).
Unprecedented human population growth has intensified the conflict between the preservation of ecological integrity and human needs for more space and resources. It is ever more important to understand the interconnecting web of micro and macroorganisms, and the basic requirements necessary to maintain functioning, resilient ecosystems. So far, the habitat requirements, population distribution and ecological niches of most lichens in the Pacific Northwest are not well understood. Some of the largest genera of crustose lichens are still taxonomically unresolved.
Lichens are also important as indicators. Lichen communities change with successional stages in vegetation (McCune 1993, Lessica et al. 1991, Neitlich and McCune 1997). Land managers can use lichens to indicate forest continuity (Kuusinen 1996, Tibell 1992, Rose 1976), to determine the distribution of specialized microhabitats and microclimates (Pearson and Lawrence 1965), to detect hotspots of biological diversity over the landscape (Karstr6m 1992, Nitare and Nor6n 1992, Neitlich and McCune 1997) and to assess water (Beck & Ramelow 1990) and air quality (see Appendix III: Lichens and Air Quality).
Finally lichens have economic value. Compounds unique to lichens are used in medicines (e.g. antibacterials and antivirals), perfumes and wool dyes. Ornate Pacific Northwest forest lichens, like
Letharia, Usnea longissima and
Ramalina menziesii are harvested for use in floral displays around the world. Traditional uses of lichens include: human survival food, poisons and tanning agents, numerous hygenic and medicinal applications, bedding, diapers and absorbents, decorations, and dyes and fiber for cloth.
How lichens work
The photosynthetic partner of a lichen produces food for the fungus, converting atmospheric carbon dioxide into carbohydrates via photosynthesis. The fungal partner forms a physical structure or "home" for the photosynthetic partner. It provides surface area for the capture of mineral elements and protection from excessive light and dessication.
Alternating periods of drying and wetting are considered necessary for the health of most terrestrial lichens and apparently ensure an adequate distribution of carbohydrates between the partners. In species studied so far, medium to high water content favors carbohydrate transfer to the fungus. At low water content, the alga retains more carbohydrate. Photosynthetically made carbohydrates are transferred from green algae to the fungal parner as sugar alcohols, and from cyanobacteria to the fungal partner as glucose (Nash 1996).
As a composite organism, lichens can grow in habitats where neither partner could survive alone. An extraordinary example is the ability of certain desert coastal lichens to thrive on daily dew or fog cycles using a kind of reverse transpiration. Under cool temperatures and high humidity, photosynthesis can be activated in green algal lichens after water vapor uptake alone, partly aided by the high osmotic content of the fungi (Nash 1996). By contrast, activation of photosynthesis in cyanobacterial lichens requires liquid water. Consequently, cyanolichens are typically found in moist microhabitats, such as mosscovered logs or tree bases, soils of shaded roadcuts, and on various substrates in riparian areas.
Growth forms and structural features
Lichens are artificially divided into three growth forms: crustose (crust),
foliose (leafy) and
fruticose (shrubby). [See the Glossary for illustrations and definitions of highlighted terms in this and following paragraphs]. Within each form there is considerable gradation and specialization. Other specializations in form include
squamulose (Cladonia, Pannaria, Psoroma) or
umbilicate (Umbilicaria, Rhizocarpon, Dermatocarpon) lichens. The latter have a central point of attachment; the former consist of tiny, leaflike scales, sometimes producing upright, fruticose stalks called
podetia. Macrolichens include foliose, fruticose and the larger squamulose forms.
The lichen body is called the
thallus. Most macrolichen thalli are
heteromerous, i.e. stratified internally. The upper or outermost layer is the cortex. The cortex can be thin to thick, pigmented or not, and contains closely packed isodiametric or elongated hyphae. Many lichens have an outermost epinecral layer of dead, collapsed and sometimes gelatinized hyphae and photobiont cells. This layer thickens in individuals growing in sunny habitats, making the individual darker and less green. Some lichens, e.g., most species of
Physconia, have surface mineral deposits which give them a powdery white appearance. These are often of calcium oxalate and are called
pruina. In fruticose and foliose lichens, the photobiont layer is found directly internal to, or below, the cortex in the upper
medulla. The medulla is a layer of loose, cobwebby, white fungal hyphae with many intercellular air spaces. In
Usnea , the medulla contains a structurally supporting central cylinder of thickwalled, closelypacked hyphae. Most foliose lichens form a lower cortex which is clearly differentiated from the upper cortex by color or by the presence of features such as hairlike, anchoring
rhizines, or a velvety
tomenturn of short, dense hyphae.
A few macrolichens are nonstratified or
homoiomerous. Nonstratified lichens (e.g.,
Collema and
Leptogium) generally have a cyanobacterial photobiont dispersed throughout the lichen body and appear dark and gelatinous when wet.
Features commonly used to identify lichens and distinguish species include shape, size and color of the thallus and
lobes (branches); the pattern and angle of branching, the degree of adhesion to the substrate, habit of the primary thallus (e.g. spreading, erect or pendant), and the structure and frequency of fruiting bodies and spores.
Other differentiating features are the presence or absence of surface wrinkles, hairlike
cilia on the lobe margins, minute bumps called
papillae, cephalodia, pseudocyphellae and cyphellae.
Pseudocyphellae are spherical pores through the upper or lower cortex which are filled with loosley packed medullary hyphae.
Cyphellae are larger, and the medullary hyphae lining the pores form rounded tips which adhere to each other. They are found only in the genus
Sticta, and look like smooth, whitish craters dotting the tomenturn of the lower surface.
Color is a very important characteristic for lichen identification. It is essential to distinguish taxonomically important color variation from environmentally induced color variation. Strong light can cause individuals to be deeper colored, browned or blackened. Shade forms, even of yellow or orange lichens, tend to be pale and more greenish. Lichens containing usnic acid are yellowish; but the yellow cast can be hard to detect, depending on how much usnic acid is present. Thomson (1967) recommends a 5% chloramineT test to confirm the presence of usnic acid in
Cladonia and
Cladina, two genera where color distinction is critical for keying.
Reproduction, growth and dispersal.
Lichens reproduce asexually and sexually. A few macrolichens, particularly the pendant epiphytic lichens,
Alectoria, Bryoria, Ramalina, and
Usnea, rely largely on fragmentation for dispersal. As an individual grows, pieces break off and become entangled in branches below or are carried short distances by wind or animals where they may establish new individuals. Although these genera include some of the fastest growing lichens, fragmentation can lead to a net increase or decrease in population biomass, depending on whether the fragments land in suitable habitats. The fraction which falls to the forest floor provides important winter survival forage for deer and other mammals, particularly in deep snow conditions (Hanley et al. 1989; Rochelle 1980). The largest biomass of these forage lichens occurs in latesuccessional and oldgrowth forests (McCune 1993; Lesica et al. 1991).
Overall, lichens grow and disperse slowly compared to vascular plants. Specialized habitat requirements, the need for continuity in the availability of substrate, and sensitivity to air pollution make many lichen species vulnerable to habitat disturbance or degradation. In the Pacific Northwest, lichen biodiversity is high along coastal and riparian areas and in high rainfall, low to mid elevation forests. Because coastal, riparian, and timbered areas receive intensive human use, land management policies will determine the survival chances of some Pacific Northwest lichens over the coming decades (Rosentreter 1995; USDAFS et al. 1993; USDAFS and USDIBLM 1994a; USDAFS and USDIBLM 1994b).
Asexual reproduction also occurs via the production and dispersal of tiny propagules containing both the fungus and the primary photobiont. Many types of vegetative propagules have been recognized in lichens. Two kinds,
isidia and soredia, are referred to most often in identification keys. They disperse in air or water and can initiate new individuals. Both types are best observed with a handlens or dissecting microscope. Isidia are tiny (30 ~tm to I mm high), stratified, usually cylindrical to globular, branched or unbranched structures. Soredia are microscopic (2050 pm diameter), loose spherical clusters of photobiont cells and fungal filaments. Soredia appear flourlike to granular and occur either diffusely spread over the upper surface or in macroscopic patches, called
soralia.
Some mystery still shrouds the mechanisms and relative importance of sexual reproduction in lichens. In principle, the fungal partner produces spores which germinate, contact freeliving algae, and develop into new individuals. Some lichen photobionts are freeliving and widespread
(Nostoc, Chlorella, Trentepohlia), but the most common macrolichen photobiont,
Trebouxia, is not known to occur in nature outside lichen thalli. And it is not known whether the freeliving photobionts such as
Nostoc are genetically the same as the lichenized populations. Recently, a few fungi have been shown able to obtain algae from other thalli or vegetative propagules (Ott 1987; Friedl 1987). However it occurs, a large number of lichens reproduce sexually and must reestablish the symbiotic state with each reproductive cycle.
Sexual structures are produced by the fungus. There are two kinds which occur in PNW macrolichens, apothecia and perithecia.
Apothecia are disklike fruiting structures and are the predominant form. A smooth, fertile layer of asci (spore containing cells) is exposed on the upper surface of the disk.
Sphaerophorus and
Tholurna have a type of modified apothecium, sometimes called a
mazaedium, in which the asci disintegrate at maturity, producing a powdery spore mass.
Perithecia occur in only one macrolichen genus,
Dermatocarpon. They are flaskshaped and immersed in the thallus. All that can be seen with a hand lens is the tiny opening of the flask at the surface.
Further Reading
For more information on lichen biology see Nash (1996), Ahmadjian
(1993), or Hale (1983).
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