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Glossary: D is for ... deciduous, dendrology, and desertification.
by Ariadne Unst

Glossary of D...

d.b.h. (n.)
diameter at breast height (1.37 m or 4.5 ft above the ground)

deciduous
Falling off or shed at maturity or in a dry season. Contrast with evergreen.

decomposer (n.)
A consumer that obtains its energy from dead and decaying materials.

decomposition (n.)
Chemical or physical breakdown of matter into smaller parts or chemical elements. Also the breakdown of organic matter by decomposing organisms into smaller parts or inorganic constituents.

decumbent (adj.)
Reclining on the ground, and with the growing end ascending.

deep time
Introduced by John McPhee (Basin and Range) to describe geological time scales. More at our deep time page.

deforestation (n.)
The removal of trees from a habitat that they dominated. "The thinning, changing, and elimination of forests", from Deforesting the Earth: From Prehistory to Global Crisis by Michael Williams. (See also our book review.)

dehiscent (adj.)
Opening spontaneously when ripe, so that the seeds are discharged.
Contrast with indehiscent.
deliquescent branching (n.)
A mode of branching in trees where the trunk divides into many branches. No central axis is left. Example: elms.

dendrochronology (n.)
The science of studying the past by looking at the growth rings on trees.

dendrology (n.)
The science of studying trees.

dense (adj.)
The congested arrangement of flowers in an inflorescence.
Compare open.

dentate (adj.)
Describes a leaf whose margins have teeth that point out.

derived (adj.)
Describes character(s) that changed from a primitive state; advanced.

derived characteristic (n.)
Evolutionarily advanced character state.

descendant (n.)
Describes a species or taxon that has evolved from another species or from within a taxon.

desert (n.)
Area that receives low precipitation (averaging less than 25 centimeters a year). Also a biome in which the plants and animals have adapted to survive severe drought conditions.

desert climate (n.)
Low rainfall.
Hot desert: no month below 6ºC.
Cold desert: one or more months below 6ºC.
Also see climate.

desertification (n.)
The creation of a region that is little cultivated, lacking in moisture, nutrients, or suitable temperature and light.
Recipe to make a desert, as used in the USA in the 1920's to make the Dust Bowl, and summarized in Ecology by John Cloudsley-Thompson:
1. Chop down the trees.
2. Plough dry, allowing much of the topsoil to blow away.
3. Sow fast-growing non-native grasses.
4. Graze cattle. Soil can be compacted and reduced to desert in 15 years.
Alternatively, produce cash crops with a minimum of fallow and natural recovery periods.
Cloudsley-Thompson comments, "It has been said that the desert is the cradle of civilization: certainly throughout their existence, people have been turning their birth place into a desert."

determinate (adj.)
A type of inflorescence. that terminates in a bud that blooms first. This halts elongation of the flowering stem.
Compare indeterminate.

disruptive selection (n.)
Natural selection where subpopulations of a single species within the same habitat exhibit different adaptations. Occurs when a habitat contains distinctly different types of soil conditions or other factors. Developing subpopulations adapt to particular features of the area. Results in evolutionary changes and new species, after which the subpopulations enter into stabilizing selection.

dichogamy (n.)
Maturation of stamens and pistils at different times, thus preventing self-pollination.

dichotomous (adj.)
Dividing repeatedly and regularly in pairs.

dicotyledon (n.)
A plant having two cotyledons or seed leaves. (dicotyledonous, adj.) Flowering parts are often in multiples of four or five; leaves are often net-veined; roots are usually fibrous.
Most flowering plants families are dicots.
A subclass of angiosperm.
Compare with monocotyledon.

dioecious (adj.)
Having staminate (male) flowers and pistillate (female) flowers on different plants of the same species.

diploid (n.)
An organism which has two sets of chromosomes (paternal and maternal) in its cells.

disclimax (n.)
A relatively stable ecological community often including kinds of organisms foreign to the region and replacing the climax because of disturbance.

disease (n.)
An impairment of the normal state of an organism, modifying or interrupting its normal function. Caused by a pathogen.

dispersion
The horizontal spacing of plants or animals. It can describe the structure of an ecological community.
Uniform dispersion results from competition for light, nutrients, and water.
Random dispersion (where individuals are distributed through a homogeneous area without regard to the presence of others) is rare.
Clumped dispersion, or aggregation, is very common. Often it results from the uneven distribution of resources.

distal (adj.)
The end relatively distant from the axis and the point of attachment.
Compare proximal.

distinct (adj.)
Having separate yet similar parts, such as the petals on a flower.
Compare united.

disturbed (adj.)
A habitat impacted by people.

diurnal (adj.)
Open (or growing) in the daytime and closed in the night.

diversity index (n.)
The relative diversity of a community.

division (n.)

DNA (n.)
Deoxyribonucleic acid, the carrier of genetic information in cells.

dominant (a crown class) (n.)
One of four main crown classes. Dominant trees are largely free-growing. They have their crowns in the uppermost layers of the canopy.

dorsal (adj.)
Referring to the outer (back) surface.

drought (n.)
Climatic condition where water loss due to evaporation and transpiration is greater than water inputs through precipitation.

drupe (n.)
A fleshy, soft fuit outside and around (making it the exocarp) that encloses a hard-shelled seed or stone (endocarp). For example, a cherry or peach.

duff (n.)
The partially decomposed organic matter (litter of leaves, flowers, and fruits) beneath plants, as on a forest floor.

dyne (n.)
The unit of force that creates an acceleration of 1 centimeter per second on a 1-gram mass. (105 dynes equals one newton.)