Glossary for agroforestry

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

natural selection

Elimination of living organisms on the basis of their inability to survive and reproduce under a particular set of environmental conditions.

necrosis

The death of (plant) tissue. Generally used in connection with localized death.

necrotic

Dead and discoloured.

nematocidal plant

A plant species that produces organic substances from the roots that adversely affect (kill) eelworms (nematodes). In some cases the substances produced stimulate the eelworm eggs in the soil to hatch, but the larvae are unable to infect the plant as it is resistant to them. Such plants can be grown as trap crops (for example, Tagetes spp).

neotropics

Tropical Central and South America. See also paleotropics

net assimilation rate

At any instant, the increase in plant material (dry weight) per unit of assimilatory material. Usually estimated as mean net assimilation rate per unit of time (for example, over a week). It is based on an assumption that there is a linear relationship between increase in leaf area and increase in dry weight.

net plot

That part of a plot that is measured. See also gross plot, guard rows

net present value

1. The present worth of the benefits less the present worth of the cost. Discounted measure of the project worth.

2. The value of a project, used for comparative purposes, derived by discounting both the forecast costs stream and the revenues stream back to the present, at a given discount rate.

net primary production

See primary production

net radiation

The difference between downward and upward (total and terrestrial) radiation; the net flux of all radiations.

neutral soil

Neither acid nor alkaline; pH 6.6–7.3.

niche

1. A place or position adapted to the character, or suited to the merit, of a person or thing. The sum total of adaptations of an organismic unit.

2. In ecology, the total range of conditions under which the individual (or population) lives and replaces itself, or the position or status of an organism within its community and ecosystem resulting from the organism's structural adaptations.

nicking

Cutting into the stem below a bud so as to prevent it from growing out. See also notching

nitrification

The oxidation of ammonia to nitrite, and of nitrite to nitrate, by microorganisms.

nitrogen fixation

The conversion of elemental N to organic combinations or to forms readily usable in biological processes.

nitrogen-fixing plant

A plant that can assimilate and fix the free nitrogen of the atmosphere with the aid of bacteria living in root nodules.

node

1. The region of the stem where one or more leaves are attached. See also internode

2. Part of a dendrogram (diagrammatic tree). The nodes are the horizontal lines linking classes of equal rank. The internodes are the vertical lines linking each class above and below it in rank.

nodule

Nitrogen-fixing root swelling of characteristic shape and size for particular leguminous species that contain rhizobia. If the rhizobial strain is effective, atmospheric nitrogen can be fixed and is readily utilizable by the plant.

nomadism

The practice of living by moving from place to place. See also pastoralism, transhumance

non-additivity

Leading to an expressed interaction. See also additivity

non-native

See exotic

non-parametric method

Methods of statistical analysis that does not rely on assumptions about the form of distribution of the analysed variables. Usually they are based on the ranks of the observations rather than their actual values. Also known as 'distribution-free method'.

non-porous wood

Wood devoid of pores or vessels. Featured by all conifers and a few hardwood species. See also porous wood

normal-aged forest

A forest composed of a series of age classes in proportions that permit sustained yield by felling under an appropriate system.

normalized data

The coordinates of a data point, or the elements of a vector, rescaled so that their squares sum to unity.

notching

Cutting a small wedge from the stem above a bud so as to promote its growth by preventing the flow of inhibitory hormones (indole acetic acid) from the stem apex. See also nicking

no-tillage

Method of planting crops that involves no seedbed preparation other than opening small slits in the soil so that seed can be placed at the intended depth. There is generally no cultivation during crop production, but chemicals are often used for weed control. Also called zero tillage. See also minimum tillage

numerical model

A model in which the governing equations are solved by means of step-by-step numerical calculations, generally necessitating the use of a computer.

nurse crop

See companion crop

nursery

See plant nursery

nursery stock

Shrub or tree species grown in a plant nursery for planting out elsewhere. See also stock plants