Glossary

A small, dry, one-seeded fruit that does not split open at maturity.

Wind-driven. Used to describe landforms, sediments and geological processes shaped by the action of wind.

A species introduced, deliberately or accidentally, by people outside its natural range. Also called non-native or exotic species.

A low, outspread, relatively flat to gently sloping mass of loose rock material, shaped like a fan or a segment of a cone, deposited by streams at the mouths of tributary valleys onto a plain or broad valley.

Arranged singly at different heights along a stem, rather than in pairs.

Having a leaf base that clasps or partly surrounds the stem.

A plant that completes its whole life cycle, from seed to flowering, fruiting and death, within one year. In AlUla, many annuals germinate only after rain.

Growing upward from a base that initially lies or spreads along the ground.

A thin living layer at the soil surface, made of cyanobacteria, algae, lichens and mosses, that binds soil particles, reduces erosion and enriches the soil.

A geographical area defined by characteristics of the natural environment. Saudi Arabian bioregions (Llewellyn, 2013) are based on physiography (geology and topography), altitude, climate, soils, and vegetation structure and composition.

Similar to a habitat but defined in relation to a suite of species.

A modified leaf associated with a flower or flower cluster.

A small, non-flowering land plant without true roots or vascular tissue: mosses, liverworts and hornworts. In AlUla, 48 bryophyte species are recorded, mostly in shaded and moist refuges.

An isolated hill with steep sides and a flat, slightly rounded top.

Falling off early or before the end of the growing season.

The outermost flower whorl, usually made up of sepals.

Covered with a fine, often grayish-white covering of hairs.

A large chasm between cliffs.

A compact head of many small flowers, as in daisies.

A dry fruit that splits open at maturity to release its seeds.

A low shrub or dwarf shrub whose regrowth buds sit close to the ground (below about 25 cm), where they are sheltered from wind and heat. Many desert subshrubs are chamaephytes.

Fragments or grains of pre-existing rocks and minerals.

A plant that climbs on rocks or other plants for support, using twining stems, tendrils or hooks.

Heart-shaped, usually with a notch at the base.

Leathery in texture.

The flower’s petals, considered together.

The top of a volcano, which is lower than the surrounding rim.

A plant whose regrowth buds are hidden below the soil surface or under water during the unfavourable season. Geophytes are the land-dwelling cryptophytes.

A flower cluster in which the central or terminal flower opens first.

IUCN Red List category for a species whose risk of extinction cannot be assessed because too little is known about its distribution or populations. Many plants of AlUla are in this situation.

Species present in areas with low to no degradation, balanced ecosystem dynamics and controlled grazing. They are often palatable, and are called "decreasers" because they tend to decrease as environmental degradation intensifies.

The process by which the quality of a natural habitat is reduced, making it less able to support wildlife. It can have human or natural causes; on this site, it refers to human causes.

Opening naturally at maturity to release seeds.

A complex of organisms and their associated physical environment, united by similar ecological processes, whose biotic structure and composition differ substantially from those of other ecosystem types.

A plant or animal found naturally only in a specific geographic region. For example, national endemics are found only in Saudi Arabia, and Arabian endemics only in the countries of the Arabian Peninsula.

Lasting for a very short time. In plants, it often describes species that grow, flower, and set seed quickly when conditions are favorable.

Growing upright.

A group of related genera sharing common features and ancestry. Plant family names end in "-aceae", for example Fabaceae (legumes) or Asteraceae (daisies).

The period during which a plant bears open flowers. In AlUla, most plants flower in late winter and spring, after the winter rains.

A dry fruit that opens along one side to release its seeds.

The period during which a plant bears fruits and seeds. Also called fructification.

A group of closely related species (plural: genera). The genus is the first part of a scientific name, for example Vachellia in Vachellia tortilis.

Fauna species with a limited distribution, defined as an Area of Occupancy (AOO) under 10 km² and/or an Extent of Occurrence (EOO) under 100 km², following the IUCN methodology.

The branch of geology concerned with the structure, origin and development of the topographical features of the earth's surface.

A plant that survives unfavorable seasons using underground storage organs, such as bulbs or tubers.

Smooth and without hairs.

Bearing glands that may produce or release sticky or aromatic substances.

The overall shape and structure of a plant (tree, shrub, herb, climber…), and the way it survives the unfavourable season. On this site, growth forms follow the Raunkiær life-form system: see Phanerophyte, Chamaephyte, Hemicryptophyte, Cryptophyte, Geophyte and Therophyte.

Plant and animal communities, as the characterising elements of the biotic environment, together with abiotic factors (soil, climate, water availability and quality, and others), operating together at a particular scale (EUNIS definition, European Environment Agency, 2023).

A plant adapted to grow in salty soils, such as those of ephemeral flooded depressions.

Describes data aligned and integrated from different sources, methodologies or classification systems to create a consistent and comparable dataset.

A lava field forming a high plateau. Harrat surfaces clearly reflect their age: older harrats are severely weathered and eroded, while younger ones show little erosion and can appear as an impassable, chaotic jumble of angular blocks.

A perennial plant whose above-ground parts die back in the unfavourable season, leaving regrowth buds at the soil surface, often protected by dead leaves. Many perennial grasses are hemicryptophytes.

A plant with soft, non-woody stems. Herbaceous plants can be annual, ephemeral or perennial.

Covered with stiff, bristly hairs.

The global inventory of the conservation status of species, maintained by the International Union for Conservation of Nature (IUCN). Its categories, from least to most at risk, are: Least Concern, Near Threatened, Vulnerable, Endangered, Critically Endangered, Extinct in the Wild and Extinct. Species not yet assessed are Not Evaluated; those lacking data are Data Deficient.

Species, possibly native to Saudi Arabia (so not alien invasives), that spread quickly with environmental degradation or overgrazing. Some are unpalatable to grazers, and some compete with established plant communities as degradation begins or intensifies.

The arrangement or cluster of flowers on a plant.

An isolated rock hill, ridge or small mountain that rises abruptly from a virtually level surrounding plain.

An alien species that spreads rapidly in its new range and harms native species, habitats or human activities.

Describes rocks, minerals or soils composed of fine, alternating, parallel layers or flat plates.

Shaped like a lance: longer than wide and tapering toward the ends.

A stream or sheet of molten or solidified lava erupted from a volcano onto the Earth's surface. Solidified lava flows can be described as fresh or eroded.

A partnership between a fungus and an alga or cyanobacterium, forming a crust, scales or small leafy or shrubby growths on rocks and soil.

A flat-topped hill with steep sides, smaller than a plateau and with much steeper sides. In AlUla, mesas are typically in sedimentary rocks, while plateaus are the harrats.

A naturally raised landform with steep, sharp edges and a peak.

Ending in a short, abrupt, sharp point.

An accumulation of wind-blown sediment around a plant, forming a raised, fertile mound.

A species that occurs naturally in a region, without having been introduced by people.

A small, usually hard, one-seeded fruit or fruit section.

Egg-shaped with the broader end toward the tip.

Arranged in pairs directly across from one another on a stem.

An area of exposed rock sticking out from the surrounding ground.

The part of a flower that contains ovules and can develop into fruit after fertilization.

Intensive grazing by animals on an area of land, particularly rangelands and grasslands, to the point that vegetation is damaged and the soil becomes liable to erosion by wind and rain, sometimes resulting in desertification.

Describes plants that livestock and wildlife readily eat. Palatable species tend to decline under heavy grazing.

A branched inflorescence in which the flower stalks are themselves branched.

A tuft or ring of hairs or bristles on a fruit, often helping it disperse by wind.

A plant that draws all or part of its water and nutrients from another living plant (its host), through specialised organs attached to the host's roots or stems.

The small stalk that supports an individual flower within a cluster.

A gently sloping bedrock surface that forms at the base of a mountain or hill, often covered by a thin layer of gravel or alluvium.

An extensive, gently sloping plain formed by the joining of multiple pediments at the base of a mountain range.

The process of soil formation.

A plant that lives for more than two years.

The flower’s non-reproductive parts; it includes the sepals and petals, or similar structures when these are not distinct.

Having a petiole, the stalk that attaches a leaf blade to a stem.

A woody plant, tree or tall shrub, whose regrowth buds are carried well above the ground (more than about 25 cm). Example: acacias (Vachellia).

The timing of recurring stages in a plant's life, such as flowering and fruiting, through the year. On species pages, the phenology chart shows in which months each species has been observed in flower or in fruit in AlUla.

Deeply divided into segments arranged along both sides of a central axis.

A large flat expanse that rises, often steeply, above the surrounding landscape.

Growing flat or close to the ground.

Covered with short, soft hairs.

A natural tube formed by flowing lava. When emptied of lava it can resemble an elongated cave. Also called a lava tube.

A sandstone formation in AlUla with honey-coloured, fairly massive layers, dominated by fluvial deposits and quartz sediment. Its cement is relatively weakened by hydrolysis and chemical weathering.

An elongated flower cluster with stalked flowers arranged along a central stem.

Refers to species known to occur only in Saudi Arabia and neighbouring countries. They are not regional endemics but have a restricted range.

A classification of plants devised by the Danish botanist Christen Raunkiær, based on where the buds that allow regrowth are placed during the unfavourable season (drought or cold): high above the ground, near the ground, at the soil surface, below ground, or only in seeds.

Areas to which components of biodiversity retreat, in which they persist, and from which they can potentially expand under changing environmental conditions.

A horizontal underground stem that can produce roots and new shoots.

A small stream or channel.

A low-relief sand surface largely devoid of dunes, although it may show low-relief aeolian features.

The youngest sandstone formation in AlUla: medium-grained, well-rounded sands with graded bedding, interlaid with shallow marine deposits. Its cement is relatively weakened by hydrolysis and chemical weathering, while the marine deposits form hard iron beds.

Rough to the touch because of short, stiff hairs or projections.

A dry fruit that splits into separate one-seeded sections when mature.

The universal Latin name of a taxon, written in italics and followed by the name of the author(s) who first described it, for example Morettia parviflora Boiss.

Having saw-like teeth along the edge, typically pointing toward the tip.

Attached directly to a stem, without a stalk.

A zone where strain forces opposing sides of the rock to slide past each other, here as a result of faulting linked to the opening of the Red Sea. The rock there is broken and more brittle, and erosion works faster.

A woody perennial plant, smaller than a tree, usually with several stems branching from the base. Dwarf shrubs and subshrubs are low shrubs, often woody only at the base.

The oldest sandstone formation in AlUla, made of several thick, medium to coarse-grained beds with large cross-bedding throughout, indicating high-energy river channels.

The basic unit of classification: a group of individuals that resemble each other and can interbreed. A species is named with two words, the genus and the species epithet, for example Rhazya stricta.

The pollen-producing part of a flower, usually consisting of a filament and anther.

The vertical layer of vegetation a plant belongs to: trees, shrubs, herbaceous plants or climbers. On this site, the stratum is used as a simple filter in the species list.

A small shrub with woody growth mainly at the base and softer stems above.

A subdivision of a species, usually a population with distinct features and its own geographic range (abbreviated subsp.). A variety (var.) is a lower rank still.

In ecology, the substance, surface or base on which biological processes occur or an organism lives.

Having fleshy tissues adapted to store water.

Another scientific name that has been used for the same taxon but is no longer the accepted one.

Rounded, smooth cavities or hollows in rock, often grouped and described as honeycomb weathering.

Any unit of biological classification, such as a family, genus, species or subspecies (plural: taxa).

A plant that survives the unfavourable season only as a seed: an annual or ephemeral. Therophytes make up a large part of desert floras.

In the IUCN Red List, a species classified as Vulnerable, Endangered or Critically Endangered.

Densely covered with soft, woolly hairs.

A woody perennial plant, usually with a single main trunk, that reaches several metres in height.

A swollen underground stem or root that stores nutrients and may help the plant survive or reproduce.

A flower cluster in which stalks arise from one point, spreading like the ribs of an umbrella.

A fresh lava flow, where the basalt has not been significantly eroded.

The dominant plant species and their structural characteristics in an area.

A hill-shaped landform that forms around a volcano.

A river channel, which can also include the floodplain and valley of a stream, usually dry except during the rainy season.

A plant adapted to survive with very little water, for example through small or absent leaves, thick cuticles, deep roots or water-storing tissues.