Click on a term for a definition of how it is used in this website.
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Acid search for term

An acid is a substance having a pH of less than 7 when dissolved in water. Ecologists apply the term acidic to plant communities growing in rather acidic and nutrient-poor soils. Acidic soils result from the presence of various inorganic and organic acids.

Synonyms: acidic

Alluvial search for term

Related to alluvium or floodplains.

Alluvium search for term

Sediment deposited by flowing streams or rivers in floodplains and streambeds/riverbeds. Different sizes of sediment particles are deposited, depending on the water velocity related to the location within the floodplain.

Ancient river terrace search for term

A former floodplain terrace, or alluvial landform that is no longer subject to flood events, but is instead in the uplands. At Rock Creek Park, some of the highest hills are topped with coarse sand and smooth stones that appear to have been rounded and polished by turbulent water in past ages. Evidence of marine sediments is lacking, so these terraces are believed to have been formed by flowing fresh water, possibly an ancestral Potomac River.

Synonyms: ancient river terraces

Aspect search for term

See slope aspect.

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Base search for term

A base is a substance having a pH above 7 when dissolved in water. A basic substance is capable of neutralizing acids. Base elements, such as calcium, magnesium, and sodium, are one class of such substances, and are important nutrients for plants. Basic rocks are composed of minerals rich in base elements. Ecologists apply the term basic to plant communities whose presence indicates that the soil contains base elements available in a form plants can use, even if the overall soil pH is not basic.

Synonyms: basic

Base element search for term

See base, basic, and element.

Base mineral search for term

A rock mineral containing base elements.

Bedrock search for term

Solid rock that underlies unconsolidated material such as soil and fragmented rock, or is occasionally exposed as rock outcrops.

Biotite search for term

A rock-forming mineral high in iron, magnesium, and potassium, dark brown or black in color. A common kind of ‘mica’ (a group of soft minerals that tend to occur in flakes or sheets.
Quoted from Stewart, K.G. and M. Roberson. 2007. Exploring the Geology of the Carolinas. UNC Press, Chapel Hill, NC.

Broadleaf search for term

Having relatively broad leaves, as opposed to needle-like leaves. Examples of broadleaf evergreen trees or shrubs: American holly, mountain laurel. Examples of broadleaf deciduous trees or shrubs: white oak, pink azalea.

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Cambium search for term

A narrow, continuous ring of growth tissues located just inside the external bark of a tree, that is responsible for transporting water, nutrients and food. As a tree ages, the cambium produces wood (to the inside) and bark (to the outside) causing the tree to grow in width.

Canopy search for term

The trees whose crowns intercept most of the sunlight in a forest stand. The uppermost layer of a forest. Adapted from Johnson, P.S., S.R. Shifley, and R. Rogers. 2002. The ecology and silviculture of oaks. Chapter 5. Page 194. CABI Publishing, New York.

Canopy gap search for term

A temporary opening in the forest canopy, caused by the death or toppling over of one or more canopy trees.

Channelize search for term

To direct the course of a stream through a manmade channel.

Synonyms: channelization channelizations

Citizen scientist search for term

A citizen scientist is a member of the general public who engages in scientific work, often in collaboration with or under the direction of professional scientists and scientific institutions; an amateur scientist.
Quoted from the Oxford English Dictionary.

Synonyms: Citizen scientists

Classification code search for term

The unique identifier for each natural community. The classification code is used to identify each natural community in the U.S. National Vegetation Classification and other classification systems. The code is written as "CEGL" followed by a series of numbers. CEGL stands for Community Element Global. For example, the classification code for the Mid-Atlantic Mesic Mixed Hardwood Forest is CEGL006075.

Clay search for term

Pulverized rock fragments; a silky-textured, extremely fine size of rock particle. In U.S. geology, clay is a size label for rock fragments below 1/256 mm. See also cobbles, gravel, sand, and silt.

Synonyms: clays

Clay loam search for term

Textural term referring to soil or sediment composed of 28-40 percent clay, and roughly equal parts of sand and silt.

Clear-cutting search for term

A system of tree harvesting that removes all the trees in a given area, as opposed to other systems that leave some trees standing. Adapted from Draper, D.L. 2002. Our Environment: A Canadian Perspective, Second edition. Glossary. Nelson, Scarborough, Ontario, Canada.

Cleaved search for term

Describes rock with planes of weakness along which it may split.

Clonal search for term

Describes colonies of plants that appear to be distinct individuals, but are genetically identical, and interconnected underground by specialized roots. Some types of shrubs in the heath family at Rock Creek Park are clonal. Above ground these plants appear to be distinct individuals, but underground they remain interconnected and are all part of the same plant.

Coastal Plain search for term

The easternmost stretch of land in the Mid-Atlantic region that lies between the Piedmont and the Atlantic Ocean. This relatively flat land is comprised of layers of unconsolidated sediments and sedimentary rock that get thicker from west to east. The western boundary of the Coastal Plain is the Fall Zone, where it overlaps the Piedmont bedrock in the vicinity of Washington, D.C.

Synonyms: Atlantic Coastal Plain

Cobbles search for term

Approximately fist-sized stones. In U.S. geology, cobble is a size label for rock fragments between 64 and 256 mm (about 2.5 to 10 inches). See also gravel, sand, silt, and clay.

Colluvial fan search for term

See colluvium.

Colluvium search for term

A blanket of loose stones and soil that moves downslope by a combination of natural processes (frost action, gravity and hillside creep, and slope wash), locally accumulating to considerable thickness on flatter areas (‘benches’ and toeslopes) in the form of ‘colluvial fans.’ At least a few inches of colluvium are present on many slopes in Rock Creek Park.

Competition search for term

The simultaneous demand by two or more organisms for limited environmental resources, such as nutrients, living space, or light. Quoted from The American Heritage Science Dictionary. 2005. Houghton Mifflin Company, Boston.

Competitive advantage search for term

The advantage that a characteristic (or set of characteristics) gives one organism over another in an environment where vital resources such as sunlight, water, nutrients, and space are limited and cannot be shared. For instance, among sun-loving plant species, the ability to grow faster and taller than surrounding plants may be a competitive advantage, giving a plant access to the greatest share of sunlight (and as a consequence, shading the other sun-loving plants, interrupting their growth).

Synonyms: competitive advantages

Concave search for term

Curved inward, like the inside surface of a bowl. Describes the shape of land on some slopes, and especially near the base of a hill (toeslope), that tends to collect moisture and fine sediment runoff.

Conifer search for term

A needleleaf tree that bears cones. A pine tree, for example.

Conservation status rank search for term

A rating of natural communities and of species that estimates their risk of elimination. Natural communities and species are given a conservation status rank to reflect the fact that while some of them are common across the landscape regionally or globally, others are not. The most rare species risk extinction. The most rare natural communities may be at risk of being severely degraded or even lost in the future unless some conservation measures are taken. (Certain plants and animals that depend on these rarest of natural communities are also at risk of extinction.) NatureServe and its natural heritage member programs have developed a consistent method for evaluating the relative imperilment of species and of natural communities, and assigning each a conservation status rank. The ranking is based on the best available information, and considers a variety of factors such as abundance, distribution, and threats. Rankings of natural communities and species across their global (G) range are as follows:
G1: critically imperiled
G2: imperiled
G3: vulnerable
G4: apparently secure
G5: secure
G4G5: uncertainty exists as to whether this community is ‘apparently secure,’ or ‘secure’

Synonyms: conservation status G1 G2 G3 G4 G5 G4G5

Convex search for term

Curving or bulging outward, like the outside surface of a ball. Describes the shape of land (on some hillsides, for instance) that tends to shed moisture.

Cut banks search for term

The steep, concave bluffs found along meandering streams on the outside of a stream bend. They are formed by the erosion of soil as the stream collides with the bank. See also point bars.

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Deciduous search for term

Referring to a plant that sheds leaves at the end of a growing season and regrows them at the beginning of the next growing season. Most deciduous plants bear flowers and have woody stems and, in this region, have broad rather than needlelike leaves. Maples, oaks, and elms are examples of deciduous trees. Compare evergreen. Adapted from The American Heritage Science Dictionary. 2005. Houghton Mifflin Company, Boston.

Decomposers search for term

Living organisms such as bacteria, fungi, ants, and worms, which are able to break down organic matter that is difficult for other organisms to digest. They typically secrete enzymes onto organic matter (dead organisms or animal or plant wastes) and then absorb the breakdown products. Decomposers fulfill a vital role in the ecosystems, returning the constituents of organic matter to the environment as inorganic nutrients that can be used again by plants. See also nutrient cycles. Adapted from Dictionary of Biology. 2004. Fifth edition. Oxford University Press.

Decomposition search for term

The breakdown of dead organisms or animal or plant wastes (organic matter) into inorganic nutrients by the action of decomposers, so that the nutrients can be used again by plants. See also nutrient cycles. Adapted from Dictionary of Biology. 2004. Fifth edition. Oxford University Press.

Detritus search for term

Debris or litter of biological origin, such as leaf litter, animal waste, or dead organisms. Adapted from Allaby, M. 1994. The Concise Oxford Dictionary of Ecology. Oxford University Press.

Disturbance intervals search for term

The amount of time that lapses between repeat episodes of a natural disturbance to a plant community. Where, for instance, long intervals between flood events are the norm, the vegetation will look quite different than where flooding occurs more frequently.

Diversion search for term

(As used here) A redirected section of a stream, either temporary or permanent (usually for construction purposes).

Synonyms: diversions

Dry-site oaks search for term

Oak species that are known for their ability to thrive on nutrient-poor, dry sites. Examples in the Mid-Atlantic region of the U.S. include: chestnut oak, scarlet oak, black oak, and post oak, and to some extent, white oak.

Duff search for term

Decaying leaves and branches covering a forest floor. Quoted from The American Heritage Dictionary of the English Language. 2006. Fourth edition. Houghton Mifflin Company, Boston.

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Ecological system search for term

A broad ecological environment in which certain natural communities tend to cluster together. (For instance, the Central Appalachian Stream & Riparian ecological system encompasses all kinds of natural communities that occur along streams in the Central Appalachian region.)

Synonyms: ecological systems

Edge habitat search for term

A boundary where two or more habitats meet. (For example, where a natural community meets another natural, semi-natural, or non-natural plant community or disturbed area.) Some animal species prefer edge habitats—where a meadow borders a forest, for instance—with access to resources that aren’t found in a single natural community.

Synonyms: edge habitats

EDRR species search for term

EDRR stands for Early Detection and Rapid Response—a strategy of watchfulness and quick eradication to keep newly-arrived species of weedy non-native invasive plants from getting established on a site. "EDRR species" is a nickname for species that are being targeted for management by this strategy.

Element search for term

A substance whose structure is made up of only a single type of atom. For example, the mineral copper, which is made up of 100 percent the element copper (and no other substances), is known as an element. The ‘periodic table of the elements’ is a layout of all the elements. Adapted from The Mineral and Gemstone Kingdom. Glossary. 2017.

Energy flow search for term

The flow of energy in an ecosystem from producers to users. Energy on Earth (in most ecosystems) is derived from the sun, which energizes plants to convert inorganic nutrients into plant tissue (photosynthesis); those plants feed (i.e., energize) animals, which feed other animals, etc. There is some loss of energy each time it is transferred—for instance, only a small percentage of sunlight that strikes plants is used by the plant for photosynthesis; and animals can’t digest every particle of plant or animal they consume. A constant input of new energy from sunlight is required to continue the energy flow on Earth; energy is not completely recycled the way nutrients are. See also nutrient cycles and food chain. Adapted from Marietta College Department of Biology and Environmental Science. 2017. Biomes of the World—Ecology Pages. Environmental Biology—Ecosystems.

Evergreen search for term

Having green leaves or needles all year. Evergreen trees lose their leaves individually on an ongoing basis, rather than losing all of them in a short period at the end of a growing season in the manner of deciduous trees. Evergreen plants may be broadleaf or needleleaf. Quoted, in part, from The American Heritage Science Dictionary. 2005. Houghton Mifflin Company, Boston.

Exotic search for term

Not native to a region; being from another part of the world. “Exotics” can be a synonym for ‘non-native plants.’ See also non-native invasive plant.

Exposed search for term

Having a landscape position (usually high or open) that heightens the effect of weather (primarily sun and wind, but also lightning, rain, ice) on vegetation. Contrasted with ‘sheltered.’

Extrusive rock search for term

Lava; magma that cooled rapidly above-ground, producing a fine-grained igneous rock. (Compare intrusive rock.) Adapted from The American Heritage Science Dictionary. 2005. Houghton Mifflin Company, Boston.

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Facilitation search for term

Species interactions that benefit at least one of the participants and cause harm to neither. (Stachowicz, J. J. 2001. Mutualism, facilitation, and the structure of ecological communities. BioScience 51: 235-246.)

Synonyms: ecological facilitation

Facilitation-biology search for term

(As used here) A natural process whereby the presence of one organism facilitates or helps another organism to grow. Both organisms may benefit from the interaction, or just one.

Fall Zone search for term

The Fall Zone (sometimes called the fall line) is the boundary between the Piedmont and the Coastal Plain; often marked by falls and rapids as rivers leave the hard rocks of the Piedmont and step down into the more easily eroded sediments of the Coastal Plain. Quoted from Stewart, K.G. and M. Roberson. 2007. Exploring the Geology of the Carolinas. UNC Press, Chapel Hill, NC.

Synonyms: fall zone Fall Zone fall line

Fault-geology search for term

A fracture in the earth which shows evidence of the movement of blocks of rock relative to one another. Earthquakes are the result of shifting along faults (‘fault motion’).

Fauna search for term

All the different kinds of animals of a particular area.

Feedback loop-ecology search for term

(As usedhere) A circular chain of cause-and-effect, in which the outcome of some natural processes affect other processes, which in turn affect the original processes.

Field layer search for term

Any low plants—including herbaceous plants as well as new tree seedlings or small shrubs. This vegetation layer provides important cover for birds and small mammals in some natural communities.

Fire suppression search for term

Efforts to control and extinguish fires, usually to prevent loss of human life and property.

Fire-adapted search for term

Describes plant species characterized by the ability to thrive as a species in spite of (or partly because of) some degree and frequency of fire occurring in the landscape. The means of survival vary considerably. Some fire-adapted species have qualities that make them less vulnerable to light or moderate fire. (Example: the thick bark of chestnut oak.) Other fire-adapted species possess qualities (such as oily sap, or low-hanging dead branches) that make mature specimens more prone to complete destruction by severe fire; but also possess qualities that will give their seeds or stump sprouts a fresh start in the charred forest that results. (Example: mountain laurel, or pitch pine)

Floodplain dynamics search for term

What happens in the natural interplay between rivers or streams and their floodplains.

Floodplain terrace search for term

The higher parts of a modern stream valley (farther from the stream channel than the floodway), which may be inundated only infrequently, e.g., a 10-year floodplain, a 25-year floodplain, etc.

Floodway search for term

That part of a modern floodplain adjacent to the active stream channel, and which is typically inundated annually or more frequently, whenever the stream overflows its banks.

Flora search for term

All the different kinds of plants of a particular area.

Food chain search for term

A series of feeding relationships in which one organism eats another organism, only to be eaten itself by another organism. Organisms such as green plants form the bottom rung of the food chain, since they do not eat other organisms, but instead convert inorganic nutrients (non-living chemicals) into food with energy supplied by sunlight. (See primary production.) The collective interactions of all the food chains in an ecosystem are called a food web (a way of representing the complex system of passing nutrients and energy from producers to users.)

Food web search for term

See food chain.

Friable search for term

Easily crumbled or broken apart into small pieces.

Frost action search for term

A type of weathering caused by the swelling and shrinking of moisture as it freezes and thaws in soil or in surface pores and cracks of rock.

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Gabbro search for term

A dark colored, coarse grained igneous rock. (The intrusive equivalent of basalt—a dark volcanic (extrusive) rock.) Composed of minerals such as biotite and hornblende.

Gap-phase regeneration search for term

A phase of forest regeneration, during which trees begin to colonize gaps created by fallen trees. Adapted from Allaby, M. 1994. The Concise Oxford Dictionary of Ecology. Oxford University Press. See also canopy gap.

Geologic substrate search for term

Underlying geologic material. For instance, the bedrock that lies beneath the soil (if any) in which a natural community grows. See also substrate.

Geomorphology search for term

The study of the form and origin of landscapes, and the arrangement of geologic materials and processes on them.

Germination search for term

The growth of a seed into a seedling.

Glenelg loams search for term

A group of medium-textured soils found throughout the uplands of Rock Creek Park, whose parent material is acidic bedrock.

Gravel search for term

Small stones or pebbles. In U.S. geology, gravel (or sometimes pebble) is a size label for rock fragments between 4 and 64 mm (about 1/6 inch to 2.5 inches). See also cobbles, sand, silt, and clay.

Synonyms: gravels

Groundwater search for term

Water that percolates down into the earth through permeable layers and cracks until it encounters a layer that water cannot penetrate. There it collects and/or flows horizontally, filling the porous spaces in soil, sediment, and rocks. Groundwater originates from rain and from melting snow and ice and is the source of water for aquifers, springs, and wells. Adapted, in part, from The American Heritage Science Dictionary. 2005. Houghton Mifflin Company, Boston. As it flows, groundwater can re-surface. It often carries with it dissolved base minerals leached from soils and bedrock. See also groundwater discharge, groundwater recharge, water table.

Groundwater discharge search for term

Groundwater that re-surfaces as seepage, or even as a flowing spring. It may surface in such places as ravines, along the bottoms of hillsides, and in stream or river channels. Where a road or trail has been cut through a hillside of layered rock, you may be able to see groundwater seeping from between the layers of exposed rock for days after a good rain.

Groundwater processes search for term

The physical, chemical and biological processes that occur in or produce groundwater.

Groundwater recharge search for term

The process of precipitation replenishing the groundwater supply as rain, melting snow, and melting ice soak into the earth.

Growing season search for term

The period of the year when climatic conditions are favorable for plant growth. Quoted, in part, from McGraw-Hill Dictionary of Scientific and Technical Terms. 2003. Sixth edition. The McGraw-Hill Companies, Inc, New York. For instance, in this part of the world, generally the time period between the last freeze in the spring and the first frost in the autumn. The growing season can vary by plant species, as different plants have different tolerances for freezing temperatures.

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Hard mast search for term

A term used to describe the hard-shelled fruits of plants such as the seeds of beech and oak. Hard mast is an especially important wildlife food in the fall and winter. It is high in fat content and is available when other plant foods (fleshy fruits and foliage) are not available. Quoted from Algonquin Provincial Park’s Online Learning Centre. 2009. The Science Behind Algonquin’s Animals. Glossary.

Hardwood search for term

A synonym for a broadleaf deciduous tree (or referring specifically to its wood).

Synonyms: hardwoods

Heath-family shrub search for term

A member of the plant family Ericaceae, made up of mostly shrubs and small trees and including azaleas, rhododendrons, mountain laurel, blueberries, and huckleberries. Botanists sometimes refer to members of this family as ‘heaths.’ Since they usually thrive on acidic soils with few nutrients, they can help indicate soil quality.

Synonyms: heath heath shrub heath shrubs 'heath shrubs'

Herbaceous plant search for term

Generally any plant which does not produce wood. Grasses, wildflowers, and ferns are all ‘herbs.’ When they die back, their stems die back, too. (This field guide does not restrict the term ‘herb’ to plants valued for flavor, scent, or medicinal qualities.)

Synonyms: Herb herbaceous plants

Herbivore search for term

An animal that feeds chiefly on plants.

Herbivory search for term

The consumption of plants by animals.

Horizon search for term

A soil layer. See also soil profile.

Hornblende search for term

A rock-forming mineral rich in iron, magnesium, and calcium. (The common form of amphibole.)

Hummocks search for term

Low hills or mounds. (A term often used for elevated bits of land in wetlands.)

Synonyms: hummocky hummock

Hydrologic cycle search for term

The continuous process by which water is circulated throughout the Earth and its atmosphere. Also known as the ‘water cycle.’ ’ Adapted from The American Heritage Science Dictionary. 2005. Houghton Mifflin Company, Boston.

Hydrology search for term

The study of water (in all its forms) on the Earth’s surface, in the soil and underlying rocks, and in the atmosphere.

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Ice Age search for term

A period of geological time ending about 10,000 years ago, when continental ice sheets seem to have repeatedly covered the high and mid latitudes of the world’s continents. Occurred during the latter part of what geologists call the Pleistocene epoch.)

Igneous rock search for term

Rock that crystallized from hot magma. Can include lava (which solidifies after reaching earth’s surface) or intrusive rock (which solidifies before reaching earth’s surface). At Rock Creek Park, most types of igneous rock (except quartz) contain minerals that become valuable plant nutrients as they weather into soil.

Synonyms: igneous

Impermeable surface search for term

Mainly artificial surfaces (e.g., pavements and rooftops, but also compacted earth) that keep water from penetrating into the ground. When the soil is thus sealed, groundwater recharge is eliminated. Sometimes called ‘impervious surfaces.’

Impervious search for term

An adjective describing a natural or man-made layer that stops rainwater from soaking in or moving through it. Examples of man-made impervious surfaces are rooftops, parking lots, and streets. Examples of natural impervious layers beneath the soil are clay and shale.

Inclusion search for term

A block of rock trapped in another kind of rock.

Indicator species search for term

Species that, when present, pretty much guarantee you are in a certain natural community, or that certain environmental conditions are present (such as nutrient-rich soils).

Infertile soil search for term

Soil that is unable to provide nutrients in a form that plants can use, or in quantities sufficient to meet the needs of many plants, is called nutrient-poor or infertile.

Synonyms: infertile

Inner Coastal Plain search for term

The westernmost part of the Atlantic Coastal Plain, closest to the Piedmont.

Insect search for term

An invertebrate animal that, as an adult, has 6 segmented legs, a three-part body, compound eyes, and two antennae. Adapted from The American Heritage Science Dictionary. 2005. Houghton Mifflin Company, Boston.

Intrusive rock search for term

Magma that cooled slowly, deep beneath the Earth’s surface, producing a medium- or coarse-grained igneous rock, e.g., gabbro, or Kensington Tonalite. (See extrusive rock for contrast.)

Invertebrate search for term

An animal such as an insect, worm, shellfish, or snail, which has no backbone. Adapted from The American Heritage Science Dictionary. 2005. Houghton Mifflin Company, Boston.

ITESC search for term

The International Terrestrial Ecological System Classification—a standard classification system for ecological systems.

IVC search for term

The International Vegetation Classification [of which the U.S. National Vegetation Classification (NVC) is a subset]—a standard used internationally by ecologists to classify (categorize) and map natural communities.

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Kensington Tonalite search for term

One of the major intrusive rock units in the greater D.C. area. Medium to coarse-grained, it includes crystals of light and dark minerals (including quartz and biotite), giving it a granite-like appearance. Named for the village of Kensington, MD.

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Laurel Formation search for term

A type of acidic bedrock at Rock Creek Park, metamorphic in origin, and containing fragments and inclusions of various exotic rocks. Widespread in the eastern Piedmont between D.C. and Baltimore, and named for the town of Laurel, MD. Restricted to the east side of the Rock Creek shear zone.

Layer search for term

A height category ecologists use to describe plants in a natural community (as in canopy layer, shrub layer, field layer).

Synonyms: layers

Leach search for term

The process by which soluble parts are dissolved out from rocks, soils, or other matter as water or other liquid passes through slowly. Adapted from The American Heritage Science Dictionary. 2005. Houghton Mifflin Company, Boston.

Synonyms: leaching leached

Levee search for term

A long ridge of sand, silt, and clay built up by a river or stream along its banks, especially during floods. Adapted from The American Heritage Science Dictionary. 2005. Houghton Mifflin Company, Boston.

Synonyms: levees

Loam search for term

Textural term referring to soil or sediment composed of roughly equal parts of sand (22-52 percent), silt (28-50 percent), and clay (8-27 percent). Sandy loam contains 50-85 percent sand, silt loam contains 50-85 percent silt, while clay loam is defined as having 28-40 percent clay.

Synonyms: loams loamy

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Mafic rock search for term

Rock with significant concentrations of iron and magnesium, making it heavy and dark.

Synonyms: Mafic

Magma search for term

Molten rock underground. Quoted from Stewart, K.G. and M. Roberson. 2007. Exploring the Geology of the Carolinas. UNC Press, Chapel Hill, NC.

Manor loams search for term

A group of medium-textured soils found throughout the uplands of Rock Creek Park, whose parent material is acidic bedrock.

Mast-fruiting search for term

A phenomenon in which large numbers of trees bear a lot of fruit in a particular year despite no seasonal change in temperature or rainfall; this does not occur every year but at intervals of two to 10 years. See also hard mast and soft mast. Adapted from Wisconsin Primate Research Center Library. 2009. Primate Info Net: Primate Factsheets Glossary. University of Wisconsin-Madison.

Mesic search for term

When said of habitats, mesic means having a moderate or well-balanced supply of moisture. When said of plants, mesic means requiring a moderate or well-balanced supply of moisture.

Mesic-site oaks search for term

(As used here) Oak species that thrive on mesic sites. Examples at Rock Creek Park include: northern red oak and white oak.

Metamorphic rock search for term

Any rock derived from other rocks by metamorphosis. See also metasedimentary rock and metavolcanic rock.

Synonyms: metamorphic metamorphic rocks

Metamorphism search for term

Metamorphism is the process of change when rock is subjected to enough heat and pressure to change the minerals, textures, or structures without melting the rock. Deep burial or stress from fault motion can cause rock to undergo metamorphism, i.e., to metamorphose.

Synonyms: metamorphose

Metasedimentary rock search for term

Shorthand for metamorphosed sedimentary rock (i.e., metamorphic rock derived from sedimentary rock). Any sedimentary rock, such as shale or sandstone, which has been subjected to enough heat and pressure to change some or all of the minerals, textures, or structures without melting the rock.

Synonyms: metasedimentary rocks metasedimentary

Metavolcanic rock search for term

Shorthand for metamorphosed volcanic rock (i.e., metamorphic rock derived from volcanic rock). Any rock of volcanic origin which has subsequently been subjected to enough heat and pressure to change some or all of the minerals, textures, or structures without melting the rock.

Synonyms: metavolcanic rocks metavolcanic

Microclimate search for term

Local climatic effects associated with or caused by a specific landform.

Synonyms: microclimates

Microorganism search for term

A microscopically small organism, such as a bacterium.

Migrant trap search for term

(As used here) A site where great numbers of migrating birds may stop over for rest during their long flights in spring and fall.

Mineral search for term

Any naturally occurring inorganic substance with an arrangement of atoms (chemical structure) that can be exact, or can vary within limits. Quartz and feldspar are examples of minerals. Elements that occur naturally as crystals are also considered minerals. A rock is mainly composed of minerals. The terms sand, silt, and clay can refer to specific particle sizes of minerals. Adapted, in part, from The Mineral & Gemstone Kingdom. Glossary. 

Mineral soil search for term

Any soil consisting primarily of mineral material (particles of weathered rock—sand, silt, and clay) rather than organic matter (decomposed plant or animal matter).

Synonyms: mineral soils

Muck search for term

Decomposed organic matter (plant and animal).

Mycorrhizal search for term

Pertaining to a mutually beneficial relationship between plant roots and fungi (plural for fungus). Plants support fungi by providing sugar and a hospitable environment. Fungi support plants by providing increased surface area for water uptake and by selectively absorbing essential minerals. Quoted from Plantlife. 2009.

Mylonite search for term

Lustrous rock, severely flattened, cleaved, and stretched, formed by the shifting of rock layers along a fault. Any kind of rock can become mylonite.

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Native plant search for term

(As used here) A plant species that occurs naturally in a particular region, and whose presence is not traceable to human actions, either directly or indirectly.

Natural community search for term

A ‘community’ of native plants that recurs in the landscape with similar species composition and physical structure. Occurrences of a natural community also tend to share characteristic environmental features such as bedrock geology, soil type, and topographic position, and to have natural processes in common such as climate, means of energy flow, nutrient cycling, and water cycling. Each supports certain kinds of wildlife. Adapted from Canada. Ministry of Forests and Range. 2008. Glossary of Forestry Terms in British Columbia, March 2008.

Synonyms: natural communities

Natural disturbances search for term

Natural events such as fire, severe drought, insect or disease attack, or wind that periodically disrupt natural communities or entire landscapes, impacting them to greater or lesser degrees, for greater or lesser periods of time. See also scale. Adapted from U.S. Forest Service. Cleveland National Forest Land Management Plan, Part 3 – Design Criteria for Southern California National Forests, Appendix L—Glossary (M–R).

Synonyms: disturbance

Natural history search for term

A descriptive study of nature, based more on observation than experimentation. It may include elements of biology, geology, climatology, ecology, and more. A ‘naturalist’ is a person who studies natural history.

Natural process search for term

A process existing in or produced by nature (rather than by the intent of human beings), e.g., evaporation, volcanic activity. Adapted, in part, from 2017.

Synonyms: natural processes

Naturalized search for term

Describes a non-native species of plant or animal that has permanently established itself in a region by successfully reproducing and living alongside native plants in the wild. Some naturalized plants become aggressive invaders. Adapted, in part, from The American Heritage Science Dictionary. 2005. Houghton Mifflin Company, Boston.

Naturalizer search for term

A plant or animal that readily establishes itself in a new region to which it is not native.

Synonyms: naturalizers

Needleleaf search for term

Having needle-like leaves, as opposed to broad leaves. Example of a needleleaf evergreen tree: Virginia pine. (Rock Creek Park lacks examples of needleleaf deciduous trees, such as bald cypress or American larch.)

Non-native search for term

(As used here) A species (plant or animal) that was not historically present naturally in a particular region. Rather, its presence can be traced to human activity, either directly or indirectly. See also non-native invasive plant.

Synonyms: non-natives

Non-native invasive plant search for term

A plant species that not only is non-native to a particular region, but also aggressively multiplies or spreads there, becoming a weedy pest and threatening the well-being of native populations of plants.

Synonyms: non-native invasive plants non-native invasive non-native invasives

Nutrient cycles search for term

The natural recycling of nutrients (chemical elements and molecules) on our planet. Carbon, nitrogen, phosphorus, and even water (hydrogen and oxygen) are just a few of the classic ‘nutrients’ whose pathways of movement are studied. As they are used in living organisms or non-living geological processes, nutrients are never ‘used up,’ but are either released and re-used elsewhere, or are held for long periods of time (such as in rock). Adapted from Marietta College Department of Biology and Environmental Science. 2017. Biomes of the World—Ecology Pages. Environmental Biology—Ecosystems. See also food chains and energy flow.

Nutrient-rich soil search for term

Soil that is good for plant growth. Sometimes called ‘fertile’ soil, or simply ‘rich.’

Nutrients search for term

(As used here) Chemical elements and molecules, in forms that plants can use. (‘Inorganic nutrients.’) At least sixteen are known to be essential to a plant’s well-being, even if in tiny amounts. Some come directly from carbon dioxide in the air and from water (hydrogen, oxygen, carbon). Others come from soil, dissolved in water (nitrogen, phosphorus, potassium, calcium, sulfur, magnesium, iron, zinc, manganese, copper, and other micro-nutrients). Groundwater dissolves some of these out of soil or bedrock.

NVC search for term

The U.S. National Vegetation Classification [a subset of the International Vegetation Classification (IVC)]—a standard used nationwide by ecologists and the federal government to classify (categorize) and map natural communities.

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Organic matter search for term

(As used here) Residue from living organisms (plants and animals) decomposing in the soil, along with living microorganisms.

Outcrop search for term

See rock outcrop.

Outer Piedmont search for term

The easternmost part of the Piedmont, closest to the Atlantic Coastal Plain.

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Parent material search for term

Rock or sediment from which a soil has formed.

Synonyms: parent rock parent rocks parent rocks)

pH search for term

The standard measure of acidity of a substance. On a scale of 1-14, 7 is neutral; anything that tests lower than 7 is technically acidic, and anything higher than 7 is alkaline (or basic). See also acid, acidic, and base, basic.

Photosynthesis search for term

The process by which a plant makes its own food. Green chlorophyll pigment in the leaves absorbs light energy, which is used to fuel sugar production. A plant ‘photosynthesizes’ during most of its growing season.

Synonyms: photosynthesize

Physiography search for term

The scientific study of the natural features of the Earth’s surface. (Physical geography.) Adapted from The American Heritage Science Dictionary. 2005. Houghton Mifflin Company, Boston. ‘Physiographic regions’ are broad-scale subdivisions based on terrain texture, rock type, and geologic structure and history. Quoted from U.S. Geological Survey. 2000. A Tapestry of Time and Terrain. Near D.C., the three physiographic regions are the Blue Ridge Mountains, the Piedmont, and the Coastal Plain. The Fall Zone separates the latter two.

Piedmont search for term

The land bounded by the Blue Ridge Mountains to the west, and the flatter Atlantic Coastal Plain to the east. This rolling terrain is underlain by solid bedrock, which is thought to be the eroded ‘roots’ of an ancient mountain range.

Pioneer species search for term

A plant or animal species that colonizes land that has been cleared or somehow severely disturbed. Typically sun-loving, fast-growing species, sometimes thought of as ‘weedy.’ Around the D.C. area, ‘pioneers’ include native plants such as wild blackberries, Virginia pine, tuliptree, and lots of grasses and other herbaceous species. Non-native plants have become some of the most aggressive pioneers in recent decades: Japanese stiltgrass, garlic mustard, Asiatic bittersweet, Japanese honeysuckle, multiflora rose, tree-of-heaven, princess tree, mimosa tree.

Point bars search for term

Crescent-shaped deposits along meandering streams, on the inside of stream bends. See also cut banks.

Primary cavity nester search for term

An animal, such as a woodpecker or chickadee, that can make its own tree cavity (hollow) in which to nest. See also secondary cavity nester.

Primary production search for term

The activity of green plants and algae that produce and store their own energy (food) from sunlight and non-living chemicals, rather than consuming other organisms to meet their energy needs. Primary producers are therefore the base of the food chain.

Propagule search for term

Any plant structure (such as a bulb, root, etc.) with the capacity to give rise to a new plant, e.g., a seed, a spore, part of the vegetative body capable of independent growth if detached from the parent. Adapted from Biology-Online Dictionary. 2017.

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Quartz search for term

Hard white rock at Rock Creek Park that weathers to an extremely acidic soil. (One of the most common minerals on earth (SiO2).) Quoted from Stewart, K.G. and M. Roberson. 2007. Exploring the Geology of the Carolinas. UNC Press, Chapel Hill, NC.

Quartz veins search for term

Fractures in rock that are filled with milky white quartz. Quartz veins form when hot silica-rich water moves through cracks; as the water cools, quartz is deposited. Quoted from Stewart, K.G. and M. Roberson. 2007. Exploring the Geology of the Carolinas. UNC Press, Chapel Hill, NC.

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Radio button search for term

A radio button is a button the user selects, to the exclusion of the other radio buttons. (You cannot select two at once.) In this website, radio buttons are circular.

Synonyms: radio buttons

Recrystallization search for term

A process in rocks that occurs during metamorphism, whereby the structure, but not the composition, of the minerals in a rock is altered due to incredible heat and pressure.

Regeneration search for term

The continuous renewal of a forest stand. Natural regeneration occurs gradually with seeds from the same or adjacent stands or with seeds brought in by wind, birds, or animals, and with stump-sprouting. Adapted from North Carolina Forestry Association. 2017. Forest Management Basics.

Respiration search for term

(As used here) The process by which an organism breathes or somehow exchanges gases, especially carbon dioxide and oxygen, with the environment. Adapted from The American Heritage Science Dictionary. 2005. Houghton Mifflin Company, Boston.

Rhizosphere search for term

The microscopically thin biologically active zone of soil and microbes on and around plant roots.

Riparian search for term

On, near, or related to the banks of a river. 

Rock outcrop search for term

Any place where bedrock is exposed on the surface of the Earth.

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Sand search for term

Tiny but gritty particles of rock. In U.S. geology, sand is a size label for rock fragments between about 1/16 mm and 2 mm (about 1/16 inch or less, but gritty). See also cobble, gravel, silt, and clay.

Synonyms: sands sandy

Sandy loam search for term

Sandy loam is a textural term referring to soil or sediment composed of 50-85 percent sand and roughly equal parts of silt and clay. (See textbox: A Speck of Difference)

Synonyms: sandy loams

Sapling search for term

A young tree.

Synonyms: saplings

Scale search for term

The magnitude of a natural process in terms of extent (how large an area affected), or frequency/duration (how frequent the recurrence, or how long the process or impact lasts). Also, the intensity (how severe the impact) of a natural disturbance.

Seasonal wetlands search for term

(As usedhere) Pools of water that periodically dry up; therefore usually lacking fish. Particularly important habitat to some salamanders, frogs, and toads.

Secondary cavity nester search for term

An animal, such as a raccoon, bluebird, or wood duck, that is unable to excavate its own tree cavity (hollow) in which to nest, and so uses cavities that other animals or natural processes have created. See also primary cavity nester.

Sediment search for term

(As used here) Loose particles of varying sizes including bits and pieces of rocks and minerals and organic matter such as shells. Includes sand, silt, and clay. Quoted, in part, from Stewart, K.G. and M. Roberson. 2007. Exploring the Geology of the Carolinas. UNC Press, Chapel Hill, NC.

Sediment transport search for term

The movement of sediment and the processes that govern their motion. Sediment transport is typically due to a combination of the force of gravity acting on the sediment, and/or the movement of the air, water, or ice in which the sediment is carried. The force of gravity is due to the sloping surface on which the particles are resting. Adapted from Wikipedia contributors, "Sediment transport," Wikipedia, The Free Encyclopedia (accessed November 17, 2009).

Sedimentary rock search for term

Rock that is formed when organic or inorganic sediments are compressed (or ‘lithified’) into layered solids by the weight of overlying material such as other rocks. It forms at pressures and temperatures that do not destroy fossil remnants. See also igneous rock and metasedimentary rock for contrasts.

Synonyms: sedimentary rocks sedimentary

Seed bank search for term

(As used here) The reservoir of viable seeds present in the soil. Adapted from

Synonyms: Soil seed bank

Seed dispersal search for term

The method by which a plant scatters its offspring away from the parent plant to reduce competition. Methods include wind, insects, animals, tension, and water. Adapted from 2017.

Seedling search for term

(As used here) A baby tree or shrub.

Synonyms: seedlings

Semi-natural community search for term

A vegetation community that, although largely comprised of native plants, owes its present form to historic human manipulation or severe natural disturbances. It is not considered a long-lasting community, but rather is giving way (or succeeding) to another, more natural community as natural ecological processes take their course over the years. May also be called a ‘successional community.’ See also succession.

Synonyms: semi-natural communities

Senescence search for term

The aging and deterioration of plants.

Serotinous search for term

Late or delayed in developing or blooming. More specifically, it can refer to a pine cone or other seed case that requires heat from a fire to eventually open and release the seed. Quoted from Natural Resources 743 - Definitions. 2009. University of Wisconsin at Stevens Point.

Shale search for term

A fine-grained sedimentary rock, consisting of compacted and hardened clay, silt, or mud. Shale forms in many distinct layers and splits easily into thin sheets or slabs. Quoted from The American Heritage Science Dictionary. 2005. Houghton Mifflin Company, Boston.

Shrub search for term

Woody plants which are typically multi-stemmed (and typically shorter than many trees). They comprise an important layer of many natural communities, providing cover for birds and small mammals.

Silt search for term

Tiny particles of rock so fine they are not gritty to the touch. In U.S. geology, silt is a size label for rock fragments between 1/256 mm and 1/16 mm. See also cobbles, gravel, sand, and clay.

Synonyms: silts

Silt loam search for term

Textural term referring to soil or sediment composed of 50-85 percent silt, and roughly equal parts of sand and clay.

Synonyms: silt loams

Silty-clay search for term

Textural term referring to soil or sediment composed of mostly silt and clay with little sand.

Slope aspect search for term

The direction a hill’s slope faces. (The orientation of a slope relative to the four points of the compass.) E.g., a slope with a northerly aspect faces north.

Slope position search for term

Refers to where something lies on a slope, e.g., high-slope, mid-slope, or low-slope.

Slope processes search for term

All processes and events by which the configuration of the slope is changed; especially processes by which rock, surficial materials and soil are transferred downslope under the dominating influence of gravity. Quoted from Canada. Ministry of Forests and Range. 2008. Glossary of Forestry Terms in British Columbia, March 2008.

Soft mast search for term

Seeds that are covered with fleshy fruit, as in holly berries or blueberries.

Soil search for term

Unconsolidated mineral and organic sediment on the surface of the earth. Soil’s pH, texture, and biological activity are shaped by soil-forming processes, and matter greatly to plants.

Synonyms: soils

Soil profile search for term

The particular arrangement of layers (or ‘horizons’) produced during the soil forming process.

Soil series search for term

The basic unit of soil mapping and classification. Each soil series is comprised of soils which have similar soil profile characteristics and share a common parent material.

Spadix search for term

A spike of small flowers growing on a fleshy axis, in plants such as skunk-cabbage and Jack-in-the-pulpit. Usually enclosed in a spathe.

Spathe search for term

Hooded appendage of a plant such as skunk-cabbage or Jack-in-the-pulpit, which encloses the flowering organ (spadix).

Spring ephemerals search for term

Any of various woodland wildflowers that appear above ground in early spring, flower and fruit, and die or return underground dormant, within a short two-month period. Quoted from 2017. Unabridged. Random House, Inc.

Stump sprout search for term

Regrowth of a felled tree by means of a sprout arising from its stump.

Synonyms: stump sprouting

Subcanopy search for term

Trees in a forest stand that are overtopped by yet taller trees (the canopy). They may be younger specimens of canopy trees, or they may be naturally smaller or more shade-tolerant species. See also layer.

Synonyms: tree understory

Substrate search for term

(As used here) The layer (stratum) that lies beneath something. For instance, the soil in which a natural community grows. See also geologic substrate.

Succeed to search for term

[as in a forest that is ‘succeeding to’ another forest type] Give way to, become.

Succession search for term

The sequence of plant communities that develops in an area after large disturbance, from the initial stages of colonization until a long-standing mature natural community is achieved. Many factors, including climate and changes brought about by the colonizing organisms, influence the nature of a succession. Adapted from Dictionary of Biology. 2004. Fifth edition. Oxford University Press.

Successional canopy trees search for term

Trees such as tulip poplar and pine that are typically the first to establish themselves on cleared or otherwise severely disturbed land. Sun-loving, fast-germinating species such as these form the canopy in the earliest successional communities in the D.C. area.

Successional community search for term

See semi-natural community.

Surficial geology search for term

A surficial geology map shows the distribution of all the loose (unconsolidated) materials such as cobbles, gravel, sand or clay which overlie solid bedrock in an area. The surficial deposits are the product of natural geologic processes such as glacier movement or water movement (such as river flood plains), or are attributed to human activity (such as road-fill or other land-modifying features), and may bear no relation to the bedrock beneath. Adapted in part from Maine Geological Survey. 2017.

Sykesville Formation search for term

A type of acidic bedrock at Rock Creek Park, metamorphic in origin, similar to the Laurel Formation in appearance and origin, but containing a somewhat different suite of exotic inclusions, some rich in base elements. Widespread in the eastern Piedmont between northern VA and MD, and named for the village of Sykesville, MD. Restricted to the west side of the Rock Creek shear zone.

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Talus search for term

Pieces of weathered, broken rock piled up on a slope

Terrace search for term

(As used here) See ancient river terrace or floodplain terrace.

Toeslope search for term

The nearly flat part at the base of a hill slope. It receives deposits of sediment—fine or coarse—that get transported downslope by gravity or other means.

Tonalite search for term

Medium- to coarse-grained intrusive igneous rock containing certain essential minerals, including at least 20 percent free quartz (acidic). Other mineral content varies widely, resulting in rocks that weather to soils of varying but fairly good fertility for plants at Rock Creek Park.

Topography search for term

The contour and shape of the land. Quoted from Stewart, K.G. and M. Roberson. 2007. Exploring the Geology of the Carolinas. UNC Press, Chapel Hill, NC.

Topsoil search for term

The uppermost several inches to one foot of soil, enriched in organic matter and containing most of the soil’s microorganisms and biological activity that support plant growth.

Transpiration search for term

The process by which water, drawn up from the ground into a plant’s leaves, evaporates into the air through tiny openings in the leaves called stomata. Part of the water cycle.

Tributary search for term

A stream or river which flows into a larger stream, river, or lake (and not directly into the ocean).

Synonyms: tributaries

Turbidity search for term

Cloudiness in water, caused by suspended elements.

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Unconsolidated sediments search for term

Sediment composed of various proportions of cobbles, gravel, sand, silt, and clay that have never been buried sufficiently deep to be compressed and consolidated (‘cemented’) into bedrock.

Understory search for term

The plants growing under a forest’s canopy. Includes small trees, young canopy trees (saplings and seedlings), shrubs, and the herbaceous layer.

Upland search for term

Non-wetlands; elevated land whose vegetation isn’t dependent on the water table.

Synonyms: uplands

Upland terrace gravel search for term

(As used here) Well-rounded, water-polished stones and sand that were likely deposited by a pre-historic river, but can now be found on some ridgetops in Rock Creek Park. See also ancient river terrace.

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Vernal pool search for term

See seasonal wetland.

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Water cycle search for term

See hydrologic cycle.

Water table search for term

The level below which the ground is saturated with water.

Watershed search for term

Natural drainage basin; all the land drained by a river or stream and its tributaries.

Synonyms: watersheds

Weathering search for term

The breakdown of rock as it is exposed to weather conditions such as heat, water, ice, or pressure, or to chemicals or biological organisms.

Wetlands search for term

Land areas that are wet enough from surface water or groundwater to be saturated at least a good part of the year. Because of the saturated soils, the plants that grow there are different than those in areas of greater elevation.

Windthrow search for term

The uprooting and overthrowing of trees by the wind. Quoted from Merriam-Webster Online Dictionary. 2017. Merriam-Webster Online.

Worm search for term

Term for many different distantly-related animals—most of them invertebrates—which have a soft, long body that is round or flattened and typically lacks legs. Adapted from The American Heritage Science Dictionary. 2005. Houghton Mifflin Company, Boston.

Wrack line search for term

A linear deposit of leaves, branches, mud, and other debris left behind after floodwaters recede, showing the maximum height of the flood. (Technically applied to the seaweed and marine debris left by high tides, but commonly applied more widely to debris from freshwater flooding.)

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