Nature & Science

Bedrock Terrace Xeric Savanna is a rare habitat with sparse plants and abundant lichen.

Virginia Department of Conservation and Recreation

The parkway's 4,580 acres are home to at least 81 plants and animals considered rare, threatened, or endangered in Virginia or Maryland. These include 37 species of vascular plants, 2 vertebrates, and 19 invertebrates.

Many of these rare species are associated with the plant communities found in few places in the Potomac River Gorge. Some of these habitats have names that make them sound every bit as exotic as they are, like Bedrock Terrace Rim Xeric Forest and Bedrock Terrace Xeric Savanna.

Natural areas within the park are extremely biodiverse, and parts of these areas have not been studied fully yet. As recently as 2011, a species of caddisfly new to science, Neophylax virginica, was discovered in Turkey Run. Each new study yields first records of species that live in the area.

 

Mammals

A total of 25 species of mammals have been recorded from GWMP. These include five species of bats, Big Brown Bat (Eptesicus fuscus), Eastern Pipistrelle (Pipistrellus subflavus), Little Brown Myotis (Myotis lucifugus), Silver-haired bat (Lasionycteris noctivagans) and Eastern Red Bat (Lasiurus borealis). Other mammals known to occur within GWMP are White-footed Mouse (Peromyscus leucopus), House Mouse (Mus musculus), Eastern Mole (Scalopus aquaticus), Northern Short-tailed Shrew (Blarina brevicauda), Southeastern Shrew (Sorex longirostris), Pine Vole (Microtus pinetorum), Norway Rat (Rattus norvegicus), Muskrat (Ondatra zibethica), Beaver (Castor canadensis), Eastern Chipmunk (Tamias striatus), Opossum (Didelphis marsupialis), Eastern Cottontail (Sylvilagusfloridanus), Eastern Gray Squirrel (Sciurus carolinensis), Raccoon (Procyon lotor), Red Fox (Vulpes fulva), Gray Fox (Urocyon cinereoargenteus), Striped Skunk (Mephitis mephitis), Domestic Cat (Felis cattus), and Whitetail Deer (Odocoileus virginianus). The Allegheny Woodrat (Neotoma magister) was historically collected in Great Falls Park (1949), Turkey Run Park (1917), and Dyke Marsh (1897), but recent search effort (1998) did not reveal the presence of this species and it is believed to be extirpated within GWMP.

Birds

A total of at least 243 species of birds have been recently recorded within the George Washington Memorial Parkway. Some of the parkway's birding hotspots include Great Falls Park from which 35 species of warblers have been reported during the spring or fall migrations and Dyke Marsh which has at least 25 species of waterfowl and is one of the few places in the Washington DC area where you have a chance of finding Marsh Wrens, and American and Least Bitterns. Another popular birding spot is the large mudflat area at the mouth of Hunting Creek (just north of Dyke Marsh). Here you can find 28 species of Plovers, Sandpipers and other wading birds during spring and fall migrations.

Fish

The Potomac River and its tributary streams within the George Washington Memorial Parkway (GWMP) are home to at least 61 species of fish (and one hybrid). Mine Run in Great Falls Park has the highest species richness of any stream in GWMP with 22 species recorded. Within GWMP, the Fallfish (Semotilus corporalis) and Longear Sunfish (Lepomis megalotis) are found only in Mine Run, the Swallowtail Shiner (Notropis procne), Fantail Darter (Etheostoma flabellare), Potomac Sculpin (Cottus girardi), and the Satinfin Shiner (Cyprinella analostana) are restricted to Turkey Run, and the Northern Hogsucker (Hypentelium nigricans) and Margined Madtom (Noturus insignis) are found only in Difficult Run. While on the Maryland side of GWMP the Sea Lamprey (Petromyzon marinus), Golden Redhorse (Moxostoma erythrurum) and Greenside Darter (Etheostoma blennioides) are known to occur only in Cabin John Creek. The American Eel (Anguilla rostrata) is found in more GWMP streams than any other species. Dyke Marsh, located along the Potomac River, is home to at least 38 species.

Amphibians

There are many charismatic animals in the park: red foxes dart across the meadows and forests, hawks and vultures circle in the skies overhead, beavers construct dams in the marshes, while massive longnose gar lurk beneath the murky water. Amphibians, though, are present in more subtle ways: frogs disappear with a splash before a glimpse can be caught; toads freeze in place when approached, seemingly melting into the forest floor; and salamanders silently crawl through the leaves on dark, rainy nights. Despite their seeming subtlety, amphibians fill vital niches in park ecosystems and are important in maintaining the health of park forests.

Amphibians are vertebrates, much like fishes, birds and mammals. From an evolutionary perspective, amphibians fall between fully aquatic fish and terrestrial birds, reptiles, and mammals. Amphibians first appeared in the fossil record over 300 million years ago and were the first vertebrates to live predominately on land. At the time amphibians had an enormous presence in terrestrial ecosystems. They expanded into a myriad of bizarre forms. Today's amphibian diversity is a shadow of their abundance during the Paleozoic era. Nevertheless, there have been over three thousand species described so far, with new species constantly being discovered. Eastern North America is home to over 150 different species (primarily salamanders) with more than twenty species occurring in the park. Taxonomically, amphibians are separated into three distinct orders: Anura (the frogs and toads), Caudata (the salamanders), and Gymnophiona (the caecilians, which will not be mentioned further).

Amphibians possess many unique anatomical features, given their relationship to other vertebrates. For example, most adult amphibians have a heart with only three chambers: two atria and a single ventricle. Freshly oxygenated blood from the “lungs” can be diluted with deoxygenated blood from the body in that single ventricle. Mammals, birds, and most reptiles have hearts with two ventricles that keep blood from the lungs and blood from the body separated.

Amphibians have evolved several different means of obtaining oxygen in response to various environmental pressures. Most adult amphibians, including frogs, toads, and many salamanders, possess a pair of lungs similar to those of higher vertebrates. However, most amphibian larvae—as well as several species of adult salamanders—possess a pair of gills, an array of vascularized, feathery projections through which oxygen diffuses from the water directly into the amphibian's blood, much like fish. One family of salamanders (plethodontidae) has, over the course of their evolution, lost both lungs and gills. Instead, oxygen diffuses directly from the air into their blood via their moist skin. The current hypothesis as to why this occurred, is that it originally evolved as an adaptation for life in mountain streams, where being buoyant would cause the salamanders to be swept away by the current.

Perhaps the most important feature of amphibian anatomy is the degree to which it changes between juvenile and adult phases. This is what gives amphibians their name: “amphibian” from “amphi” and “bios,” or two lives. Most amphibians lay large numbers of gelatinous eggs in a body of water, with the developing embryos easily visible through the translucent coating. After a variable period of time, the eggs "hatch" (the gelatinous coating dissolves), and free swimming larvae are released into the water. These larvae, often called tadpoles, possess a long, finned tail for swimming; a pair of gills for respiration; initially lack limbs; and, in the case of frogs and toads, possess a completely different digestive system than their adult counterparts. Hormones, genetics, ad environmental cues guide the transformation from larvae to adult.

After a period of development that ranges from weeks to years, the fully metamorphosed larvae leave the water to begin the second part of their life on land. However, there are many exceptions to this process. One genus of salamanders and an entire family of frogs omit the aquatic larval stage. Instead, they lay their eggs in damp, sheltered places on land where the developing embryos complete their metamorphosis before hatching as fully formed adults. At the other end of the spectrum, many amphibians never complete their development and live out their entire lives as permanent larvae, a condition known as neotony. An entire suborder of salamanders, plus several distinct families, are exclusively neotonic, and many other families have individual genera or species that are so. Even more curious are species of salamanders which, in response to environmental conditions, are capable of suppressing their metamorphosis. These salamanders are capable of breeding and producing offspring while neotonic, but their offspring are still capable of metamorphosing. It is believed that the developing larvae respond to cues such as the presence of fish predators or drought conditions, to accelerate or suppress their transformation.

Frogs and Toads

Thirteen species of frogs and toads, divided among four families, make their home in a diverse array of habitats in the park. They are active from early spring to late fall.

The family Pelobatidae is represented in the park by a single species; the Eastern spadefoot Toad. This toad is notable for the presence of a sharp spade on each of their back feet, which aids them in burrowing into loose sandy soil. They are the only species of toad in the park with vertically oriented pupils in their eyes, making them easily distinguishable from other toads.

The family Bufonidae contains the “true toads,” represented in the park by two closely related species, the American and Fowler's toads. Unlike frogs, toads have dry warty skin that is better at maintaining moisture, allowing toads to live in drier upland areas further away from water. Although they lack the powerful legs of frogs, toads are capable of quick movement through a series of short hops. When they feel threatened toads typically will freeze in place, allowing their natural camouflage to hide them; this behavior makes them relatively easy to approach and observe.

The family Hylidae contains the tree frogs and their relatives, the cricket and chorus frogs. Four species of this family can be found in the park. Staring in the early spring, and lasting into the summer, the calls of these frogs create a continuous din that echoes throughout the nights. These frogs tend to hide in dense vegetation near water, and are well camouflaged. You are significantly more likely to hear these frogs calling than you are to ever see them.

The family Ranidae consists of the “true frogs.” In this park it is represented by six species, including many familiar species, such as the northern and southern leopard frogs, and the bullfrogs. These large frogs are voracious predators that spend much of their time near the edge of shallow water, and will not hesitate to dive in at the first sign of danger. The meat of these frogs is a fairly popular food source, and bullfrogs are farmed commercially for their meat.

Salamanders

In addition to frogs and toads, the George Washington Memorial Parkway is home to ten species of salamanders, distributed among four families. They become active during the early spring, but will disappear during the summer. They will emerge again during the fall, but will vanish yet again when the first frost comes. Rarely, they may hunt during warm spells in the winter.

The family Ambystomidae is represented in the park by the Jefferson's and spotted salamanders. This family is frequently referred to as the "mole salamanders" due to their habitation of underground retreats dug by other animals. These salamanders spend much of their adult life underground, but will emerge in large numbers to breed in quiet pools during the spring. Famous members of this family include the brilliantly marked tiger salamander, and the neotonic axolotl.

The family Salamandridae is represented here by a single species, the Eastern newt. This is primarily an old world family of salamanders consisting of the newts and “true salamanders,” with many species across Eurasia and North Africa, but only six in North America. The Eastern newt is notable among park salamanders because it undergoes three distinct life phases: an aquatic larvae, a brilliant orange terrestrial eft phase, and an aquatic mature phase. Many predators avoid the efts due to their toxic skin secretions.

The family Sirenidae, while not documented in the park, does inhabit the Washington, DC region, and is represented by a single species. These large, nocturnal, eel-like salamanders live out their lives as permanent larvae that inhabit shallow, weed-choked stretches of standing or slow moving water.

The family Plethodontidae is represented by seven species, spread amongst the dusky, red, brook, and mountain salamander generas. This family of small salamanders is known as the “lungless salamanders,” due to the absence of lungs. These are salamanders typical of woodland areas. They seek shelter under rocks, rotting logs, loose tree bark, or in animal burrows during the day. They emerge at night to hunt for insects and small invertebrates on the forest floor.

The Role of Amphibians

While amphibians may not be the most flamboyant group of animals that inhabit this park, they do much to make their presence here felt. All amphibians are voracious carnivores, with appetites that are only sated by dozens of insects each time they hunt. The insects they eat would otherwise graze on plants, lowering the amount of carbon dioxide that would have been released by these insects as they breathed. The amount of carbon sequestered by salamanders this way can really add up: over 200 kg/hectare. The tadpoles of frogs eat countless mosquito larvae, helping control the spread of mosquito-borne illnesses in humans.

In addition to their insect control services, amphibians serve as a dietary staple for many of the park’s shorebirds and reptiles. Great blue herons, egrets, water snakes, and many others depend on frogs as a reliable source of food.

Aside from predators, amphibians face a variety of threats from humans. The misuse of pesticides in agriculture and landscaping, as well as the improper disposal of chemicals, pose a serious threat to amphibians, especially salamanders, whose larvae often require pure water to develop normally. Introduced exotic species such as the Northern snakehead—and even domestic cats—put additional stress on amphibian populations. It is probable that the most imminent threat to amphibians, a fungal infection called chytridiomycosis, was introduced as humans transport specimens from place to place. It is killing incomprehensible numbers of animals. Many amphibians, from the Shenandoah salamander of Virginia, to the golden frog of Panama, to the Paghman stream salamander of Afghanistan are edging closer to extinction as a result.

On a larger scale, amphibians face a combination of suburban development and climate change, both of which are shrinking and altering their habitats. Although they face a formidable combination of threats, there is still hope if ordinary people take action and make simple changes. For information on simple things you can do to help amphibians in your area read How Tos and Tips from the Chesapeake Bay Program.

Crustaceans

Most of the crustacean inventories at GWMP have focused on amphipods, thus little is known of the other crustacean species that likely occur here. Nine species of amphipod and two species of Caecidotea isopod have been documented from GWMP. One of the amphipods found within GWMP is a species new to science, currently known as Stygobromus sp. 15, until it is given a species epithet. The other eight species of amphipods known from GWMP are Crangonyx shoemakeri, Crangonyx stagnicolous, Crangonyx palustris, Synurella chamberlaini, Gammarus fasciatus, Gammarus minus, Stygobromus pizzinii, and Stygobromus tenuis. A Grass Shrimp (Palaemonetes sp.) is occasionally found in Dyke Marsh. Eighteen species of copepod have been documented within GWMP. Two non-native crayfish, the Red Swamp Crayfish (Procambarus clarkii) and the Virile Crayfish (Orconectes virilis), and three native species, the Devil Crayfish (Cambarus diogenes), Appalachian Brook Crayfish (Cambarus bartonii) and Spiny-checked Crayfish (Orconectes limosus) have been found in streams and wetlands of GWMP.

Reptiles

The woodlands, streams and riverbanks found within the units of the George Washington Memorial Parkway (GWMP) provide habitat for a total of 21 species of reptiles that live here. These include eleven species of snakes, two species of skinks, and eight species of turtles.

The eleven species of snakes known from GWMP are, Northern Brown Snake (Storeria dekayi), Northern Copperhead (Agkistrodoncontortrix mokasen), Eastern Worm Snake (Carphophis a. amoenus), Northern Ringneck Snake (Diadophis punctatus edwardsi), Black Rat Snake (Elaphe obsoleta), Northern Watersnake (Nerodia sipedon), Queen Snake (Regina septemvittata), Rough Green Snake (Opheodrys aestivus), Eastern Hognose Snake (Heterodon platirhinos), Corn Snake (Elaphe guttata), and Eastern Garter Snake (Thamnophis s. sirtalis). Only one of these, the Northern Copperhead, is venomous. Northern Copperheads have been reported from Turkey Run and Great Falls Parks, where they are fairly common.

The two species of skinks known from GWMP are the Broadhead Skink (Eumeces laticeps) and the Five-lined Skink (Eumeces fasciatus). These two species can look quite similar at certain stages of their life cycles. However, the Broadhead Skink matures at a larger size than that of the Five-lined Skink.

A total of six species of native turtles and two non-native (introduced) species have been recorded from GWMP. The six native species are Box Turtle (Terrapene carolina), Common Musk Turtle (Sternotherus odoratus), Eastern Painted Turtle (Chrysemys p. picta), Eastern Snapping Turtle (Chelydra s. serpentina), Red-bellied Turtle (Pseudemys rubiventris), and Wood Turtle (Clemmys insculpta). Feral, introduced, species include the Red-eared Slider (Trachemys scripta elegans) and Chinese Shoftshell (Pelodiscus sinensis).

The Wood Turtle is known in GWMP from a single record from Great Falls Park in 1994 and a record from Turkey Run Park in 2005. The Chinese Shoftshell is known only from a 2006 record from Dyke Marsh. The Wood Turtle is a state listed (rare) species in Virginia. Any observations of Wood Turtles or Softshell Turtles, or other species not listed here, within GWMP should be reported to the park's Natural Resource Management Staff.

Trees and Shrubs
A total of 99 species of trees have been documented from the George Washington Memorial Parkway (GWMP). These include 14 species of non-native trees. Shrubs are represented by 92 species, 25 of which are non-native species. The most common genus of tree or shrub is Quercus (oaks) which are represented by 15 species. Five of the trees or shrubs known from GWMP are listed by the States of Virginia or Maryland as rare, threatened or endangered.

Ferns

The woods, marshes and swamps of the George Washington Memorial Parkway (GWMP) provide habitat for 28 extant species of ferns and seven other vascular crytograms such as horsetails, quillworts and lycopoid clubmosses. Six additional species of ferns historically documented from GWMP are now belived to be extripated along with the spikemoss, Selaginella rupestris.

Mosses and Liverworts

An inventory of mosses and liverworts is currently underway at Great Falls Park, Virginia. To date a total of 24 species of liverworts and 94 species of mosses have been documented from the park.

Wildflowers

A total of 591 species of wildflowers have been documented from the George Washington Memorial Parkway. The spring wildflower show is no more brilliant anywhere in Virginia than in Turkey Run Park.

Grasses

A total of 87 extant species of grasses are known from within the George Washington Memorial Parkway. An additional nine species are known only from historic voucher records.

Others

The sedges (Cyperaceae) make up one of the the most diverse groups of vascular plants within the George Washington Memorial Parkway (GWMP). Currently, 94 species in eight genera are known to be extant within GWMP and five additional species are known from historic specimen records. Undoubtedly, even more will be discovered as future inventory efforts increase. As recently as 2003 a species of native sedge new to the State of Virginia (Carex davisii), was found within GWMP.

Additionally, the rushes and wood rushes (Juncaceae) are represented by eight species within GWMP.

Mollusks

The George Washington Memorial Parkway is home to forty species of mollusks: twenty-seven species of terrestrial snails and slugs, five species of aquatic gastropods, and eight species of freshwater mussels. This list includes rare species such as the fine-ribbed striate, and the Appalachian springsnail.

"Mollusks" describes all organisms within the invertebrate phylum Molluska. They are an extremely diverse group of organisms, with living members in eight classes. Their anatomical features vary greatly, although generally they possess a shell and a fleshy mantle which excretes it. (There are numerous exceptions.) Two classes of mollusks inhabit the park: the gastropods, which consist of several families of snails and slugs, and the bivalves, which are represented by freshwater mussels.

Gastropods

Gastropods in the park are primarily nocturnal and are most frequently seen on spring and summer nights crawling across the forest floor or on rocks and vegetation after fresh rainfall. They can also be seen during the day hiding under loose leaf litter and rotting logs or—in the case of slugs—wedged under loose tree bark. They may occasionally be seen moving across the rocks of exposed stream beds. Aquatic gastropods may be found in bodies of water throughout the park. They may be moving across submerged rocks or even suspended upside-down from the surface of the water, feeding on algae at the surface.

Gastropods—as primary consumers of algal growth—occupy an important niche in the ecosystem. Algae are most readily found growing on hard surfaces like rocks and wood. In order to exploit this food source gastropods have specialized mouthparts consisting of several rows of chitinous teeth (a "radula"). The radula functions as a sort of tongue that gastropods use to scrape off algae. Muscular contractions then force the food backwards down the esophagus into the stomach.

While the feeding habits of gastropods ensure that they have access to a plentiful food source, it also leaves them vulnerable to a variety of predators. To compensate for this, gastropods have evolved a shell composed of calcium carbonate embedded in a protein mesh. As in all mollusks, a gastropod's shell is secreted by a structure called the mantle. The mantle continuously secretes the shell throughout the snail's life, causing the shell to grow larger and wider with age. Slugs, on the other hand, have lost their shells at some point in their evolution in a tradeoff between the added protection afforded by a shell and the ability to compress their bodies to maneuver into tight spaces, such as a crack on a rock face or under the bark of a rotting logs.

Bivalves

While walking along one of the park's many waterside trails, you may notice large quantities of broken, discarded shells strewn about the shoreline and accumulating along the water bottom. This attests to the large populations of freshwater bivalves that inhabit the waterways of the George Washington Memorial parkway. These mussels are most frequently found partially buried in sandy gravel in slow moving sections of the river.

The wide distribution of our bivalves is due to their unique juvenile dispersal strategies and the relationships they maintain with native fishes. After fertilization, the mussel larvae develop within the gills of the parent mussel. This causes the mussel's gills to distend. (These elongated gills are termed "gill marsupia.") After a brief period of development. the larvae (called "glochidia") are released into the water column through the mussel's exhalent siphon. Then, the glochidia attempt to attach themselves to the gills of passing fish. Once attached, they derive sustenance from the blood plasma of their host until they complete their development. Then they drop off of the fish's gills and settle on the bottom. This relationship with native fishes allows mussels to distribute both upstream and downstream.

Bivalves feed by drawing water into their gills through their inhalant siphon. This is accomplished by rhythmically beating tiny hairs, or cilia, that line the labial palps near the mussel's internal mouth. The incoming water passes over the gills where tiny organic particles become trapped in a net of mucus strung between the gills. Beating cilia gradually draw the mucus-entangled food towards the mouth. By doing this shoals of mussels are capable of filtering large amounts of water, removing suspended particles, and improving water clarity.

Mollusks as a Source of Food

Mollusks occupy an important niche in park ecosystems. They serve as an important food source for wildlife: waterfowl, raccoons, turtles, and snakes frequently eat mollusks, as do several species of fish, including American shad, channel catfish, and striped bass. Historically, mussels served as a food source to both indigenous peoples and early colonists in the region.

Conservation Status

Although mollusks are abundant in the park, they face many threats. Overuse of pesticides for maintaining lawns, gardens, landscaping, and agriculture threaten gastropods because they often feed on affected surfaces or live in water contaminated with pesticide laden runoff. Increasing land development and urbanization throughout northern Virginia over the past few decades has increased the amount of water runoff from impervious surfaces like roads, sidewalks, buildings, and parking lots. This increase in runoff decreases the amount of time water has to infiltrate into the ground. Large quantities of fast moving water suspend more sediments in the water column. This can overwhelm mussels' ability to sort particles and interfere with their respiratory functions. If you would like to learn more, visit the Chesapeake Bay Program website.

Linchens

Lichens are the product of a unique symbiotic relationship between a fungus, and either a green algae or a cyanobacterium. In this association, the fungal member is termed the mycobiont while the algal component is termed the photobiont.

Lichens obtain their energy through photosynthesis by the photobiont, sugars produced by the photobiont are then absorbed by the mycobiont as its food source. In exchange, the fungal cells form a protective layer that screens the photobiont from damaging ultraviolet radiation, and aids in water retention. Because of this, lichens are able o survive in some of our most inhospitable environments, such as exposed, wind-swept rock faces and monuments.

Park staff currently have no data on the lichen species that occur within the George Washington Memorial Parkway, however inventories of this group are currently underway in Great Falls Park.

Mushrooms and Other Fungi

The George Washington Memorial Parkway is home to lush forests, teeming wetlands, and verdant meadows. However, none of these spectacular ecosystems would function without the vital roles filled by the fungal community. This enigmatic group of organisms not only possess an incredible level of diversity in both form and function, but also plays a key role in maintaining the health and diversity of plant and animal communities.

Although they may superficially resemble plants, fungi occupy a separate taxonomic kingdom, "fungi." During their long, but ongoing, evolution; the ancestors of fungi diverged from the ancestors of plants sometime around one billion years before present, and from animals only slightly more recently. In addition to true fungi, there are also fungus like organisms that (through convergent evolution) have evolved a lifestyle similar to that of fungi; such as slime moulds, and a group of organisms represented by the microorganism that causes potato blight disease in potato crops.

A billion years of evolution has produced many unique features that characterize fungi. Two of their most important features pertain to the way in which fungi grow, and the composition of their chromosomes. Firstly, fungi exhibit two distinct forms of growth. They can grow as hyphae, which are a series of branching root-like filaments that expand by dividing behind the leading tip of individual hypha. The mass formed by these branching hyphae is termed the mycelium. Fungi can also grow as free living yeasts, which grow by fission of individual cells. A few fungi, called "Dikaryotic Fungi" are even capable of switching between these two types of growth. Secondly, the cells of fungi contain haploid nuclei. Unlike the cells of plants and animals (excluding special reproductive cells), fungal cells only contain half the organism's chromosomes at any given time. Despite these unique characteristics; fungi, much like animals, are incapable of producing their own food, and must obtain their food by absorbing it from their environment. Whether it's from the soil, or from the dead or living tissues of plants, animals, or even other fungi, the search for energy continues to drive the evolution of fungi within our ecosystems.

What Lives Here

Since they first appeared a billion years ago, fungi have undergone an impressive degree of speciation. The legacy of their evolution can be found in the five living phyla within the kingdom fungi, all of which are represented within the ecosystems of the George Washington Memorial Parkway.

Chitridiomycota
Chytrids live in close association with water, and can be found in almost any body of water within our parks, as well as in damp soil. Chytrids are unique among fungi due to the fact that their zoospores (single-celled seeds or larvae) are free swimming and are capable of "searching" for an appropriate location to encyst and grow. In this park chytrids are inconspicuous decomposers of dead organic matter, or as parasites of plants and freshwater invertebrates. The chytrids familiar to most people are the ones that cause the black crusts that occasionally show up on grocery store potatoes, and the invasive chytrid that has been implicated in the devastating worldwide loss of amphibians.

Glomeromycota
These fungi are closely associated as symbiotes of plants, and can be found growing between and into the root cells of herbaceous plants. Like the chytrids, they are highly inconspicuous, but play an important role in maintaining the health of park ecosystems.

Zygomycota
The Zygomycota are a group of fungi typically associated with unpleasant images of animal feces and rotting fruit. Some species of Zygomycota are able to grow at unusually high temperatures (about 600C) associated with compost piles. Many Zygomycota are important parasites of insects, and are capable of causing spectacular crashes in their host populations.

Basidiomycota
Basidiomycota are probably the phylum of fungi familiar to most people when they picture a fungus, since they produce the familiar mushrooms. Many species in the Basidiomycota are symbiotes with large forest trees, making them important to the overall health of forest ecosystems. Many others are decomposers growing in the soil or on decaying logs. A few are virulent pathogenic parasites that cause many plant diseases.

Ascomycota
The Ascomycota are the most diverse and speciose phylum of fungi found in this park, occupying an enormous variety of niches within our ecosystems. They fill everything from decomposers of organic material, to parasites or symbiotes of plants and insects. They can even be found growing harmlessly as yeasts within the respiratory tracts of humans and other animals. Lichens, which are ubiquitous on exposed rock, usually contain an Ascomycete fungus as partner in the relationship.

Finding Fungi

A bioblitz sponsored by the George Washington Memorial Parkway in 2006 turned up 55 different species of mushrooms; representing a tiny fraction of the possible number of species present in the park. The best times to find and observe fungi are between late spring and mid fall, preferably after a recent rainstorm. If you're looking to identify individual species, it helps to also identify what they are growing on or near.

The Role of Fungi

Fungi are important components of our ecosystems, and are vital in maintaining the health and stability of this park by occupying three primary niches: as saprotrophs, as parasites, and as symbiotes.

Saprotrophs

The George Washington Memorial Parkway includes a diverse array of habitats, from rocky cliffs, to forests, to swamps, to grasslands. Every year, millions of trees shed their leaves, animals die, and organic sediments are deposited by storms. Nearly all of this material is eventually recycled back into new growth due to the activity of fungi in making these nutrients available.

Saprotrophs are organisms that participate in the decomposition and recycling of dead organic material at the cellular level.

A good example of saprotrophs are the fungi that are capable of breaking down lignin, the component in wood that presents the greatest obstacle to its decomposition. Lignin's strength as a molecule comes from its shape, which is composed of aromatic hydrocarbons. (Imagine six atoms of carbon attached to each other in the shape of a hexagon). Aromatic hydrocarbons are very stable, making them very difficult to break apart. In order to break apart these hexagons, lignin-decaying fungi secrete a molecule called laccase, which has two "sticky" hydroxyl ions that bind to two adjacent corners of the hexagon. As a result, the two corners of the hexagon want to stick to the hydroxyl groups more than they do to each other. Once one side of the hexagon is removed, the lignin is weak enough to allow the whole molecule to be broken down and recycled. Without this process and many others like it, dead wood would just continue to accumulate until the entire landscape was buried under a thick layer, suffocating plant life, and providing fuel for fires of cataclysmic proportions.

Parasites

Parasites are organisms that get their nutrients by continuously absorbing them from a living host organism, negatively affecting it in a measurable way.

Fungi are common parasites of plants, but they can also parasitize insects, other fungi, and even higher animals such as humans (for example, athlete's foot). Although they can cause great harm to the organism they infect, parasites are important in maintaining a balanced ecosystem by helping to control the population size of their hosts. However, exotic parasites can destroy entire populations of their hosts, since their hosts have not had enough time to evolve any resistance to the parasite. Every year the staff of the George Washington Memorial Parkway put tremendous effort into controlling exotic fungal infections in elms, chestnuts, and ashes.

Fungal Parasites can be subdivided into two categories: biotrophic, and necrotrophic parasites.

Biotrophic parasites grow into the tissues of their plant hosts, absorbing nutrients from the host's cells without killing their host. Several species of powdery mildews fall into this category.

Necrotrophic parasites, on the other hand, slowly kill their hosts; absorbing nutrients from the host's cells, pre and post mortem. American Chestnuts and American Elms have experienced severe losses from the invasive necrotrophic parasites Cryphonectria parasitica and Osphiostoma ulmni respectively.

Symbiotes

Mycorrhizas are a special type of symbiotic nutritional relationship shared between plants and fungi that confers advantages to both organisms.

All living things are composed of six primary elements: sulfur, phosphorus, oxygen, nitrogen, carbon, and hydrogen. In most natural systems plants have access to an adequate supply of carbon and phosphorus. Nitrogen, on the other hand, is usually in short supply, thus becoming a limiting factor for plant growth. However, certain species of fungi are very good at obtaining nitrogen from the soil through their hyphal network. At some point early in their evolution, plants entered into a symbiotic relationship with these fungi. The fungus supplies the plant with nitrogen in exchange for some of the sugars that the plant produces through photosynthesis. Plants that possess this type of relationship tend to grow faster and are more tolerant of variable environmental conditions than plants which do not. This is why the vast majority of plants in natural communities possess mycorrhizas.

In forests such as those of the George Washington Memorial Parkway mycorrhizal fungi can grow and associate with several different trees at once, acting as a conduit for nutrients between trees. Seedlings that are able to tap into this fungal network can obtain nutrients from nearby mature trees, allowing them to grow in the shade of the forest floor until they are tall enough to reach the light. There are actually two different types of mycorrhizas that occur within our parks: ectomycorrhizas and arbuscular mycorrhizas.

Ectomycorrhizas are typical of large forest trees, such as tulip trees, pin oaks, sycamores, beeches, and others. These are the basidiomycete fungi that produce many of the familiar mushrooms such as amanitas, russales, chanterelles, and others. Ectomycorhizal fungi grow by forming a sheath around the growing tips of the roots of their plant hosts. Individual fungal hyphae grow inward from the sheath into the intercellular space between root cells, but do not actually penetrate the cell walls of the plant. (Hence the "ecto" in ectomycorrhiza.)

Arbuscular Mycorrhizas grow in association with herbaceous plants, grasses, some trees, and especially cacti. Unlike the ectomycorhizal fungi, arbuscular mycorhizal fungi do not form a sheath around the root tip. Instead, they grow directly into the host's tissue, penetrating the cell wall (but not the inner cell membrane) of the root cells and form a branching structure called an arbuscule, through which nutrients are exchanged.