- A horizon
- The topsoil layer in a soil profile that commonly contains organic matter.
- abiotic
- Not related to life, purely physical-chemical processes/conditions
- ablation
- In glaciers, refers to melting, erosion and evaporation which reduces the area of the ice.
- ablation area
- The termini of glaciers where loss of ice occurs through calving, melting or evaporation
- accumulation area
- The snowfields or cirques of mountain glaciers and the snowfields of continental glaciers are called the zone of accumulation because it is here that new snow falls to nourish the glacier.
- active layer
- The layer of ground that is subject to annual thawing and freezing in areas underlain by permafrost
- adaptation
- Act of changing to fit different conditions. A change of structure, form, or habits to suit conditions, inherited within a species and increasing rates of reproduction and survival.
- advective
- The transfer of air mass properties by the velocity field of the atmosphere
- aeolic
- Wind borne, e.g. dust deposited by wind
- agamospermy
- The formation of seeds without sexual reproduction. The seeds are genetically identical to the mother plant.
- albedo
- or surface reflectivity is the ratio of the amount of electromagnetic radiation reflected by a body (surface) to the amount incident upon it, commonly expressed as a percentage. Ice may reflect up to 90 percent of incoming solar radiation (albedo = 90), while blacktop asphalt may absorb up to 95 percent (albedo = 5).
- allophane
- Clay mineral of the kaolinite group, whitish, amorphous, non-crystalline
- alpine
- of or relating to a region that occurs above the tree line and below the (permanent) snow line on temperate and tropical mountains. The vegetation is characterised by an absence of trees and varies greatly with its aspect, the greatest contrasts being between the wet side and the dry leeward side of the mountains concerned. The elevation of the lower limit of the zone increases from about 1000 m above sea level in Scotland to over 2000 m in the Swiss Alps and to 3700 m in the western Himalaya.
- Alpine
- (a) of or relating to the Alps or their inhabitants
(b) (Geology) of or relating to an episode of mountain building in the Tertiary period during which the Alps were formed
- alpine, the
- in its widest sense, any area in the belt between the high elevation treeline and the snowline. In a narrower sense, restricted to the alpine belt proper, i.e. the treeless land above the treeline with a full vegetation cover. The term applies globally just like "the tropics".
- altiplano
- High altitude plane in Latin America
- androecium
- Male part of flower
- annoxia
- Lack of oxygen
- anthesis
- The opening of a flower, sometimes used to denote the stage when the flower first donates pollen, or is receptive to pollen, whichever is earlier.
- anthocyanid
- Blue/red water soluble pigment
- apomixis
- Asexual reproduction, includes both vegetative reproduction and agamospermy.
- apoplast
- The part of a plant tissue outside the outer protoplast membrane. Commonly the sum of cell walls (cellulose) and intercellular spaces.
- asthenosphere
- A layer in the mantle that is relatively weak and viscous and lies directly beneath the solid lithosphere.
- autogamy
- Within-flower fertilisation
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