Glossary for agroforestry

Compiled and edited by Peter Huxley and Helen van Houten, International Centre for Research in Agroforestry - 1997

gallery forest

Vegetation, with trees and shrubs, growing alongside or close to a watercourse, lake, swamp, or the like, and often dependent on its roots reaching the watertable. Also called a riparian forest.

gamete

A male or female reproductive cell, typically the product of meiosis, capable of uniting in the process of fertilization with one of the opposite sex.

gapping

See beating up

gene

The unit of inheritance.

gene bank

For plants, any place established with the appropriate facilities and trained staff where plant germplasm can be maintained in the form of seeds or tissues or as growing plants.

genecology

A combination of ecology and genetics to study the genetic variation among populations of a species that is correlated with habitat. See also autecology, synecology

gene complex

The balanced system of genes constituting the internal environment within which each gene acts, so balanced that a change in one affects the operation of other genes.

gene fixation

The condition in which a particular allele becomes the only allele present in a population, because of selection or genetic drift.

gene flow

1. Exchange of genetic traits between populations by movement of individuals, gametes or spores.

2. The spread of genes through crossing.

gene frequency

The proportion in which alternative alleles of a gene occur in a population.

gene pool

The total genetic information possessed by the reproductive members of a population of sexually reproducing organisms.

general combining ability

See combining ability

genet

The ramets derived from one seed.

genetic base

The aggregate of genetic variability available within a cultivar or species; may be broad or narrow.

genetic drift

The change in gene frequency and population characteristics caused by chance rather than selection, usually most pronounced in small populations. Random fluctuations in gene frequency caused by sampling error. See also gene fixation

genetic engineering

The alteration of the genetic composition of a cell by various procedures (transformation, protoplast fusion, and so on) in tissue culture.

genetic gain

The increase in productivity following a change in gene frequency.

genetic plasticity

In genetic terms, the capacity for adaptation caused by genetic changes; as in natural selection. See also phenotypic plasticity

genetics

Deals with the causes of resemblances and differences among organisms related by descent; takes into account the effects of genes and environment.

genome

One complete set of genes containing all the genetic information to produce an individual.

genotype

1. The entire genetic constitution of an organism.

2. An individual's hereditary constitution, with or without phenotypic expression of the one or more characters it underlies. The genotype is determined chiefly from the performance of progeny and other relatives. Genotype x environment = phenotype. See also selection

genotype x environment interaction

1. The degree to which relative ranks (levels of differences) are found when sets of genotypes are tested in different environments (or cultural conditions).

2. The tendency for the size of a difference in performance between two genotypes to depend on environment. See also plasticity

genus

A rather arbitrary category in the taxonomic hierarchy between that of family and species. Genera consist of one or more closely related species; plant genera are defined mostly on characteristics of the flower, fruit or both.

geographic variety

A subdivision of a species with distinct morphological characters and a distinctive geographical range, given a Latin name according to the strict rules of the International Code of Botanical Nomenclature. A taxonomic variety is known by the first validly published name applied to it and that nomenclature tends to be stable. See also variety

geophyte

A herbaceous plant, perennial or biennial, that periodically reduces its shoots to storage organs embedded in the ground. This often occurs during the season that is unfavourable to the whole shoot system.

geotropism

Plant growth movements induced by gravitational stimulus.

germination

Growth of the embryo in the seed until the emergence of the embryonic radicle through the seedcoat. In seed testing, the capacity of the embryo to emerge from the seedcoat with the essential structures indicates a potential to produce normal plants. In dry seeds, germination follows imbibition (absorbing water and swelling).

germinative capacity

Percentage of seeds that germinate during the whole of the germination period, or percentage of seeds that germinate during a specified time interval, determined by the maximum rate of germination.

germplasm

1. The material constituting the physical basis of inheritance (seeds, cuttings, tissue cultures). The sum total of the hereditary materials in a species.

2. The sum total of the genes and cytoplasmic factors governing inheritance. The hereditary material transmitted to offspring through the germ cells.

3. The (sometimes multiple) sets of genes that constitute an individual or a cultivar; a source of genetic material for breeding work.

girth

The size or measurement around an object, such as the circumference of a tree trunk.

gley

Soil with impeded drainage. Used in soil classifications based on profile leaching.

global radiation

The total of direct solar radiation and diffuse sky radiation received by a unit horizontal surface.

graded terrace

A terrace having a constant or variable grade (slope) along its length. See also level terrace

gradoni bench

Small bench terrace or narrow shelf cut along the contour (usually with an inward slope).

graft incompatibility

When the graft union between the stock and the scion fails. This can occur after some years of apparently healthy growth. See also grafting

grafting

1. Placing a portion of one plant in close cambial contact with another with the object of obtaining vegetative union between the two.

2. A method of plant propagation by transplantation of a bud or a scion of a plant onto another plant. Also, the joining of cut surfaces of two plants so as to form a living union.

grassland

Land covered with grasses and other herbaceous species. Woody plants may be present, but if so,they do not cover more than 10% of the ground. There are many different types of grassland designated by ecozone, topography, climate, soil conditions, and so on. Derived grassland is maintained in that condition by regular burning; edaphic grassland arises on particular soil types, for example, those found in or around permanent or seasonal swamps. See also rangeland, veld

gravelly

Soil texture when soil mass contains 15–90% pebbles by volume.

gravimetric method

Density determination (eg of wood) by weight and volume, as opposed to densitometric determination by beta-ray or x-ray analysis.

grazing

A method of feeding by herbivores characterized by repeated removal of only a part, generally the leaf, of the plant, which is most commonly called herbage. Defoliation by herbivorous animals. See also browsing

green manure

1. A crop that is grown for soil protection, biological nitrogen reduction, or organic matter and ploughed, disked or hoed into the soil.

2. Any crop grown for the purpose of being turned under while green, or soon after maturity, for soil improvement.

gregarious

Species that occur in groups within a community and that may form pure stands of species in the natural state.

gross energy value

Of animal feed, the energy released when a known weight of food is oxidized in a bomb calorimeter, that is, the total heat produced. Proteins have a total energy value around 24 MJ kg–1, fats around 38 MJ kg–1 and carbohydrates around 17.5 MJ kg–1, so that feedstuffs average about 18 MJ kg–1. No account is taken of losses that occur when an animal actually feeds (digestibility, palatability). See also digestible energy

gross plot

The full plot size, including guard rows. See also net plot

ground cover

1. A crop planted to provide a covering over the soil.

2. Any vegetation producing a protective mat on or just above the soil surface. In forestry, low-growing shrubs and herbaceous plants under the trees. See also herbaceous layer.

ground flora

The grasses, herbs and other plants growing on the forest floor.

ground truth

Data collected by field observation to compare with, or support, the interpretation of remote sensing imagery. Also ground validation.

ground validation

See validation

groundwater

The water below the land surface that has drained through the upper soil layers. See also water table

grove

A small wood or group of trees. See also coppice

growing on

The re-establishment of seedlings or young plants into a more spacious site or container than that in which they were growing previously. After germinating seeds the sequence is often pricking out, growing on (perhaps more than once), planting out. See also hardening off

growing season

Period(s) of the year during which water is available for plant growth and temperatures are favourable (not too high or low). It is generally poorly defined, and usually used only with regard to crop plants. See also cropping season

growing stock

Volume of standing timber or number of standing trees in a forest.

growth correlation

The interrelationships between the various growth processes that control the rate, extent and kind of growth that occurs in an organ or plant.

growth habit

The basic pattern of structural development or symmetry attained by a plant, for example 'shrubby', 'treelike' or 'viney', 'erect' or 'spreading'.

growth promoter

A plant growth regulator that stimulates growth.

growth regulator

A natural substance that regulates the enlargement, division or activation of plant cells. See also plant growth regulator

growth retardant

A chemical substance used to slow down the growth of crop plants, for example, maleic hydrazide. Can also be a naturally occurring substance. See also plant growth regulator

grubbing out

Felling trees and shrubs by exposing and cutting the roots. Same as 'grub felling'.

grub felling

See grubbing out

guard row

Line of plants along the edge of a research plot that is not measured. Its object is to ensure that the measured part of the plot (net plot) is not affected by external influences. See also gross

plot

gully

A channel cut by concentrated runoff but through which water commonly flows only during or immediately after heavy rains, or during the melting of snow. Conservationists distinguish a rill from a gully by its depth. A gully is sufficiently deep not to be obliterated by normal tillage operations.

gully erosion

The erosion process in which water accumulates in narrow channels and, over short periods, removes the soil from this narrow area, sometimes to considerable depths.

gum

Complex polysaccharidal substances formed by cells in reaction to wounding or infection.

gummosis

Production of gum by or in plant tissue. Sometimes caused by bacterial infection.

guttation

Process by which plants expel liquid water from the leaves in excess of transpiration.